DDWFTTW - Directly Downwind Faster Than The Wind

[Rick Willoughby]

There has been suggestions to dedicate a thread to this interesting concept for boat or land vehicle propulsion.

Jack Goodman created quite a stir with the demonstration of a small cart achieving the objective:
http://www.youtube.com/watch?v=aJpdWHFqHm0

Goodman published the attached article which explains the vehicles operation quite well along with some actual test data from a controlled test environment.

Rick W

[Rick Willoughby]

Spork and JB made significant contribution to the general understanding of the concept by reproducing different versions of the Goodman cart and demonstrate the principle independently. They compiled a video for the Mythbusters challenge:
http://www.youtube.com/watch?v=xHsXcHoJu-A

Rick W

[Rick Willoughby]

Mark Drela has provided the theory of operation of a DDWFTTW boat using an air propeller and a water turbine. In the attached papers he provides the necessary conditions on efficiency and drag to achieve the objective.

Rick W

[Rick Willoughby]

I have attached 4 slides that provide the basic principle of operation of a DDWFTTW cart for those who struggle with the possibility of it actually being done.

The numbers provided are consistent with the requirements for a large scale radio controlled vehicle having a total mass of say 20kg. The propeller required to achieve the nominated efficiency is quite large, around 4m diameter for the given windspeed, even on this size vehicle.

I hope this helps those who struggle with the concept.

Rick W

[tom28571]

Rick,

Very interesting experiments going on here but I have to be skeptical until one of these things runs into me. It does appear to violate some basic laws.

A question about the trial video of the cart. When the cart is running at a steady pace in wind, what happens if the wind suddenly drops to zero? The claims here seems to say that the cart would continue to run forward continuously. That can't happen. Sometimes we don't really see what we are looking at. If the total power of driving the car forward is derived from the propeller and the total power on the propeller is derived from the following wind, where does the power come from if the cart is running faster than the following wind?

If we assume that the cart is running faster than the following wind then the propeller sees a headwind on the propeller. Such a headwind will tend to drive the propeller backwards and slow the cart down.

We all want to believe in perpetual motion but I just don't see the logic of this argument as yet. We are promised wormholes, warp drive and time travel, but I am certain that I will never see any of them. I can readily believe that there can be momentary speeds faster than the wind downwind but just can't accept the argument of such sustained speed, as yet. I stick in the "as yet" to cover my a** in the case that Newton and other discoverers of basic laws were wrong. In this event, the laws of conservation of energy (energy out cannot exceed energy in) and the law of equal and opposite reaction.

[yipster]

only see this now and must admit its even harder to grasp than straight upwind
i'll have it sink in first a bit more before gettin more into blades than foils

[tom28571]

only see this now and must admit its even harder to grasp than straight upwind
i'll have it sink in first a bit more before gettin more into blades than foils

yipman, Had any more thoughts lately on our old option one?

Straight upwind driven by propeller in water or wheels on land is not a problem for me because the energy of the wind is never zero in that case. How fast you can go is limited only by the different forms of drag and the efficiency of the power train. Going faster than the windspeed upwind must be difficult but not theoretically impossible.

[yipster]

Ricks well meant illustarations got me confused and sorry Rick
and oh god let me learn how to keep my big mouth shot before i know what i'm talking about
sailing faster tahn wind has been discussed here a couple of times before and has nothing, well nothing, to do with sails vs windmills
its this simple http://socrates.berkeley.edu/~wprinz/Images/Down3.gif

[TeddyDiver]

I believe the main obstruction in our minds against this "phenomen" is what seems to be the mysterious source of the energy. It's a lot easier to comprehend when we'll try to see all movement's of the elements around with the eyes of the passanger, not as a by stander..
After all, everything needed are the different forces btw 2 elements and the vehicle capable to take some advantage of the force vectors involved...

[Jimbo1490]

The concept that made it clear to me is that the propeller can be a source of 'simple' drag while SIMULTANEOUSLY spinning and producing thrust. This SEEMS impossible, but is what actually happens. So there's no mystery source of additional energy. It's that the cart continues to extract energy from the wind from the drag on the propeller and the clever gearing causes that extracted energy to be converted to thrust, even at speeds greater than the wind. That the cart does not continue to accelerate to light speed shows that an equillibrium is reached (albeit not the one many of us expected), and thus no laws of physics are viloated.

Jimbo

[Rick Willoughby]

Rick,

Very interesting experiments going on here but I have to be skeptical until one of these things runs into me. It does appear to violate some basic laws.

A question about the trial video of the cart. When the cart is running at a steady pace in wind, what happens if the wind suddenly drops to zero? The claims here seems to say that the cart would continue to run forward continuously. That can't happen. Sometimes we don't really see what we are looking at. If the total power of driving the car forward is derived from the propeller and the total power on the propeller is derived from the following wind, where does the power come from if the cart is running faster than the following wind?

If we assume that the cart is running faster than the following wind then the propeller sees a headwind on the propeller. Such a headwind will tend to drive the propeller backwards and slow the cart down.

We all want to believe in perpetual motion but I just don't see the logic of this argument as yet. We are promised wormholes, warp drive and time travel, but I am certain that I will never see any of them. I can readily believe that there can be momentary speeds faster than the wind downwind but just can't accept the argument of such sustained speed, as yet. I stick in the "as yet" to cover my a** in the case that Newton and other discoverers of basic laws were wrong. In this event, the laws of conservation of energy (energy out cannot exceed energy in) and the law of equal and opposite reaction.

As soon as the cart starts to move, the wheels turn in forward motion and begin powering the prop. So it is always drawing air backwards through the vehicle. This is akin to a propeller on an aircraft on the tarmac with a tail wind or a boat at a dock with a current from astern, in all these situations the propeller is able to reverse the direction of airflow/waterflow in the local field.

The propeller will turn in the forward direction because the gearing enables the wheels to overcome the prop torque. If the gear RATIO was reversed then wind blades (turbine) would drive the wheels and the vehicle would move into the wind.

I am sorry to disappoint you but If the wind stops, the vehicle stops. There must be relative motion between the wind and ground to get the necessary velocity difference that makes up for the losses in the vehicle. No wind, no go.

I am an engineer, I have no desire to believe in perpetual motion because I know it cannot be done. This vehicle is simply a clever use of propellers.

Rick W

[Rick Willoughby]

Ricks well meant illustarations got me confused and sorry Rick
and oh god let me learn how to keep my big mouth shot before i know what i'm talking about
sailing faster tahn wind has been discussed here a couple of times before and has nothing, well nothing, to do with sails vs windmills
its this simple http://socrates.berkeley.edu/~wprinz/Images/Down3.gif

You are right about it having nothing to do with sails v windmills. We are not talking about a windmill here (a power extracting device more accurately called a wind turbine). We are talking about a simple propeller driven by the wheels of the cart.

The difference with this vehicle is that it can exceed windspeed DIRECTLY downwind rather than having to tack. The propeller simply takes a little power from the wheels (at low force and high velocity) to push the veicle a bit faster than the wind (at higher force and lower velocity).

One of the first steps in understanding is to know we are talking about a propeller not a turbine. If the gear RATIO was reverse then there is a turbine driving the wheels and the vehicle would go directly upwind.

Rick W

[tom28571]

As soon as the cart starts to move, the wheels turn in forward motion and begin powering the prop. OK, Rick, I’m dense here. What made the cart start to move? I thought you had the prop acting to drive the wheels in the first place. If that is the case, then the prop is turned by the wind like a windmill, the energy from the prop goes to the wheels by gearing and the force from the turning wheels then makes the prop turn and that makes the wheels turn faster and then-------. I hope you see my problem here. Are you saying that the prop is not acting like a windmill to drive the wheels in a forward direction?? If so, what does start the cart rolling? So it is always drawing air backwards through the vehicle. This is akin to a propeller on an aircraft on the tarmac with a tail wind or a boat at a dock with a current from astern, in all these situations the propeller is able to reverse the direction of airflow/waterflow in the local field. This analogy escapes me too. I must be slow today.

The propeller will turn in the forward direction because the gearing enables the wheels to overcome the prop torque. If the gear RATIO was reversed then wind blades (turbine) would drive the wheels and the vehicle would move into the wind.

I am sorry to disappoint you but If the wind stops, the vehicle stops. There must be relative motion between the wind and ground to get the necessary velocity difference that makes up for the losses in the vehicle. No wind, no go. I’m not disappointed, with this I agree completely.

I am an engineer, I have no desire to believe in perpetual motion because I know it cannot be done. This vehicle is simply a clever use of propellers. I am also an engineer (not mechanical) and relate all such experiments to the basic laws drummed into me in freshman physics. If it really works, it’s more than clever. If we look at the proposed steady state with the cart running faster than the wind, where is the energy coming from to keep it doing so? In this case, the only energy source I can find is the kinetic energy of the cart and that will soon run out and the cart will slow down to an equilibrium speed where the driving force from the tail wind balances the losses from drag and rolling resistance.

Rick W

All in the venture to learn, not to agitate anyone.

[Rick Willoughby]

The energy comes from the air movement relative to the ground. There is nothing fantastic about it. It does not disobey any fundamentals of physics.

If you understand power is the product of force and velocity you have all you need to know.

The prop generates a relatively large force at low velocity relative to the air. The thrust it generates is enough to overcome the rolling friction and wind drag on the vehicle plus the force on the wheels to drive the propeller.

The wheels are collecting power at low force but high velocity because the wind is already providing some of the forward component.

The vehicle is moving quite fast relative to the ground but only just making forward progress through the air. The best practical system would get about twice wind speed with a cart. This is still well behind what is possible with a land yacht only it must tack.

I have difficulty how readily people say it is possible to exceed windspeed downwind tacking without breaking fundamental principles but then they see this as perpetual motion. Simply shows they do not take the time to think it through or do not really understand basic physics.

Not being antagonistic here just that most people jump in without really thinking about it. Go through my 4 slides and understand what they demonstrate. There is no trick.

For the propeller; Pp = large FORCE x small velocity
For the wheels; Pw = small force x big VELOCITY

Pp will be less than Pw because there are transfer losses to overcome.

The power input to the vehicle is Force on the propeller times the windspeed. (Note that the prop is producing a reverse stream that reacts against the wind) Think of a plane with an airspeed of 100kts in 50kts of wind. The ground speed is 150kts. But the plane has not needed to do any more work to achieve this extra speed. The wind is doing the extra work.

The power dissipated in the vehicle is the air drag on the vehicle, the rolling drag on the wheels and the conversion losses of power from the wheels transferred to the prop.

These little demos show how the gearing works with wheels being used to drive against the forcing media. In both cases the vehicle moves faster than the solid media being used to propel them.
http://www.youtube.com/watch?v=ZVSjA7Rhccs
http://www.youtube.com/watch?v=k-trDF8Yldc

The only difference with the propeller driven vehicle is that the propeller is reacting against a fluid medium not a solid medium. If you deny this possibility then you are denying the possibility of boats and planes ever working with propellers.

Rick W

[Rick Willoughby]

This video was done to show the cart self starts. You will note that the prop turns in the forward direction as soon as the vehicle begins to move:
http://www.youtube.com/watch?v=QTAd891IpRs&feature=related
The gearing is such that the wheels develop more torque than the prop. Just a matter of having the pitch of the prop, mechanical gear ratio and wheel diameter set appropriately so this can happen. If the ratio was the other way the blades become a turbine and it climbs into the wind.

The cart goes upwind or downwind simply by changing the gear ratio. Albeit the optimum configuration of vehicle for either circumstances is vastly different.

Rick W

[tom28571]

Quote: "The propeller will turn in the forward direction because the gearing enables the wheels to overcome the prop torque. If the gear RATIO was reversed then wind blades (turbine) would drive the wheels and the vehicle would move into the wind."

I did not read this as meaning that the propeller was forced by the wheel torque to turn such as to produce thrust toward aft. Obviously this makes a lot of difference to all the arguments. I assumed that it turned as a windmill blade would. Bad assumption. In my case, I wish this had been made more clear since I did not catch the true intent and led myself astray.

[Rick Willoughby]

Quote: "The propeller will turn in the forward direction because the gearing enables the wheels to overcome the prop torque. If the gear RATIO was reversed then wind blades (turbine) would drive the wheels and the vehicle would move into the wind."

I did not read this as meaning that the propeller was forced by the wheel torque to turn such as to produce thrust toward aft. Obviously this makes a lot of difference to all the arguments. I assumed that it turned as a windmill blade would. Bad assumption. In my case, I wish this had been made more clear since I did not catch the true intent and led myself astray.

I do not know how it could be made more clear. All of the first four posts discuss the operation of a propeller. Mark Drela talks about a turbine but only in relation to a boat where it would be used to extract power from the water to power the air prop.

None of the four slides I prepared mention a turbine. All discuss the operation of a propeller.

Like I say most people jump in with a preconception of how it works and get it badly wrong. At least, Tom, you are one of the few who quickly recognised your error in their understanding and can appreciate there is no magic involved. Others have taken months of argument to show a glimmer of understanding but remain sceptical. So much so that I just give up because you need basic maths and physics knowledge to work it out and some lack this ability as is apparent on the three other threads that discuss this means of propulsion.

Rick W

[Fanie]

Very neat Rick. If you think about it it is almost logical. The key is in the prop being able to turn faster than the wind's speed.

We have played with wind power gen blades some time ago, and a prop turned by the wind 'flies' in the wind and exceeds the wind speed, hence the noise these things make and their ban for using close to domestic areas.

In this case the aparent wind enhances the power available to the prop and packs enough energy to overcome the mechanical drag, taking into consideration there is almost zero wind resistance from ahead.

What we have to do now is to convert this principle to the flying saucer by overcoming gravity using momentum. Aparently it has been done too and I for one is looking foreward to it. Sick of driving in trafic :D

Yipster -
only see this now and must admit its even harder to grasp than straight upwind
I think it is very doable, I'm currently getting things together to sail directly upwind, no tacking.

[tom28571]

In my defense Rick I will say that all of the wind machines seen in the past have worked the way I assumed this one does. The sailboats driven by a water propeller and powered by a wind propeller (more accurately called a turbine by you and a windmill by me) work in the way I assumed the cart does. Here is how it became clear to me:

There is a 1:1 coupling between the wheels and the road surface.

The wheel torque drives the propeller through gearing that provides thrust in the aft direction.

Propeller coupling to the air is weak and so the wheels are able to turn it in the opposite direction to that the wind would if the propeller were free on the shaft.

Your definition of propeller and turbine is not a clear one. Propellers propel but turbines can either drive or be driven. At least that is the way industry uses the term.

I'm sure all the doubters are a trial to you and it is difficult to be patient. Perhaps many are stuck on the same assumptions that I was. Experience can be a good thing but it can also mislead, I have a lot of experience, 77+ years, much of it in piddly useless things but do enjoy looking at puzzles like you brought here. Thanks.

[yipster]

thanks all i will read again to make sure couse this one kept me awake

[Rick Willoughby]

In my defense Rick I will say that all of the wind machines seen in the past have worked the way I assumed this one does. The sailboats driven by a water propeller and powered by a wind propeller (more accurately called a turbine by you and a windmill by me) work in the way I assumed the cart does. Here is how it became clear to me:

There is a 1:1 coupling between the wheels and the road surface.

The wheel torque drives the propeller through gearing that provides thrust in the aft direction.

Propeller coupling to the air is weak and so the wheels are able to turn it in the opposite direction to that the wind would if the propeller were free on the shaft.

Your definition of propeller and turbine is not a clear one. Propellers propel but turbines can either drive or be driven. At least that is the way industry uses the term.

I'm sure all the doubters are a trial to you and it is difficult to be patient. Perhaps many are stuck on the same assumptions that I was. Experience can be a good thing but it can also mislead, I have a lot of experience, 77+ years, much of it in piddly useless things but do enjoy looking at puzzles like you brought here. Thanks.

Tom
A few points.

The coupling between the propeller and air is not necessarily weak. The important element is the gear RATIO. You want the effective pitch on the propeller to be larger than the mechanical ratio combined with the wheel diameter. Typically the prop pitch is twice the distance the vehicle would move in one rev of the prop. This ensure the wheels have the meachanical advantage. If the advantage was the other way then you have a wind turbine driving the wheels and the vehicle goes up wind.

The propeller and turbine distinction is important. Turbines might be used to propel something but not directly. All turbines extract power from a moving medium in the first place so the distinction I make is correct. It was something Windmaster pointed out a while ago on another thread and is the only way to make the correct distinction. You could talk about the air twisted blades that propel or the air twisted blades that extract power but this is cumbersome terminology. The turbine is not a windmill because there is no grinding (milling) being done unless some gears get mashed of course. So using the word windmill is poor terminology and takes us back a couple of hundred years when such things actually existed.

The reason I started this thread was because there are others that discuss the same topic but this one is dedicated to it. I understand the idea has been around since the 70s at least but it was Goodman's YouTube video that started the debate. There are many other forums that discuss this topic. Most people can work out, and know of, the upwind case so almost nearly all make the same assumption you did without reading what has been stated. I spent about 6 hours trying to work it out before I got it. It was Goodman's paper and the demo on the exercise machine that Goodman did that provided the key to understanding. I then did the analysis to prove it. It would be very difficult to achieve on a small boat but not impossible.

Rick W

[yipster]

last night i woke up thinking apperant wind wasnt the answer eighter
but reading early morning posts and back i got to understand it
so bicycles in the wind wont have to be pedalled anymore someday
clever ingeneering indeed and thanks for bringing it to our attention
sorry for misreading the post, pdf's and your drawings yesterday

[Rick Willoughby]

.........
so bicycles in the wind wont have to be pedalled anymore someday
........

Not quite bicycles, most are tricycles, but wind powered road vehicles are getting serious attention right in your backyard:
http://www.windenergyevents.com/Picture%20gallery.html
Very slow link and is said to be a security risk!

This link is much faster and shows the winner and runner up:
http://www.symscape.com/blog/wind-turbine-powered-tricycle


The problem is that DDWFTTW is not really compatible with the turbine powered case because the turbine and prop operate in different regimes. For the sake of going a bit slower downwind using the turbine rather than a prop the complexity is dramatically reduced.

For a single person boat the propeller needs to be around 12m diameter. For a land cart to go DDWFTTW you need at least 3m propeller and then some decent wind and very low friction vehicle and efficient transmission.

Rick W

[Fanie]

When the prop starts turning, all the power is towards the center of the blade which is wider than the outside, using force to get movement going, then as it spins up, the power area is moved towards the blade tips which exceeds the wind speed. As Rick mentioned, the coupling between the propeller and air is not weak, and increases as the prop spins up and the blade's power area moves outward.

The outward shift of the blade's power area also increases the leverage on it's rotation shaft, hence can produce enough torque to spin up more, but there are limitations.

Not continuous motion, you still need wind power. Would have been nice to just spin the prop up and it goes faster and faster :D

[tom28571]

The first problem I had was that none of the proponents described the relationship between the wheels and propeller thrust direction well enough for most people, including me, to get what was driving what. Thinking that these guys are just pulling your leg by cheating gets us no where. Once Rick got me on the right track about the propeller, it looks OK.

To me, it's the pressure differential produced behind the cart by the air thrust from the propeller and the following wind that provides the energy input to keep it going. No following wind, no pressure differential and it slows down until it stops.

When the wind starts, it tries to turn the propeller backwards but can't because the prop is locked to the wheels which require more torque than the prop can supply at the selected gear ratio. Raise the wind speed enough and it overcomes rolling resistance and turns the wheels, which forces the propeller to turn in a direction to generate thrust backwards which makes the cart speed up, and so on, and on until equilibrium is reached, which is not limited by the ambient wind speed. Speed is limited by the force of the aft pressure differential balancing the drag of the cart plus energy required to drive the propeller. This may or may not be faster than the ambient wind.

Change the gear ratio so the wind can turn the prop before the rolling resistance is overcome to turn the wheels and it will go up wind.

That is, unless I'm wrong. Again.

[Rick Willoughby]

Tom
I believe you have provided an accurate description of operation.

I do not know a simpler way of proving the feasibility than I have in the four slides I produced. Even demonstrations are a waste of time unless the skeptic actually builds a vehicle and tries it. Many have now done this so the weight of numbers is favouring science over blind skepticism. You can get some idea of the changing tide by going through this thread:
http://www.boatdesign.net/forums/boat-design/wind-powered-sail-less-boat-24669.html
It will give you an idea of the difficulty many have in understanding it.

The videos that Goodman and others produce do not convince many. If you look through the YouTube comments on the videos you get an appreciation of how difficult it is for most to grasp. It was the clear logic in Goodman's article that gave me understanding. The fact that the article was in an ARYS publication was incentive to take it seriously and work through it.

Rick W

[tom28571]

Rick,

You are most patient. I has to be difficult to get past all of us skeptics, especially the ones who look for tricks and cheats. Thanks to you and others who stick with it some of us doubters can come on board.

That said, I will remain a skeptic on things that look too good to be true because there are Amish out there selling magic house heaters to gullible people. I keep thinking that someone will stop these slickers.

[robherc]

Rick,

I've been thinking about it a LOT recently (wish you guys hadn't gotten me started on this in the middle of a build...my boat'll NEVER get done at this rate...lol), and it seems to me that it just MIGHT be feasible to create a cart that could go both DUW & DDWFTTW with the same symmetrical-foil bladed prop/turbine and a good transmission. I know you'd lose some efficiency in the air-screw by going with a symmetrical section, instead of one optimized for either a turbine or a propeller, but if you got it spinning fast enough, it would seem to me that you could dramatically reduce the size of the prop. needed for the DDWFTTW cart, maybe you'd have to duct it though, but I think it might be able to work. What do you think?

[Rick Willoughby]

You can do it with cart. I am not sure if the best result would come from a symmetric foil or one optimised for downwind. The downwind part is the most challenging because the prop is not operating in the ideal regime. It has low Re# and low velocity through the prop disc until the cart builds speed into the wind.

To do what you want you need a gearbox with variable ratio to get it through the low apparent wind condition when you lose the thrust from tail wind on the vehicle. At this point the propeller has to take over and provide all the thrust while operating in unfavourable conditions.

I have reliable models for the boat case but I have not bothered modelling the cart. Goodman has already stated that he could get his DDWFTTW cart to go upwind with the downwind prop just by changing the gear ratio.

You do get advantages with scale. So a manned vehicle would be better than Goodman's cart. I expect it will not be too far off before we see a manned cart operating DDWFTTW. Spork and JB are working on something. They may have to push their cart to get it going because they are aiming for impressive performance from a single gear ratio. It may get stuck in getting going because it could be overgeared. Simple to alter the ratio but then you derate the top end performance.

The question really is - why bother. What practical purpose does such a vehicle serve?

Rick W

[robherc]

lol, I dunno....but now you've gotten me thinking....instead of finishing my boat. So, that's what I thought up so far....trying to figure out where you'd be able to get peak efficiency at all points combined....fo no better reason than to figure it out, since I'm not really interested in sailing a wind-turbine anywise. Although, a DDWFTTW cart big enough to ride on might be good fun in a strong gale!

[Rick Willoughby]

rob
To give you an idea of what the challenge is I looked at a NACA0010 profile on a bicycle type vehicle weighing 30kg plus 75kg rider. I assumed drag equivalent to an upright bike. This is probably optimistic given the size of the prop.

This vehicle is intended to really perform. The design objective was to do 10m/s (say 22mph) in 5m/s wind (say 10kts). This wind should be reasonably easy to come across. The vehicle drag at this speed is 17N when apparent wind is 5m/s.

The check condition is zero apparent wind and vehicle moving at 5m/s. The vehicle drag is 6N.

The prop needs to 5m (16ft) in diameter and 3-bladed with 190mm chord (almost 8 inches) operating at 100rpm. (Will need to be made from CF to keep the weight down of course). Efficiency at design condition is 76%.

The design point requires a prop input power of 390W. This will be a bit over 400W required at the wheels allowing for transmission losses. So extra drag with a bit of wheel slip is say 42W to drive the prop when vehicle is at 10m/s. Total drag is now 59W. The prop will generate thrust of 60N in 5m/s wind with 400W. So it does the job.

When the vehicle is doing 5m/s the apparent wind is zero and prop is doing 50rpm if the gearing is fixed. The prop power absorbed is 96W - say 100 at the wheels. This will add about 21N extra drag at the wheels so total drag is 27N. The prop will be producing 37N so the vehicle will be accelerating - no problem here.

If you were to use an asymmetrical blade - eg NACA16-610. The diameter can be reduced to 3.5m. Prop power reduces to 310W running at 110rpm. Still 3-bladed but with slightly larger chord. It also gets through the zero apparent wind condition. This prop would probably be OK for going upwind as well and would automatically depower because it would stall early if it was loaded up when working as a turbine.

So for a duel function prop/turbine I think something like a 3.5m asymmetric prop/turbine would work OK.

Rick W

[Fanie]

The question really is - why bother. What practical purpose does such a vehicle serve?

Perspective.

The seeming impossible has been overcome, how ? - perspective. Opens the next small door to the next invention that can be usefull in a sensable way.

Would the same apply when you are floating down a current using hydrodynamics ?

[robherc]

Would the same apply when you are floating down a current using hydrodynamics ?

Assuming you're floating down a current, and you have an apparent wind, then yes, you could use the same principles to go DDCFTTC. Assuming, of course, that you can achieve the necessary efficiency.
At that point, you'd be using an air turbine, and a water prop. ... and it's actually the exact same thing as a current boat going DUW, just from a different frame of reference. (Imagine there is no current, but there is wind; or there is no wind but there is current...same idea, just looking at it from a diff. perspective)

[robherc]

Thanks Rick, I'm gonna stew on that for a little while before I respond...very useful (for solving this problem at least) information.

[Rick Willoughby]

Would the same apply when you are floating down a current using hydrodynamics ?

The relativity is reversible however most currents would be too slow. A bit like operating the cart on a treadmill.

Rick W

[Fanie]

That and the differential in speed between the vessel and the liquid. Liquid is not compressable, and you won't be able to use it as the wind.

[Rick Willoughby]

That and the differential in speed between the vessel and the liquid. Liquid is not compressable, and you won't be able to use it as the wind.

Under the conditions required to achieve DDWFTTW the air is not compressible either. It is operating at very low pressure differential. The pressure difference to achieve 60N of thrust from a 5m or even 3.5m diameter disc is minute.

Rick W

[robherc]

Fanie, if you're trying to go downstream faster than the stream, just go directly upwind once & think about the relativity of it:
When you're going directly upwind in a turbine (windmill) boat, you're actually already going downstream faster than the water flow, relative to the wind. So there's really no difference at all there. ;)

[Rick Willoughby]

I thought I should post this link here to give Goodman proper recognition for the treadmill demonstration. It was this video that enabled me to grasp the physics behind DDWFTTW:
http://www.ayrs.org/MOV05703.mpg

The fact that it is on the ayrs web site lends credence to its scientific merit so encouraged me to spend time to understand it.

This video was done well before any of the cart on treadmill videos appeared on YouTube.

Rick W

[eponodyne]

Let's hope this gets through to those frenetic dickheads on Sailing Anarchy.

[Rick Willoughby]

To add to the reference sources I have attached a couple of scientific papers that spork posted on another thread on this topic. You will see the physics involved was well documented 40 years ago although these two papers concentrate on the upwind boat. Credit to Dr Bauer for his efforts.

Rick W

[spork]

Let's hope this gets through to those frenetic dickheads on Sailing Anarchy.

I think there's only one dickhead left on S/A that has a problem. It took years, and cost lives, but it seems everyone there has now been convinced, or at least wandered off shaking their heads.



Credit to Dr Bauer for his efforts.

Indeed. I wish though that I could find the paper that inspired Bauer to build his cart

[sailor2]

To add to the reference sources I have attached a couple of scientific papers that spork posted on another thread on this topic. You will see the physics involved was well documented 40 years ago although these two papers concentrate on the upwind boat. Credit to Dr Bauer for his efforts.

Rick W
In the second pdf on your post:
And in page 112 on Bauers paper in formulas (A-2) & (A-3)
Assumed air- & water- speeds related to blades in air & water respectively, is calculated ignoring any induced velocities at the actuator disk in any direction.
This assumption doesn't seem to hold water with any realistic swepted areas for the rotating part in air. This causes a major overestimation of the gained speeds for the vessel compared to the traditional boat.
Trying to use lower L/D to compensate this is not easy task at all and don't think it as a good idea anyway.
If one interprates the results on page 115 with water- & air- speeds related to swept areas rather than as those related to real wind & boatspeed related to water, the graphs don't necessarily extend to ddwfttw at all in my opinion.

That of course does't mean acheaving ddwfttw would be impossible on the water, quite the contrary, just not so easily as those papers estimate.

[Rick Willoughby]

In the second pdf on your post:
And in page 112 on Bauers paper in formulas (A-2) & (A-3)
Assumed air- & water- speeds related to blades in air & water respectively, is calculated ignoring any induced velocities at the actuator disk in any direction.
This assumption doesn't seem to hold water with any realistic swepted areas for the rotating part in air. This causes a major overestimation of the gained speeds for the vessel compared to the traditional boat.
Trying to use lower L/D to compensate this is not easy task at all and don't think it as a good idea anyway.
If one interprates the results on page 115 with water- & air- speeds related to swept areas rather than as those related to real wind & boatspeed related to water, the graphs don't necessarily extend to ddwfttw at all in my opinion.

That of course does't mean acheaving ddwfttw would be impossible on the water, quite the contrary, just not so easily as those papers estimate.

The swept areas I get are larger than Bauer calculates but then he makes simplifying assumptions that he notes. Remember when this study was done the availability of computers was limited and engineers of his vintage were usually not trained in their use at that time. I did not get my first programmable calculator until I started work and I am a generation after Bauer.

I believe his analysis at least highlights the similarities between props/turbines and sails.

Rick W

[sailor2]

The swept areas I get are larger than Bauer calculates but then he makes simplifying assumptions that he notes. Remember when this study was done the availability of computers was limited and engineers of his vintage were usually not trained in their use at that time. I did not get my first programmable calculator until I started work and I am a generation after Bauer.

I believe his analysis at least highlights the similarities between props/turbines and sails.

Rick W
I have downloaded that pdf years ago, and certainly haven't find any indication on Bauers assumed swept areas. How can you compare yours to something that isn't defined at all ? If you think it is, please state the page number and quote a few lines in the neighborhood so I can find it.
I think this is the biggest simplification by far and is not notified at all as far as I can see. I sure hope I'm wrong in this one, but can't see how.

Sure, no computers back then, but momentum theory is much older than Bauers' papers on this subject. So involving swept areas & at least axial induced velocities from momentum theory should have been there as vital part of the analyses. It can easily be done by hand calc at least for one case, if not for all speeds & gear ratios & ...

I believe his analysis at least highlights the similarities between props/turbines and sails. Agreed. But leaves out most of the differencies.
Like in a normal sailboat whole appendage has same aoa, while in a turbine what do you do if it converts too much shaftpower from linear forces & flow ?
If you reduce pitch, the blade-element near hub goes quickly aoa=0 in soon after becomes an inefficient prop if pitch is changed anymore. Having blade_elements at tip on different aoa than those near hub reduces efficiency.
If you dimension turbine for max speed case so this doesn't happen, the turbine tips will stall easily at lower boat speeds by overloading. This kills efficiency very suddenly. Stalling blades near hub like props do has much less effect on efficiency. If avoided by suitable gear ratio, power level is way below the optimum as required for max excess thrust according the ddw2.pdf Drela posted and my test runs with it.
I guess one could use more than one turbine and put them on drive legs like outboards and lift one of them completely off water once max speed with 2 of them has been accheaved. This would cut too much power off, so over all power level could increase at more suitable rate with boatspeed instead as a third power of Vb.

[Rick Willoughby]

The figures at the back of the papers provide the areas. He compares them with sail area actually used on the boat he based the calculations on. Figure 7 gives the DDWFTTW case. You should look more closely - like all engineering papers of the time the drawings/figures were provided at the back of the report. Took me a second read to realise he actually did calculations for the DDWFTTW. I can do better than reference it I have attached the figure. If you actually read the entire paper you will find it.

My initial impression of the DDWFTTW principle was that it would need a continuously variable transmission to get the best result. This may be true but for any given windspeed I find the cart, at least, has no problem accelerating up to the set design condition. I have not looked at all possibilities but seems to work fine with fixed pitch and fixed gearing. One thing that can be overlooked is that the prop speed is linked to vehicle speed. The angle of attack is higher at low speed than at the design condition but if the prop is big enough to achieve DDWFTTW then it will have ample thrust to accelerate up to this speed. There is no dead-spot in the range.

Likewise with the boat the turbine and prop are linked so their operating regimes at any speed are similar - the prop is never in a really good regime for a boat of pleasure craft proportions and even worse for a model to demonstrate it. A manned boat needs a very large prop even for a slender hull like a rowing scull. Admittedly my design speed is a higher ratio of windspeed than Bauer has used but he is discussing a 2.5t boat of only moderate length.

Rick W

[sailor2]

The figures at the back of the papers provide the areas. He compares them with sail area actually used on the boat he based the calculations on. Figure 7 gives the DDWFTTW case.
Rick W
Sorry you are very wrong. The areas in that figure are blade areas not swept areas. He uses that definition in just about every page.
Please read it again and see for youself, he is very clear on this issue, no place for misunderstandings.
A headline taken from page 110 3.5 Windmill Performance at heading of 180 degrees. Third sentence below heading reads: Such angles could be used at gamma=180degs, but this would result in much larger blade areas than given in figure 7.
Also see page 112 equation (A-15)
That's a standard equ. relating Cl & lift_force using Blade area, flow speed, density just like for a solid wing used on aeroplane. Nothing to do with swept areas.

[Rick Willoughby]

Sorry you are very wrong. The areas in that figure are blade areas not swept areas. He uses that definition in just about every page.
Please read it again and see for youself, he is very clear on this issue, no place for misunderstandings.
A headline taken from page 110 Third sentence below heading reads:
Also see page 112 equation (A-15)
That's a standard equ. relating Cl & lift_force using Blade area, flow speed, density just like for a solid wing used on aeroplane. Nothing to do with swept areas.

I see. This means his figures are now at least closer to what I would expect. Glad you cleared that up. I still think the areas are small for such a heavy boat exceeding the hull speed.

I have a swept area of 201sq.m and a blade area of 3.2sq.m for a single person craft 7.2m long displacing 100kg. These are high aspect blades with average L/D well over 20 in most conditions.

You could work out the swept area for a given foil section by taking the lift to drag ratio that he provides. There will be differences based on number of blades of course but at least you could determine if he is in the ball park. I personally think it would be exceedingly difficult to get his Gimcrack above windspeed of 11kts in DDWFTTW mode but I have not done any calculation on this.

Rick W

[Rick Willoughby]

If you take the DDWFTTW case shown in figure 7 Bauer determines a blade area of 100sq.ft with the L/D of 12 to do 12kts.

A 10% cambered foil to achieve this at low Re# needs to have a blade aspect up around 4. So prop radius is determined to be 20ft. Swept area is hence a little over 1200sq.ft.

I have grave doubts that a prop of these proportions would drive that heavy boat with a far field velocity of 1kt.

Rick W

[sailor2]

If you take the DDWFTTW case shown in figure 7 Bauer determines a blade area of 100sq.ft with the L/D of 12 to do 12kts.

A 10% cambered foil to achieve this at low Re# needs to have a blade aspect up around 4. So prop radius is determined to be 20ft. Swept area is hence a little over 1200sq.ft.

I have grave doubts that a prop of these proportions would drive that heavy boat with a far field velocity of 1kt.

Rick W
I'm afread I have to disagree with you again.
A 2D l/d from xfoil or similar is one thing and 3D L/D is another with a different meaning. When calculating 3D from2D case you need distribution of lift in addition of span loading. To get your values with blade aspect of 4 your assumed lift distributions are not realistic at all. Do you realise that lift is dependent on flow speed squared and at halfway between tip & root there is just 1/2 of the tangential speed and approximately 1/4 of dynamic pressure if axial flow is neglible compared to tangential. To get same lift there you need 4 times greater chord*Cl halve way in from tip than near tip. That is possible with this very simplified theory, but further closer to axis this approach falls apart. If you would use a more sophisticated theory involving both tangential & axial induced flow in addition of the axial induced flow things get worse, not better. I would be very surprased to have any better than even L/D=6 with aspect of 4 for individual blade if L & D are defined in relation to assumed flow direction (without induced velocities) at r/R = 0.7 as Bauer did in his analyses. If you define L & D related to local flow things are obviously a lot different, but you still overestimated the L/D. Keep in mind L is oriented more towards creating torque near hub than near tip.

[Rick Willoughby]

I'm afread I have to disagree with you again.
A 2D l/d from xfoil or similar is one thing and 3D L/D is another with a different meaning. When calculating 3D from2D case you need distribution of lift in addition of span loading. To get your values with blade aspect of 4 your assumed lift distributions are not realistic at all. Do you realise that lift is dependent on flow speed squared and at halfway between tip & root there is just 1/2 of the tangential speed and approximately 1/4 of dynamic pressure if axial flow is neglible compared to tangential. To get same lift there you need 4 times greater chord*Cl halve way in from tip than near tip. That is possible with this very simplified theory, but further closer to axis this approach falls apart. If you would use a more sophisticated theory involving both tangential & axial induced flow in addition of the axial induced flow things get worse, not better. I would be very surprased to have any better than even L/D=6 with aspect of 4 for individual blade if L & D are defined in relation to assumed flow direction (without induced velocities) at r/R = 0.7 as Bauer did in his analyses. If you define L & D related to local flow things are obviously a lot different, but you still overestimated the L/D. Keep in mind L is oriented more towards creating torque near hub than near tip.

You seem horribly bent in discrediting Bauer's work. I just accept it for what it is - a simplistic approach.

I do my own prop and turbine analysis that I know gives reliable results. I consider his numbers highly optimistic. If you go back through the myriad of pages on this topic on the various threads you will see discussions on the size of the prop and turbine required for specific applications.

If you want to build a boat that will work I can give you a design. I would not rely on Bauer's numbers for design purposes but I do give him credit for his early work on the science.

Rick W

[Rick Willoughby]

Don't be sorry or afraid to disagree with me. It sounds like a put down - intended or otherwise. I don't think you are sorry or afraid at all. Just say what you mean. If I am wrong it won't be the first time. I can remember I was once wrong but it rarely happens.

Rick W

[sailor2]

I would be very surprased to have any better than even L/D=6 with aspect of 4 for individual blade if L & D are defined in relation to assumed flow direction (without induced velocities) at r/R = 0.7 as Bauer did in his analyses. If you define L & D related to local flow things are obviously a lot different, but you still overestimated the L/D. Keep in mind L is oriented more towards creating torque near hub than near tip.
Also keep in mind the use of r/R = 0.7 to represent the all blade_elements is based on assumption that centre of pressure of prop disk is located there. This gives some idea of the lift distribution along span assumed in his analyses.
Also note that by simply using narrower chords & greater revs to get better AR nothing is gained in terms of L/D as span loading doesn't change at all. Only reducing disk loading will have positive effect on that. Using narrower blades might propably do the opposite if RE-numbers are brought down as well. So high blade AR by it self is no solution at all in improving things.

Ps. I'm not any prop- or aero- expert, just an amateour in the field unlike Drela or Speer. I have carefully analysed what is easily available in web on the subject (as widely interprated) with presumably good enough engineering skills thoug.
If you have doubts on any of this due to that, don't hesitate to ask from more knowlidgeable, I'm always glad to be proven wrong as that allows me to learn more. Seems like you share that approach.

[sailor2]

You seem horribly bent in discrediting Bauer's work. I just accept it for what it is - a simplistic approach.

I do my own prop and turbine analysis that I know gives reliable results. I consider his numbers highly optimistic.
Rick W
No I don't want to discredit his work, I just don't want it to be used for something it can't be used due to the error it had, resulting over-estimating how easy the ddwfttw on the water would be according to his analyses for even heavy cruising boats. Clearly a false result.

He did a fine job of being the first to try scientific analyses on prop & turbine propulsion and deserves all the credit for that. That doesn't change the fact it had one vital error in the first formulas used and everything else was based on that formula. I mean the ignoring induced velocities part. The other simplifications even brought together are much smaller inaccuracies compared to the ignoring induced velocities part.

If you want to call this discrediting be my guest, but that's not how I see this issue. I consider far more important to present accurate analyses that recognizes it's own inaccuracies. That's a mandatory requirement on scientific papers as far as I'm conserned. Someone failing to do that in doctors theses would have bad result of not becaming a doc at all for sure. It was my understanding those papers were intended for scientific audience and that they were used for that too. I don't know what the reaction from AIAA was after the presentation. Is my point of view clear now ?

[sailor2]

Don't be sorry or afraid to disagree with me. It sounds like a put down - intended or otherwise. I don't think you are sorry or afraid at all. Just say what you mean. If I am wrong it won't be the first time. I can remember I was once wrong but it rarely happens.
Rick W
Not being afraid at all, just trying to be polite as required on the rules of this forum. Not being a native english speaker might lead to over do that to avoid being accused of personal attacs leading to be thrown out of here by forum policy. That won't have any effect what I say, just how I say it. Have been lurking for years, but joined just yesterday after weaks of trying. So a newbie here, but certainly not a newbie on the substance.
:cool:

[Rick Willoughby]

No I don't want to discredit his work, I just don't want it to be used for something it can't be used due to the error it had, resulting over-estimating how easy the ddwfttw on the water would be according to his analyses for even heavy cruising boats. Clearly a false result.

He did a fine job of being the first to try scientific analyses on prop & turbine propulsion and deserves all the credit for that. That doesn't change the fact it had one vital error in the first formulas used and everything else was based on that formula. I mean the ignoring induced velocities part. The other simplifications even brought together are much smaller inaccuracies compared to the ignoring induced velocities part.

If you want to call this discrediting be my guest, but that's not how I see this issue. I consider far more important to present accurate analyses that recognizes it's own inaccuracies. That's a mandatory requirement on scientific papers as far as I'm conserned. Someone failing to do that in doctors theses would have bad result of not becaming a doc at all for sure. It was my understanding those papers were intended for scientific audience and that they were used for that too. I don't know what the reaction from AIAA was after the presentation. Is my point of view clear now ?

Bauer has simply made assumptions that he discusses. I did not bother too much with his analysis because I knew his assumptions would cause underestimations. I believe he aimed to prove a point that a turbine/prop boat can go directly to windward and DDWFTTW.

If his experience was with powered props he would not appreciate the high velocity ratio required to generate the thrust needed to propel such a large boat in very low far field wind speed. For the low power water props I design and build I neglect near field conditions because the velocity ratio is so low.

Rick W

[sailor2]

If his experience was with powered props he would not appreciate the high velocity ratio required to generate the thrust needed to propel such a large boat in very low far field wind speed. For the low power water props I design and build I neglect near field conditions because the velocity ratio is so low. Do I understand correctly that the velocity ratio you are talking about is defined as induced speed / free stream speed ?
If so what about swirl losses ? If you size prop diameter up to low down that ratio, you also lower the revs for a given power meaning you increase torque.
That means you increase swirl losses, there is optimum diameter in real world due to that even ignoring Re-number effects. Not sure if you are anywhere near that diam, but if you are ...

[Rick Willoughby]

I have done some design work on a manned boat to achieve DDWFTTW.

It is easy to look at numbers and not appreciate what they mean. Something showing the proportions provides greater meaning.

I was intending to show a bike chain drive between the turbine and prop but that would be too unwieldy. I expect a carbon shaft up the prop mast would be the best.

This is not something I would ever contemplate building and I doubt anyone else will. It might be possible to reduce the prop size but it means it will need higher wind to operate.

The exercise of looking at a large model boat might yield something more practical but it will still need quite a large prop.

I have included a performance curve to give an idea what this could achieve. In the current configuration power transfer becomes a serious consideration over 10m/s. I did not go above 15m/s wind because the power is over 10kW. This would require much heavier components than I have allowed for in the displacement.

Rick W

[sailor2]

I have done some design work on a manned boat to achieve DDWFTTW. Rick WWow, surprising shape on that curve except the lower end.

How does assumed rpm of turbine change with boat speed ?
How does assumed torque from turbine change with boat speed ?
If not by square law, turbine efficiency changes. If by square law prop efficiency changes. I just can't see how both could be kept reasonably high for so wide operating range dispate of pitch & gear changes.

How does assumed propeller efficiency change with boat speed ?
How does assumed turbine efficiency change with boat speed ?

Or did you simply assume different prop & turbine combination for each boatspeed optimized for the conditions and was pitching moment assumed to be balanced by shifting weight in form of airprop & tower aft to compensate ?
Considering how little boatspeed/windspeed ratio changes seems to indicate just that.

[Rick Willoughby]

I have modeled all elements of this with reasonable accuracy - all within 2%. So I have a VPP where I set the windspeed, and automatically increase the boat speed from 80% of windspeed till I get overall thrust and power balance.

I assume the boat can be trimmed so it sits on the waterline with the outriggers just skimming. I have not determined how the pilot weight would need to be moved around to achieve this. I expect it may need a trampoline between the outriggers

The turbine has quite low slip through out the speed range so its rpm is almost liner with speed. At 13.4kts it does 193rpm.

The model only works with a narrow range of mechanical gear ratio. The optimum at all windspeed is 5.1:1. I cannot get the model to start if the ratio is below 4.9 and higher than 5.6. This could be just an issue with setting the starting conditions but I have played quite a lot to increase the range. I have not yet determined what limits it. The 5.1:1 mechanical ratio gives an effective ratio of 0.6 allowing for the geometric pitch of both turbine and prop.

The power input to the prop at a few wind speeds is:
3m/s, 97W
6m/s, 849W
10m/s, 4273W

The overall efficiency of power transfer from water to air improves considerably as the prop improves its operating regime. At windspeed of 5m/s it is 37% and increases to 63% at 10m/s. Both relate to the steady state boat speed for the given windspeed.

The wave drag on this hull is about 10% of total drag throughout the speed range. I use Michlet data for this that I know is good up to about 5m/s on this particular hull. Error may increase as speed increases but it will not be huge as it is dominated by viscous drag.

Rick W

[InetRoadkill]

If the prop is producing 9N at 4m/s, it's energy demands are 36W, not 15W. If you allow for a prop efficiency of 80%, the demands are closer to 45W.

[Rick Willoughby]

If the prop is producing 9N at 4m/s, it's energy demands are 36W, not 15W. If you allow for a prop efficiency of 80%, the demands are closer to 45W.

The prop only has an airflow of 1m/s. The same as it did when there was no tail wind. At 9N the power output is 9W which results in a battery drain of 15W allowing for the losses.

If you apply your logic to an airplane with 100kts airspeed in a tail wind of 100kts it would require to double its power to achieve 200kts over the ground.

The propeller on the cart is not reacting against the ground. It is reacting against the air. This is exactly the same condition as an aircraft flying at 10,000m. It does not matter if it is in the air or on the ground. Increasing ground speed with a tail wind does not increase the power required by the prop.

So if you can prove that an aircraft increases power to maintain its airspeed in a tail wind you may be able to mount a plausible argument against DDWFTTW.

Rick W

[InetRoadkill]

Your diagram clearly shows that 9N are required to push the cart 4 m/s. Therefore, 36W are required. It doesn't matter what the airspeed thru the prop is. You indicated that there is 9N of drag. So 36W of power are required to overcome it at 4 m/s.

FYI: I happen to be a pilot and I do understand indicated airspeed vs. ground speed. I've actually flown a plane backwards in level flight according to the GPS.

[sailor2]

Your diagram clearly shows that 9N are required to push the cart 4 m/s. Therefore, 36W are required. It doesn't matter what the airspeed thru the prop is. You indicated that there is 9N of drag. So 36W of power are required to overcome it at 4 m/s.
In ground IRF cart needs 36W to go 4m/s with 9N drag force.
That fact has nothing to do with how much electrical power is needed to rotate the prop. You are mixing too very different things. In craft IRF the craft doesn't move at all meanig speed is zero, and power is 9N*0m/s = 0 Watts.
That doesn't tell anything about needed shaft power.

FYI: I happen to be a pilot and I do understand indicated airspeed vs. ground speed. I've actually flown a plane backwards in level flight according to the GPS. If you do, why do you use any other than airspeed to determine shaft power needed by the prop ?
Or do you claim your power consumption was negative while going backwards related to the ground ? If not, why don't you just say so.

[Rick Willoughby]

Your diagram clearly shows that 9N are required to push the cart 4 m/s. Therefore, 36W are required. It doesn't matter what the airspeed thru the prop is. You indicated that there is 9N of drag. So 36W of power are required to overcome it at 4 m/s.

FYI: I happen to be a pilot and I do understand indicated airspeed vs. ground speed. I've actually flown a plane backwards in level flight according to the GPS.

The 36W is correct. 9W comes from the electric motor and 27W from the wind.

If I was to replace the propeller with a spinnaker in 3m/s tail wind I could get maybe 2m/s over the ground. The power input from the wind in this case is 18W.

My cart is capable of generating 9N of thrust with an airspeed of 1m/s. If I give it a tail wind of 3m/s then it will generate 9N with an airspeed of 1m/s and a ground speed of 4m/s. The 9N is enough to maintain the ground speed of 4m/s.

Go back to your aeroplane. If it takes 100HP for level flight at 100kts you do not double horsepower to achieve 200kts over the ground in a 100kt tail wind. You only need the same 100HP. Exactly the same with my cart. The conditions for the prop have not changed. It sees a headwind of 1m/s. It is not bothered by what it happening at ground level. The prop operates in air.

Rick W

[InetRoadkill]

You are pushing against the ground and surrounding environment with 9N of force at 4 m/s. That's 36W. You can't escape that by arbitrarily juggling the frame of reference until you find one that makes your energy demands go away.

As far as the airplane goes, its frame of reference is the free air stream. Ground speed is meaningless in flight. And before you say that cart's frame of reference should be the free airstream like the airplane, let me remind you that the cart's wheels are still attached to the ground. Even if we allow for zero aerodynamic drag in spite the fact that the cart has a headwind, you are still in need of 20W of power just to turn the wheels against the ground which has busted your budget of 15W.

[Rick Willoughby]

You are pushing against the ground and surrounding environment with 9N of force at 4 m/s. That's 36W. You can't escape that by arbitrarily juggling the frame of reference until you find one that makes your energy demands go away.

As far as the airplane goes, its frame of reference is the free air stream. Ground speed is meaningless in flight. And before you say that cart's frame of reference should be the free airstream like the airplane, let me remind you that the cart's wheels are still attached to the ground. Even if we allow for zero aerodynamic drag in spite the fact that the cart has a headwind, you are still in need of 20W of power just to turn the wheels against the ground which has busted your budget of 15W.

My propeller is the only source of propulsion. The wheels carry the weight and have constant rolling drag. The propeller is accelerating the incoming 1m/s airstream to something a bit faster relative to the vehicle to generate that thrust. That is all it sees - air incoming at 1m/s. Why do you think it needs to increase power when it is already producing the 9N of thrust required to keep the vehicle rolling at any speed? I have said the rolling drag does not change with ground speed so that 9N is enough to maintain 1m/s or 4m/s across the ground.

When it is operating in a tail wind of 3m/s and doing 4m/s across the ground the wind provides the extra power. I have not busted the budget because the power demanded by the prop has not changed. It sees 1m/s wind and is producing 9N. The wind provides the rest of the power.

You have not responded to how a spinnaker could actually provide energy to the cart doing 2m/s in a 3m/s tail wind. How much power is the spinnaker generating. By your analysis you are saying the spinnaker needs to produce at least 18W. What is the energy input to the spinnaker. Where is its battery connected?????

I also disagree on your point that ground speed is not important to flight. If you can get a 100kt tail wind in a plane capable of 100kts then you get from point A to point B in half the time using half the fuel. The wind has saved you all that energy. My cart is simply exploiting the tail wind the same way.

If it makes life easier for you assume there is no wheel drag. Will it speed up in this case with a tail wind?

Rick W

[spork]

FYI: I happen to be a pilot...

Thanks for the warning. We'll try not to use any complicated aero terms or formulas.


As far as the airplane goes, its frame of reference is the free air stream.

That's why you're a pilot and not an aero engineer. The plane's frame of reference is whatever inertial frame we choose to perform the analysis in (or even a non-inertial frame if we're particularly sadistic).

you are still in need of 20W of power just to turn the wheels against the ground which has busted your budget of 15W.

Something must be wrong with all the carts I built then - because they can all achieve DDWFTTW steady state.

[InetRoadkill]

My propeller is the only source of propulsion. The wheels carry the weight and have constant rolling drag.

Fine. they have constant rolling resistance. But the power demands increase linearly with velocity.

The propeller is accelerating the incoming 1m/s airstream to something a bit faster relative to the vehicle to generate that thrust. That is all it sees - air incoming at 1m/s.

That's ok too. You've exceeded the wind speed and now you have a headwind. No problem.

Why do you think it needs to increase power when it is already producing the 9N of thrust required to keep the vehicle rolling at any speed?

I'll come back to this.

I have said the rolling drag does not change with ground speed so that 9N is enough to maintain 1m/s or 4m/s across the ground.

But power needed to overcome drag does increase with velocity. This is an important point.

When it is operating in a tail wind of 3m/s and doing 4m/s across the ground the wind provides the extra power. I have not busted the budget because the power demanded by the prop has not changed. It sees 1m/s wind and is producing 9N. The wind provides the rest of the power.

Houston, have a problem. You're saying that the prop is drawing energy from the wind and then turning around to say the prop is providing thrust (expending energy) against the wind. Umm, that doesn't make sense. Which is it?

You have not responded to how a spinnaker could actually provide energy to the cart doing 2m/s in a 3m/s tail wind. How much power is the spinnaker generating. By your analysis you are saying the spinnaker needs to produce at least 18W. What is the energy input to the spinnaker. Where is its battery connected?????

As long as the air is behind the spinnaker, it can power the boat. I don't think I've seen any spinnakers deployed with a headwind.

I also disagree on your point that ground speed is not important to flight. If you can get a 100kt tail wind in a plane capable of 100kts then you get from point A to point B in half the time using half the fuel. The wind has saved you all that energy. My cart is simply exploiting the tail wind the same way.

You're twisting my point. Ground speed may be important to the pilot as a matter of convenience by saving some time, but it has no bearing on the mechanics of flight. A tail wind may save you time on your trip, but it doesn't change the efficiency of the aircraft.

If it makes life easier for you assume there is no wheel drag. Will it speed up in this case with a tail wind?

Rick W

Wait, weren't the wheels providing power to the prop? How can the wheel drag go to zero then?

[spork]

Fine. they have constant rolling resistance. But the power demands increase linearly with velocity.

We're not talking about rolling resistance. We're talking about the torque that must be provided by the road's force on the wheel to turn the axle, and thus the prop-shaft.

Houston, have a problem. You're saying that the prop is drawing energy from the wind and then turning around to say the prop is providing thrust (expending energy) against the wind. Umm, that doesn't make sense. Which is it?

This is a brain-teaser; it's not supposed to be trivial. Questions of torque and force will be frame invariant. Questions of energy (such as "is this vehicle powered by the wind or the ground?") will depend on the frame chosen for the analysis.

As long as the air is behind the spinnaker, it can power the boat. I don't think I've seen any spinnakers deployed with a headwind.

Yes, and as you may be aware, a traditional sailboat can never achieve steady-state DDWFTTW - not even with a really big spinnaker.

You're twisting my point. Ground speed may be important to the pilot as a matter of convenience by saving some time, but it has no bearing on the mechanics of flight.

True.

A tail wind may save you time on your trip, but it doesn't change the efficiency of the aircraft.

I guess that depends on how you choose to define efficiency. It does if you're interested in miles/gallon (along the ground).

[InetRoadkill]

We're not talking about rolling resistance. We're talking about the torque that must be provided by the road's force on the wheel to turn the axle, and thus the prop-shaft.

Sorry, but that torque has to come from somewhere and it's not free. Since the wheels are turning against the torque, power is being expended. The force needed to produce the torque shows up as rolling resistance against the ground.

This is a brain-teaser; it's not supposed to be trivial. Questions of torque and force will be frame invariant. Questions of energy (such as "is this vehicle powered by the wind or the ground?") will depend on the frame chosen for the analysis.

You dodged my point. What is driving the prop? Where is the energy coming from? Is the prop driving the air to produce the thrust needed to exceed the wind speed?

[Rick Willoughby]

.


Wait, weren't the wheels providing power to the prop? How can the wheel drag go to zero then?

We may be talking about different conditions.

I have an electric powered vehicle as shown in these slides. I am only considering battery power here.

Do you agree with what I am saying in Slide 1 and Slide 2 as depicted? If not tell me where my analysis is flawed.

Rick W

[spork]

Sorry, but that torque has to come from somewhere and it's not free.

No need to apologize. We know you're confused. The torque comes from the tangential force of the road against the rolling wheels. And no one said it's free. It's always the sceptics that like to talk about perpetual motion. We like to explain where the energy is coming from, and the sceptics like to ignore us.

Since the wheels are turning against the torque, power is being expended.

In the frame of the road there is no work being done by the wheels on the road or by the road on the wheels. From the frame of the wind, the road does work on the wheels.

The force needed to produce the torque shows up as rolling resistance against the ground.

I recommend you avoid terms you're not familiar with.

You dodged my point. What is driving the prop?

You failing to understand an answer does not constitue me dodging your point. If you want to actually understand this thing rather than tell me I'm flat wrong about an object I've built and tested many times over, we can chill and explain it carefully and thoroughly. Your call.

What's driving the prop? The prop shaft. What's driving the prop shaft? The axle (through a 90 degree bevel gear set). What's driving the axle? The wheels. What's driving the wheels? Well, that depends on how you look at it. We can say it's the road (which isn't the most pleasing answer) or the propeller thrust (which SEEMS like it would constitute perpetual motion, but it does not for reasons we can easily explain).

Where is the energy coming from? Is the prop driving the air to produce the thrust needed to exceed the wind speed?

Yes, the prop is producing the thrust needed to exceed the speed of the wind. The energy is extracted from the wind/ground interface. The cart leaves a wake of slowed wind in its path.

[InetRoadkill]

We may be talking about different conditions.

I have an electric powered vehicle as shown in these slides. I am only considering battery power here.

Do you agree with what I am saying in Slide 1 and Slide 2 as depicted? If not tell me where my analysis is flawed.

Rick W


I notice that you're mixing rolling resistance with rolling power. Slide 1 shows the energy required at this point is 1.75W and that you have a surplus of power available from a battery. Cool.

Slide 2 shows that the power needed to roll the cart is now 3W. So far so good.

[Rick Willoughby]

I notice that you're mixing rolling resistance with rolling power. Slide 1 shows the energy required at this point is 1.75W and that you have a surplus of power available from a battery. Cool.

Slide 2 shows that the power needed to roll the cart is now 3W. So far so good.

In Slide 3 I have ditched the motor and propeller and fitted a generator to my buggy. I am using another vehicle to push it.

Do you agree with my analysis here?

Rick W.

[Rick Willoughby]

I notice that you're mixing rolling resistance with rolling power. Slide 1 shows the energy required at this point is 1.75W and that you have a surplus of power available from a battery. Cool.

Slide 2 shows that the power needed to roll the cart is now 3W. So far so good.

In Slide 3 I have ditched the motor and propeller and fitted a generator to my buggy. I am using another vehicle to push my buggy.

Do you agree with my analysis here?

Rick W.

[InetRoadkill]

Your power demands in slide 3 are now 33W.

Windage power; 1N * 1 m/s = 1W
Rolling power; ((2N + 5N) + 1) * 4 m/s = 32W;

Total power = windage + rolling = 32W + 1W = 33W

[Rick Willoughby]

Your power demands in slide 3 are now 33W.

Windage power; 1N * 1 m/s = 1W
Rolling power; ((2N + 5N) + 1) * 4 m/s = 32W;

Total power = windage + rolling = 32W + 1W = 33W

This is good. It is less than I calculated and you are correct.

In Slide 4 I have put back the motor and propeller and left the generator in place and operating.

Do you agree with Slide 4 and if not why not.

Rick W

[InetRoadkill]

The prop is only producing 9W of power. You're still short 24W.

(9N thrust * 1 m/s apparent wind)

[Rick Willoughby]

28499The prop is only producing 9W of power. You're still short 24W.

(9N thrust * 1 m/s apparent wind)

OK. I can see what you are saying. I was thinking slightly differently along these lines. Lets say I have fitted the generator to the vehicle and it has a clutch so it can be taken in and out of operation.

I am accelerating as we agreed in Slide 2 using the battery and at the instant that the vehicle hits 4m/s road speed (1m/s air speed) I engage the clutch and have a sudden 5N increase in drag due to the generator. You are saying it is obvious that the vehicle will not only stop accelerating but will actually slow down because there is not enough power - that ever present conservation of energy.

Can you give me a more detailed explanation of what will happen as I engage the clutch. I would like some rough numbers so I have included the performance curve for the prop in the data sheet attached. The design point was 12.94W at 1m/s and the motor is a small outrunner synchronous motor operating at fixed frequency so rpm is constant with load. What vehicle speed and prop power will it balance at. Or will it just grind to a halt?


Rick W

[InetRoadkill]

28499

OK. I can see what you are saying. I was thinking slightly differently along these lines. Lets say I have fitted the generator to the vehicle and it has a clutch so it can be taken in and out of operation.

I am accelerating as we agreed in Slide 2 using the battery and at the instant that the vehicle hits 4m/s road speed (1m/s air speed) I engage the clutch and have a sudden 5N increase in drag due to the generator. You are saying it is obvious that the vehicle will not only stop accelerating but will actually slow down because there is not enough power - that ever present conservation of energy.

Can you give me a more detailed explanation of what will happen as I engage the clutch. I would like some rough numbers so I have included the performance curve for the prop in the data sheet attached. The design point was 12.94W at 1m/s and the motor is a small outrunner synchronous motor operating at fixed frequency so rpm is constant with load. What vehicle speed and prop power will it balance at. Or will it just grind to a halt?


Rick W

You need 33N of thrust which is not on the chart. We can extrapolate from the parameters given and get an estimate of 55W of input power needed to produce the necessary thrust. What's going to happen is that since the prop is not producing the required thrust due to the input power being insufficient, it's going to surrender its angular kinetic energy to the thrust it is producing and also to drag and back into the wheels. It will wind down until the energy equations balance again. Where that point happens is hard to tell. Given the interdependency between the wheels and the prop with the possibility of the wheels either trying to over or underdrive the prop I can't make a prediction to the final speed. It will be less than the DW speed. From the videos, I'll assume you found a sweet spot where the prop is able to capture enough energy from the wind to cancel its drag thus allowing the cart to move at near the DW speed.

I can tell you from experience that a windmilling prop is rather draggy. The prop may well start acting as a small spinnaker in this situation.

[Rick Willoughby]

You need 33N of thrust which is not on the chart. ...n.

Where the hell does the 33N come from. My vehicle will accelerate like crazy if I apply that much thrust. It only needs 9N to do 4m/s as you agreed in Slide 3. What has changed all of a sudden.

You already agreed that I only needed 9N of thrust to push the vehicle at 4m/s with the generator connected. Then when I add a propeller that is able to provide 9N of thrust you are saying I actually create a brake that is somehow needing an extra 24N of thrust. What has suddenly changed with my prop that it becomes a brake when I engage the generator. Conditions at the prop haven't changed.

Rick W

[InetRoadkill]

We determined that the energy demands were 33 Watts. The air is moving past the prop at 1 m/s apparent. So ...

Power / velocity = thrust

33 / 1 = 33 N thrust.


edit: the reason it's not 9 is because your pushing against a moving airstream.

[Rick Willoughby]

We determined that the energy demands were 33 Watts. The air is moving past the prop at 1 m/s apparent. So ...

Power / velocity = thrust

33 / 1 = 33 N thrust.


edit: the reason it's not 9 is because your pushing against a moving airstream.

Where is all that extra thrust going. I do not want to accelerate I just want to hold 4m/s?? I only need a thrust of 9N not 33N. Unless you can tell me that I can only conclude your power calculation is nonsense.

Pushing against the moving airstream gets us back to the more power required in an aircraft with a tail wind. I do not think this is the case.

If we have extra force than we need the buggy will accelerate so anything more than 9N means we are accelerating whether the prop is acting on a "moving" airstream or whatever. If the forces do not balance the vehicle is accelerating or decelerating. I think this is Newtons first law. You need to tell me what all that extra force is doing. THe vehicle does not need to accelerate just hold steady speed at 4m/s.

If you fly your plane along at 100kts and then suddenly triple the force it will accelerate like crazy. Why is my buggy any different.

Rick W

[InetRoadkill]

Once again, we determined that the power required was 33W from a couple of posts back. Going back to the car pushing at 4 m/s the required thrust is actually 8.25N (not quite 9N) to achieve this (33W / 4m/s). Note that the speed is 4 m/s with the car pushing. Your airspeed is only 1/4 of that (1 m/s) so the thrust must go up accordingly to produce the required power.

[Rick Willoughby]

Once again, we determined that the power required was 33W from a couple of posts back. Going back to the car pushing at 4 m/s the required thrust is actually 8.25N (not quite 9N) to achieve this (33W / 4m/s). Note that the speed is 4 m/s with the car pushing. Your airspeed is only 1/4 of that (1 m/s) so the thrust must go up accordingly to produce the required power.

Just to be clear on this you are saying that the propeller thrust is going to have to be 33N but the vehicle drag is only 8.25N. By my calculation the excess force is therefore 24.75N. Have I got this right?

You are also saying that extra force is doing nothing because the vehicle is in steady state at 4m/s.

Actually it is Newton's Second Law that needs to be considered:
Second law
Observed from an inertial reference frame, the net force on a particle is proportional to the time rate of change of its linear momentum: F = d(mv)/dt. Momentum mv is the product of mass and velocity. Force and momentum are vector quantities and the resultant force is found from all the forces present by vector addition. This law is often stated as, "F = ma: the net force on an object is equal to the mass of the object multiplied by its acceleration."

How do you reconcile Newton's Second Law with what you are saying?

Rick W

[spork]

Rick, I think there's a critical part you're missing. If you simply start with the assumption that the cart cannot possibly go directly downwind faster than the wind (as inet has), you'll find that you can move numbers around (somewhat randomly) until it satisfies you that your conclusion/assumption is correct.

Don't get too wrapped up with doing the calcs correctly. That could lead to the unacceptable conclusion that DDWFTTW is possible. While that agrees with the physical evidence, it does not agree with inet's assumption - and he's a pilot.

[Rick Willoughby]

Rick, I think there's a critical part you're missing. If you simply start with the assumption that the cart cannot possibly go directly downwind faster than the wind (as inet has), you'll find that you can move numbers around (somewhat randomly) until it satisfies you that your conclusion/assumption is correct.

Don't get too wrapped up with doing the calcs correctly. That could lead to the unacceptable conclusion that DDWFTTW is possible. While that agrees with the physical evidence, it does not agree with inet's assumption - and he's a pilot.

spork
We have to satisfy a few fundamental laws here and I think Inet knows enough to appreciate that.

We cannot have conservation of energy at the expense of Newton's laws. There has to be a consistent solution that satisfies both.

I just have to get Inet to realise that the wind can provide energy to the system as it does with an aeroplane when flying in a tail wind or when it acts on a spinnaker. After all HE IS A PILOT you know.

Rick W

[Rick Willoughby]

We determined that the energy demands were 33 Watts. The air is moving past the prop at 1 m/s apparent. So ...

Power / velocity = thrust

33 / 1 = 33 N thrust.


edit: the reason it's not 9 is because your pushing against a moving airstream.

IRK
I have tried to set down your theory on forces in the attached slide. Have I got it accurately described?

Rick W

[spork]

I just have to get Inet to realise that the wind can provide energy to the system as it does with an aeroplane when flying in a tail wind or when it acts on a spinnaker.

Unfortunately, my cynicism gauge is pegging. He put me off from the start. You're clearly doing much better with him, but I still think there's a very high probability he'll prove to be fully logic-proof.

After all HE IS A PILOT you know.

Yeah, me too. I've been both an aero engineer and a pilot for decades. That's why I suggested we avoid any technical terms with him.

[Rick Willoughby]

We determined that the energy demands were 33 Watts. The air is moving past the prop at 1 m/s apparent. So ...

Power / velocity = thrust

33 / 1 = 33 N thrust.


edit: the reason it's not 9 is because your pushing against a moving airstream.

IRK
I am trying to determine if the plane in the attached diagram will accelerate or slow down. I am having trouble with the AST and ASD factors.

As you can see the prop is producing 2000N thrust in the airstream, the air drag is 1000N and the wheel drag is 200N. I do not have a problem with the 200N from the wheels as this is not in the airsteam but the prop thrust of 2000N is pushing on an airstream and the aircraft drag of 1000N is being applied by the airstream.

You will note that the plane is forced to take off in a slight tail wind of 3m/s and the ground speed of the aircraft is currently 10m/s. From my understanding of the IRK Law of Forces Pushing on Air (ILFPA for short) I believe these are essential data to work out the force derating factors.

What are the AST and the ASD in this case? If there is not enough thrust with 2000N how much would the prop need to produce to keep accelerating.

[I hope I am accurate in giving you credit for the ILFPA. I am not a pilot so it might be part of pilot training that I am completely ignorant about. Please advise a reference if you have one on this important part of flight training whereby forces are derated when acting on an airstream.]


Rick W.

[InetRoadkill]

Well clearly there is a problem. We can either balance the forces or we can balance energy. So how do we get both to balance?

[InetRoadkill]

IRK
I am trying to determine if the plane in the attached diagram will accelerate or slow down. I am having trouble with the AST and ASD factors.

As you can see the prop is producing 2000N thrust in the airstream, the air drag is 1000N and the wheel drag is 200N. I do not have a problem with the 200N from the wheels as this is not in the airsteam but the prop thrust of 2000N is pushing on an airstream and the aircraft drag of 1000N is being applied by the airstream.

You will note that the plane is forced to take off in a slight tail wind of 3m/s and the ground speed of the aircraft is currently 10m/s. From my understanding of the IRK Law of Forces Pushing on Air (ILFPA for short) I believe these are essential data to work out the force derating factors.

What are the AST and the ASD in this case? If there is not enough thrust with 2000N how much would the prop need to produce to keep accelerating.

[I hope I am accurate in giving you credit for the ILFPA. I am not a pilot so it might be part of pilot training that I am completely ignorant about. Please advise a reference if you have one on this important part of flight training whereby forces are derated when acting on an airstream.]


Rick W.
Oops. we cross posted.

Actually, I'm not familiar with those abbreviations. But that's ok.

Since the plane is taking off with a 3 m/s tailwind, the indicated airspeed would be 7 m/s.

The derating comes in from the need to accelerate to a higher kinetic energy before taking off. In other words, if we say that the plane can lift off at 10m/s, we have a certain amount of kinetic energy. But with the tailwind, we now have to accelerate to 13 m/s with a correspondingly higher KE. The power needed to spin the wheels has increased too since they are now traveling faster across the ground. The engine can only supply the KE so fast (limited by its horsepower), so it takes longer to get off the ground.

[Rick Willoughby]

Oops. we cross posted.

Actually, I'm not familiar with those abbreviations. But that's ok.

Since the plane is taking off with a 3 m/s tailwind, the indicated airspeed would be 7 m/s.

The derating comes in from the need to accelerate to a higher kinetic energy before taking off. In other words, if we say that the plane can lift off at 10m/s, we have a certain amount of kinetic energy. But with the tailwind, we now have to accelerate to 13 m/s with a correspondingly higher KE. The power needed to spin the wheels has increased too since they are now traveling faster across the ground. The engine can only supply the KE so fast (limited by its horsepower), so it takes longer to get off the ground.

All I would like to know in the case with the airplane is what does the propeller thrust of 2000N reduce to because it is acting on an airstream and likewise what does the 1000N become because it is being applied by an airstream. Show how you use the KE and horsepower to derate the forces.

I have no pilot training so I am interested in how pilots calculate derating factors for forces acting on, or applied by, an airstream. You did it for my buggy model but I am having trouble applying the same principle to an airplane. I understand this is an area where pilots have a lot of training and knowledge.

Rick W

[Rick Willoughby]

Well clearly there is a problem. We can either balance the forces or we can balance energy. So how do we get both to balance?

The problem is that you refuse to accept that the air can apply power to the system. Once you get over this point it all works out. Irrespective if it a sheet of nylon with the wind applying a force to it or the wind applying a force to a propeller the air is doing work on the system. The power developed by the wind is the force times the windspeed.

To be honest I have never heard anything so absurd as having to derate a force because it is acting on an airstream. A force is a force. It has nothing to do with what it is acting on. If a plane propeller is producing 2000N it is always 2000N. Exactly the same with my buggy 9N is 9N. It does not magically need to increase because it is acting on an airstream. Why did it not have to increase in the earlier examples where it is also acting on the airstream?

Also how can 1N reduce to 0.25N because it is being applied by air.

You are so convinced that it cannot work that you are coming up with stupid calculations. Go back and review what you have written. You will see it is utter stupidity.

[InetRoadkill]

Actually, I did go back and review. And I believe I have solved problem with the forces and energies not balancing. I did make an error with the 33N of thrust. But that doesn't get the cart out of trouble with energy.

Let me ask a couple questions:

* How much energy is the cart demanding? Are we still in agreement that it's 33W? (Actually, it's 36W. The windage should be multiplied by 4m/s like it originally was. Not 1m/s. Another minor mistake.)

* Were is the energy coming from?

[spork]

You are so convinced that it cannot work that you are coming up with stupid calculations. Go back and review what you have written. You will see it is utter stupidity.


Now you're approaching my level of frustration.

Inet. Let me ask you a serious question. Are you open minded enough to believe this cart may be doing exactly what it appears to be doing? If so, we have all the time in the world to explain how it can and does do that. If not, no worries. We won't burden you with facts.

Gratuitous 2nd question: do you believe that ice-boats can achieve and maintain a downwind velocity component of 3X to 4X the wind speed?

[InetRoadkill]

Now you're approaching my level of frustration.

Inet. Let me ask you a serious question. Are you open minded enough to believe this cart may be doing exactly what it appears to be doing? If so, we have all the time in the world to explain how it can and does do that. If not, no worries. We won't burden you with facts.

Gratuitous 2nd question: do you believe that ice-boats can achieve and maintain a downwind velocity component of 3X to 4X the wind speed?

According to the ice boat people I've talked too, they can only obtain that kind of speed on a reach with the downwind component less than windspeed. I'll take their word for it.

Regarding the problem at hand, as I've pointed out the numbers presented aren't adding up which means there's a flaw in the proposed model. Once I had a chance to get some sleep and revisit the problem, I was able for see what's going on.

As was pointed out earlier, the cart in the example requires 36W of power to meet the demands of driving the prop and overcoming the other drag factors. The wind is providing 27W under the given conditions which means there's an energy shortfall. That's a problem.

As it turns out, once you resolve the energy discrepancy, you'll find that you can theoretically exceed wind speed and meet the the demands of conservation of energy and balanced forces. (The maximum speed works out to be twice the wind speed.) However, it requires that the prop have 100% efficiency and that the cart have zero drag. That's not likely to happen.

The equations suggest that you can achieve a fraction over wind speed with some drag and prop losses. However, the drag budget is pretty small. It would be difficult to achieve in practice.

[spork]

According to the ice boat people I've talked too, they can only obtain that kind of speed on a reach with the downwind component less than windspeed. I'll take their word for it.

So that's a "no" - you don't believe they can. However, there is plenty of physical evidence and very simple analysis that says otherwise. I'm not sure if you're one for facts or data, but you should consider the GPS data in the attached files.

As it turns out, once you resolve the energy discrepancy, you'll find that you can theoretically exceed wind speed and meet the the demands of conservation of energy and balanced forces.

As it turns out, I resolved any such discrepancy years ago. I've since built several vehicles that go directly downwind faster than the wind, steady state, by a pretty fair margin.

The maximum speed works out to be twice the wind speed.

Wrong. Given arbitrarily small losses, you can achieve an arbitrarily large multiple of the wind speed.

However, it requires that the prop have 100% efficiency and that the cart have zero drag. That's not likely to happen.

Wrong again.

The equations suggest that you can achieve a fraction over wind speed with some drag and prop losses. However, the drag budget is pretty small. It would be difficult to achieve in practice.

And still wrong - even you can do it if you follow my build videos closely; and you'll find you can do better than "a fraction over wind speed".

I'm really curious to know why you trust your flawed analyses over real world results.

[Rick Willoughby]

Now you're approaching my level of frustration.

Inet. Let me ask you a serious question. Are you open minded enough to believe this cart may be doing exactly what it appears to be doing? If so, we have all the time in the world to explain how it can and does do that. If not, no worries. We won't burden you with facts.

Gratuitous 2nd question: do you believe that ice-boats can achieve and maintain a downwind velocity component of 3X to 4X the wind speed?

Actually IRK amuses me tremendously. Have you seen his theory on force adjustment factors when the force acts on an airsteam. A force of 33N must be applied to create an "adjusted" force of 9N because the force is being applied to an airsteam. Someone has actually given this guy a pilot's licence if you believe what he says although I have my own view on that.

Rick W

[spork]

Actually IRK amuses me tremendously.

Admittedly, he amuses me as well. But it's in sort of an "oh my god" kind of way.

Someone has acrually given this guy a pilot's licence if you believe what he says although I have my own view on that.

I have little doubt he has a pilot's license. Having a pilot's license qualifies you to discuss aerodynamics and engineering like driving a bus qualifies you to design an F1 engine. Did you happen to see the pilot on Mythbusters that felt his plane could not take off from a moving treadmill!?

[Rick Willoughby]

... Did you happen to see the pilot on Mythbusters that felt his plane could not take off from a moving treadmill!?

I came into that story at the end of the model testing and wondered what they were on about. Then when I saw the test set up and heard the pilots comment I was flabbergasted. That guy probably went to the same pilot training course as IRK.

I have been trying to get IRK to explain the force derating factor that applies to a force that happens to be acting on an airstream. It seems to change with different conditions of velocity. Did you get to the force derating lecture during your pilots training course? If you have the lecture notes I would like to see them. I am just an ignorant electrical engineer not versed in all those key aspects of physics that enable an airplane to fly. Maybe if I could just grasp the force derating factor for a force acting on an airstream I would have a better understanding of how planes fly.

Rick W

[spork]

Did you get to the force derating lecture during your pilots training course?

I tried pretty hard not to listen to the aero descriptions in my flight training, and I must have been absent the day they covered that in my undergrad and/or grad aero programs.

[Rick Willoughby]

...
* Were is the energy coming from?

The energy literally comes out of the air. The ground speed of the undisturbed airflow is 3m/s. After the vehicle passes there will be a cylindrical airstream immediately behind the cart prop that has been slowed down. Over time this will gradually be entrained into the general environment as happens with any propeller or wing.

Let me provide some actual numbers. The prop has an apparent wind of 1m/s. The airflow is being accelerated as it passes through the prop. In my particular example the airflow reaches 1.28m/s. The mass flow rate to achieve my nominated prop efficiency with thrust of 9N is calculated as 44kg/s.

So in this case the far field velocity for the prop is 1m/s and the near field is 1.28m/s. When the cart is doing 4m/s ground speed the far field velocity with respect to the ground is 3m/s and the near field velocity in the vicinity of the prop is 2.72m/s.

The kinetic energy reduction in the propeller airstream due to the cart passing can be determined as:
Energy/sec = 0.5 x 44 x (3^2 - 2.72^2) = 36W.

I have done some rounding here but it shows where the energy comes from. If you set up a smoke trail you would see the air speed over the ground is slowed down in the vicinity of the prop as the cart passes by. This energy goes into overcoming the losses in the cart.

Rick W

[InetRoadkill]

Present a general set of equations describing your device.

[Rick Willoughby]

Present a general set of equations describing your device.

There are two papers that Mark Drela produced that I included at post #3 on this thread that gives the generalised equations of motion and energy balance. These are actually for a boat using a water turbine to power the prop. To apply them to the buggy situation you need to equate his turbine and transmission efficiency to my generator efficiency. Other than this the boat and buggy have the same generalised set of equations.

I expect as you are a pilot and all you will at least know of Mark Drela.

Rick W

[spork]

I expect as you are a pilot and all you will at least know of Mark Drela.

I doubt it very much. But I'm sure he's familiar with Google.

[Richard Atkin]

A wind tunnel test will show you guys where you went wrong in your equations.

Sorry to drag up an old thread....couldn't resist

[spork]

A wind tunnel test will show you guys where you went wrong in your equations.

Actually, both the outdoor tests and treadmill tests show that we were exactly right with our equations. If you understood the concept of going downwind vs. upwind, you might realize how silly it is to suggest testing this in a wind tunnel.

Perhaps you don't understand how a wind tunnel works. It's like a tunnel - for lack of a better word - with wind in it. It really isn't likely to be able to show us where an equation went wrong. Of course if you even thought you knew what you were talking about, you could try and show us where our equations went wrong.

Sorry to drag up an old thread....couldn't resist

No worries. If you want to go that far out of your way to be wrong - be my guest.

[Richard Atkin]

Hi Spork. My point is, you need to be able to make very ACCURATE measurements to provide the proof in a real-world test. You will need a highly controlled wind source to eliminate variations, and when you discover your theory doesn't work, the result will encourage you to look for the flaw in your equation. I can't be bothered because I already know it can't work. I was just offering a challenge. If someone can prove it does work, that person should recieve the Nobel Prize.

No sarcasm intended.

[spork]

Hi Spork. My point is, you need to be able to make very ACCURATE measurements to provide the proof in a real-world test. You will need a highly controlled wind source to eliminate variations, and when you discover your theory doesn't work, the result will encourage you to look for the flaw in your equation.

And yet we've already done the tests that show it DOES work. Putting the cart on a treadmill provides a perfectly controlled wind source. Using a tell-tale on a pole is all the instrumentation needed in an outdoor test to show the cart is going faster than the wind.

I can't be bothered because I already know it can't work.

Interesting - you just "know" this without even understanding the equations and arguments presented. Of course knowing everything is the greatest impediment to learning anything.

I was just offering a challenge.

I guess I missed it. What is the challenge exactly?

If someone can prove it does work, that person should recieve the Nobel Prize.

Unfortunately that just shows how poor your understanding of this thing is. There will be no Nobel prize for this simple brainteaser.

No sarcasm intended.

I understand no sarcasm is intended. Was the stupidity intended?

[spork]

Hey, here's a challenge for you:

I'll bet you $100K that it does work. I presume you could be bothered to take my $100K - no?

Do you really KNOW it can't work? If so, take my money.

[spork]

Incidentally, if you want to understand how and why this thing does work, rather than simply tell me that you think it shouldn't, I'll be more than happy to explain it.

[Richard Atkin]

are you prepared to enter the bet?

[FAST FRED]

"I'll bet you $100K that it does work."


I would be happy to hold the CA$H for this contest!

No ca$h in ADVANCE , not a serious challenge .

FF

[Stumble]

Tell you what, if you guys really want to make this bet I will hold the money in my clients escro account, and hold it for the winner. Though it may be better to find an attorney in Vegas to do it for you.

[spork]

are you prepared to enter the bet?

Of course I am. That's why I offered it. And yes, we both have to put up the money up front. And no, we're not going to leave it with someone we've never met on the internet. We will put it in a joint escrow account.

Sounds like you're up for it?

[Rick Willoughby]

Of course I am. That's why I offered it. And yes, we both have to put up the money up front. And no, we're not going to leave it with someone we've never met on the internet. We will put it in a joint escrow account.

Sounds like you're up for it?

Spork/Richard
Is this one for boat or land based vehicle?

Spork
Did you ever collect on your land vehicle bet?

Have you progressed with the version you can ride?

Rick W

[spork]

Spork/Richard
Is this one for boat or land based vehicle?

The original post is yours, and it mentions both a boat and a land vehicle. Richard seems to think the concept is fundamentally flawed, so either one succeeding should suffice to prove him wrong. As I've made several of the land variety, that's what I would be inclined to have decide the wager. Of course there is no theoretical reason it can't be done with a boat, but it'd clearly be more difficult.

I think I've managed a tack on my kiteboard with downwind VMG faster than the wind (as recently as 2 days ago). But I wouldn't bet on it - at least not until I convinced myself.

Spork
Did you ever collect on your land vehicle bet?

I'm not certain which one you're refering to. I've offered this, and similar, bets MANY times. There was only one guy that I recall that ever seemed serious about taking me up. He was in Germany, and wanted a bit of time to raise the money from people that would obviously be well paid for their investment. It seems they insisted he dig a little deeper before they invested with him, and he emailed me off the list asking to be let out of the bet. He claimed that he tried it himself, and found to his surprise that it actually worked. I suspect he did not actually do this, but did become convinced by someone rational that he was about to lose $100K. He was one of the most beligerent and insulting critics we've ever had.

Have you progressed with the version you can ride?

You can never go wrong betting on my laziness. We've done some design work and found the key components. JB and I also presented this as a seminar at both Stanford and SJSU in the interest of finding some Aero students that might be interested in taking it on as a project. One student came forward, and is trying to find 3 more for his team. I somewhat doubt that'll get any traction. I've suggested to JB that maybe we should just suck it up and do it ourselves once the kitesurfing, hang gliding, paragliding season ends (and I get bored). Perhaps we'll do that - but you'd still be pretty safe betting on my laziness if history is any guide.

One thing is for sure... if someone actually puts up the money for this bet, I'll run out and build a full-scale version in a hurry. We figure it'll cost a couple grand, and I'm an unbelievable cheapskate (which should be a warning to Richard - I would NEVER put up $100K on a bet that I might lose).

[Rick Willoughby]

A wind tunnel test will show you guys where you went wrong in your equations.

Sorry to drag up an old thread....couldn't resist

Richard
Before you jump any deeper into this you should Google DDWFTTW. Also take a look at Mark's equations in post #3 and check on who he is if you do not know.

What you are challenging Spork to is not particularly difficult to demonstrate on land.

Most people have difficulty appreciating that the blades become a propeller driven by the wheels - the opposite to going upwind. What you are arguing is the same as saying an aeroplane cannot produce any thrust in a tail wind - it is nonsense.

Rick W

[spork]

What you are challenging Spork to is not particularly difficult to demonstrate on land.

Yo - ixnay on the infopay! Daddy needs a new airplane. :D

[Rick Willoughby]

Yo - ixnay on the infopay! Daddy needs a new airplane. :D

I think Richard would be better off spending his hard earned on his dream boat than donating to your new airplane. But if he has the money to put behind his convictions then I would not mind a slice of it as well.

Here's an idea. Line up all those who say it is impossible against those who say it is. Each put up USD50 - say at least 100 people either side. The believers build a vehicle at their cost - has to be something that a person can ride. Rather than holding a single pool the individuals pair off so they decide their own way of settling.

The believers select a builder and provide funding based on an estimate. Set a time frame. I guess the builder needs to be trustworthy and competent with some means to collect the funds.

Once proven the believers take their individual booty from their partnered non-believers. The vehicle can be donated to a museum.

Any word from the mythbusters?

Rick W

[spork]

I think Richard would be better off spending his hard earned on his dream boat than donating to your new airplane. But if he has the money to put behind his convictions then I would not mind a slice of it as well.

Here's an idea. Line up all those who say it is impossible against those who say it is. Each put up USD50 - say at least 100 people either side. The believers build a vehicle at their cost - has to be something that a person can ride. Rather than holding a single pool the individuals pair off so they decide their own way of settling.

The believers select a builder and provide funding based on an estimate. Set a time frame. I guess the builder needs to be trustworthy and competent with some means to collect the funds.

I think that would be great. Although to be honest I doubt you could find more than a handful of non-believers that would put up even $50. In my experience, they all say "I KNOW it can't be done - I don't have to make any bets". Then I say, "if you KNOW it can't why not take my money?". Inevitably, it ends up that none of the non-believers have even the slightest need or interest in money.

That being said, I would definitely be up for it, and would volunteer as the builder. I have no doubt at all that JB and I could build a full-scale working version.

Any word from the mythbusters?

Here's the latest...

One of our friends is an Aero PhD, and a consultant to the show. He pitched the idea to them, and they sounded interested. He said we might hear from them. We still haven't. :(

[Rick Willoughby]

I did not see this test unit till now:
http://www.youtube.com/watch?v=GC-EoGbdyzE&NR=1
It is creative and much harder to dispute. There are a series of the clips.


It gets over the treadmill frame of reference problems.

Rick W

[spork]

I did not see this test unit till now:
http://www.youtube.com/watch?v=GC-EoGbdyzE&NR=1
It is creative and much harder to dispute. There are a series of the clips.


It gets over the treadmill frame of reference problems.

Rick W

Yes, very interesting case. This guy gave us LOADS of crap about not doing the right tests. He said he was too busy to do it, so we should (since JB and I aren't busy I suppose). But he came through and did two good tests - the turntable test, and the circular wind-tunnel test. He gets 1000 points for being in the extreme minority and getting off his butt to do the tests.

He loses 100 points for still not being convinced it works even after his own turntable tests proved it did. He also loses 328 points for lying to us about how some of his tests were performed until he was cornered into 'fessing up.

[Rick Willoughby]

Explain the fessing up bit!

Rick

[spork]

Explain the fessing up bit!

Rick

I honestly don't remember the exact details. As I recall, I think it was related to his turntable not being powered, but rather coasting gradually to a stop while a steel ball rolled on it. When he was called on it, he eventually claimed that he intentionally lied about his experiment description because he was playing "spot the flaw in my story" or something of that sort. We explained that if you don't tell people to "spot the flaw in your story", you're simply lying. He claimed that if you told them to spot the flaw, it ruins the "spot the flaw" game.

In summary, he was sloppy and simply refused to admit it.

But like I said, he's still way ahead of 99.92% of the people because he actually did some experiments.

[spork]

I just noticed from one of your posts that you're an Aussie Rick. This guy is one of yours as I recall.

[Richard Atkin]

I am not prepared to enter the bet at this stage. I need to think about this a bit more. At first glance, I thought there was a flaw in the energy coming from the treadmill. I will need to think about it harder when I have the time and mental energy.

I am still skeptical but I have become intrigued by the efficiency of propellers as shown in those videos.

[Rick Willoughby]

Sorry spork. The new plane will have to wait!

Actually why don't you set up a web site for a DDWFTTW cart. Have you got some way to collect donations. Maybe the DDWFTTW foundation!!

Set out a plan with timeline and budget. See how much interest you can scare up. Maybe produce a little sponsorship memento. I would be prepared to donate. If some people are serious about $100k bets surely they would be prepared to give a donation to see it proven beyond doubt.

Go back to all the old forums and see what sort of response you can get.

You may even be able to get things like props and gears donated or at least at cost. if you publicise it. I can give you some contacts in this regard. No point doing unless it is well publicised.

Rick W

[Richard Atkin]

Spork, my brother took a look at it (I was too lazy - or stupid) and he thinks it does work. He is almost always right when it comes to things like this. Rick is always right aswell. I should've seen this coming....aaaaaaaagh...egg on my face. I will get my brother to explain it to me later.

ALL BETS ARE OFF, DUDE!!

I was never serious about taking your money anyway (that's the truth). I just wanted to see your level of conviction. If it turns out that the idea doesn't actually work (not likely) I would still never bet $100K unless I was betting with a rich person and I knew I couldn't lose.

It would be a fun toy to use on the cross-country cycleways currently being built around New Zealand if it is used in conjunction with pedals so the propeller doesn't need to be so big. Wind resistance is the biggest downer for cyclists.

[Richard Atkin]

Rick, his new website should be SPORKvsDORK.com

[spork]

Richard, I give you big points for keeping an open mind about this. There have been LOTS of people that insist this is impossible, and then simply become more and more insulting. You're on a very short list of folks that have said "hey, maybe I was wrong".

That being said, let me ask you if you're aware that ice-boats can tack their way downwind faster than the wind (and some sailboats can as well)?

This is a pretty well documented phenomenon. For the ice-boat guys, it's just an everyday thing.

I'm not saying that tacking downwind is the same thing - but it's the first step.

[spork]

Actually why don't you set up a web site for a DDWFTTW cart. Have you got some way to collect donations. Maybe the DDWFTTW foundation!!

Set out a plan with timeline and budget. See how much interest you can scare up. Maybe produce a little sponsorship memento. I would be prepared to donate. If some people are serious about $100k bets surely they would be prepared to give a donation to see it proven beyond doubt.

Go back to all the old forums and see what sort of response you can get.

You may even be able to get things like props and gears donated or at least at cost. if you publicise it. I can give you some contacts in this regard. No point doing unless it is well publicised.

Rick W


I suggested this approach to JB as well. He doubts that we could get donations. I also did some research on blades, and have a good line on a set that I think would work great. Normally they run $2K, and I've got a guy with a set in his garage that will take $1K. Maybe I just have to commit to the idea of building this thing once the boredom sets in this winter.

[Richard Atkin]

Well Spork and Rick, my brother drew a diagram (very similar to Rick's explanation) and I didn't understand it because I am not used to thinking in physics equations. I never listened to my physics teacher at school.
But then I saw the 'wheels under the ruler' demonstration and noticed it looked very similar to my brothers diagram. BAM!!! My brain suddenly twisted itself in the right direction.

The ruler (wind) is trying to turn the big wheel (propeller) clockwise which in turn, tries to turn the little cotton reels (wheels) anticlockwise, but the ground exerts an opposing force on the cotton reels. Because the ground is effectively 'pushing' the lowest gear, it wins as far as the rotational directions are concerned. The end result is that while the ruler is pushing against the big wheel, the big wheel is ALSO pushing against the ruler. This must cause the vehicle to accelerate while the ruler remains at a constant speed. No matter how fast the big wheel travels in relation to the ruler, the ruler will always be able to push against it. As the vehicle continues to accelerate, it must be trying to slow the ruler down, but the ruler continues to collect kinetic energy from the person pushing it.

The wind is just like the ruler. The cart's propeller is actually trying to slow down ALL of the wind existing over it's travel path, but the wind's speed is maintained by the wind's source. The wind is slowed AT FIRST by the cart's resistance, but the wind will MAINTAIN it's own slower speed after that, without depleting (like the man who continues to push hard on the ruler which is moving slower than expected). I think this is where many people are getting confused. The cart's energy doesn't come from nowhere. It's nothing more than gearing.

I will post my brother's explanation soon (much better than mine).

By the way, I don't think it's necessary to include winnie the poo!! :D Are you trying to insult people to encourage them to bet $100K?

[spork]

By the way, I don't think it's necessary to include winnie the poo!! :D

Perhaps not, but Michael C. (who made that video) is very sharp, and gifted when it comes to explaining subtle or complex concepts.

Are you trying to insult people to encourage them to bet $100K?

Not at all. I try very hard to have the discussion and explain how and why it works. I only offer the bet to people that are absolutely certain it doesn't.

I'll take a look at your brother's explanation. There are actually several correct ways to look at it - and some very interesting and counter-intuitive insights that come from understanding those explanations.

[Richard Atkin]

My brother re-posted his explanation with a correction. He gives his apologies

[Richard Atkin]

Probably the best way to first explain it to unscientific people like me, is to say "The propeller is never speeding up an apparent head wind - it is always slowing down a tail wind." (a scientist would rather use the term 'pressure differential' but my wording is better for us teddy bear huggers).
That ought to push a few critics in the right direction. From there they can learn how the whole system is based on gearing - not free energy.

Spork, I was thinking, if you actually build one that you can sit in, it will be impractical and dangerous without a big heavy protection cage, and all the non-believers will continue to non-believe. If you're going to do it, you will need to do official speed trials (no cage) in a spectacular way, on a windy day with lots of jounalists and hype so it gets in the magazines. I agree with Rick, publicise it. It is a popular story - 'Free Energy' and all that. Be a rock star....it's the only way.

[Rick Willoughby]

Peter Sharp kindly gave me reference to a proven device that clearly demonstrates how DDWFTTW craft operate. In this case the wind is replaced with a wire that is pulled from the aft of a torpedo. It is exactly the same concept as DDWFTTW only the wire, pulled from shore, replaces the wind.

If you can grasp how this works then you can appreciate how a DDWFTTW craft or vehicle operates.

It proves that the idea did serve a practical purpose. The details are given here:
http://en.wikipedia.org/wiki/Brennan_torpedo
As can be seen the performance was impressive.

Rick W

[spork]

That torpedo had been mentioned on other forums. I agree completely of course. I assure you it goes nowhere with the deniers.

If you're interested in seeing the truly hard of thinking, we have two such people that are astonishingly so here:

http://talkrational.org/showthread.php?p=610758#post610758

You should try the torpedo argument on them - but be prepared for extreme frustration.

[ThinAirDesigns]

San Jose State University Aero Professor and Stanford Phd Dr. Nikos Mourtos along with a team of students and advisors have taken on a project to construct and document DDWFTTW in a more thorough way than ever before. Their goal is to achieve a documented 2x windspeed DDW.

Follow their blog at www.fasterthanthewind.org

Remember that in the blog format the latest posts show up on top. To view the entries from the start, click on the "2009" link on the right (below the list of followers) and scroll to the bottom.

Enjoy.

JB