DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. tom28571
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    tom28571 Senior Member

    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.
     
  2. Guest625101138

    Guest625101138 Previous Member

    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.
     
  3. Fanie
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    Fanie 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 -
    I think it is very doable, I'm currently getting things together to sail directly upwind, no tacking.
     
  4. tom28571
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    tom28571 Senior Member

    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.
     
  5. yipster
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    yipster designer

    thanks all i will read again to make sure couse this one kept me awake
     
  6. Guest625101138

    Guest625101138 Previous Member

    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.
     
  7. yipster
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    yipster designer

    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
     
  8. Guest625101138

    Guest625101138 Previous Member

    Not quite bicycles, most are tricycles, but wind powered road vehicles are getting serious attention right in your backyard:
    http://www.windenergyevents.com/Picture gallery.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.
     
  9. Fanie
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    Fanie 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
     
  10. tom28571
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    tom28571 Senior Member

    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.
     
  11. Guest625101138

    Guest625101138 Previous Member

    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.
     
  12. tom28571
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    tom28571 Senior Member

    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.
     
  13. robherc
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    robherc Designer/Hobbyist

    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?
     
  14. Guest625101138

    Guest625101138 Previous Member

    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?
     

  15. robherc
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    robherc Designer/Hobbyist

    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!
     
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