The Wind Powered Sail-less Boat

Typically the propeller efficiency is defined by

Eta = F * V /(T*w),

where F is the thrust force (N), V the velocity at the propeller ("without prop", m/s), T torque (Nm) and w rotational speed (rad/s). So the Cessna prop has zero efficiency and is not doing any work for the Cessna. Just like a weightlifter is not doing any work while holding the weights up or a car spinning wheels with the brake on.

Efficiencies can be defined in a different way, but the definition above is the one that makes most sense when dealing with propulsion. E.g. the Cessna prop is doing work for the air accelerated by the prop and would have a non-zero efficiency, if the job of the prop would be to move air and not the Cessna.

Joakim

 

Thanks for the clarification. I assumed it must be a definitional matter, and you're right (of course) that your definition makes the most sense from the point of view of the Cessna. I was thinking in terms of pure propeller efficiency. In this case I think it's more meaningul to look at work done by the prop on the air divided by work done on the prop by the prop-shaft.

 

For prop efficiency at zero airspeed it is advisable to use the figure of merit instead of eta which is zero at zero airspeed.

F.O.M. = T(Va + Vi)/P

T is thrust , Va is airspeed , Vi is induced velocity at propeller.

It is also important to define what is meant as large when talking about the prop diameter - large in relation to what? I would be inclined to say large compared to the designed thrust force ..

<<<<The total resistance is not well at all modeled with D=c * V^2.>>>

For a rough calculation it is not so bad and it is actually quite accurate for certain velocity windows , if you have a suggestion that is also relatively simple but would model D better please post it.

I am not going to attempt a detailed model straight off the bat , rather i am going to create a rough model that gets the larger terms in their correct relation first and then later build on , and refine that model.

So for starters i am not going to use e = e(W, V,....) but instead assume the prop is infinitely variable in its pitch, diameter, airfoil , etc. Then later i can put the refined value of e in and later still see what kind of special engineering problems we might be facing and see what if any might be viable solutions.

 

Like you Tcubed, I found it incredibly intriguing. I've done a full analysis as well and have posted a lot of it right here on this thread. In the end, for critics of the device the most important analysis of all should relate to "does it work as promised?". That particular question is a simple one for the critics to answer for themselves -- in less time than it takes to write computer simulations one can just build it for yourself with simple tools.

http://www.youtube.com/watch?v=T-ArigMKhi4&feature=PlayList&p=B9620506705D182C&index=0&playnext=1

This whole "academics over application" attitude that the critics take has always mystified me. We're not talking origin of the universe here -- only a simple toy originally made from the RC Heli scrap heap.

I greatly respect those who have wrapped their heads around the workings of the device and say "Wow - that's very interesting, I want to work out and understand the math behind it". What I don't get are those who ignore the absolutely independently repeatable results that the device affords and say in effect "show me the math the way *I* want to see it or it never happened".

Here's a perfect example from Guillarmo to spork:
>Mmmmmm, it could seem you're just avoiding a
>road which perhaps can come to a not desired
>end to you

There is no math done properly that "can come to a not desired end" for this device. It doesn't care about the math -- it just goes faster than the wind driving it straight downwind. Try it for yourself and see.

I'm going to steal two statement from a very wise but uninvolved friend of mine regarding this device:

"This has been independently repeated and confirmed so many times now that it's no longer a physics experiment, but rather a sociological one."

"Sure it works in practice, but can you prove it in theory?"

JB

 

Sorry masalai -- fixed.

JB

 

I think he meant to post links to my series of "build" videos:

Build video 1 of 3:
http://www.youtube.com/watch?v=T-ArigMKhi4&fmt=18

Build video 2 of 3:
http://www.youtube.com/watch?v=p0rhgop5wEM&fmt=18

Build video 3 of 3:
http://www.youtube.com/watch?v=gSHNqrF93MU&fmt=18

 

Thanks, Tcubed.
I'll wait for your results. I'll then ask you a couple of further questions in the wake of this quiz game.

Keep it simple for me, please, just energy related reasoning, abstracting the apparatus itself and how it takes energy from the wind, which are secondary to demonstration purposes.

Cheers

P.S.
By the way, some of the kinematics or mechanics maths used in this thread as 'proving explanations' of the DDFTTW thing, are incorrect formal logics using premises which assume the conclusions (begging the question fallacies).

 

I don't think Tcubed is working on a "energy related reasoning" and he is certainly not going to keep it simple, since he is working on all angles, not just DDW.

Could you please be more spesific what you mean by your P.S?

Joakim

 

The analysis you asked for is anything but simple. However, I think what you're asking for now is simple and I'd be happy to oblige if you like.

Would you be kind enough to point me toward those "proofs"?

I did see some stuff earlier in the thread attempting to support DDWFTTW that I feel is in error, but I don't recall seeing any that beg the question.

 

<<<<<are incorrect formal logics using premises which assume the conclusions (begging the question fallacies).>>>>

I do know exactly what Guillermo means , however i do not know if i am qualified to attack the problem from that angle. However , i will add that to my list of goals in my analysis. I studied mathematics and fluid dynamics. Within my maths i leant heavily towards applied maths and the more concrete pure maths rather than logic theory, proof theory etc.

Personally i am satisfied with a classical mechanics analysis. Balance of force eqs for steady state and conservation of energy eqs. Dealing with unsteady state means dealing with differentials of everything so that would be at the very end of the analysis. First things first.

Spork , do you have any equations you can post instead of links?

 

Then please point me to those posts that beg the question - just for my personal interest. I don't recall seeing such.

Agreed. I feel the same way.

Agreed again. The most interesting aspect to me in that regard is the question of whether the prop-cart can self-start, achieve greater than wind speed, and maintain greater than wind speed directly downwind. For my purposes I'm satisfied of these facts both from the experiments we've done, and from my basic analysis. My basic vector analysis of an ice-boat on a 45 degree downwind tack shows me that an ice-boat can do exactly this - and I can show that the sail on the ice-boat is an exact analog of the prop blade on our cart. Therefore, if the transmission and wheels of the cart can serve as a kinematic analog of the blades on my ice-boat, we know the cart can do this as well (and I think that can easily be shown).

Sure, tell me what you'd like to see. Just the basic energy analysis for the cart at greater than windspeed is quite trivial. Is that what you'd like?

 

This may be way too late in the discussion to be of any use, but the way I wrapped my brain around the DDWFTTW concept was to remember qualifying for my helicopter license. One test was to shut down the engine and glide to a landing. The rotor falling down must be functionally equal to air rushing up through the rotor disc which keeps it spinning (if I'm careful) and not incidentally offers considerable resistance to the earthward plummet of the 'copter. Turning the whole thing sideways, it it no stretch for me to imagine that the spinning rotor disc of Goodman's "toy car which proves nothing" would allow the wind to help push the car along, and add enough energy to the process to overcome the mechanical losses in the wheel-to prop gearing, allowing the wheels to drive the car at a speed that increases until the additional push of the wind is reduced enough to allow the mechanical losses to limit the speed. So there really is a continuous energy input without which the system would not work.
Does that sound even remotely reasonable to those of you who really know why it works?
I highly recommed Goodman's original article in Catalyst, whic gives some details of his cart's construction and performance.
http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

 

<<<<Sure, tell me what you'd like to see. Just the basic energy analysis for the cart at greater than windspeed is quite trivial. Is that what you'd like?>>>>

Yes that would be nice. Just post everything you got while you' re at it , if you can. Thanks.

 

Hi Timothy22:

Different things make it click for different people, but as you requested I do have a couple comments regarding your heli analogy:

First, in autorotate mode, the heli blades are operating as turbine blades until that last moment. In DDWFTTW operation, the blades on our cart never act as a turbine but rather as a simple propeller.

Second, at one point in your post you say " ... allowing the wheels to drive the car ...". The force on our wheels is always a braking force and not a force that drives the car directly. It is the thrust from the propeller that drives the car forward against the resistance on the wheels.

Best wishes.

JB

 

The helicopter autorotation thing is quite interesting too and is another separate (but somewhat related) subject. Autogiro sailboats have been experimented with too.

Just to clarify here for readers;
Auto-rotation requires negative collective pitch in order to sustain itself. The axial thrust is orders of magnitude greater when using auto-rotation than if the blades were locked and stalled. This is why it is better to lock your aux. motor prop when sailing instead of letting it freely spin. However , the difference is not nearly as great as in the case of a helicopter or similar , due to the solidity and pitch of a typical engine prop.