Downwind Faster Than The Wind - update

[Regis-de-giens]

Once again, I am fully in line with all your statement, and I believe it is still coherent with my view, that I maybe had bad explained ; at least from your explanation I think so, let me detail:

Many people claim that the propeller is pushing air aft, creating a sort of air cushion that the tailwind can apply a force to (since the tailwind can't catch up with the propeller). That's sort of what I took from your description. Let me offer mine in a different way...?

I fully agree with your below description, and also consider that it does not goes against mine ; about "air cushion" , I never presented it like this, and I prefer to speak about where comes the air particules: tailwind or in front of the blades; all is about the blade angle. But I personally think that yes wind could come from back of the blades in certain configuration (at least): with a very little AoA or ratio (or even 0% to illustrate), or at the very start, the wind is pushing the blade by tailwind, right? do you agree at that 0 speed step ?

Consider how a propeller works on a small plane. First let's do it with no wind. The plane is flying along and the propeller is interacting with only the air it encounters in its path. It produces forward "lift" by thrusting that air aft. Now let's imagine that same plane flying with a tailwind. In this case absolutely nothing changes about the physics. It's just a change of reference. Galilean relativity tells us that the two situations are actually identical. So the prop in this case is not interacting with the air any differently than in the first case. It's not interacting more with the air from behind the plane etc. It's just that the entire experiment is now moving along with the wind. And if you (the observer) happen to be moving along with the wind (maybe you're floating in a hot-air balloon), you literally couldn't see or measure anything different (except to note the ground beneath seems to be moving by you as if it's a conveyor belt.

200 % agree and I must say you explain well and concisely your point ; fully agree, nothing to add ;
just a small remark I am too passionated; sorry . To me this is the case where the Angle/speed of the blade is high ... higher than the wind speed related to the blade mast... do you agree ?

That ratio is everything.

I also think this is and the beauty to build one

For the vehicle to go DDWFTTW the prop has to be trying to advance through the air more slowly than the wheels advance over the ground. Keep in mind the two are connected by a drive chain. The closer that ratio is to 1:1, the faster the "design speed" of the vehicle as a multiple of wind speed. But at the same time, the more efficient the vehicle needs to be to work at all. A relatively crappy vehicle can go DDWFTTW at 1.1X wind speed, but it requires a pretty efficient vehicle (i.e. low rolling resistance, low aerodynamic drag, low rolling resistance, low transmission losses...) to make a vehicle that goes 3X wind speed DDWFTTW. The AoA of the blades is a tricky question as AoA varies along the span of the blade and depends too on the vehicle speed relative to the wind speed, etc. But the pitch of the prop, as described above, must be such that it's trying to advance through the air more slowly than the wheels advance over the ground. The pitch is positive at all times. Our prototype used a fixed pitch prop, but we upgraded it to a variable pitch prop before attempting to establish a record.

So happy to hear this story about 1:1 (this is why i use the particular case of ratio 100% in my demo, to cancel the effect of apparent wind up to tis maximum) and then 50 % to say it would be easier to cover the frictions and accelerate but with an assumpte at (1 / 50% = 2), so twice the wind speed ;
(*) side note : when thinking again at your good energy point of view and Galilea relativity, I am wondering if this is still true actually , but let discuss this later one to first conclude ou my analysis please;

So, I have a last question ... which is a bit a trap but which could help ensure of our both understanding ... what happens if the ration is superior to 1 in your opinion ? And have you tried that ?

[kiteykitekite]

I don't think there's anyone left arguing it really. Just a friendly discussion.

No I am amazed it is continuing, though thinking on the problem it does ponder as interesting. How is it something of a low velocity can impart a greater velocity on something else? In aerodynamics we see that this is possible when L/D is greater than 1. It must be the sum of the lower velocity equals a greater velocity or a pressure effect. If you look at something like rockets you can have both an exhaust velocity that is slower than the rocket as a speed relative to earth measuring the rocket and faster than the rocket relative to it. Interesting perspectives to think with.

Yes, with a sufficiently efficient craft you can make downwind progress faster than the wind. But you have to do it by tacking. A traditional sailing craft can never maintain *direct* DWFTTW in steady state.

The propeller of these special machines are actually tacking through the air in the same way a sail craft is.

Yes - sort of. It's not pushing off the wind behind it. It's pulling itself through the wind ahead of it.

Well to me yes that is a valid thought, just as valid as the rear one. Valid for the reason that the effects continue out into the gas and back. There has to be though a momentum exchange at the blades, and it gets this momentum from the air in front. So if it runs into dead air it will slow and stop.

Thinking more on this the momentum exchange is tied to the ground. It must be building an acceleration between the differences. Like dynamic soaring does I think. Watch this model glider go 514mph in a circle, powered by just the wind



Oh and Regis you thought turning was so bad?

[eyytee]

For the vehicle to go DDWFTTW the prop has to be trying to advance through the air more slowly than the wheels advance over the ground. Keep in mind the two are connected by a drive chain. The closer that ratio is to 1:1, the faster the "design speed" of the vehicle as a multiple of wind speed. But at the same time, the more efficient the vehicle needs to be to work at all.

So, I have a last question ... which is a bit a trap but which could help ensure of our both understanding ... what happens if the ration is superior to 1 in your opinion ? And have you tried that ?

If the advance ratio (A) is greater than 1, the vehicle can go upwind (against the true wind):

A < 0 : downwind, slower than true wind
0 < A < 1 : downwind, faster than true wind (if efficient enough)
A = 1 : no movement (forces from wheels and rotor balance)
1 < A < 2 : upwind, faster than true wind (if efficient enough)
2 < A : upwind, slower than true wind

[eyytee]

If the advance ratio (A) is greater than 1, the vehicle can go upwind (against the true wind):

A < 0 : downwind, slower than true wind
0 < A < 1 : downwind, faster than true wind (if efficient enough)
A = 1 : no movement (forces from wheels and rotor balance)
1 < A < 2 : upwind, faster than true wind (if efficient enough)
2 < A : upwind, slower than true wind

Here an analogy to simpler gear models:

[spork]

I personally think that yes wind could come from back of the blades in certain configuration (at least): with a very little AoA or ratio (or even 0% to illustrate), or at the very start, the wind is pushing the blade by tailwind, right? do you agree at that 0 speed step ?

First off - sorry for my delayed response. You're right - when the vehicle is going downwind well below wind speed, the air is hitting the back of the blades and pushing the entire vehicle as a bluff body. For what it's worth, at this point the acceleration is painfully slow.

To me this is the case where the Angle/speed of the blade is high ... higher than the wind speed related to the blade mast... do you agree ?

I'm sorry, I don't understand the question.

when thinking again at your good energy point of view and Galilea relativity, I am wondering if this is still true actually , but let discuss this later one to first conclude ou my analysis please;

Fair enough. But again I'm a bit confused by the question. In any event, I'll mention that at steady-state you can always apply Galilean relativity to change reference frames without any of the physics changing (if we ignore Special and General relativity).

So, I have a last question ... which is a bit a trap but which could help ensure of our both understanding ... what happens if the ration is superior to 1 in your opinion ? And have you tried that ?

Ah yes. We did in fact try that. And we established a direct upwind record of 2.1X wind speed in that configuration. eyytee has a nice little chart showing how different values of that ratio will impact the performance of the cart. The way I like to look at it is this...
When the cart is sitting still and the wind is hitting it from behind, the wind is trying to blow the whole thing downwind as a bluff body. But at the same time it's trying to turn the blades as a turbine - in the opposite direction from what the wheels are trying to do with them. When the ratio is less than 1, the wheels win the battle and the cart goes downwind. When the ratio is greater than 1, the blades win the battle (acting as a turbine) and the cart goes directly upwind.

[eyytee]

If the advance ratio (A) is greater than 1, the vehicle can go upwind (against the true wind):

A < 0 : downwind, slower than true wind
0 < A < 1 : downwind, faster than true wind (if efficient enough)
A = 1 : no movement (forces from wheels and rotor balance)
1 < A < 2 : upwind, faster than true wind (if efficient enough)
2 < A : upwind, slower than true wind

Here a simplified diagram of the wheel-airfoil coupling:

[spork]

How is it something of a low velocity can impart a greater velocity on something else?

In the talk I gave I offered a few examples to try and illustrate how this can happen. Probably my favorite is the notion of pushing on the middle of a tall pole such that you move it at 5 mph. At that point the top of the pole (which is being tipped over) will be moving 10 mph - twice the speed you're pushing it with. So you can then imagine this pole is actually one spoke of a giant wheel and the hub is at the top. That gets us to the yo-yo analogy, in which you can pull on the string of the yo-yo, and the yo-yo will wind up the string and go faster than your hand that's pulling it until it hits your fingers.

In aerodynamics we see that this is possible when L/D is greater than 1.

Yes, but in this case the force and resulting speed tend to be nearly perpendicular to one another.

If you look at something like rockets you can have both an exhaust velocity that is slower than the rocket as a speed relative to earth measuring the rocket and faster than the rocket relative to it. Interesting perspectives to think with.

Rockets are a really interesting case for a number of reasons. If you consider the rocket in a vacuum, it doesn't even make sense to talk about the speed of the rocket. What's it moving relative to if not the atmosphere around it. To create thrust, all that's necessary is that you expel mass out the aft end (at any speed relative to the rocket itself). This leads to some incredibly counter-intuitive results when it come to orbit transfers.

The propeller of these special machines are actually tacking through the air in the same way a sail craft is.

Exactly. The only trick is that the blade is on a continuous helical broad reach tack. It doesn't have to keep jibing.

There has to be though a momentum exchange at the blades, and it gets this momentum from the air in front. So if it runs into dead air it will slow and stop.

Yup!

It must be building an acceleration between the differences. Like dynamic soaring does I think.

Yup! Except when going directly downwind, a traditional sailing craft is exploiting the energy available at the interface of the air and ground/water/ice. The DDWFTTW cart does the same thing. In this case it can continue to exploit that energy source even when it's going directly downwind at exactly wind speed. Dynamic soaring does the same thing - in a different way. It's more intuitive (at least to me) when you consider the way an albatross does it. It operates in the wind gradient near the surface. It gains altitude into the faster moving wind and pushes against that wind, then dives into the slower wind near the surface and pushes in the opposite direction. You can think of this exactly like a keel and sail in the wind and water, except that the bird's momentum is what "attaches" the two rather than a hull and mast.

[Regis-de-giens]

love the analogy with Albatros ; I was also thinking of putting a glider on a highway road : making it fly in the air, with non wind by just swaping from one line to the line in the opposite direction ... using its inertia and elevation reserve to go from one line to the other, then be lifted up by the opposite air flow created by car drags, and when sufficient high again, go back to the other line, ect ...

[Regis-de-giens]

If the advance ratio (A) is greater than 1, the vehicle can go upwind (against the true wind):

A < 0 : downwind, slower than true wind
0 < A < 1 : downwind, faster than true wind (if efficient enough)
A = 1 : no movement (forces from wheels and rotor balance)
1 < A < 2 : upwind, faster than true wind (if efficient enough)
2 < A : upwind, slower than true wind

Here a simplified diagram of the wheel-airfoil coupling:

rotor_carts_01.png

Well , I understand that a very high ratio could be used to go upwind; but if Spork has experimented the DDWFTW with a ratio of 2 , I do not understand ... maybe both work in a certain range, depending on initial velocity of vehicule and frictions amount ?
Also for the A=0 = no move ? : I would like to challenge that . To me the cart, with a 'neutral' thruster like A=0 will still be pushed when immobile : as if the blades were not turning ... no ?

[eyytee]

If the advance ratio (A) is greater than 1, the vehicle can go upwind (against the true wind):

A < 0 : downwind, slower than true wind
0 < A < 1 : downwind, faster than true wind (if efficient enough)
A = 1 : no movement (forces from wheels and rotor balance)
1 < A < 2 : upwind, faster than true wind (if efficient enough)
2 < A : upwind, slower than true wind

Well , I understand that a very high ratio could be used to go upwind; but if Spork has experimented the DDWFTW with a ratio of 2

Spork went upwind faster than the wind too, with a vechicle advance ratio 1 < A < 2. Note that this ratio is not the windspeed multiple achieved, which was more than 2.

Also for the A=0 = no move ?

No:
A = 1 : no movement (forces from wheels and rotor balance)