Downwind Faster Than The Wind - update

[Regis-de-giens]

I must say i am fully and even 200% in line with spork analysis.
And let me rephrase my "rotating spinacker" story, i was unclear in my analogy, sorry:

- i agree , it is abolutelly not possible with a classic sail boat or a kite , even by looping the kite or whatever (to loop, the kite need wind comming from backward, so direct down wind is not possible. Definitelly.

- i was just refering to a spinacker because a spinacker is just "pushed " by the wind fully perpandicular, hence absolutelly no mention to L/D ratio is possible here. In other word, it is like a door in front or the wind that wants to open... no AoA at all, or say 90 degree if we can say like that...

Nothing to do with apparent wind (in the sense of vehicule speed). Nor inertia, nor momentum...

Blade are turning with an angle just to compensate part of the vehicule movement and BAD influrnce of apparent wind , by still "presentinh a surface" on which the wind coming from the back of the vehicule can still push the blade likeca door (exactky as it does it at the start of the vehicule, easier to understand at that 0 instant).

Lets see it like this, theoretically (and once "visualuzed, it is very easy to understand):

- imagine the angle of blade is such that in the blade surface goes backard exactly as much as the vehicule goes forward during one turn of the wheels (it is just a mattter of choosing the good angle).

- then, you will agree, at start , 0 speed, the wind pushes on the blades, immobile. So the vehicule starts moving downwind.

- then at very low speed, the whhels impose the mouvement of the blades (with no or very few counter reaction (*) ) , and it still works the same: the wind pushes the surface of the blade, even if the blade turns (but we just do not care that it turns, it it still a surface that "opposites" to the wind, amost perpandicularly, even if rotation is not mechanically used and necessary at that stage.

- (*) explanation here : the blade surface rotating exactly at the speed of the vehicule, there is no additional draft of the blade due to apparent wind (negative) of the vehicule. On contrary ... anyway.

- when the vehicules reaches wind speed : i do not see any change in the principle nor on the forces in place (only the structure of the vehicule stops beeing pushed by the wind): the angle of the blades is still the same = calculated to go "backward" at sale speed as the wheel of the vehicle = if you take now the ground as reference, the blade surface is immobile ! So it still catches the wind from backward (and not from frontward like would a classic rotor do).

- so in theory there is no limit and the vehicule can accelerate always and always ...

- but you have frictions that you cannot totally emiminate (wind frontward on the structure, wheels, mechanical drivings, wing thickness surface hat cuts the air), so you stop when they become too high

This example of angle exaclty compensating the speed is for illustration. It also works at lower angle, it just leads to better performance at low speed (higher acceleration, easier to tackle frictions), but final speed will be limited . If angle is only 50 % of wheel, then the assymptote of speed will be twice the wind...

Please let me know if i was clear. And very interested by Spoke comment on this vulgarisation

[kiteykitekite]

Interesting you are all still arguing this thread...Without reading it new, I will add my bit.

You can go dwfttw with a kite or sailboat for the same reason you can go upwind. That is you can ride into the wind. In the case of going dwfttw you approach a speed and direction that causes the apparent wind to come more from downwind, you then ride into this wind. It would be something you progressively build speed and direction into, the faster you can go the better this will work. For more speed you need higher L/D, but particularly at higher speeds. Which means having not just high L/D gear (kite and board), but gear that is appropriately sized for the target apparent wind speed. This is so your AoA on either board or kite is for best L/D and so goes fastest.

As to the propeller machine, it is a little different. It is generating a thrust that pushes off the wind. The propeller generates the thrust. The traction with the ground spins the propeller, not the wind. The thrust off the propeller against the wind pushes the device which allows it to generate the traction.

Don't get me wrong though a kite does generate a thrust too, it's just a kite and this propeller setup are very very different. You have no mechanical connection to the traction to generate movement of a kite. As well you can not loop a kite well while going upwind can you? then such motion won't help going dwfttw. You could maybe sine the kite but that would just be for a phase and if you had to do it constantly it would mean you are on the wrong size I think, being too small.

[Regis-de-giens]

I think you just forget one thing : we are not talking about dwftw but Ddwftw. Meaning DIRECT dwft, at 180 degree from the wind.

With a kite you cannot, whatever the LD ratio. You cannot count on your apparent wind only , with no "mechanics system " to force the kite, because the wind drives the kite.... you cannot...

Ok, you can have a negative vmg higher than wind , yes agree. But not a direct downwind (i.e. with 180 degree to the wind).

and ...Counting on your apparent wind only is .... impossible (it would be a case of perpetual movement which physics and energy conservation does not allows on earth )

[spork]

Regis-de-giens - if I understand your analysis, I think you're caught in one of the very common misconceptions. The wind from behind never actually reaches the blades at all once we're going faster than wind speed. The reality is that the blades are now pulling us into new air that's in front of the vehicle. In fact, on some of our runs we would wait for a wind gust, and then wait another 30 seconds or so before starting. We could then go and catch up to that wind gust and take advantage of it when we did. The only reason this can work is because we're moving more slowly through the air than we are over the ground. This means we can harvest more energy at the wheels than we need to pull ourselves through the air.

[spork]

Interesting you are all still arguing this thread...Without reading it new, I will add my bit.

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

You can go dwfttw with a kite or sailboat for the same reason you can go upwind.

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.

As to the propeller machine, it is a little different. It is generating a thrust that pushes off the wind.

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

[Herman]

Just for info for Spork! Some of the last few posts have been a little cross threaded. I was quoted from the "what does apparent wind bring" a Regis thread, if memory serves. Not sure it's worth the read though! Loved your presentation on the DDWFTTW machine.

[Regis-de-giens]

Regis-de-giens - if I understand your analysis, I think you're caught in one of the very common misconceptions. The wind from behind never actually reaches the blades at all once we're going faster than wind speed. The reality is that the blades are now pulling us into new air that's in front of the vehicle. In fact, on some of our runs we would wait for a wind gust, and then wait another 30 seconds or so before starting. We could then go and catch up to that wind gust and take advantage of it when we did. The only reason this can work is because we're moving more slowly through the air than we are over the ground. This means we can harvest more energy at the wheels than we need to pull ourselves through the air.

Ok i get your point about the vehicule going faster vs ground while slower vs wind. It brings a efficient approach based on energy to proove it works : may i reformulate, by saying that power is equzly to force x speed, hence at steady states, for a given force, you get more power from the wheel turning faster than from the blades turning slower (vs their respective contact point, So the difference can compensate for all frictions and drags or generate acceleration downwind. Wonderul, clean and it opens my eyes smartly. Ok thanks again for your time for this answer.

However , to me it does not goes against my approach , which is more based on a "particule point of view " rather than global energy balance....

So I have 2 questions:
1- what is wrong in your opinion in my analysis ? it works theoretically i think, and would like to catch where is the misunderstanding in your opinion. Maybe my explanation was not clear enough, on the word "surface" maybe.

2- what is the ratio of the blade rotation vs wheel speed and what is the AoA of the blades ? I am curious to know if the blade surface goes faster or slower than the vehicule (excuse my english, not easy to explain). I think it should work whatever the blade angle , superior or inferior to my illustration case set at "100%" then 50 % . Just to know your thought please.

Ps: maybe my word "surface" is confusing but i do not know how to say better : i speak of the "projection" of the blade on X axis during the rotation. Nog more clear, but ...
Merci

[spork]

So I have 2 questions:
1- what is wrong in your opinion in my analysis ? it works theoretically i think, and would like to catch where is the misunderstanding in your opinion. Maybe my explanation was not clear enough, on the word "surface" maybe.

It's possible that I didn't understand it properly. 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...

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.

Are we saying the same thing?

2- what is the ratio of the blade rotation vs wheel speed and what is the AoA of the blades ?

That ratio is everything. 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.

[PrfctChaos]

Not entirely sure why this is a theoretical discussion. Racers currently reach downwind VMG in excess of wind speed, so if he does a downwind leg to the left, then jibes and does a downwind leg to the right, he gets to the point directly downwind of his starting point before the air that he started in. Effectively directly downwind faster than the wind. Now imagine him just redoing this 5 or 10 or 100 times and you see he will be moving in a channel (of certain width) directly downwind faster than the wind.

That means the only thing to define is how narrow you want to make this "direct downwind" channel he is allowed to move in. If you make it 100m wide, then he needs to jibe every 100m to stay in the channel. If you make it 50m wide, it is just a question of jybing twice as often. Sure at some point the channel gets practically too narrow, but it is just a question of scale. A tiny scale model of a kite racer could operate in a very narrow channel.

Sporks machine does this (with the wheels standing in for a very effective keel / foil) and rotating wings, so can go in straight line, no jibing (Or the wings are constantly jibing, however you want to look at it).

[spork]

Not entirely sure why this is a theoretical discussion. Racers currently reach downwind VMG in excess of wind speed

I think it's just a discussion about how that works, and what other possible ways one could do it.

That means the only thing to define is how narrow you want to make this "direct downwind" channel he is allowed to move in.

Sort of. I mean if the objective is to get downwind faster than the wind in an arbitrarily narrow channel - then sure. The original brain-teaser I posed was to see if you could actually go *directly* DWFTTW. Just as an exercise. It wasn't intended to be practical.