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 ?spork wrote: ↑Sun Mar 22, 2020 6:18 amMany 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...?
200 % agree and I must say you explain well and concisely your point ; fully agree, nothing to add ;spork wrote: ↑Sun Mar 22, 2020 6:18 amConsider 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.
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 ?
I also think this is and the beauty to build one
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 ;spork wrote: ↑Sun Mar 22, 2020 6:18 amFor 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.
(*) 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 ?