I don't think there's anyone left arguing it really. Just a friendly discussion.
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.You can go dwfttw with a kite or sailboat for the same reason you can go upwind.
Yes - sort of. It's not pushing off the wind behind it. It's pulling itself through the wind ahead of it.As to the propeller machine, it is a little different. It is generating a thrust that pushes off the wind.
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.spork wrote: ↑Sun Mar 22, 2020 1:23 amRegis-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.
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...
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.2- what is the ratio of the blade rotation vs wheel speed and what is the AoA of the blades ?
I think it's just a discussion about how that works, and what other possible ways one could do it.
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.That means the only thing to define is how narrow you want to make this "direct downwind" channel he is allowed to move in.
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