I "GET" it now !
Thanks : Bille
I "GET" it now !
No, but if you have a couple of hours to waste on a really boring video, you can watch me try to explain it at the St. Francis Yacht Club...Are the physics behind it explained somewhere in this thread?
Yes and no...I understand that when you are going the same speed as the wind, the apparent wind is 0, but because of the kinetic energy of the cart driving the propeller, it's basically the same as driving the propeller in no wind...
It's true that it becomes harder to achieve multiples of wind speed in higher wind. This is because it requires a very efficient overall process to get high multiples, and you lose lots of efficiency when you begin to get the tips of the propeller in the realm where the flow is compressible (around Mach 0.3). I think we achieved about 3.1X wind speed in 10-15 mph wind unofficially, and 2.8X wind speed in 10-12 mph wind for the record runs.I can imagine this only working in very little wind to reach more than 2x windspeed, am I right?
Interestingly we have run the vehicle in two modes - upwind and downwind. In the downwind mode the propeller is *always* a propeller (which is to say that the wheels turn it) below, at, and above wind speed.
Initially we did not implement variable pitch on the propeller. It still worked fine going directly downwind faster than the wind, but the startup is very slow (which was to be expected). We later added variable pitch, and that saved us a lot of runway in getting up to speed. But we never move the pitch from pusher to tractor or vice-versa on any run....figured you could change the angle of the propeller to pull instead of push...
Yes - that's exactly what I said (now that you caught it and I changed it )I assume in the post above you meant “ ‘upwind’ mode, the big spiny thing acts as a turbine”?