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  1. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Yes, that is pretty much how it works. By pushing the nose down when entering a downdraft you will gain more kinetic energy than you loose in potential energy, because you accelerate the sinking air upwards taking kinetic energy from it. As soon you leave the downdraft you pull up again, and...
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    Sailing trivia (get in here if you like airplanes and treadmills)

    M'kay. The version for the smartass: Can a glider flying in an area with only still air and some downdrafts, but no updrafts, reach the same altitude and groundspeed that he had 1 hour earlier during the same flight Let's stay in the lower atmosphere for now. :D
  3. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Where exactly do they say this?
  4. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Correct. Most sail boats wouldn't make it. But the USA-17 that BlackJack mentioned could do the following trick: Start at the downwind mark and simultaneously release a balloon at the upwind mark. While the balloon floats: upwind mark -> downwind mark the boat goes: downwind mark ->...
  5. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Next question: Can a glider (plane without engine) stay airborne permanently in an area with only still air and some downdrafts(sinking air), but no updrafts(rising air)?
  6. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    :thumbsup: Correct, efficient sail boats could do it. It is symmetrical to the downstream race: Downstream race : Relative to the airmass B is going downstream, faster than the stream. Upstream race : Relative to the water B (and C) are going downwind, faster than the wind.
  7. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Yes, thanks for the picture. No motors. Just sailing upstream. Which one(s) can do it?
  8. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    No takers on this?: Boat A: Current : 10mph (relative to ground) Wind : 10mph (relative to ground, same direction as current) Boat B: Current : 10mph (relative to ground) Wind : 0mph (relative to ground) I will add a bonus case Boat C: Current : 20mph (relative to ground)...
  9. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    They are probably just lazy programmers, and didn't nomalize the velocity vector. So when you press UP+LEFT you run 1.41 times faster than just with UP pressed. That's how it was in DOOM IIRC.
  10. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    No, he just changed the perspective. This is what an obsever floating in the water will see. And all observers must agree which boat wins.
  11. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    To make it clear for the others: Boat A: Current : 10mph True wind : 10mph down the current. Boat B: Current : 10mph True wind : 0mph Can any of the boats go against the current (up the river)?
  12. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Okay, but what about going upstream in the same wind conditions as in your question. Can one of the boats reach a point up the river from the starting point?
  13. R

    Sailing trivia (get in here if you like airplanes and treadmills)

    Yes A is not making any progress downstream relative to the water. So any progress downstream relative to the water that B makes will bring it ahead of A. It is trivial for a boat to make some minimal progress relative to the water, in the opposite direction than the airmass moves relative to...
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    D d w f t t w

    http://forums.anandtech.com/showthread.php?t=2151815
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    Wind powered vehicle, or perpetual motion machine?

    There are no different principles utilized by different foils. Just different laws (based on models of different abstraction level), which are satisfied by all foils. Every foil accelerates the fluid opposite to the force it creates (Newtons 3rd). Every foil obeys Bernoulli's Laws. But if by...
  16. R

    Wind powered vehicle, or perpetual motion machine?

    Yeah, I guess you mean the difference between attached flow at the blades and separated flow. But keep in mind that even with propellers the aim is to have attached flow, like on a wing.
  17. R

    Wind powered vehicle, or perpetual motion machine?

    It is exactly the angles a sailcraft would use on a broad reach with downwind VMG > windspeed. Look at the diagram below and replace: boat velocity -> blade velocity downwind VMG -> vehicle velocity As you see vehicle velocity(downwind VMG) is 1.5 x true wind, and the blade is still pulled...
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    Wind powered vehicle, or perpetual motion machine?

    It's not wrong. But the transition between the cases 1-3 is smoother than you describe it. The airfoils are completely stalled at first, and then with increasing speed they gradually become unstalled (from the tips inwards) and start to work more efficiently. Some of the differences you try...
  19. R

    Wind powered vehicle, or perpetual motion machine?

    A point on the propeller blade moves in a different direction than the vehicle . It is simple vector addition: blade_velocity = cart_velocity + tangential_blade_velocity And since tangential_blade_velocity is perpendicular to the cart_velocity & wind, the blade_velocity is not parallel to...
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    Wind powered vehicle, or perpetual motion machine?

    Propellers spin like that. Turbines spin the other way: http://www.youtube.com/watch?v=nudBjrOF3LE The propeller blades are going at an angle to the wind: http://www.youtube.com/watch?v=UGRFb8yNtBo
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    Wind powered vehicle, or perpetual motion machine?

    Nope. Perpetual motion would be an isolated system, that keeps going. This is not isolated at all. It takes energy from the wind (air motion relative to ground) which is slowed down.
  22. R

    Wind powered vehicle, or perpetual motion machine?

    That means a backwards force on the air, which slows the air down relative to the ground. Not out of nothing. Out of the true wind.
  23. R

    Wind powered vehicle, or perpetual motion machine?

    No, it is taking energy from the true wind, which is slowed down by the vehicle.
  24. R

    Wind powered vehicle, or perpetual motion machine?

    Start here: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good Example vector diagram: Animated diagram: http://www.youtube.com/watch?v=63hvQABLOaE The air has always a relative velocity to the rotating airfoils, Nope: But I'm sure he could do...