On Wed, 22 Sep 2010 16:01:42 +0100, you wrote: >This must be a scam. As Jed said, at the point where the craft is going >downwind at the speed of the wind, the relative wind across the propeller >would be zero so it could not accelerate from this point on. If it did, the >force from the prop would reverse anyway. Even more obviously, if it can >accelerate from a position of zero relative wind then one could start it off >in no wind conditions and it would accelerate - perpetual motion just isn't >that easy!
--- I've attached a drawing which shows basically how Thin Air Design's "Blackbird" vehicle works. Note that with the wind pushing the cart and the pitch of the propeller as shown, the wind would, intuitively, be forcing the propeller to rotate counter-clockwise as viewed from the rear of the cart. However, such is not the case. What's really happening is that the wind is pushing on the prop, forcing the cart to move forward, and the torque generated by the wheels is coupled to the prop in such a way as to cause the prop to rotate clockwise when viewed from the rear. This direction of rotation makes the prop a "pusher", and will increase the apparent force of the wind. As long as the wind is blowing from the rear, the cart will accelerate until it reaches wind speed, when the wind speed will effectively be zero. However, because of the prop's action as a pusher, the cart will be going a little faster than wind speed, at wind speed. Then, as soon as the prop feels the headwind it'll stop being a propeller and will become a turbine, driving the wheels and accelerating into the headwind until, eventually, everything settles out and the cart reaches its speed limit. --- --- JF
