Originally Posted by
mollusk
The direction of the drag force is understood to be aligned with the freestream velocity. Much like weight (a force) is in a known direction in everyday applications so one need not explain the direction of the weight of an object on the surface of the Earth. It is understood. Since the direction of the drag force is known the specification of a single scalar quantity will define the force vector. The communication problem that we are having is that you want to make the drag force go in a direction other than in the direction of the freestream velocity. You want to align the drag force with the wheel instead and that is not the correct direction for the drag because the wheel is yawed and not aligned with the freestream. You are talking about a component of force, but it isn't the drag.
So, it is theoretically possible that there is an force in the plane of the wheel that pushes it forward if the wheel is yawed with respect to the freestream? Sure. Is it due to negative drag? No way. That is my main complaint. If it is true it must be due to lift.
The joy of semantics. Though I am not a rocket scientist, physicist, or any one else who should have any say on this thread, in great BF tradition I'll post anyway. It seems to me, based on my lay knowledge, that the language used in the articles on the wheels to describe what is going on is imprecise and probably a bit of a result of trying make it sound good to the reader. It is, however, possible, that these wheels may act in a manner that (as I read it) causes something
other than rider input to contribute to forward motion (forgive my imprecise language). Simply described, at certain yaw angles above a certain speed, the interaction of the wheel and the air contributes slightly to forward motion. My actual vision is for a human powered airplane. In a very loose way, a more appropriate analogy than a sail is that Zipp's wheel is acting like a propeller, not a perpetual motion machine, generating lift, which acts against drag, but is not itself negative drag. Unless, of course, it is a sail, transmitting wind energy into forward motion.
But then again, I am just a social scientist. I'm not even sure what I just wrote, but I am happy to say that both Mollusk and asgelle are right. But not DocRay. Never DocRay.
