Negative Aerodynamic Drag (was: Help wanted with bicycle wheel design) - rec.bicycles.tech | Google GroupsAt the University of Saskatchewan, this concept was used for developing a body style for a small 3 wheel car, designed for very high fuel economy. The car was designed so that it was a vertical airfoil (it had a specific number, I believe). The airfoil was symmetric. When faced with a cross wind, the shape of the car could produce *negative* drag. The drag is negative because the component of lift in the direction of travel (along the axis of the airfoil, rather then the attack angle of the wind), which overcomes the aerodynamic drag. The angles of attack for this situation to exist are small. I think it was something like 10 to 15 degrees from the direction of travel.
..... I intend to fix a limited-angle rotatable aerofoil onto each spoke of the wheel which would be tilted by the crosswind to allow the wind to pass through the wheel. The aerofoils would be approximately 250mm long and 40mm wide at the top, tapering to 10mm at the bottom.....
In the specific competition (1986), the car (Canadian Challenge I think) obtained around 5600 mp(imperial)g. The day of the competition was sunny, and quite windy ;-). The SAE at UofS experimented with varying the angle of attack the next year of the competitions. They did this by introducing rear wheel steering in the car. They found that it was quite difficult for the driver to adjust the angle to its optimum in practice, although theoretically it gave a much bigger range of negative drag attack angles.
Your application with the bicycle spokes is similar in many ways. If each spoke has less drag, then the resistance of the bicycle will decrease, and people will be able to go faster! I would recommend first selecting an airfoil shape that works without rotation, if that is possible. One problem is that the airfoil will usually be in choppy air, so you may not be able to avoid separation at the best of times. A static airfoil would be able to provide a component of thrust when hit by a cross wind in either direction.
If any one wants references, I could dig some up. I know one student did his Masters on this concept, and also looked into applying it to heavy transport vehicles (trucks).