David Damerell wrote:
> Shock <
[email protected]> wrote:
>
>>drag than all those spokes whipping up the air. There has even been some wind tunnel data to
>>suggest that disc wheels and wheel covers may even provide some small amount of propulsion in
>>certain crosswind situations...
>
>
> Which is evidently nonsensical - the wheel is symmetrical fore-aft.
It may be counter-intuitive but it's not nonsensical. Disc wheels, and some other wheels such as the
tri-spoke wheels with airfoil spokes and deep section rims, can generate some forward thrust in a
crosswind. This has been measured in wind tunnels and does not contradict theory. The thrust is not
nearly enough to overcome drag of the rider and the rest of the bike, but it is enough to make a
difference. FWIW, vertical-axis wind turbines typically use symmetrical airfoils, with the airfoil's
plane of symmetry tangent to its circle of rotation. They won't self-start, but generate power after
being started by external power.
An airfoil, aligned with the direction of travel, can generate forward thrust whenever
arctangent(Drag/Lift) < angle of attack. Angle of attack depends on the vector difference of the
true wind velocity and the bike velocity. For example, a bike going 25 mph with a pure side wind of
7 mph has an angle of attack of 15.64 degrees. If the wheel has a Lift/Drag ratio of better than
3.57 at this angle of attack, it will generate net thrust rather than net drag. Wheels don't make
very good airfoils, but L/D of 3.6 is very low. For comparison, an old thick NACA 0025 symmetrical
airfoil has an L/D of about 12 at this angle of attack. Rotation of the wheels complicates things,
especially for spoked wheels, compared to a simple fixed airfoil, but it still doesn't take much of
an airfoil to generate some thrust in the right side wind conditions.
Dave Lehnen