# Thread: downwind vehicle, can it go faster than the wind

1. That makes sense.

The torpedos especially explain things pretty well.

2. Originally Posted by megafiddle
The torpedoes especially explain things pretty well.
Glad that helped. The interesting thing is that the only external force on the torpedoes is a backwards force applied to the wires, yet because of the effective gearing between wires and propellers ( |propeller thrust speed| < |wire speed| ), those torpedoes ended up moving forwards.

One more analogy where the two things moving at different speeds are streams of air (as opposed to air and ground, or wires and water). Take the case of a strong wind blowing over the edge of a reasonably long ridge. On the upwind side of the ridge, you have an updraft. At some point above the ridge, you have a strong, nearly horizontal wind, and at some point downwind and below the ridge you have turbulent air with nearly zero net velocity. The "shear" boundary between the strong horizontal wind and the nearly non-moving air below is fairly thin. A radio control glider can be flown conventionally using the updraft on the upwind side of the ridge, then the glider is flown in the downwind direction (strong tailwind) towards that slow air behind the ridge and a half loop is performed so that the middle part of the half loop occurs in that slow air. Then the loop is continued back into the fast moving air, again with the idea that middle part of the occurs in the fast moving air, resulting in a large amount of acceleration in the direction of that fast moving air. The acceleration is related to the lift of the model, which is perpendicular to the velocity of the model (with respect to the relative wind), and during the period where there's a significant component of lift from the model in the direction of that strong wind, the model accelerates a lot. As a series of loops between the streams of air moving at different speeds is done, the glider picks up a lot of speed, called "dynamic soaring". In this video a reading of 468 mph (749 kph) is captured while it's dynamic soaring. It's hard to see, but in the second half of the video, the guy with the helmet cam flies the glider so it's much easier to see and he stops once he reaches a reading of 405 mph (648 kph). Link to youtube video:

Spencer's World Record 468mph Kinetic100DP flight video!!! - YouTube

3. Awesome little question.

The input power to the "fan" can be calculated by (Wind turbines and the wind energy - how much power is in the wind? http://www.raeng.org.uk/education/di...nd_turbine.pdf)

The output power is
forward_driving_force * vehicle_velocity
(otherwise know as P=Fv)

A few notes -
The input power is not linear with air velocity
The output power can be increased by increasing the blade diameter - If you are not getting enough power, increase the blade diameter
The maximum velocity is achieved when the force from the air resistance equals the forward driving force
No energy is being created - In fact a loss is guarenteed. You just need a "fan" large enough to provide enough power.

4. Getting back to the original subject, a downwind vehicle, here again is the link to a youtube video of the vehicle enclosed in a composite shell reaching about 50 mph (80 kph) in a 20 mph (32 kph) wind: