Saturday, August 28, 2010

TIL: you can travel a wind powered vehicle faster downwind than the wind speed

TIL (today I learned)

I still can't get my head around how it works, but it has been proved that it does.

Background article that I can make little sense of.

1 comment:

annom said...

This problem postponed the graduation of 4 to-be-engineers with a day.

It is definitely counter intuitive, but I do understand the reasoning. One of my fellow students is still not convinced though.

The easiest way to see this is to look at energy. Work is the energy transferred by a force. work = force * distance. The distance here is relative to the medium the force acts on. This distance is larger for the ground with respect to vehicle than for the air with respect to vehicle (because the relative speed with the air is slower than the relative speed with the ground).

Equal energies can thus exert a larger force on the air (over a shorter relative distance) than the ground on the vehicle (over a longer relative distance).

This is also why the force of a propeller engine (turboprop) drops with increasing speed while the power of the engine does not change. At least that's what we think here.

Net force is thus in the direction of movement and will accelerate the vehicle (or combat drag).