[sanddrag]

Well, take for example an ordinary fan. The faster you spin the motor, the faster the air flow is coming out of it. Now reverse the idea. Instead of a motor, put some sort of encoder on the fan blade. The faster you are moving through the air, the faster the fan will spin (and the encoder will read this). I'm guessing it is a linear relationship but I could be wrong. In any case, with a little trial and error you should be able to come up with a simple equation that relates fan RPMs to airspeed. The only thing you need to make sure of is a very low friction encoder or encoding system (slotted wheel and photogate perhaps). I'm willing to bet you could do it with a relatively small fan blade, something like what's in the fan on top of a Victor 883. Just make sure everything is low friction so it spins freely.

Now, remember this will only give you your speed relative to the air, not to the ground. If there is wind, the readings will be off.