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Originally Posted by mechanicalbrain
How about a volt meter?  The faster the motor spins the higher the voltage! And it can act as an alternator to charge the battery! However as to the aerodynamicness (yes i know its not a real word!  ) of it all.... |
You're thinking of a ram air turbine; those were used to drive the ECM pods on EA-6s and can be found as emergency power devices on many large aircraft (they deploy if all engines fail, for instance). On the EA-6B, I believe that combat radius decreased by 5% for each RAT-equipped ECM pod carried, so at least in that application, they were non-negligible. Plus, conservation of energy dictates that you're just going to waste energy by trying to charge the battery, if the battery is what's causing the forward motion of the plane (it's different story if it's powered separately, such as with a gas engine). And then, of course, there's the issue of charge rate; I doubt you can charge the battery fast enough to make up for a significant portion of the power output (again assuming it's an electrically-propelled aircraft).
Also, a plane moves in 3-D space; if there is pitch or yaw relative to the axis of the turbine (e.g. high-angle-of-attack or flat-turn maneouvres), it might not read as expected. In any case, I'm guessing that the relation between fan motion and airspeed has something to do with the drag force on the fan blades, which ought to be roughly proportional to the square of the velocity. If this is the case, though, C
d changes with the angle of the fan's axis relative to the direction of travel, so you'd need additional equations to model that dependency. (Yuck!)
If you don't like GPS, or a turbine, what about INS? Couldn't you set up some sort of 3-axis accelerometer, and continuously integrate its outputs over some short time periods? Actually, it seems like I'm thinking, once again, of groundspeed, rather than airspeed. I'd have to say that groundspeed is much more useful for navigation, anyway....
Edit: Here's an interesting idea I just dug up: GPS
and INS. However, it sounds bulkier than you'd anticipated, difficult, and computationally intensive. Give it a few years:
Some sensor designers even foresee that by 2010, it will be possible to obtain navigation performance of 0.01 deg/h using a MEMS IMU only 2in
3in size.