[Al Skierkiewicz]

Greg and Josh,
Since speed is going to be relatively slow and I am guessing that this competition will take place indoors there is not likely to be much variation in barometric pressure and little in cross or head winds there is an easier way to do this. I would just use a reflective photo detector to count prop revolutions. You can find a number of reflective devices from Digikey. Add a little piece of audio splicing tape, (from Radio Shack, silver adhesive backed tape) to the back of the prop and you will not affect it's balance or drag coeeficient. Attach the device to the side of the motor and calibrate by running the airplane over a fixed distance to determine prop speed vs. disstance traveled. Unless a strom comes through while flying, the ambient pressure would stay relatively constant and regardless of battery voltage, the prop RPM would still give a good indication of air speed.
It's hard to tell from the picture, but you may be able to add something to the motor shaft on the side opposite the prop and count revs that way as well.

Al