ornitz@kodak.UUCP (barry ornitz) (12/08/88)
In article <5084@boulder.Colorado.EDU> ciciora@tramp.Colorado.EDU (Ciciora) writes: [A method for cheaply determining the airspeed of a model aircraft.] > > -a) Bust a small light bulb. > -b) Run a constant current through it. > -c) Measure the voltage accross it vs air speed. > >I once wanted to measure air speed and this was the method sugested. >Never really calabrated it, though. Not sure how. It was interesting >to send the output to a O'scope and look at the fluctuations of a fan, >person walking by, blowing, etc. Made a nice lunch-break hack. This is a good lost cost alternative to my suggestion on using thermistors. Bulb resistance is a strong function of temperature (although probably not as sensitive as thermistors). The thermal mass of the bulb filament is small giving a high speed of response (if you use thermistors, use the smallest bead types to get a short thermal time constant too). The problem with lamp filaments is their low resistance. Appreciable current must be drawn to make this method work; battery life may be a problem. The subminature 28 volt lamps used in military equipment such as a #327 would be a good choice. Using two filaments in a bridge as I suggested for thermistors will reduce the effects of ambient temperature changes on the speed readings. Temperature drop with altitude is probably not a problem with model planes, however. Calibration is really simple. Place the sensor on the end of a long stick. Hold it out the window of a moving automobile on a calm day. Calibrate the output versus the speedometer reading. The stick insures that the slipstream of the auto does not affect the flow by the sensor. You might want to read the discussions on radar detectors in wreck.autos if you plan to calibrate at speeds over the local limits! ;-) 73 de WA4VZQ, Barry