air speed sensor for rc plane

[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

[Greg Needel]

this is not really for a competition, it is a spin off project from Josh's employment that will probably be the ground work for a senior design project. The device is going to be used outside. and that picture is just an example not our final product. Although i think that the propeller counting might work outside still because of our low altitudes we are running there still wont be that much pressure difference, hence the reason we don't want to use pressure sensor to do speed.

[Chris Hibner]

Originally Posted by Greg Needel

...because of our low altitudes we are running there still wont be that much pressure difference, hence the reason we don't want to use pressure sensor to do speed.

Airspeed measurement has nothing to do with what altitude you're at. Well, as the air gets thinner, the conversion constant to convert your measured pressure difference into true airspeed is different, but the altitude makes no difference in the ability to measure it. If your airspeed is too low, that might be a problem, but the altitude is independent.

Your low change in altitude will make it easier and cheaper - you should be able to do a sufficient job with only one pressure transucer (the one at the pitot tube). Since your altitude changes will be small, you won't need to know the ambient pressure.

[Al Skierkiewicz]

For outside runs, counting prop rotation will not give you repeatable results. Head winds and cross winds vastly effect air speed vs. ground speed. Without a correlation there you will not be able to run a course in auto. Is there a possibility for triangulation from the ground to be transmitted to the plane for course changes? That would allow you to save weight and computing by performing all the functions on the ground.

[Gdeaver]

One problem your going to have is that with that size airframe and power you have almost no electronics payload budget. If you go up in scale and to gas power, there are some off the self options. What I' m thinking off is a pocket PC with a GPS add on. The position, altitude, velocity, heading are available. Use the pocket PC as the main brain. Visual studio Ide can be used to program the pocket PC. The Intel processor has more power than a microcontroller. With the proper cable a pocket PC can out put serial commands. There are servo controller chips based on pic16's that can take the serial command from the pocket pc and generate the hobby servo PWM signals. A rs232 converted is needed. the next part is to use gyro's designed for helicopters and planes. There are single axis units and 3 axis units. They are what stabilizes the RC helicopters. With control of 3 axis, stable straight level flight is possible. These gyro's are designed to take PWM input from the RC receiver and mix these commands with the stabilizing out of the gyro to give smooth control. Release the joy sticks and the plane will return to straight level flight. The Pocket pc controlled servo chip replaces the RC receiver in auto pilot mode. Don't underestimate the magnitude and cost involved with this project.

[seanwitte]

www.eagletreesystems.com

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