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Originally Posted by Gdeaver
A Freescale pressure sensor with the proper ADC can easily give altitude with in 5 ft. To measure air speed use a tube perpendicular to the airflow and measure the pressure drop within the tube. I forget the name for this device it begins with a p. The MPX4115a is the most used sensor for these apps.
There are GPS units about the size of a quarter that interface with a PIC by serial. There are allot of sources on the web on how to decode the packet. A 3 axis digital compass would also be a nice addition but they are expensive. You should be looking on the RC airplane forums. You'll find allot more specific how to's.
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The perpendicular tube you mention is called a pitot tube. You determine airspeed by measuring the difference in pressure between the impact air pressure (at the pitot tube) and the ambient pressure (same source you use for your altimeter). Any fluid mechanics book has good explainations on how pitot tubes are used to measure fluid speeds. You will need two pressure transducers.
Someone earlier mentioned that a drawback to the fan (and pitot tube) is that it only measures speed relative to the air and not the ground. This is NOT a drawback. An airplane flies relative to its surrounding air - NOT the ground. The airplane does not care whatsoever what is happening with the ground unless the plane is within ~1 wingspan above the ground (i.e. only takeoffs and landings).
I think you should get rid of the accelerometers and replace them with angular rate sensors (i.e. solid state gyros). Acceleration is not important in an aircraft and can lead to false information. It is the gyroscope that allows pilots to fly in the blind. I guess one accelerometer can be useful as a slip/skid indicator, but that's not important for an autopilot.
You'll have to let me know how it goes. I've long been wanting to develop an RC autopilot. My big goal is that I wanted to do an auto-land feature.