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The tri rotor airframe is made from the following:

3 Hextronik DT700 motors
3 SuperSimple 30A ESCs
3 12x6 propellers
1 Futaba S3102
2 * 14x12x750mm CF rods
14 guage wire for power cable
1/16" plywood for fuselage & motor mounts
1/2" x 1/2" poplar for rod brackets
Mounting tape to keep rods from turning


Originally it ran the full autopilot. This was the same basic VTOL autopilot you would find in any commercial copter. It used the following sensors:

Gyros: ADXRS613 * 3
Accelerometers: ADXL330 * 1
Magnetometers: Micromag 3 * 1
Barometer: SCP1000
GPS: uBlox 5 Helical antenna

It used 3 gyros for short term rotation sensing, 2 accelerometers for long term tilt sensing, 3 magnetometers for long term heading sensing, a high quality GPS module for horizontal position, a barometer for altitude sensing. A 3rd accelerometer was too sensitive to vibration & altitude changes to be any good. Because of the long term sensors & the very stable gyros, the orientation was always pretty accurate & didn't drift.

Although GPS can provide altitude data, it depends on the satellite positions, so we went to a barometer for altitude. The barometer didn't work on the single rotor vehicles because of the rotor wash, but works well on the tri rotor because of the large space between propellers.

Position sensing uses no ordinary GPS module but a uBlox5. We haven't flown a helical antenna but recommend it instead of a patch antenna because steep angles of attack reduce satellite count.

This was the 1st module under $100 which sensed sub meter/sec velocity, output ECEF coordinates at 4Hz, & had very little latency. 4Hz & low latency were necessary to get autonomous hovering to work. There are probably cheaper alternatives now.


As algorithms improved, the autopilot was converted to fewer sensors to decrease cost and improve reliability.

Gyros: ADXRS613 * 3
Barometer: SCP1000
GPS: uBlox 5 Helical antenna
This autopilot must take off pointing North, can't do maneuvers involving constant turns but is a lot easier to get configured. The tilt reading constantly drifts. Only changes in tilt are used for flight. Heading is learned by sensing horizontal movement.


The absolute cheapest autopilot flown used these components:

Gyros: IDG300 for the XY gyros & a LISY300AL for the heading gyro
Position: 4 * Matsushita 0D24K2
The minimal autopilot could fly inside or outside in calm air. It used 3 gyros for all rotation sensing & 4 sonar transducers for position. Keeping the sonar transducers unobstructed by landing gear was a problem.

The IMU used an IDG300 for the XY gyros & a LISY300AL for the heading gyro. Best results require the IMU to reach a stable temperature before flight.

The sonar system consists of an array of 3 receivers on the ground and 1 transmitter on the airframe. The time of ping reception at each array sensor is converted into distance for triangulating the position of the transmitter.

Sonar uses 4 Matsushita 0D24K2's from The lower the frequency the better, to get more omnidirectional range. The minimal autopilot can fly in a 2x2x12 meter box directly over the array.

(C) 2012 Flat broke, unemployed programmers