View Full Version : AHRS / C of G

24th Feb 2008, 18:05
This is probably one of those design secrets at the likes of Garmin and Avidyne.

How does the AHRS unit (thinking DA42) compensate for its distance from the C of G, when calculating the attitude of the aircraft. It would be easy if the C of G was at a fixed station but it's not.

If someone has the answer I would be very pleased and you could also consider yourself a tech geek!

Mad (Flt) Scientist
24th Feb 2008, 18:12
Simple answer: it doesn't need to.

The ATTITUDE of the aircraft (bank, heading, pitch) can be measured anywhere on the aircraft, because it is constant throughout the aircraft, unless the aircraft is made of putty.

So if the gyros sense a 5 degree shift in pitch, the whole aircraft has (to any reasonable approximation) moved 5 degrees in pitch. As long as the sensors are firmly fixed to the structuire, it won't matter where they are.

now, sensing accelerations is a different issue. Those WILL vary with location in the aircraft relative to the cg. Any accelerometers will be put as close to the nominal cg as they can be; things like FDR accelerometers are located specifically.

If anyone ever needs to correct to the true cg accelerations, you'll use the sensed angular rates to strip out the terms due to the offset sensing 9from a known or calculated cg position) but that's more an analysis task than a task for the AHRS.