Lateral center of rotation and remote electronic gyro

[Brian Vickers]

Final details on my RV-4 project and I'm analyzing locations to mount an electronic gyro. My glass panel unit uses a remote three axis gyro (not magnetic sensitive) that is to be mounted as close as possible to the center of rotation to reduce centrifugal forces. The instructions devote a whole page to this so I'm trying to comply. In flight, does an airplane actually rotate on the CG? Doesn't the center of lift have an influence, and therefore change with speed? My Jeppesen training book says, ??the airplane always moves about its CG, regardless of which axis is involved.? Obviously the pilot and passenger sit in the theoretical ideal spot so the choice becomes; below floor panels, behind the baggage area, or somewhere behind the panel. Mounting below the floor panels would probably put it closer to the lateral, but below the longitudinal. I?m assuming the longitudinal axis is in the stomach area. If the true lateral axis when flying is forward or aft of the CG then this would be good to know. Mounting the gyro immediately behind the baggage compartment is a long way back, but that is where the compass module is located, and mounting them together would keep it simple. Also, they can be linked together and the compass module can be set to use the gyro data, thus increasing heading accuracy during and after turns.

I'm assuming that 95% of my flying will be solo. I have never flown in an RV-4, or any tandem configuration so I don't have experience. Maybe I?m over thinking? Any thoughts?

Thanks

[penguin]

Brian,

I think you are over thinking the issue. Although the position on the chord of the wing that the lift force might actually act through may change with angle of attack or flap position, as the aircraft does not start spinning around means that the change in pitching moment about the cg is counteracted by force generated by the tailplane and elevator. On the ground the aircraft tends to rotate around the main wheels, in the air rotation is about the cg.

If your gyro pack is mounted too far away from the cg (in all 3 axes) a sudden gust or control movement will cause the aircraft to rotate and apply an acceleration to the gyro pack greater than that felt by the aircraft as a whole. How far is too far? Difficult to guess, but if you mounted it in the tailcone you might see the attitude instruments swing about as you pitched or yawed the airplane (but an RV-4 can't generate huge angular accelerations for very long). Would you get away with the baggage locker? My feeling is probably as its not that far from the cg. I suspect under the floor panels would be fine for roll indications. Have you considered the wing root? Not ideal for roll indications, but if under the floor boards is not possible might be a reasonable second. Behind the panel is probably a reasonable place also (or perhaps just aft of the battery).

You could always ask the gyro pack supplier the maximum distance from the cg that is permissible - I would have thought in pitch and yaw 2 or 3 feet away would be OK, and less in roll (roll rates are higher). But that is a complete guess, especially as you haven't mentioned whose gyro pack you are using.

I would mount the gyro pack and compass as close to the cg as you can comfortably achieve, without making installation and removal a complete bear. If that were the panel or the baggage compartment in an RV-4 I would be surprised if you could discern the acceleration effects on the instrument display (but I am guessing to some extent).

Hope this helps,

Pete

Military avionics integration & test in a previous life

[Sam Buchanan]

The Dynon D10-A and related EFIS's have the AHRS in the panel-mounted display unit and they work fine. I would mount your AHRS box as close to your display as possible making sure it is easily accessible.