Draker
Well Known Member
I recently "derailed" this thread with my report of extreme vibration issues with the Garmin GSU 25 mounted remotely, aft of the baggage compartment. My device was attached via four bolts/nutplates to Van's AHRS mount, which consists of a pretty thin sheet of aluminum riveted to the exterior skin and the center brace between the bulkheads.
The symptom: AHRS display malfunction (incorrect ~20 degree roll depicted) at high RPM, with CAS messages indicating ATT MISCOMP and AHRS REVERT. Upon analysis, AHRS Dev (%) as logged by the G3X was spiking up to 700% during these episodes. Analysis across multiple flights indicated AHRS Dev (%) correlated most strongly with Eng. Pwr and RPM. Pretty likely vibration related.
It looks like the VAF consensus is that installing these ADAHRS units on the back of a GDU display is acceptable and indeed produces the best results for most RVs due to the strength and thickness of our panels. Well, I'm just too lazy to re-route my tubes and wires forward through the wing spar, so here we go.
After looking at a few examples sent to me, I set out to build a chonkier mount for this thing, while allowing me to keep it in its current location. After a trip to Metal Mart and a couple of designs I settled on a thick 1/4" 6061-T6 aluminum angle with another 1/8" angle attached. This guy is thick and beefy:
It fits right under the GSU unit, sandwiched between it and the original Vans mount. The 1/8" angle gets bolted to the long angle that goes down the center between the bulkheads:
So it adds some mass and is cross-braced to a solid, structural angle in the plane. I'm not a mechanical engineer, but I convinced myself it at least wouldn't hurt to try.
Well, three flights later results are so far so good! After 6 full-power takeoffs and 7 full-power climbs from 2500MSL to 5500 (tested every 5kt between between 65 and 95), I did not record any AHRS failures or CAS messages. Great. So let's analyze the logs.
Here, you can see plots of RPM (blue) vs AHRS Dev (red) before the new mount:
Observe the large spikes > 400 whenever full power was applied. These corresponded to the AHRS failures and CAS messages.
After installing the GSU on the thicker, braced mount:
As you can see, it's not perfect, but extreme spikes during full power ops are gone, and visually, the attitude display has been corrected. I'm going to spend another 10 or so hours testing this mount, and if I don't see any failures in that time, I'm going to consider this problem corrected!
The symptom: AHRS display malfunction (incorrect ~20 degree roll depicted) at high RPM, with CAS messages indicating ATT MISCOMP and AHRS REVERT. Upon analysis, AHRS Dev (%) as logged by the G3X was spiking up to 700% during these episodes. Analysis across multiple flights indicated AHRS Dev (%) correlated most strongly with Eng. Pwr and RPM. Pretty likely vibration related.
It looks like the VAF consensus is that installing these ADAHRS units on the back of a GDU display is acceptable and indeed produces the best results for most RVs due to the strength and thickness of our panels. Well, I'm just too lazy to re-route my tubes and wires forward through the wing spar, so here we go.
After looking at a few examples sent to me, I set out to build a chonkier mount for this thing, while allowing me to keep it in its current location. After a trip to Metal Mart and a couple of designs I settled on a thick 1/4" 6061-T6 aluminum angle with another 1/8" angle attached. This guy is thick and beefy:
It fits right under the GSU unit, sandwiched between it and the original Vans mount. The 1/8" angle gets bolted to the long angle that goes down the center between the bulkheads:
So it adds some mass and is cross-braced to a solid, structural angle in the plane. I'm not a mechanical engineer, but I convinced myself it at least wouldn't hurt to try.
Well, three flights later results are so far so good! After 6 full-power takeoffs and 7 full-power climbs from 2500MSL to 5500 (tested every 5kt between between 65 and 95), I did not record any AHRS failures or CAS messages. Great. So let's analyze the logs.
Here, you can see plots of RPM (blue) vs AHRS Dev (red) before the new mount:
Observe the large spikes > 400 whenever full power was applied. These corresponded to the AHRS failures and CAS messages.
After installing the GSU on the thicker, braced mount:
As you can see, it's not perfect, but extreme spikes during full power ops are gone, and visually, the attitude display has been corrected. I'm going to spend another 10 or so hours testing this mount, and if I don't see any failures in that time, I'm going to consider this problem corrected!