SP9 Crash on TakeOff???

[hpmicrowave]

I finally have my RV8 flying but have a 100% consistent SP9 Serious issue at Takeoff!! Every time I takeoff my SP9 crashes as seen in the picture below. Once airborne I reset the attitude on the EFIS and it appears to work ok after that throughout the flight (so far). But Still early in the test program. I don't think there are a lot of MGL SP9 users but has anyone seen this issue before? Real question is How can I fix this? Yes it goes to pretty much straight up on Takeoff, about 9 straight times now?

Thanks
Cecil

[rainier]

Couple of hundred of these are in the field.
My guess is that you have a mounting issue that is causing the SP-9 to exceed its maximum gyro rates on take off - it could be due to runway bumps or engine vibrations with an infirm mounting. The SP-9 will loose track like any AHRS if gyro rates are exceeded even for very small (but fast) movements.

Rainier

[hpmicrowave]

Thanks Rainier, Ill go back and look at the mounting, I of course thought it was mounted rigid and had 4 little shock absorbers under it.

Will see what I can do.

[PaulSS]

One of the snags of having a good performance aircraft like an RV8. The acceleration rate causes the gyro to pitch up and exceeds its limits to compensate. I bet if you accelerated more slowly it wouldn't happen. It's the same as the pitch-up illusion experienced by the inner ear when being catapulted off an aircraft carrier. Accelerate down a runway in an F18 and you don't get it.

I'd be interested to see what happens if you accelerated slowly in your RV8 and only slowly added power, instead of going straight to full power.

Quite how the problem is solved I don't know. Maybe mounting the unit 'nose low' a bit would help compensate for the pitch up experienced.

[Netconn]

Mine does this every so often in my Lancair during take off. I've tried different mounting options but it doesn't help. I have to reboot the system to put it back straight. Maybe the software could be updated to buffer the acceleration, if that's the cause.

[apachejoe]

I've had the same issue with my SP-7 pitching up during takeoff for several years in my RV-10. Just bought an SP-9, it did the same thing on takeoff. It did recover quickly on its own without having to restart. I've only flown it once so I'll do more testing to see if the issue continues.

[rainier]

These kinds of issues are typically caused by a sloppy mount of the AHRS. It is allowed to move relative to the airframe. This typically affects the device on take-off (acceleration and bumps from runway) or during turbulence.
These kinds of movements can dramatically exceed the measurement range of the gyros - this directly results in the AHRS loosing track of your attitude. During take off this tends to affect pitch mostly but bank can also be affected during normal flight.

Other favorites causing similar issues are shelves holding the AHRS or mounting the AHRS on the actual skin of the aircraft - like under a seat etc (this tends to beat like a drum from the prop wash and can severely affect the AHRS).

The very best mounts for an AHRS are typically on a sturdy airframe member like a wing spar box and mounted using velcro tape - this works extremely well filtering out high frequency vibrations while being firm enough to prevent the AHRS from moving independent of the airframe.

[rainier]

BTW that shelf shown in the picture is not a good idea.
It may look like it will do the job - it won't.
The AHRS is also mounted solidly to the shelf. Not a good idea. No filtering of engine vibartions.

You will only realize how badly that shelf performs when you place your hand on it (if possible) during various engine speeds - the two straps you have to hold it will do little to prevent it from shaking like a leaf.

Been there - done that - got the T-shirt as they say.

Seen really well made, professionally created shelves that simply do not do the job once put through the test. Resonances are a bitch to control.

[PaulSS]

I see the box is mounted on rubber bushes. Would you recommend Velcro over those?

[rainier]

I see the box is mounted on rubber bushes. Would you recommend Velcro over those?

Those bushes will be way too stiff - the SP-7 has very little mass so they will do little.
In my experience simple Velcro wins every time ! I just use the self adhesive variety. Two trips below the SP-7, no screws and you are done.

Just make sure you have it on a really firm platform - the AHRS must follow the airframe's movements without anything corrupting it. Even small vibrations are bad - particularly so as they tend to be high frequency. It may seem like nothing to worry about but every single period of mechanical oscillation cannot be measured accurately and these measurement errors build up very quickly.

The AHRS uses three gyros at right angles to each other to carefully and as accurately as possible measure your airframes rotations in three dimensions and integrates these rotations (with a bit of maths thrown in to work out any angular movement relative to the three axis) - this way it tracks how your attitude changes. Even the smallest measurement error here quickly causes an incorrect result - which just gets worse as every new measurement is based on a false assumption.
Of course - no system is perfect no matter how much money you spend on it so there are unavoidable errors - but these tend to have certain characteristics that help the system to elliminate them based on other measurements, chiefly those from three accelerometers and your aircraft's velocities.