Seemingly unrelated troubles

[Dave Withers]

Seemingly unrelated troubles

Lancair IV. New installation of dual G3X touch configured PFD/MFD, GMC507 autopilot, 2ea GSA28 AP servos.

When on autopilot at 130kts, when I extend my landing gear, the airplane pitches down and loses 60’-80’, followed by a drastic pitch up to regain the altitude. Following the AP adjustment instructions in the G3X installation manual appendix A, I am unable to adjust this out. Additionally, the airplane is a bit unstable at 130kts. I am constantly adjusting power in an attempt to maintain 120kts or 130kts.

When I shoot a coupled approach clean, or with flaps 10°, the airplane works great. If, on approach, I extend the landing gear with the autopilot engaged, my approach sensitivity degrades, my AP kicks off, and I get a message which states that my approach sensitivity has downgraded, use VNAV minimums. If, when configuring, I disconnect the AP, extend the gear, stabilize the airplane and reengage the AP, it will fly the approach fine.

The G3Xperts are stumped on this one.

Anyone got any ideas on either of my issues?

Thanks!

[George Wehrung]

I am beginning to wonder if it could be some sort of antenna interference? Maybe from the hydraulic pump running? I wonder how your antenna wires are running. The only thing wrong with my theory is that you can configure the plane yourself and then reengage auto pilot and it goes away, just some random thoughts before I get my day started


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[Rock Barchfeld]

If I’m reading you correctly, I would look at the auto trim. If pitch is out of trim it could cause the autopilot to “hunt” in pitch, if it gets too far out of trim the autopilot will disconnect. Just a thought as gear extension in my IV has a minimal trim change, flap extension somewhat more, but slowing from about 140 to 110 kts requires quite a bit of trim up, as I slow. The CG of my plane is 88” about 2 “ forward of mid-point further aft would amplify pitch trim sensitivity. They are all different so your mileage may vary.

I’m not familiar with your particular system, but many have a sensitivity/speed setting for the auto trim.

Just a thought, as with all things “Rock”, consider the source….

[George Rosel]

Rock,

Another thought... You may have your static ports in a poor location and when you change flap settings and/or gear up/down/airspeed changes, etc. this can create turbulent air over these ports causing minute fluctuation in airspeed. This condition can be hard to detect but with huge confusing results.

FYI... This happened to me in my turbine years ago during flight testing. Give me a call and I can explain how we diagnosed the problem.

Regards,

... George

[Dave Withers]

I am beginning to wonder if it could be some sort of antenna interference? Maybe from the hydraulic pump running? I wonder how your antenna wires are running. The only thing wrong with my theory is that you can configure the plane yourself and then reengage auto pilot and it goes away, just some random thoughts before I get my day started


Sent from my iPhone using Tapatalk

I know at least one, and possibly both of my GPS antennas are under my glareshield. Thanks George!

[Dave Withers]

If I’m reading you correctly, I would look at the auto trim. If pitch is out of trim it could cause the autopilot to “hunt” in pitch, if it gets too far out of trim the autopilot will disconnect. Just a thought as gear extension in my IV has a minimal trim change, flap extension somewhat more, but slowing from about 140 to 110 kts requires quite a bit of trim up, as I slow. The CG of my plane is 88” about 2 “ forward of mid-point further aft would amplify pitch trim sensitivity. They are all different so your mileage may vary.

I’m not familiar with your particular system, but many have a sensitivity/speed setting for the auto trim.

Just a thought, as with all things “Rock”, consider the source….

Hi Rock,

I've had to look hard at many things. It's a long unoriginal story having to do with my buying a "project". One of the issues was auto trim which took forever for us to get working. turns out, the elevator servo wasn't sending a signal to the elevator trim servo. I had a bad AP servo (GSA28). folks need to know that Garmin had a bunch of these that were bad. In the certified world, they are paying for replacements. In the experimental world, they are replacing the servos. My servos were -1's. The replacements they sent me were -22's (I think). This fixed my auto trim issue. So - why is gear and flap extension so different in our airplanes of the same model? I didn't want to suggest CG as I didn't want to lead anyone, but, they added an air-conditioner in the tail of my plane. This ran my CG WAY aft. To the aft limit - perhaps even slightly aft of that. My airplane is slightly unstable at 120kts-130kts. I am suspecting CG may be a factor.

Thanks!

[Rock Barchfeld]

Dave,

I can't say if, or how much of a factor, your aft CG may be, but I would say it doesn't help. Won't beat the "Aft CG = Bad" pony, however I will say they fly WAY better the further forward the CG is, (not to mention the CG moves about 2" aft when you retract the gear). There are lots of time consuming and expensive ways to work the CG forward, but I can say for sure yours would not the the first or only IV flying around with lead weights on top of the nose gear mount. Inelegant, but a quick, simple and relatively cheap way to attack this issue to eliminate the CG as the culprit.

Been mulling this one over and not being an G3Xpert, I can't tell if it's an autopilot or GPS issue. Reading you initial post it appears you're talking about RNAV approaches. How does it fly a Nav/ILS approach? Wondering different inputs equal the same results? Or, during an RNAV approach do things go sideways at the same point on the approach, say at the final approach fix? Lots of things happen at the FAF that could result in downgraded performance.

I'd bet these questions have already been asked and answered, just putting out thoughts from the peanut gallery.

Rock

[konstantin_azarov]

When you fly without autopilot, is there a significant pitch trim change as you're extending the gear? In my airplane there is no significant change of the pitch forces on the stick while the gear is being extended, just some minor yaw oscillation.

Does this behavior only happen in the approach mode? What if the autopilot is in altitude hold or pitch mode?

Also, could you share G3X data log from when it happens?