Autopilot twitchy on glide slope only

[Solana]

Dynon support,

I am down to one last issue with setting up the autopilot. The pitch gets twitchy on glide slope. It is an obvious change in behavior as soon as it intercepts and begins to track the glide slope. The accuracy in tracking the glide slope is good.

My current setting in pitch are:

sensitivity 24
o pitch gain 3.0
o altitude gain 1.1
o pull rate 1.2
o VSI gain 2.4
o G error gain 1.0
o G error limit 0.25

Thanks,

Rick Solana

 

[Solana]

Dynon support,

Let me add what behavior I am defining as “twitchy”. The autopilot will pitch up abruptly enough to feel a little G pressure, then about one second later pitch down so that you again feel a little opposite pressure, and continue this oscillation all the way from glide slope intercept to missed approach. All this time there is minimal deviation from the glide slope.

 

[led]

Hi Rick,

The most likely cause of twitchiness is that both pitch sensitivity and pitch gain are maxed out. Usually that's not needed for good performance.

Because I don't know all the details of your airplane/install, I can't promise it will work on the first go round, but I'd try reducing pitch sensitivity the next time you try the glideslope. Back it down until the twitchiness stops but glideslope tracking is still good.

You'll want to recheck that altitude and airspeed hold performance is still good afterwards.

If that doesn't help we can take a deeper look at your setup.

--
Lawrence Doan
Aeronautical/Autopilot Engineer

 

[rfazio1951]

I’m flying an RV6 and mine was doing the same thing on the glide slope. My gain was too high. You are set higher on gain and sensitivity. Like Lawrence said!

Just so you know I’ve got mine flying the glide slope really nice now.

 

[Raymo]

My RV-7A pitch is currently:

Sensitivity: 15
Gain: 1.2

I agree, you should reduce gain to 1 and see how it does.

 

[Solana]

Thanks for all of the advice! I worked the pitch sensitivity down until the twitching was 95% gone on the glide slope. The sensitivity is now at 20 and the gain is still at 3.0.

This has not significantly affected altitude and airspeed hold. Airspeed hold will overshoot by 2 knots (sometimes it jumps around and will overshoot by 3-4). Altitude capture is good.

If I remember correctly, I needed to increase the pitch gain because at default setting I did not track the glide slope. Actually, I think I needed to increase Altitude Gain and VSI Gain to help the glide slope performance.