Pitch servo hunting on Lanacir 360

[coryadams]

I have the servos and AP74 operating very well but when I am at higher altitudes above 8,500 feet the pitch servo will hunt and start to oscillate such that the aircraft will go through vertical ossciallations by as much as 1,200 FPM or more which makes for a very uncomfortable ride.

I will take it out of AP just to make sure the pitch is balanced before re-engaging the AP but the problem still persists.

I have tried every torque and sensitivity combination with the same results.

Could anybody offer any suggestions?

Is this affected by the AOA at higher altitude perhaps? Also I did see a cross post on the AP discussion board about static pressure as being a possible cause of this?

 

[dynonsupport]

Nothing immediately comes to mind, but, one thing that will help us see what you're seeing will be to enable datalogging under the EFIS>SETUP>DATALOG menu. Set it for 1/sec, on at boot, and then fly the profile you've described below with the AP. Download that log with the support program and email it to support at dynonavionics dot com with a link back to this thread.

 

[PhantomPholly]

Hi Cory,

The Lancair is an extremely close-coupled airplane and difficult to get "stable" with any of the known autopilots.  Difficult, but not impossible.  I've had APs on both my Lancairs; my 235 was much worse in what you describe than my current 320.

There are two things you can do to improve it:  

a) Make the plane more stable - move CG forward, perhaps by relocating a heavy component, and augment the spring loaded pitch trim system with a trim tab on the elevator bent down so increasing airspeed tends to cause the nose to climb. Both of my airplanes experienced pitch "destabilization" with the AP on if I was too heavily loaded in the baggage area. A trick you can use to see if stability improves is to nudge in a couple of degrees of flaps - this has the effect of making the airplane fly as if it were more nose-heavy, adding some stability. However, you don't want to have to fly that way all the time!

b) Make some changes to the static system to reduce "static lag"

With the Dynon you do not have the option of switching your autopilot to cabin static air as you do with an independent AP (or, at least not without causing errors in your altimeter), so you will need to pursue the more difficult path.  The static lines act as a "reservoir" of air pressure causing there to be a delay between the time the aircraft actually changes from "climb" to "descend" and the time the AP senses it.  The things you can do to reduce this lag are to reduce the static line run length; reduce the diameter of the tubing to the absolute minimum possible; and make the static port hole larger.  This last should be the last thing you try as it can foobar your readings.

Pitch sensitivity is part of the price we pay for superior speed; like everything else about the Lancairs you need to spend extra effort getting your setup just right if you want to have both the speed and the ease of flying.

Good luck!

Bill

 

[DBRV10]

Make sure all your linkages are tight and no slop in the rod ends etc.

We had a sloppy rod end in the Dynon pack, I ordered another pack as it was cheap and easier than asking for a warranty job. Also make sure your shear screw is loctite secured.

DB

 

[dynonsupport]

Hi all,

We're for folks that have autopilots installed and flying, particularly D10/D100 series customers, who are running the latest firmware (5.3 currently), and have tried everything they can to dial in the autopilot - particularly in pitch - but aren't quite seeing the performance they would like. If this is you and:
-you're fairly technical - you understand how to use the Dynon Support Program to load firmware into your unit and you're comfortable with that process
-you're available to fly and iterate firmware and report back at least a few times over the coming weeks and months
-you're patient and fault-tolerant, and understand that testing "beta" firmware is an experimental thing

...then we want to hear from you. Please email betatest at dynonavionics dot com with the following information:
-airplane type
-dynon equipment, particularly, which servos you have
-roll axis setup:
servo type
short description of mounting method
all settings from the roll axis page in the ap setup menu
your performance notes/issues (in turbulence and smooth air if they're different)
what you've done so far (settings tweaked, results,etc) to try to dial it in
-pitch axis setup:
servo type
short description of mounting method
all settings from the roll axis page in the ap setup menu
your performance notes/issues (in turbulence and smooth air if they're different)
what you've done so far (settings tweaked, results,etc) to try to dial it in

Thanks!