Pitch servo jittering

[MP-UNIK]

Just installed SV32s/D100 in my original homebuilt. After config, the pitch servo and, to a lesser degree also the roll servo, jitter almost continuously while holding ALT and HDG on the ground, and the pitch servo continues to do so in the air regardless of sensitivity and Q. This happens static both on the ground and in trim in the air, with the yellow UP trim indication appearing soon after AP selected ON. Trimming up tail heavy in cruise makes the Yellow msg disappear.

The pitch servo is mated via generic install kit to a 2-ft long  intermediate push-pull elevator tube which moves a bellcrank to which are attached the elevator actuating cables. I have stick actuated controls. The roll servo is likewise linked to the actuating arm of the aileron torque tube running outboard from the center section.  

In flight, the jitters are always present while the AP correctly controls the plane. The instances of jitters are every 2 seconds to half a second in pitch and more intermittently in roll. No free play in the ailerons and only cable slack for the elevator, and control stick inertia at the other end. I also detect occasionnal roll servo jitters in flight.

Also, the AP only tracks the actual course made good of the 296 GPS at the time of TRK engagement, but does not keep the lateral track bar centered. Of course, it doesn't intercept either and won't turn over a turning point enroute. GPS settings are AVIATION IN/NMEA AND VHF OUT Normal 9600 baud rate. Yes, I get the loss of GPS tracking info to the HSI and reversion to the HDG mode.

Any fixes?

Michel

 

[dynonsupport]

Hi Michel,

Sure like to see a picture of your plane!

Just to be clear, you're seeing small motions of the stick every 1/2 to 2 seconds both in the air and on the ground?

Is the AP holding ALT and TRK ok while this is happening? Some small motions of the stick are normal. Larger motions resulting in poor altitude and heading hold are of concern.

As to the trim, you're seeing the UP indication both on the ground and in the air? If you trim until the indication goes away and then turn the AP off , is the airplane in trim?

TRK mode will hold the current TRK indicated by your GPS; NAV mode will intercept and center the needle. Does the AP allow you to select NAV mode, or just HDG and TRK?

 

[MP-UNIK]

Attached are pictures of the plane and the AP installation. Also, I have made 3 short videos today which I will post on the video sharing site - VIMEO asap.

Overall motions of the controls are normal. It is while it is doing its fine work that this problem shows. The very small stick mouvement is like a quiver and feels like tapping on the stick. This happens while it is holding a steady, referenced position. The amplitude is quite small, the stick kicking instantenously maybe 1/16 to 1/8th of an inch back and forth. Not only does it do so static on the ground(with ALT ON) but also while effecting a stick motion like initiating a climb or descent, or correcting for a gust upset.

I get some of this with the roll servo on the ground (with HDG ON), but can't feel anything in hdg only mode in the air flying straight or turning.

As for the UP trim, the signal only goes away with a small amount of up trim applied. When the AP is selected off, there is positive nose up force present. In other words, the signal goes away when the servo exerts a small downward force on the stick to keep the plane level or in a steady state climbing or descending. On the ground, a forward force must be held on the stick in order for the UP trim signal to stay out or go away. The signal comes back soon after if this force is released. In other words, a neutral trim condition produces an UP signal both static and airborne.

Disregard my remarks about the lateral modes. They are all available and fonction properly.

 

[MP-UNIK]

2nd photo

 

[dynonsupport]

Very small movements of the stick, such as you describe, are normal even on the ground. As long as the autopilot is controlling the airplane satisfactorally, they are not cause for concern.

The trim indication should go out when the force that the servo is exerting to maintain level flight is small. That is, if the servo is pulling back on the stick to hold altitude, the UP indication will light, until up trim is added to reduce that force. (The trim indication is related only to the *force* on the controls - not to the position of the elevator.)

If the AP is then disconnected the force the pilot needs to hold on the stick should be small (the airplane should be in trim.)

If the airplane is significantly out of trim, then the trim sensor may not be working correctly.

An UP trim indication on the ground would be expected if the AP is engaged with the stick neutral in an airplane where the stick wants to go forward when released. The servo would then have to be pulling back on the stick to hold the position it was engaged in. However, pushing forward should not make the UP indication go out; pulling back (helping the servo) should make it go out.

If the AP is holding level flight, and you trim till there is no UP or DOWN, and the AP is disengaged, what climb or descent rate do you see?

 

[MP-UNIK]

I will flight test the AP again and try documenting with video. I have already videoed the stick jitter that I reported.

 

[MP-UNIK]

Thanks you for your promt replies.

On my flight test this evening, ground activation of ALT with the stick centered and neutral( the elevator is statically balanced and the stick remains centered by itself) produced within a few seconds the AP UP signal. Servo clacking is present. Application of a down force extinguishes the warning but it returns a few seconds after removal of this pressure.

In flight, with the plane stabilized in trim and in cruise at 103 mph IAS and 2400 rpm, ALT ON produced an AP UP warning within 15 seconds. Trimmed incrementally upwards concelled the warning while altitude was held steady by the AP. Selection of ALT OFF, gave a immediate climb rate of 1000ft/min. The elevator is also aerodynamiclly balanced to almost neutral.

A 2nd identical scenario was done except that, instead of deselecting ALT, incremental trimming towards nose down reduced the up stick force virtually to neutral without the UP warning coming back.

Third identical scenario except that trimming back down was continued until the UP warning returned. When ALT OFF selected, trim was essentially neutral again.

Settings on the pitch servo were 90% and sense 8. There was a little less of what I call 'jittering' at this sensitivity setting.

Will advise when I have posted my videos.

 

[dynonsupport]

We're getting the impression that we might want to take a look at at least the pitch servo. But, if you could post the videos, that might help us get a handle on what's going on.

 

[MP-UNIK]

Thanks, I will advise.

 

[MP-UNIK]

Hi,

As a precaution, I replaced my original twisted wire with a shielded company made one with the shielding grounded at each end. No change observed.

I uploaded 4 short videos, 3 made on the ground and 1 in the air.

1.   http://vimeo.com/5830045 Pitch servo static

2.   http://vimeo.com/5830168 Disconnected

3.   http://vimeo.com/5830307  Jitters in flight with AP trimmed up and positive forward push by AP and UP light out.

4.   http://vimeo.com/5830347  For comparaison, a short one showing some clacking when roll servo is engaged on the ground. This seems to be insignificant in flight.

 

[dynonsupport]

Hi,

The ticking noise and motion you are seeing is normal.

The UP trim inidcation with the servo disconnected from the controls suggests that the trim sensor in that servo may not be working properly. Please send us an email at support @dynonavionics.com or give us a call at 425-402-0433.

 

[MP-UNIK]

Got replacement servo within 24 hours. That's being expeditious!

New servo works perfectly. Autopilot trim messages occur as per config instructions.

This is a nice VFR AP for smooth to lightly turbulent air. For a straight combination of EFIS/AP, it navigates very well with direct Garmap 296 input.

The characteristics of this AP are impressive for the price. I fly Learjet 45 and SuperKingAir 350.