Dynon AP Servo Issue- maybe

[Derek]

I've be chasing an AP altitude hunting issue for a while, i won't bore you all with the details, but I will say it's caused me a phenomenal amount of frustration trying to fix it, more so because for the first 30 hours of my aircrafts life the AP worked faultlessly. Additionally, I can trick the AP into holding altitude by trimming in 3 clicks of pitch trim.

Today I decided to check for slop in the linkages, once again. All my linkages are good ( no slop) not surprising since my aircraft is new 100hrs TTIS, however on closer inspection of the AP Servo arm I found some slop.

I'm guessing (hoping) that this amount of slop is unacceptable ??

http://www.youtube.com/watch?v=g2zZxFoLaj0&sns=em

[Jesse]

There should be absolutely no slop in that position at all. It should be tight. I don't know that that would cause it to porpoise in flight, though. Usually a porpoise is caused by an activity setting being a little off. If it is a "twitchy" hunting, then turn down the activity. If it is a "wallow" hunting, then turn it up a little.

Sorry for the use of such technical terms.

[blueflyer]

did the shear pin break in the servo arm (giving you the slop)?

[Jesse]

No, you can see the shear screw in the video. The nut holding the arm on the servo is loose.

[dynonsupport]

As Jessie correctly says, there should be no slop there. That will cause hunting for sure, since when the servo changes direction, the first few moves are just taking up that slop. So I think you found your culprit.

We've looked at the video, and we're pretty sure that shear screw is broken. The head is stuck in the output arm, so it's still there, but it's likely sheared. It appears to tilt with the arm which it should not. The simple test is to engage the AP on the ground and see if you can move the pitch controls up and down without the servo really fighting you.

Once the shear screw breaks, then other wear occurs, and you get some of that tilting.

Give us a call or email and we can get you a new shear and instructions on how to tighten it all up.

[Derek]

for all the replies to my AP issue.

Here are my next steps:

Perform the AP ground test as suggested by our good friends at Dynon Support and if it fails, remove the AP arm and replace the shear pin.

Thinking out loud here. We all know now that the servo castle nut needs tightening, whilst I'm down there doing this can I just remove the servo output arm and visibly check if the shear pin is broken ?

[blueflyer]

Yes, you can do that to check if the shear pin is broken. When you take the servo arm off, the head of the shear pin will fall out (or come out with the servo arm). Then, replace the shear pin in a new hole, reinstall the servo arm, rotate it all by hand to be reconnected in same orientation as before, tighten the castle nut, install cotter pin and you're done.

[Derek]

I removed the servo output arm today, and yes the shear screw has snapped.

The fix / solution is quite simple, but I'm wondering what has caused the screw to break in the first place ?

[Jesse]

The shear screw most likely broke at least partially because the arm was loose. Beyond that sometimes they just break. I don't know how many I have replaced, but it's dozens probably, and sometimes they just break. If/when you break your second or third, it's time to really try to figure out why. On the first, just out a new one in and move on.

[Derek]

I've just just reset all the Skyview AP settings back to their defaults. Once the new shear screw turns up fingers crossed my altitude hunting issue will be a thing of the past.