Slight friction on control column after installing Dynon auto pilot servos

[fviyuela]

Hello everyone,

I´m 6 years into the build of a legacy -12 (rotax ULS) and I´m currently installing the Dynon autopilot servos (Dynon AP SV32. Section 44A). After installing the one on the back for roll/ailerons I noticed a slight resistance/friction appeared when moving the control column side to side (ailerons). I tried moving by hand the other AP servo arm, the one I was going to install for the elevator control, and it had the same kind of small steps friction/resistance as what I was feeling on the ailerons.

I proceded to install the elevator servo, and there it was! I could now also feel that same friction/resistance when moving the elevator. I seems this added friction is due to the electric motor of the servos but I wanted to make sure with anyone else that has already installed this servos to make sure they had the same "issue". The friction is very small, you can still move the control column with a very slight pressure but it´s a pity because before it had barely any friction at all.

Also, I had to change the AN3-7A for a AN3-10Aon the servo arm of the elevator as if I used the called-for -7A It ended up being slightly short and I didn´t get the 3 threads sticking out of the nut. Anyone else had this issue? On the push-rod to roll servo installation, instructions call for using a longer AN3-10 length for a very similar set-up (it has 1 more washer to hold).

Another issue/thing I noticed on this section:

The nut that holds the elevator/pitch servo arm to the push rod is a AN365-1032, an elastic stop nut. Has anyone tried using a longer drilled bolt and a castellated nut?

I´m asking because I just read the post from 4Kilo titled " Check Your Autopilot Servo Arm Screws!" and I tested to see what would happen if that elastic stop nut gave away. What happens is the push-rod falls down under the servo when using the elevator in its max up position and then it locks there as it is pressed against the bulkhead under the seats, creating quite a problem because the locked pushrod stops you from moving the elevator past that position. Is this really an issue or is the elastic stop nut reliable enough for such a critical set-up? I tried doing the same thing on the roll servo but over there it seems the push rod doesn´t get locked anywhere if they become loose from any one side.

I think the Dynon servo has an over-torque protection that releases the servo arm if it becomes stuck but that won´t prevent the push-rod from locking the control column if it has become loose from the servo arm and is still attached to the control column.

Or am I worrying too much??

[NinerBikes]

I proceded to install the elevator servo, and there it was! I could now also feel that same friction/resistance when moving the elevator. I seems this added friction is due to the electric motor of the servos but I wanted to make sure with anyone else that has already installed this servos to make sure they had the same "issue". The friction is very small, you can still move the control column with a very slight pressure but it´s a pity because before it had barely any friction at all.




I think that is normal.

I've compared my RV-12 which does have the servos for auto pilot to my neighbors, which was built without the servos, and there is a little bit more resistance in the motions, due to the servos.

[azonslow]

See FlightDEK D-180 Installation Guide, page 8-17 for instructions on how to set the torque values.

[DHeal]

Slight servo drag is normal — you will feel/hear this even when the AP is electronically disengaged. In fact, if you do not feel/hear this very slight drag when you preflight exercise your stab and flaperons, you may find that one of your servo shear pins has sheared.

[Piper J3]

The “friction” is actually back-driving the servo motor through a series of reduction gears. Back-driving a gear reducer is always a tough chore because mechanical advantage is in the wrong direction.

I have flown RV-12 with AP and control feel is heavier than my plane with no AP. My plane is much more “sports car” feeling in the air. Very light on the stick for both roll and pitch…

[fviyuela]

It´s great to be able to check this things out with other people that have the same plane!

[rondawes]

I have that feel with my Dynon servos also. Just curious, is it there for Garmin servos? I would imagine that it is but still curious.

[iwannarv]

Originally Posted by rondawes

I have that feel with my Dynon servos also. Just curious, is it there for Garmin servos? I would imagine that it is but still curious.

It is not present with Garmin servos. Garmin servos include electronic clutches that completely disengage the servo from the rest of the control system when not being used. So, no control system friction like traditional stepper-type motors.

The clutches can also 'slip' if the pilot needs to override the autopilot. Nothing breaks like a shear pin. Huge advantage. Override of the autopilot is actually a preflight check with a Garmin servo. Don't do that with other brand servos, or you'll be pulling them for repair.

[rvbuilder2002]

Originally Posted by iwannarv

The clutches can also 'slip' if the pilot needs to override the autopilot. Nothing breaks like a shear pin.

The Dynon servos have a clutch that also allows slippage, depending on what the programed torque value is.
It is true that they also have a shear pin, a shear screw actually, (and the Garmin servo has one as well) but over riding the torque slip of the servo does not automatically mean the shear screw shears. Also, Dynon redesigned the shear screw many years ago which seems to have pretty much eliminated random shearing events.

It is true that the Garmin servo induces no system feedback friction. The friction induced by the Dynon servo is a low enough in percentage compared to the control forces felt on an RV-12, that most people would not consider it at all objectionable. Particularly when maneuvering at lower speeds when the control forces are reduced by quite a bit.

[mburch]

Originally Posted by rvbuilder2002

they also have a shear pin, a shear screw actually, (and the Garmin servo has one as well)

Hmm, might want to double check that...