Yoke with extra weight

I don’t think so: the autopilot is supposed to work on the trims, not on the yoke/stick, as I was taught. So I can’t see what interaction there could be. Then again, I am not exactly an authority on autopilots.

Jan_Olieslagers wrote:

I don’t think so: the autopilot is supposed to work on the trims, not on the yoke/stick, as I was taught.

All the autopilots I have come across work with servos acting on the normal control surfaces. They also use the trim to keep the servo forces low. So changing the control balance by adding masses will affect the autopilot in exactly the same was as it will a human pilot.

what_next wrote:

All the autopilots I have come across work with servos acting on the normal control surfaces. They also use the trim to keep the servo forces low. So changing the control balance by adding masses will affect the autopilot in exactly the same was as it will a human pilot.

Agreed. Normally a pitch servo and a pitch trim servo plus a roll servo in light GA. I would be surprised however if a less than 1kg weight in the centre of a yoke could make a realistic impact.

JasonC wrote:

I would be surprised however if a less than 1kg weight in the centre of a yoke could make a realistic impact.

If you don’t feel it, then the autopilot won’t feel it either.

What exactly the servos drive does vary – e.g. here is a good read.

There are some bizzare arrangements around, and some work better than others in turbulence.

Driving the trim on a control surface should be OK but you need a much faster running servo because the control surface position is basically an integral of the servo capstan angular movement, whereas with a directly driven control surface the control surface position is directly proportional to the servo capstan angular movement.

In reality when you make a retrofit AP (like the DFC90) you are constrained by existing mechanics and existing (already approved) servo mounts and even servos.

On the TB20 autopilots, AFAIK all of them drive pitch and roll directly, and if you have a pitch trim servo that drives the elevator trim mechanism.

As regards the OP’s question, I vaguely recollect reading about such a thing before. It may have been a post from him…Years ago? It is maybe possible but it would take a very marginally stable AP and the King ones are normally pretty good because when they were doing the STCs they actually flight tested the stuff – unlike STEC which are often unstable, in certain loading configurations.

What does “deviation of track” mean, Thomas? Is it an oscillation, or a steady error?

That’s probably a different issue, to do with the autopilot gain in the roll control loop. I think @wigglyamp knows these boxes.

It can’t be a weight on the yoke otherwise e.g. an imbalance in the fuel tanks would do the same.

Thomas wrote:

I fly a TB 20 and have mounted at the yoke left and right a garmin 795.
Can it be that this extra weight impact the function of my autopilot KAP 150?

Autopilot is one thing, but adding mass to the yoke could also affect flutter characteristics. If the best remedy against flutter is letting go of the stick, then adding mass will do the same as not letting go of the stick completely. I would not add stuff to the stick. The stick/yoke is for controlling the airplane, not a hanger for “stuff” IMO.

Adding weight to the yoke will affect a KAP 150 autopilot, but perhaps not in the manner noted above.

I flew once with a fellow pilot who fitted his own aged, heavy, GPS device to the right hand yoke. I already had a 795 on my yoke. The plane porpoised for the whole journey. I only realised afterwards what had caused it – I think someone here made the diagnosis. It was definitely correct. (Reproducible and cancelable by adding / removing the heavy GPS.)