Stick tends to 'Pulse'

[AlanR]

Skyview D1000 x2 + Autopilot button panel.Currently in Simplified mode.

My recently installed autopilot works perfectly and it holds the altitude just fine. However the stick tends to 'pulse' in pitch slightly as though taking up some play in the control linkage but having checked on the ground there doesn't seem to be any undue moment in any control joints.
A setting adjustment maybe?
All thoughts appreciated.

 

[Dynon]

The servo has small but discrete steps that it moves in, and in some aircraft you can see that in the stick or yoke while in flight. It can be evidence of a small amount of play in the servo to control surface interface, but, if it's sufficiently small, it won't translate into either pulses that you can feel through the airframe or any hunting or other sub-optimal autopilot performance.

 

[AlanR]

Ok, thanks for the input Dynon. The aircraft is a Bristell NG5 with 60hours from new.
There is no hunting of pitch and the pulsing can't actually be felt through the airframe. It is just the stick that can be felt and also seen to give small jerky pulses in the pitch plane. This I might add is in smooth air. There is no play in any control joints.
Slightly disconcerting.

 

[Robski]

Might the length of the control arm on the servo, or selection of hole in said arm, or too powerful a servo (such as an SV42 where an SV32 might be more appropriate) cause something like this?

Disclaimer.
This is just working from basic geometric and engineering principles.
I have no experience in this area - yet.
I monitor the forum as I plan on fitting the AP servos to my D100/120/AP74 setup in a few months (when the bank balance recovers).

 

[mmarien]

You may have the pitch sensitivity set too high. Page 4-1 of the AutoPilot In-Flight Tuning Guide discusses twitchy pitch movements?

 

[AlanR]

mmarien,Thanks for the pointer. I did have to increase the Pitch sensitivity from default 10 to 18 during setup, maybe I have overdone it a little although the altitude hold currently is very stable and there is no airframe movement just the stick.
I will try reducing it slightly.

 

[Raymo]

The part that doesn't add up, Alan, is that the stick is moving but the airplane is not. If there really is no play in the control system, the airplane would have to move with the stick, don't you think?

 

[AlanR]

Hmmmm....I see where you are coming from however it is a small jerky pulse.
Just thinking though there are a number of joints and pushrods between the stick, all the way back to the elevator so whilst there seems to be nothing worn or excessive play I wonder if there is naturally a minutely small amount of overall movement enough to create a pulse if the settings are not right, but not enough to be felt through the airframe.
I dunno.....Your line of thinking means I will now check the control system over once again.
Thanks for the input.

 

[DHeal]

As a data point, my RV-12's SV AP does "pulse" somewhat during flight (mainly the pitch axis) -- it is barely noticeable when holding onto the stick. I have manual trim so I can minimize the pulsing effect by keeping the trim in balance thus reducing the work the servos have to do. Pulsing is not uncommon on other AP systems -- its the servos doing their thing.

 

[Dynon]

So even in smooth air, the AP is going to make small constant adjustments. It's normal for the stick to be active (but not moving much) pretty constantly when the AP is engaged. I suspect that's what you're seeing. If you want to send us a video, that might help too.

 

[AlanR]

As a data point, my RV-12's SV AP does "pulse" somewhat during flight (mainly the pitch axis) -- it is barely noticeable when holding onto the stick.  I have manual trim so I can minimize the pulsing effect by keeping the trim in balance thus reducing the work the servos have to do.  Pulsing is not uncommon on other AP systems -- its the servos doing their thing.

Thanks for that. Reassuring to know I am not alone with this and that a slight pitch 'pulsing' of the stick just may be normal.
I think I am still going to experiement with sensitivity settings and also recheck control linkages yet again just to be absolutely sure there are no physical control issues.

 

[Allinore]

Hello Friends.
I have similar problem on WT-9 Dynamic with D180 avionics. In my case - "pulsing" on a roll channel. After first appears, I recalibrate servos, and some time all being ok. But after 2.5 hour of flight, "pulsing" appears again. Look as a short 1-2 degree fast movenemt of ailerons, about 1-1.5 time per second. Almost nothing effect to flight traectory.
Servo - SV32. Electrical connections is ok. Maybe, need to examine bearings in servo connections?
Airplane has good flighttime, but this sympthomes is new for me.
Thanks