REFLECTOR: Aileron Rigging

[Douglas Holub]

In a closed loop control system, if there is too much gain in the feedback 
loop the output will oscillate. With a gain of 15k the output (airplane 
rolling) was changing faster than the autopilot could control. The wings 
were probably constantly doing a little dance. It looks like the first thing 
he did was reduce the electronic gain from 15k to 9.2k, and when that wasn't 
enough he moved the aileron control cable closer to the axis of the bell 
crank so that the same amount of bell crank turning commanded by the 
autopilot would result in less aileron movement.

I would guess that lowering your ailerons so that they are 1/16" up in level 
flight has reduced the gain in your autopilot's feedback loop since the air 
is moving slower closer to the wing so that the same amount of aileron 
movement would have less effect on roll. This would explain why the 
autopilot isn't tracking as well as it was.

So I guess you could increase the gain again either electronically or 
mechanically and observe the results. But I don't know how dangerous that 
roll dance is or how linear the effect of increased gain will be on the 
magnitude of the oscillations. Too bad there's not a way to adjust it on the 
ground. If it was me, I would call the autopilot company and chat with a 
knowledgeable person before I started making changes.

Doug Holub
2009 Standard FG

----- Original Message ----- 
From: "John Dibble" <aminetech at bluefrog.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Friday, December 30, 2011 10:37 AM
Subject: REFLECTOR: Aileron Rigging


> After the discussion a few weeks back I decided to try a different
> aileron position.  Mine are 1/4" up with 1/8" total play.  To have them
> in line with the wing TE during straight and level flight, I would need
> to have them 1/16" down.  I decided to try an intermediate position of
> 1/16" up.  Now I have to use more stick force to enter a roll.  It feels
> like I'm flying a bus.  On the plus side, the roll is more stable
> meaning when I let go while flying straight, it tracks better.  I like
> this better for IFR flying.  I had 5 kn more airspeed on a recent flight
> from MS to NY, but this may be due to the colder temperatures.  I plan
> to keep it this way for a while and see if I can get used to the "bus"
> feel.  One other thing, the A/P doesn't track as well.  Before it would
> maintain the heading to within a degree or so.  Now it's more like 5-10
> degrees.  I set my gps CDI scaling to the smallest (0.3 mi) range to see
> if that helps when tracking the gps.  Of course that won't help when
> flying the heading bug.
> The builder modified the roll gain.  Here's the entries he made in the
> logbook.
>
> "Change A/P roll gain from 15k to 9.2k & test
> Change A/P roll gain mechanically & test
> A/P wing rock problem gone"
>
> Does this mean the A/P was too responsive before?  If so, can I un-do
> this modification to get more response with the A/P?
>
> Wondering if I have universal joints......
>
> John
>
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