Today's test
Jaime (and DS):
Did the following today:
Ran the servo test and checked for slop in the control surfaces and at the servo arm. Found the following:
With very light fingertip pressure at the outer corner of the aileron, it moved about 1/16th to 1/8th inch in each direction (eyeballs only, no protractor, so I don't have degrees, but it was small). With the same very light pressure on the corner of the elevator, it did not move. However, if I applied a little more pressure at the center of the aileron or elevator (just enough pressure to keep the servo from slipping...a little more and it ratcheted), each surface moved about 1/4th to 3/8th of an inch in each direction (more than I had hoped for, but the flight test went OK, as I describe below).
Then I looked at the roll servo arm through the viewing window I made for the floor, and with the servo in test, I could move the stick enough to make the servo arm move about 1/4th inch in each direction, and the ailerons moved about the same as the second test above (all without servo slipping). Pitch movement at the stick seemed about the same...just a bit of slop in the stick...but I didn't pull the baggage bulkhead to look at the P servo (couldn't bring myself to pull those screws again!
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I did try it in HDG/ALT Hold on the ground, and the results were about the same (as in both of the first tests). I did note the hunting you described, and it made the test less conclusive, just as you described.
I then did a compass cal, and then flew a test hop (and Dynon was right, the cal did wonders for HDG hold). Things went pretty well on the test. Felt a little more twitchy in roll than in pitch, and I ended up decreasing the sesitivity to 7 for roll and 8 for pitch, and it performed pretty well. A little overshoot and hunting in roll, and an occasional pitch occilation, but overall pretty good. Turns to heading were pretty good, with a little twitchiness in the turn and a few corrections to nail the new heading. Climbs went pretty well too. One thing I noticed is that if I left the pitch trim alone if it was hunting the altitude a bit after level off, and didn't chase the UP/DN annunciators with manual trim, it did better than if I reacted to each trim annunciation and trimmed (unless the annunciation was on for a while). I also noticed that having the ball in the center and being smooth with the rudders made a big difference in HDG hold (go figure!
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I won't call it locked in, as it still seems a bit jiggly (good technical term), but it's probably just me getting used to the difference in a light duty AP on a nimble little plane, versus the heavy iron with the high dollar AP.
Some background info for comaprison:
- SV-32 servos
- RV-6, with clipped wings and a big gas guzzler up front.
- My seller and F1 Rocket driver said it's lighter on the controls than his F1
- Test at 8,500 to 9,500 MSL (3-4K AGL), with light winds, very occasional light chop.
- Flown at about 1,700# (1900# GW Airplane)
- Speed was about 165 KIAS / 180 KTAS (a bit less in climbs)
Today's position and force readings were:
Ground:
Roll...Left -90, Neut +11, Right +110
Pitch...Fwd +116, Neut +30, Aft -71
In Flight, Straight and Level, HDG and ALT Hold engaged:
Roll...Pos +15 +/- 5 (+10 to +20); Force +18 +/- 5 (+13 to +25)
Pitch...Pos + 25 +/- 5 (+20 to +30); Force -10 +/- 5 (-5 to -15)
Those in-flight numbers are a bit of a SWAG, as they move around a lot, as you know. Looks like the inflight positions were close to neutral in S&L flight, so that's probably a good indication (and the off-neutral numbers were probably a combination of whatever deflection the controls have at that speed, as well as perhaps an indicator of a very slightly out of trim elevator, and maybe an indicator of when I did not have the ball centered or was too touchy on the rudder).
I'll keep working it, especially if I get a chance to fly on a little X-C, and see how I can dial it in. Not sure how my control surface movement results compare with yours, but I'll keep an eye on it as I work to tweak it all.
Best of luck with your new servos, very interested to hear if the servo arm and surface movement remains, or decreases/goes away!
And DS, thanks for the tips. Will try various settings and see how they each work. And my compliments on your manuals, they do a great job throughout. If the last DS poster was Ian, thanks to you and Mike for all the help along the way, and Ian, congrats on the new baby!!
Cheers,
Bob
Bob,
Couple of quick setup tips:
- Try 15 to 18 on pitch sensitivity.
- HDG mode requires a good compass cal, but you can set up the AP in TRK mode if your compass isn't in cal.
- The trim up/trim down annunciations arent necessary for the autopilot to hold altitude so long as the servo isn't slipping. If you follow them however, the airplane will be in trim when you turn the AP off.
Hi Bob...excellent info...thanks!
Yes, the AP TEST mode puts it in the extreme positions, but the reason to do it that way is because if you just engage the autopilot on the ground it will continuously 'hunt' and will be changing steps occasionally so it isn't really a good test. If you use the test mode it doesn't hunt. What I did was just engage the test then using the stick put the control surfaces trail.
Yes, the servo is sliding slightly on the shaft. Of course a tiny movement at the shaft equates to a much larger movement at the end of the arm where the pushrod connects. The pushrod then connects to the elevator bellcrank which then magnifies the slop.
Also, as mentioned in a previous post I taped the elevators into trail and tried to move the servo pushrod by hand and it wouldn't budge...so that leads me to believe that the 4 degrees of travel I'm seeing at the elevators is the result of the arm slipping on the servo itself. I can also visually observe the slipping, so it seems to me like a reasonable conclusion to come to.
It's as if the sheer pin does not provide close enough tolerance and is 'wallowing' in the hole through the shaft.