Uncoordinated flight-aileron trim tab

imwithtuxedo

imwithtuxedo

Pre-takeoff checklist
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My Comanche has been flying crooked since some modifications including a new engine and prop. Not much, could just screw the rudder trim adjustment knob, but I’m wondering if I can get it fly straight without doing that. Here’s how the ball is oriented when flying on a no wind day on a three-way track. It’s pretty consistently just outside the lines:

1706532718190.jpeg

My Comanche has an aileron trim tab. I’m not excited about trial and error and would rather leave it as is and just screw with the rudder trim if it won’t address this issue. I’m wondering if a little tweaking of the tab will help. Thoughts?


1706532536267.jpeg
 
If it's flying on a straight track with the ball off, that's exactly what the aileron trim tab is for. Trial and error is how you adjust it.
 
I wouldn't fart with it given that the reason you bought a Comanche is that you don't care about speed. Otherwise I would suggest that you get it straightened out since flying all cattywampus is going to cost a knot or three. But that's me.
 
It was rigged before the new engine and prop. Always have had to use the rudder trim adjustment knob in my Comanche but I’m attempting to see if I can make adjustments to make the plane fly straighter to alleviate as much knob twisting as possible. Maybe it isn’t possible.
TCABM said:
Was that before or after the modifications and new engine/prop you referenced in the OP?
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Hey Russ. I’ve flown in pretty severe crosswind scenarios where lots of rudder trim was required. I flew a three leg track to see how the ball would react to differing wind conditions.
RussR said:
I'm curious what effect you believe the wind, or the necessity of flying a 3-leg track has to do with it.
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How do you know the TC is leveled properly?
How do you know the rudder trim is centered properly?
 
You may have forgotten some basic aerodynamics. Cross winds are adjusted for with headings, not rudder trim. Rudder trim is for centering the ball. Aileron trim is for level wings or no turn with a center ball.

Now it's possible to have an airplane out of rig, and have a center ball with zero turn, but might still have a wing low.
 
imwithtuxedo said:
I’ve flown in pretty severe crosswind scenarios where lots of rudder trim was required. I flew a three leg track to see how the ball would react to differing wind conditions.
Click to expand...
Rudder trim has absolutely nothing to do with compensating for a crosswind. You're flying in the air, the ground track is irrelevant.
 
imwithtuxedo said:
Hey Russ. I’ve flown in pretty severe crosswind scenarios where lots of rudder trim was required. I flew a three leg track to see how the ball would react to differing wind conditions.
Click to expand...
Only relevant if you're talking about coming down to land, or some other ground-reference maneuver. I assumed you were talking about cruise flight, and 3 legs at cruise altitude. In cruise, crosswind has no effect on your ability to hold a heading.
 
imwithtuxedo said:
My Comanche has an aileron trim tab. I’m not excited about trial and error and would rather leave it as is and just screw with the rudder trim if it won’t address this issue. I’m wondering if a little tweaking of the tab will help. Thoughts?
Click to expand...
The airplane's Service manual has the best, and most authoritative, advice. And yet, many mechanics ignore it and just do their own thing, often making the airplane way out of rig.

You have an aileron problem. If that ball is off-center, and the TC is level as it should be, the flaps are rigged correctly, and the wings are level in flight, adjusting rudder trim will get the ball centered but now you will be in a banked turn that you have to counter with opposite aileron. That's what that ground-adjustable tab on the aileron is all about. The wings themselves are imperfectly aligned. But all the other stuff needs to be confirmed first.
imwithtuxedo said:
Hey Russ. I’ve flown in pretty severe crosswind scenarios where lots of rudder trim was required. I flew a three leg track to see how the ball would react to differing wind conditions.
Click to expand...
As the others have said, rudder is not used to maintain track. Crabbing to counter a crosswind does not move the ball off-center. The flight should be coordinated. The only time you need to slip is in a crosswind landing or takeoff.
 
Need to understand what causes your aircraft to fly out of coordination in the first place.

An airplane does not naturally fly straight. In level flight, the prop slipstream presses on the left side of the fuselage, causing it to yaw left. Torque also causes it to roll left. Most aircraft have built in means to offset those tendencies. For example, the vertical stabilizer on my Decathlon is offset to the left, which causes the aircraft to yaw right to counteract the slipstream. Other aircraft accomplish this with an offset engine mount. In either case, the counteraction may be fined tuned with fixed or adjustable rudder and aileron trim.

But the counteraction provided by the offset is only correct for one power setting. Add more power and the aircraft will still yaw and roll left. Reduce power and the aircraft will yaw and roll right. This is easy to verify by yourself. Go up and trim for coordinated S&L flight. Then make large power changes and watch the ball slew left or right. And if the aircraft yaws one way, then you have to roll the other way to maintain heading.

So, it could be your aircraft is not flying any more or less crooked than before. Could be you are just flying at different power settings, or your new engine is making more power at the old settings, and you have not tweaked your trim accordingly.

Could also be fuel management. More fuel in one wing causes the aircraft to roll towards the heavy wing. Slight down aileron is required to counteract the roll. Down aileron increases AoA of wing, which increases drag, which causes aircraft to yaw in that direction, requiring rudder to offset.

So before you get spun up about rerigging, play with different trim and power settings. Could be your airplane is just doing normal airplane stuff, and you didn't notice it before.
 
imwithtuxedo said:
I’ve flown in pretty severe crosswind scenarios where lots of rudder trim was required. I flew a three leg track to see how the ball would react to differing wind conditions.
Click to expand...

I know you've already been beaten up over it but until you get past this fundamental misconception of how wind affects an airborne craft you're going to be chasing shadows. As suggested above try using different power settings with rudder trim centered and see if you can find one that puts the ball in the center. Since the only thing altered was engine and prop that's the most likely cause of the change.
 
imwithtuxedo said:
My Comanche has been flying crooked since some modifications including a new engine and prop.
Click to expand...
What does flying crooked mean? In the photo, the plane appears to be flying straight (not turning). Albeit uncoordinated. Were the wings level relative to the horizon when this photo was taken? What happens if you center the ball and keep the wings level?
 
imwithtuxedo said:
My Comanche has been flying crooked since some modifications including a new engine and prop. Not much, could just screw the rudder trim adjustment knob, but I’m wondering if I can get it fly straight without doing that. Here’s how the ball is oriented when flying on a no wind day on a three-way track. It’s pretty consistently just outside the lines:

View attachment 124843

My Comanche has an aileron trim tab. I’m not excited about trial and error and would rather leave it as is and just screw with the rudder trim if it won’t address this issue. I’m wondering if a little tweaking of the tab will help. Thoughts?


View attachment 124842
Click to expand...
My question is why are you reluctant to use the rudder trim? That is what it is made for. My old Cherokee had a rudder trim and I would use it to trim out skid error when flying a long trip. That way I didn't have to use the rudder constantly while cruising. I found that if I cruised at different airspeeds or density altitudes I would sometimes re-trim. It was very subtle, but it made a big difference on a long XC. Rudder trim isn't like pitch trim. You don't need it for pattern work or maneuvers, but it works really well to eliminate control forces during cruise. I don't think my Mooney has one. I wish it did.
 
Rudder with a fixed tab will be trimmed properly for ONE speed. Just like the elevator.

Most planes need some left rudder at cruise speed, as rudder tab is set to help on takeoff and climb with a LOT of right rudder is needed.
 
Does the AI show your airplane out of level in cruise with the ball out of center?
 
Looks like your turn coordinator has been adjusted a few times. I found with a out of trim situation on a Cherokee 6 for a while until I actually levelled the plane and found my TC was turned slightly.

Might be worth a quick check before rigging.
 
The marring on the screw slots k9medic noted shows the TC has been previously adjusted.
The Cherokee procedure for leveling the TC is to put a level across the spar box assembly under the rear seat and deflate the tires appropriately to center the level's bubble. If the adjustment is beyond the tire technique, then get out the wing jack.

I imagine this is done with two people. One person places the level and sits in the pilot's seat and loosens the TC. From the pilot's seat they monitor the level. When level, lock down the TC.
 
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I thought he said it was okay and then it wasn't.
 
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