Incidence adjustment for trim

The problem with Cubs is the aileron hinge line is both below the bottom of the wing and below the bottom surface of the aileron. A teeny difference between left and right will allow a significant difference in torque due to wind force on the aileron face.

I believe the Decathlon hinge surface is above the bottom edge, and the top of the Dec ailerons have a gap seal. I can look today -

So in the case of hands-off asymmetrical aileron deployment in a Decathlon - start with the spades. Make sure both are aligned equally with the ailerons. The other (remote) possibility is mismatched ailerons themselves. They build them in precise jigs.

There is no "cable adjustment" that can fix this sort of problem - don't even try to center the stick until you can fly hands-off with ailerons equally trailing. And for the Cubs, the asymmetric force is so strong that even a two foot long trim tab couldn't cure it.

Difficult to believe a factory test pilot would not notice this . . .
 
Bartman said:
He bought the 8KCAB Super D I restored. ACA did the final assembly and test flights. New wings with the new style ailerons.
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Can’t wrap my brain around it. At 135mph cruise, to fly straight with no roll requires left aileron and left rudder. If you let go it rolls fairly aggressively to the right. If you level the wings with aileron it’s an uncomfortable amount yaw with the ball fully deflected to the left. At 120 it’s more or less flying straight with a very slight right roll.
 
Derek Lawless said:
Can’t wrap my brain around it. At 135mph cruise, to fly straight with no roll requires left aileron and left rudder. If you let go it rolls fairly aggressively to the right. If you level the wings with aileron it’s an uncomfortable amount yaw with the ball fully deflected to the left. At 120 it’s more or less flying straight with a very slight right roll.
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Talked to Chris at ACA and he read me the steps off the FAQ from their website. This is the left rear strut attachment. The right one, which I could have sworn I took a picture of but can’t find, only shows 2 threads. That was my A&P’s first try, but doesn’t make sense. I did try to counter the roll with rudder but there’s not enough rudder authority.
 
Bob Turner said:
The problem with Cubs is the aileron hinge line is both below the bottom of the wing and below the bottom surface of the aileron. A teeny difference between left and right will allow a significant difference in torque due to wind force on the aileron face.

I believe the Decathlon hinge surface is above the bottom edge, and the top of the Dec ailerons have a gap seal. I can look today -

So in the case of hands-off asymmetrical aileron deployment in a Decathlon - start with the spades. Make sure both are aligned equally with the ailerons. The other (remote) possibility is mismatched ailerons themselves. They build them in precise jigs.

There is no "cable adjustment" that can fix this sort of problem - don't even try to center the stick until you can fly hands-off with ailerons equally trailing. And for the Cubs, the asymmetric force is so strong that even a two foot long trim tab couldn't cure it.

Difficult to believe a factory test pilot would not notice this . . .
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Chris from ACA suggested they only rig them to 120mph.
 
I thought you said above that when you let go of the stick one aileron goes up, the other down. If that is not the case, then my suggestions are not applicable.

(Left instantly drops an inch and a half, two inches and the right goes up a corresponding amount.)

The spades can be adjusted with a slight wedge where the flat plate meets the strut. The pivot point for the aileron is about 3/4" above the bottom surface, making these ailerons far less critical than those on a Cub.
 
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This plane has the new style ailerons, no spades required. They're hinged about a third of the chord back from the leading edge and fatter than the trailing edge at the aileron cove. They are tecchnically referred to as "aerodynamically boosted ailerons", the thickness of the aileron forces the air to reattach as it passes over them making them more effective at greater deflections.

Hard to say what's going on without being able to check wing incidence. When I had the plane I didn't do anything more than really basic acro so I didn't see the issues at higher airspeeds that Derek is describing but the plane did have what seemed to be out-of-rig ailerons. It flew mostly straight, not as bad as Derek is describing, so I'm wondering if anyone tried unsuccessfully to improve it.
 
If what he said about one going up 2" and the other going down 2" was in reference to the aileron trailing edges, then no need to check rigging. You simply cannot rig enough differential wash to overcome that kind of offset. The ailerons need to be changed.

If he was not talking about ailerons, then my suggestions are obviously off the mark.
 
Tangogawd said:
Derek, has the wing wash been checked, and adjusted?
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Me and an A&P who owns a Champ gave it a shot, but it seems pretty fuzzy trying to find a reliable point at which to measure the semi symmetrical wing. Based on what we could tell, it didn’t look like there was any difference between the two sides, but the left wing strut was already adjusted about 10 threads and the right strut was bottomed out. So he lengthened the right strut (which didn’t make sense to me) a couple of turns to see if that made a difference, but I’m not able to tell that it did. It’s still rolling right and requires constant left aileron. What makes zero sense to me is wings level with the stick left, the left aileron is down and the right is up. My brain says it should be the opposite. And yes, the ailerons are connected correctly (stick left to roll left, stick right to go right). Feet on the floor wings level with the stick also results in the ball being all the way to the left. To fly straight you need both left stick ( but left aileron down) and a decent amount of left rudder.
 
Tell is one more time - where do the ailerons go when your hand is not on the stick?
 
Make a rigging board, piece of 1x2 about as long as the chord with an L shape at one end to go against the leading edge and then two posts at the spars to make the board about parallel with the chord line. It doesn't have to be perfect if you use a digital level and just remember it's all relative to the root incidence on each side. Check that the incidence on each side is equal at the root, then check about halfway out and near the tip. Make sure the underlying aluminum layers around the aileron cove are not changing the measurement.

You can raise the tail to level the chord line of the wing or leave the tail down but try to level the plane laterally. I believe the tube under the instrument panel would work as a level reference or a level across the window sills behind the front seat.

That frame was reworked at the factory in the original jig so it should be perfect.
 
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Bob Turner said:
Tell is one more time - where do the ailerons go when your hand is not on the stick?
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Hands off, the left aileron is down and the right is up. Had a chance to go fly this morning in nice calm conditions. Has a real nice snap entry to a spin to the right from a power off stall with ball centered (no pilot supplied pro spin input). Does not want to spin to the left.
 
Treat your wing like it's the Wright flyer where the only way to make the airplane roll is to adjust the angle of incidence of the wings. In level flight hold the ailerons in trail with the trailing edge (TE) of the wing and observe what happens. Based on your previous comment stating that the LH aileron is down from the TE when you are holding level, I suspect that once you align the aileron with the TE the airplane will roll left. This indicates that the RH wing incidence at the outboard strut attachment is too high and needs to be reduced (RH wing is producing too much lift), or conversely, the LH wing incidence at the outboard strut is too low and needs to be increased (LH wing lift needs to be increased). The problem is, this correction will actually make your right roll tendency worse! I believe the underlying issue is a problem with one of the ailerons causing them to deflect in flight, LH aileron down, causing a right roll. The ACA FAQ talks about using two hands to bend the TE of the ailerons; Think of this as adding a small trim tab to the TE of the aileron. In your case you would bend the TE of the LH aileron down to defect the aileron up in flight. The goal is to get the ailerons to fly 'in trail' with the TE of the wing with no stick pressure. I wasn't comfortable bending the ailerons so rigged my spades to create this pressure. A small balsa wedge taped to the TE of the aileron may have the same effect?

I highly suggest you fabricate a rig board and use a decent digital level to get accurate readings. Zero your level at the LH wing root and take relative readings at the RH root and a rib outboard of the outboard strut attachments on the LH and RH wings. You want to make sure your are adjusting the correct wing; You want zero washout on both. When I went through this I needed to adjust both wings; The LH more than the RH.
 
Once again - then I am going to let you struggle without my input.

Bad (mismatched) ailerons cannot be fixed by changing the wing wash angles. You can partially compensate, but not completely, and when you do your aircraft no longer matches the factory specification. Further, some aircraft will exhibit non-standard stall characteristics when wing warp is not equal- we have a Super Cub like that (owner is aware) that violently drops the left wing in a gentle power off stall. She needs new ailerons.

Ailerons that trail unevenly put powerful roll inputs into the system. Fix that first. As I understand it, the Decathlon is supposed to have zero wash. Both wings. Borrow a Smart Level, and make a cradle as advised above, but do not bother until you can take your hand off the stick and see both ailerons trailing identically.

I am now out of this discussion.
 
The idea I see trending on this thread is that maybe the ailerons are not creating equal force and so an imbalance exists between them. One has a bit more lift and is pulling itself up and driving the other aileron down.

I'm a lurker and have no experience on ACA products. I'll throw this out there as a data point/experience I had. You decide what to do with it. Its experience from another type of aircraft that might not apply to your situation. I'd try everything else first if I were you.

1) First Check the incidence on the Horizontal stab to make sure it is not influencing any roll moment on the aircraft.

2) If a fabric covered aileron's trailing edge skeleton is fabricated from aluminum that is bent in a brake, then the trailing edge will have a radius to it. My aileron (not an ACA product) is about 1/4" radius. (Cessna 140's are not made like this) That trailing edge radius can be slightly pinched to a tighter dimension and doing this will put a lifting force on the aileron due to its center of lift moving slightly aft, or maybe this pinch increases the camber on the aileron, and lifts it up a bit due to a slight increase in the ailerons lift. Some experimental aircraft (Vans RV-7) are trimmed in roll this way during the test phase. I pinched just 3 inches of my trailing edge with a pair of seaming pliers so that the trailing edge compressed just .020" and it fixed my imbalance. It was visually invisible. I pinched the aileron on the wing I wanted to lift....said another way, I pinched the aileron on the lighter wing.

Pinching the aileron trailing edge cannot be reversed. I used this techniques as a last resort. My experience may give you hope and if the aileron's construction meets the parameters it would not surprise me if you can get yours to fly hands off without replacing both ailerons with a new matched set..
 
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my .02, which I'm not sure is worth more or less now that pennies are out of minting;

I have found that it is better to just undo it all, and rig everything from nominal. Especially at this point where "tweaks" have been made to "just see what it'll do".

Rigging isn't a black art, but it does take a bit of experience and knowledge.

Find a shop that does large repairs that require reassembly of whole aircraft,they will have a guy with rigging experience. Not one that just does annuals and oil changes.

Good luck
 
Tangogawd said:
my .02, which I'm not sure is worth more or less now that pennies are out of minting;

I have found that it is better to just undo it all, and rig everything from nominal. Especially at this point where "tweaks" have been made to "just see what it'll do".

Rigging isn't a black art, but it does take a bit of experience and knowledge.

Find a shop that does large repairs that require reassembly of whole aircraft,they will have a guy with rigging experience. Not one that just does annuals and oil changes.

Good luck
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I exercised the atomic option. Flew up to ACA this morning. Right wing rear strut increased an additional 5 from the 2 threads originally showing. Left rear strut was adjusted out another 2 threads from the 10 showing. All components of the empennage were in spec. All cable tensions were wildly out of spec. Aileron cables were 13lbs. Elevator was 20. Will test fly in the morning. Not going to play test pilot with 16 gusting 26 at 40 degrees off the runway not knowing if it’s going to aileron roll on takeoff. The ailerons did pass muster on travel, alignment, and droop. Chris said they’re advising less droop now even to flush. Says they fly better.
 
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