Aircraft rolls with up elevator

Aircraft rolls with up elevator
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Aircraft rolls with up elevator - 8/13/2007 2:01:30 AM

KurtS

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From: Roseville, CA, USA
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Part 2 of my trimming dilemma (see my Trimming and CG posting) is another airplane of mine that is doing weird things. It is a freshly built Midwest Das Little Stik and in addition to being unable to trim it out to fly hands off inverted as described in my trimming thread, it has a strange characteristic of rolling right when you input up elevator.

If you pull moderately back on the stick it will pitch up and at the same time start a roll to the right that will end up with the aircraft heading 90 degrees to the right at the completion of an attempted loop. The less elevator you add the less it does this.

The aircraft is balanced at the aft end of the CG range, will not snap roll even with a lot of rudder throw, lands smoothly, slows to a crawl, doesn't drop a wing when stalled, and flies straight and level upright with zero aileron or rudder but just a bit (maybe 1/8"

of up elevator.

It too is built straight, has a bit of down thrust but no side thrust in the engine, yet when it comes off the ground on takeoff I have to add left rudder to fly runway heading if I pull the nose up to a high pitch angle. I think the two characteristics here are somehow related.

My only thought is that when building this thing, I noticed the wood of one aileron was a lot softer than the other. Could a sudden pitch up be flexing the soft aileron and causing a roll?

And as asked in my previous thread, the inverted level flight thing. I've attempted to cure this by slowly moving the CG aft yet it still requires a lot of down elevator.

One thought did just come to mind though and that is the down thrust in the engine. At lower speeds it requires down trim to fly level and at higher speeds it requires up trim. Too much down thrust, which requires the 1/8" or so of up elevator which in turn is causing it to pitch down when rolled inverted?

Curiouser and Curiouser...

Kurt

RE: Aircraft rolls with up elevator - 8/13/2007 3:15:31 AM

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From: La Vergne, TN, USA
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I'd be inclined to check a simpler cause first...elevator halves either out of alignment (one riding higher than the other), or out of synch (one comes up faster/farther than the other).

Either would be a fairly easy fix if they're each on their own servo, otherwise a bit more difficult, but still doable.

That's the first thing I'd check, anyway.

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(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/13/2007 3:45:40 AM

KurtS

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From: Roseville, CA, USA
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I would certainly agree with you if it weren't for the fact that this airplane has a solid, one-piece elevator...

(in reply to gboulton)

RE: Aircraft rolls with up elevator - 8/13/2007 4:02:03 AM

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From: Burnaby, BC, CANADA
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If the rudder and ailerons are a little cross trimmed you can end up with this. Try playing with some various combinations.

Also if the wing is warped and you chose to "trim" it out with aileron trim that doesn't always do as good a job as it may appear that it should. I know you said it is aligned but another common cause of this sort of rolling in loops can be a skewed wing to fuselage angle or a slightly "bannana'ed" fuselage and resulting offset fin. Try checking it again but try using a different method. It's best to assume that nothing is straight until you check it against some known straight and perpendicular lines.

One way to do this is set the model up with one wingtip touching a wall checked for straight and plumb. Using a big carpenter's square set the leading edge of the wing so it's perpendicular to the wall. Now measure to the center of the wing and use that measurement to double check the tail surfaces for being at least parallel to the wall if not exactly on the centerline. Check the fuselage as well to make sure it's not skewed. And as silly as it sounds make sure the wing span on each side is the same.

Also if you don't have the wing keyed so that it sits down in the exact same orientation each time then that's another place to start. Even if you center it if the wing is skewed by even a couple of degrees it'll do funny things to how it tracks through various maneuvers. It may even be shifting in flight if you're just relying on rubber bands to hold it in place.

That should at least get you started.

Oh, and your stabilizer isn't sitting one tip high and one low so the stab is tilted is it?

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RE: Aircraft rolls with up elevator - 8/13/2007 4:02:20 AM

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Any chance you have some aileron/elevator mix involved? One other thing, does it do this at all speeds? Could you being experiencing a bit of tip stall? Could your wing have a little twist to it?

Don

(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/13/2007 11:20:46 AM

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From: western, NC, USA
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quote:

ORIGINAL: KurtS
It is a freshly built Midwest Das Little Stik and in addition to being unable to trim it out to fly hands off inverted as described in my trimming thread

When the horizontal tail and the wing are not on the same line, the downwash angle that tail sees is different from upright to inverted. So the amount of elevator needed could differ from upright to inverted. Also, you really should not expect to be able to trim for for hands off inverted. Very few airplanes will. Matter of fact, some of the trim guides say that hands-off-inverted is a sign of tail heavy.

quote:

If you pull moderately back on the stick it will pitch up and at the same time start a roll to the right that will end up with the aircraft heading 90 degrees to the right at the completion of an attempted loop. The less elevator you add the less it does this.

Does the airplane pass the side-thrust tests in your trim guide? Most of them have a test for it. Fly straight and level toward you. Pull to straight vertical. Does the airplane veer left or right in the pullup?

quote:

The aircraft is balanced at the aft end of the CG range, will not snap roll even with a lot of rudder throw, doesn't drop a wing when stalled,

To snap the wing has to be stalled. Airplanes that have "safe" stall response, i.e. don't drop a wing when stalled, stall only at a very slow speed, are very hard to snap. My Ultra Stiks were very hard to snap. They were also lightly loaded and that can contribute to the difficulty to snap.

quote:

My only thought is that when building this thing, I noticed the wood of one aileron was a lot softer than the other. Could a sudden pitch up be flexing the soft aileron and causing a roll?

More apt to have a weak aileron servo that's being overcome.

quote:

One thought did just come to mind though and that is the down thrust in the engine. At lower speeds it requires down trim to fly level and at higher speeds it requires up trim.

Trim guides allmost always have up/down thrust tests. The one that comes to mind is:
Fly straight and level with full power and trimmed for hands off. Chop power. If the airplane dives, it has upthrust. If the airplane climbs, it has downthrust.

When you're trimming a model, you really shouldn't expect to be able to do all the tests in a mishmash and expect each one to give sensible results. You really need to sort each step before going to the next. For example, the first real test is for CG. CG affects all the rest. If you've not sorted CG you can't really expect the very next test, Up/Down Thrust, to give valid results. BTW, the trimming guide that's laying here on this desk does 1. Control Neutrals, 2. Throws, 3. CG, 4. Up/Down thrust. And the CG test says "a lot of down required" in the CG test. I underlined the word "lot", because the author of that guide used it to get across the fact that you should expect some down when inverted.

Follow a guide in order. And trim each step before going on.

(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/13/2007 3:20:51 PM

KurtS

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From: Roseville, CA, USA
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Good replies everyone, thanks. I figured I was probably being overly optimistic in my interpretation of "a lot of down" elevator and was just hoping to be able to roll inverted without an immediate stab at the stick to hold altitude. Sure would be nice though.

I did start at the top of the trimming chart and began working my way down, but since it rolls so badly with up elevator I pretty much got stuck when trying the side thrust test - it doesn't just come out of a loop with one with low, it changes heading about 90 degrees. The harder you pull up the more drastic the roll. It's not a snap, it doesn't fall out of the loop when it rolls, it's a rolling motion as if I was adding aileron along with the elevator.

As for the up and down thrust testing, it flies fairly level at full throttle/high speed, and when reduced to idle continues to do so until the speed starts to decay at which point it will begin to descend. So I think it's fairly ok in that department but if it is trimmed to fly at 3/4 to full throttle level flight and then I reduce to 1/3, I'd expect to have to add up trim to fly level due to the lower speed. This airplane is the opposite, I need down trim at low speeds and up trim at high speeds which is why I think I have too much down thrust.

It does make sense that the more tail heavy the airplane the less up elevator it needs. My other airplane that I've been attempting to trim out has suddenly become much nicer in the landing flare now that the nose isn't so heavy.

I will keep working with these two airplanes and the trimming chart to see what I can make of them. This is the first time I've ever tried to properly trim an airplane, I've always just flown it as is in the past.

Thanks!

Kurt

(in reply to da Rock)

RE: Aircraft rolls with up elevator - 8/13/2007 3:38:53 PM

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The up/down trim is like everything else, it's subject to your interpretation. That's where you develop your "experience".

The up/down thrust test has you interpret the loss of thrust. Whatever the airplane does almost immediately after losing the thrust can be what you're looking for. However, if you don't have a balanced CG already established in the airplane, the test result is going to be clouded by the airplane's trim change, and the trim is going to be biased by the CG location.

So sort out your CG and then do the up/down thrust test.

BTW, the way I got my Ultra Sticks to snap halfway decent was after moving the CG back and being less than satisfied with that, I increased the elevator throw. The elevator has to have the strength to stall the wing. Moving the CG back increases the elevator's efficiency/strength. But if just moving the CG doesn't help enough, then simply rig the elevator to get that strength. Sticks are lightly loaded. Lightly loaded airplanes are less easy to stall. The horizontal tails on sticks are very low aspect ratio. Stubby tails are less efficient, less powerful. They not only look square and fat, but they act the way they look.

(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/13/2007 5:28:54 PM

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How does the wing balance? You may need to add weight to one wing tip, heavy wing will be rolled into on both inside and outside loops. Also if the gap between the wing and ailerons is greater on one side, that can cause problems. Seal the hinge line. Also, is the wing aligned to the tail?

Also sounds like you have too much down thrust. Remove some or most and the pitch trim problems should go away.

(in reply to da Rock)

RE: Aircraft rolls with up elevator - 8/14/2007 1:15:45 AM

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As the last post stated, check the pitch of the motor, that's the first thing that jumped out at me considering you are having to trim it differently based upon how fast you are flying. I take it when you are just gliding, it requires yet more trim to keep straight and level right? Get some serious altitude, do a powered dive, chop the power and pull straight up, you likely are going to stall the plane, but might get a loop out of it, that will difinitavely tell you if it's a control surface problem, or a power setup problem.

(in reply to HighPlains)

RE: Aircraft rolls with up elevator - 8/15/2007 10:47:33 PM

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I have had the exact same problem with my CAP 232 by HANGER 9 I had it for a while
And it had been rebuilt due to a severe crash and major strengthening was put into the
Tale but this threw the balance way out. But I used the tapered wing cg formula to
Find the cg and it balanced great so when I went to fly it had the exact same problem as you describe. So I downloaded the manual and found the cg to be much further forward
And I either had to put 500grams on the nose or set the engine 4 inches further out
So I am in the middle of making a ply engine mount to fit a 61 mds rear port engine
Instead of an IRVINE 46 by the way it is only a 60 inch wingspan hope this help

(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/16/2007 7:04:25 PM

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Is the plane heavier on one side than the other?

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RE: Aircraft rolls with up elevator - 8/16/2007 7:34:23 PM

KurtS

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From: Roseville, CA, USA
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Not that I know of but I haven't balanced it lateraly yet. To fly straight and level it requires no aileron or rudder trim/input and if it was heavy enough on one wing to cause it to roll when pitched up I'd expect it to require some aileron trim/input to hold the wing up in level flight.

Kurt

quote:

ORIGINAL: Bosch232

Is the plane heavier on one side than the other?

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RE: Aircraft rolls with up elevator - 8/16/2007 8:46:18 PM

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Not really, it may have been compensated for during your initial trim flights.

Generally a good way to tell if that is the case is that you'll notice some change in roll trim as you alter the flying speed. Try seeing how it is from slow flight ot rippin'. If you find that you need to compensate for level flight speed changes to avoid some rolling then it's out of whack by at least a little.

The only other thing I can think of offhand is that you're looping quite tight and one wing leading edge or other building factor is different from the other and one side is reaching a stall or near stall and the extra drag is pulling the wing back and causing what appears to be a roll in the loop.

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(in reply to KurtS)

RE: Aircraft rolls with up elevator - 8/16/2007 9:45:56 PM

KurtS

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From: Roseville, CA, USA
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Hmmmmm...

Well, I tend to disagree. If the airplane flies with ailerons and rudder perfectly straight, then there is/was no flight control compensation. They are dead even, no up, no down, no left no right. If there were a twist in the wing or if one wing was longer than the other or if one was heavier, the airplane would have started a roll immediately after takeoff that would haved needed aileron input to stop. That correction would have to have been maintained to fly hands off level flight, aka trimmed. Since no correction was needed to me that rules out heaviness, warp, or unequal length.

I do tend to agree with the idea that on a sudden pullup, critical angle of attack is being exceeded and perhaps the gyroscopic action of the engine, p-factor, torque, what-have-you is causing the consitant roll in one direction. And since I haven't now flown the thing for a bit over a week I'm confusing myself on which way it actually does roll. Since the less the elevator input the less the roll, perhaps it is just an accelerated stall of sorts.

Have to go fly it again now just to make sure.

Kurt

quote:

ORIGINAL: BMatthews

Not really, it may have been compensated for during your initial trim flights.

Generally a good way to tell if that is the case is that you'll notice some change in roll trim as you alter the flying speed. Try seeing how it is from slow flight ot rippin'. If you find that you need to compensate for level flight speed changes to avoid some rolling then it's out of whack by at least a little.

The only other thing I can think of offhand is that you're looping quite tight and one wing leading edge or other building factor is different from the other and one side is reaching a stall or near stall and the extra drag is pulling the wing back and causing what appears to be a roll in the loop.

(in reply to BMatthews)