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Aircraft rolls with up elevator

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Aircraft rolls with up elevator

Old 08-12-2007, 08:01 PM
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KurtS
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Default Aircraft rolls with up elevator

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
Old 08-12-2007, 09:15 PM
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gboulton
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Default RE: Aircraft rolls with up elevator

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.
Old 08-12-2007, 09:45 PM
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KurtS
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Default RE: Aircraft rolls with up elevator

[] I would certainly agree with you if it weren't for the fact that this airplane has a solid, one-piece elevator...
Old 08-12-2007, 10:02 PM
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Default RE: Aircraft rolls with up elevator

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?
Old 08-12-2007, 10:02 PM
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Campgems
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Default RE: Aircraft rolls with up elevator

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
Old 08-13-2007, 05:20 AM
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Default RE: Aircraft rolls with up elevator


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.


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?


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.


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.


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.



Old 08-13-2007, 09:20 AM
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KurtS
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Default RE: Aircraft rolls with up elevator

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

Old 08-13-2007, 09:38 AM
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da Rock
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Default RE: Aircraft rolls with up elevator

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.
Old 08-13-2007, 11:28 AM
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Default RE: Aircraft rolls with up elevator

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.
Old 08-13-2007, 07:15 PM
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Default RE: Aircraft rolls with up elevator

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.
Old 08-15-2007, 04:47 PM
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Default RE: Aircraft rolls with up elevator

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
Old 08-16-2007, 01:04 PM
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Bosch232
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Default RE: Aircraft rolls with up elevator

Is the plane heavier on one side than the other?
Old 08-16-2007, 01:34 PM
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KurtS
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Default RE: Aircraft rolls with up elevator

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


ORIGINAL: Bosch232

Is the plane heavier on one side than the other?
Old 08-16-2007, 02:46 PM
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Default RE: Aircraft rolls with up elevator

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.
Old 08-16-2007, 03:45 PM
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KurtS
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Default RE: Aircraft rolls with up elevator

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





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.
Old 08-16-2007, 06:16 PM
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Default RE: Aircraft rolls with up elevator

Keep in mind, none of us have had his plane in our hands, what we consider perfectly straight may vary, to some, they can't see a 1/32 difference, to other's, it's very clear. I know from experience flying rough prototypes, any number of things can cause the plane to behave differently. The underlying factor, it's usually only one element that needs to be fixed, but sometimes more then one, but I just want the original poster to realize, do not try and make all of these mods and revisions at once or you'll find it counterproductive.

Just start at the top of the list, see what happens, and move down it, posting the progress along the way.
Old 08-23-2007, 02:08 PM
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Default RE: Aircraft rolls with up elevator

Here's another possible item to check that I haven't seen mentioned yet.
Seal the aileron gaps, using strips of tape or film-covering material.
If one aileron gap is larger than the other, the plane will roll with elevator input. The reaction is similar to a plane that is not balanced laterally, side-to-side.
Old 08-23-2007, 05:04 PM
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KurtS
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Default RE: Aircraft rolls with up elevator

Interesting idea. I haven't sealed the gaps on the control surfaces of this airplane, I'll give it a try.

I have made some progress however. I reduced the down thrust of the engine a little and flew it again a couple days ago. That change had a positive effect in that less up elevator trim was needed and that I feel helped it fly inverted with probaly half the down elevator input that it required before, not to mention it maintained altitude with less up elevator trim. It still rolls off in a loop though but only if it is a tight loop. Again, the less up elevator input the less it wants to roll.

I also dropped $43 today on a Robart Incidence Meter (finally). It was hard to go down the trim chart from top to bottom when I didn't have to tool to properly check wing and tail incidence. I've now leveled the fuselage and checked that angles and while the horizontal stab was 0 degrees to the fuselage the wing was about -.5 degrees. I'd read that a fully symmetrical airfoil wing should be +1 degree compared to the tail and so a little tweaking and adjusting has it now at +.75 degrees - not perfect but much better. I think I posted originally that the model seemed to fly tail low and required 1/8" of up elevator to maintain altitude. Certainly the wt shift aft helped that a bit as did the reduction in down thrust. I think this wing incidence change should provide the greatest effect since with the wing set at a positive angle now the fuselage should fly level. As it was before, the tail had to fly the fuselage in a tail-low attitude in order to get the wing at the proper angle of attack.

I need to go test fly this latest change, hopefully tomorrow.

Kurt


ORIGINAL: gyrocptr

Here's another possible item to check that I haven't seen mentioned yet.
Seal the aileron gaps, using strips of tape or film-covering material.
If one aileron gap is larger than the other, the plane will roll with elevator input. The reaction is similar to a plane that is not balanced laterally, side-to-side.
Old 08-23-2007, 05:58 PM
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Default RE: Aircraft rolls with up elevator

On one of my planes, it's set up so that you can adjust the angle of attack on the forward and aft wing, "it's a tandem wing design" I have to be very careful when I adjust it's configurations to make sure the incidence is correct or it won't fly at all. The last time I took it up, one of the mounts came loose causing it to lose it's angle, it immediately went completely out of control, so looks like I gotta build a new fuse for it. So, yeah, very important you make sure those are lined up correctly.
Old 09-23-2007, 12:12 AM
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Default RE: Aircraft rolls with up elevator

I have the same problem as this. Did you fix the problem?

For my plane, I have tried everything. All surfaces are square and true. CoG is good front/back, left/right. I re-hinged the elevators, one flight I even tried over compensating with making one elevator move farther to compensate for the right roll on up elevator, it didn't work. If I keep looping it keeps rolling... over and over.
I replaced the elevator servos thinking one might have a dying motor. I'm out of ideas. I've read this thread and have looked at everything. It only rolls with significant up elevator. The elevator halves move in unison, with equal strength.

Any more ideas?
Old 09-23-2007, 12:24 AM
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KurtS
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Default RE: Aircraft rolls with up elevator

I wish I had more to report but thanks to moving and working I haven't had a chance to go do any more flight testing. Hopefully soon thought. Glad to hear someone else has the same issue, perhaps we have the same problem. Naaa, they're airplanes, and all different.

K


ORIGINAL: Kreag

I have the same problem as this. Did you fix the problem?

For my plane, I have tried everything. All surfaces are square and true. CoG is good front/back, left/right. I re-hinged the elevators, one flight I even tried over compensating with making one elevator move farther to compensate for the right roll on up elevator, it didn't work. If I keep looping it keeps rolling... over and over.
I replaced the elevator servos thinking one might have a dying motor. I'm out of ideas. I've read this thread and have looked at everything. It only rolls with significant up elevator. The elevator halves move in unison, with equal strength.

Any more ideas?
Old 09-23-2007, 09:19 AM
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da Rock
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Default RE: Aircraft rolls with up elevator

If it slow rolls, it might have a warped wing. In a loop, the wing is pitched into a greater AOA to get the additional lift that offsets the centrifugal force that's going to load the airplane. And warps work more than they do at cruise.

BTW, how square are your ailerons? Did you have to click any trim into the sucker on it's maiden? Aileron trim works great for level cruise but becomes flaps in a loop. And one flap is down and the other is up.

If you've got a lot of aileron differential to trim for level flight, it might bite you in a loop or any pitch change. That's why precision aerobatics flyers don't like wings that take aileron offset to fly level. They usually find a twist to remove that gets the ailerons back equal. And the airplane flies everything better from then on.
Old 09-23-2007, 01:10 PM
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Default RE: Aircraft rolls with up elevator

The roll isn't that slow, in fact its quite fast... almost snappy. Sooo many people at the field have looked, prodded, lined up, and generally oggled at the wings.. they are very square and true.
Old 09-23-2007, 01:36 PM
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Default RE: Aircraft rolls with up elevator

I've not been able to find where you identify the model. What is the airplane?

When a model snaps out of a loop, it's time to do some judging of the speed and sharpness of the loop.

Almost any will snap out if they slow enough and you don't ease up on the elevator. And if you're trying to force too tight a loop, some will snap out when they've had enough of your pushing.

And almost every model of an aerobatic fullscale will have a very strong inclination to snap out of loops until you've tuned your elevator throws. Tapered wing airplanes especially like to snap out. That's why Edge, Extra, Sukhoi, Yak, Cap and the like fullscales were designed with significantly tapered wing planforms. So they will do the snap maneuvers and the tumbles and such that are part of their competitions. And our semi-scale models of them repeat that behavior really good.

There are a lot of threads filled with the elevator tuning that lots of experienced flyers do to their snap-capable models. It's basically to fly and try. Each flight tries with less elevator throw until the airplane stops snapping out of loops it shouldn't snap out of. And then the modeler puts back the last bit of throw he just removed, and uses the final setting in the airplane for his High Rate Elevator setting. And sets the Low Rate so the deflection won't trigger that snap. And then has good control over a better flying model. The model no longer has excessive, wasted elevator throw that only made the model twitchy and risky to fly.
Old 09-23-2007, 01:54 PM
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Default RE: Aircraft rolls with up elevator

ORIGINAL: Kreag

The roll isn't that slow, in fact its quite fast... almost snappy. Sooo many people at the field have looked, prodded, lined up, and generally oggled at the wings.. they are very square and true.

That sure sounds like one wing is stalling in the loop. Especially since you mentioned in your earlier post that it only happens when you're doing tighter loops.

Try some straight ahead stalls and see if you can detect a definite tendency for it to drop to one specific side. If it does then you'll want to study the shape of the wing for any differeences from one side to the other. For example a leading edge that is improperly or differently shaped from one side to the other can easily induce an earlier stall on one side compared to the other.

You may need to use some sort of templating techniques here. THis is a time where small differences cause big results. Something like 1/8 plumbing solder or #14 copper electrical wire carefully bent around one wing's leading edge and then used as a template to compare to the other side can work well. 1/32's count for the world here and even 1/64's can make a big difference. The first 1/5 of the airfoil on the "upper" surface, depending on if you're pulling positive or pushing negative G is the most critical portion of the airfoil in terms of maintaining similar airflow. And the first 10%, basically the leading edge, is even more critical.

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