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Unstable in Roll

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Unstable in Roll

Old 04-28-2015, 07:49 AM
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crjame
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Default Unstable in Roll

Hey there,

I originally posted this in the ARF forum but have not been getting much feedback, so I thought I would try it here. I am looking for some input into what I feel is an issue. The aircraft (GP Extra 300SP) is trimmed and cg is in the recommended range. When I fly the aircraft, it does not really want to stay wings level. Its almost like its balancing on a ball always wanting to tip to one side. I make an input back to wings level and shortly thereafter, it falls off the other way. If I was building it I would say it needs a bit of dihedral. Obviously I can not put any in.

Does anyone have any ideas or recommendations? Reflex both ailerons?

Thanks
Old 04-28-2015, 09:18 AM
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Hydro Junkie
 
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What you need to remember is that an Extra, being an aerobatic plane, is inherently unstable. This is due, in part, to it's midwing design that facilitates rolling. Without seeing how the plane is set up, internally, it's hard to really give input. One thing that may be happening is the CG could be too high. That could be helped by lowering the radio gear and battery pack(s) if they are not already mounted in the bottom of the fuselage
Old 04-28-2015, 11:58 AM
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speedracerntrixie
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What servos are on the ailerons? There is nothing inherently unstable about an Extra. It should remain wings level without issue. You could have a bad servo or two. Binding linkage. Or a warped wing that only trims level at one airspeed and drifts when going slower or faster.
Old 04-28-2015, 12:11 PM
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I have a GP Extra300SP and Ii's a great flyer. I have mine set up on rates and CG per the manuel. Be sure to check those.
Karl
Old 04-28-2015, 12:13 PM
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vertical grimmace
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I would say cheap servos as well. With as large as those ailerons are, poor centering would be very apparent.
Old 04-28-2015, 02:46 PM
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All Day Dan
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Check for balance around the longitudinal axis (nose to tail). Dan.
Old 04-28-2015, 02:48 PM
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RBACONS
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Another possibility is a heavy wing that has necessitated putting in some aileron trim to compensate for. Again, a speed sensitive setting.
Old 04-28-2015, 03:12 PM
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dasquirrelisme
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I had that same plane and after the first flight I wanted to sell it. It was my first highly aerobatic plane and it was a lot different than a sport plane. That was the first plane I used Expo on and after I dialed 30% expo it was an awesome flying plane. That plane is sensitive to inputs and the expo will really help out. You might want to set the rates down a bit on the low rates.
Old 04-28-2015, 03:17 PM
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dbacque
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I've read through this thread a couple of times now and had nothing to offer. Then it dawned on me that this sounds like a control surface centering problem. I think Vertical Grimmace is right. But it isn't necessary the fault of the servo. Even a high dollar, high torque, super accurate servo can have centering problems if the linkage isn't right.

If centering isn't dead on accurate it will cause exactly what you describe, this is frequently caused by stiff linkage or hinging. Remove the pushrod clevis from the horn and make sure the surfaces are very free to move. No glue in the hinges, no binding torque rods, no off-axis hinges. Lots of things can lead to binding. Also check the linkages. Is everything free? Quality servo and good centering?

How are the ailerons set up? Torque rods? 1 or 2 servos? Servos with direct pushrods to surface? All of these answers change the equation. What kind of hinges? Details help the diagnosis.

My bet is on a little bit of control surface binding that is causing centering problems. Especially since you say it rolls one way, you correct for it (moving the control surface the other way) and then it rolls the other way.

Take a look at the ailerons and see if they always returns back to the exact same center position if you move the stick to the left or right. But return the stick to the center position very slowly in both cases. If you see any difference in aileron position when you return to center , you've got binding and the result will be exactly what you described. Centering is so critical that sometimes the centering looks good enough to the eye but it's not accurate enough for flight conditions. So just because it looks right, doesn't mean it is. Do you feel any friction at all in the hinging?

Dave

Last edited by dbacque; 04-28-2015 at 03:53 PM.
Old 04-28-2015, 04:08 PM
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aeronately
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It has been years since I read RCUniverse. I hope that I can still handle a response.

First I must know if the model is exhibiting the following characteristic. What I am thinking of is when the acft likes to fly stably in either a right wing down yawing left or flops over to fly left wing down and yawing right. As an aero engineer I have witnessed this phenomena in the Langley free flight wind tunnel and can address the exact cause of the lateral instability and can suggest a simple fix. Are we talking about the same thing?

Nathan Rambo. [email protected]
Old 04-28-2015, 07:44 PM
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flycatch
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You need to put an incidence meter on each panel. Check to ensure that from root to tip they are both the same. If they are than bolt them to the fuselage and check again. You have either a bad wing panel or the alignment pin. It may also be were the alignment pin enter the fuselage. Misaligned wing panels act like ailerons. If your lucky it will be the alignment pin and that is an easy fix. I have been there with ARFs. Another thing to look at is the horizontal stabilizer for proper alignment in relationship to the trailing edge of the wing panels.
Old 04-28-2015, 08:19 PM
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crjame
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Hey guys,

Thanks for all the input. I have 5245's on the ailerons with ball links on aluminum servo arms and the stock clevis's on the aileron end. At this point the linkage would be at the bottom of my list to be honest. I have expo programmed in already and am happy with that set up. I think the first step will be checking all the incidences and lateral balance. Thanks for all the ideas and I will let you guys know.
Old 04-29-2015, 02:21 AM
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When reading the described problem, I thought... yep, I experienced that once. It was caused by a CG much too far forward. I'd balanced the plane (Spacewalker II) inverted on a CG machine, to discover doing so did not produce the same results as doing it upright at the wing tips as the directions outlined. After the maiden, I said the exact same thing to the guys... ya can't keep the wings level, it wants to roll off one way or the other. Correcting the CG solved the problem.
Old 04-29-2015, 08:00 AM
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cublover
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Balance it just a tad nose heavy!!...oh.... and ad 30% expo on surfaces,,,then,,,after each flight,,take out 5% till you get comfy...

Last edited by cublover; 04-29-2015 at 08:03 AM.
Old 04-29-2015, 12:07 PM
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speedracerntrixie
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CG has nothing to do with this issue. Either the ailerons are not centering, the wing is warped or reverse expo has been adjusted in.
Old 04-29-2015, 12:52 PM
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AA5BY
 
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While I've never noted normal amounts of CG unbalance to cause what is described, I stick with my claim that the grossly forward balance Spacewalker acted as described. The problem cleared when properly balanced. I can even be more specific, the plane required four ounces of tail weight added to the 1/2 oz that had been given it during the first and evidently faulty balance.

The only other time I've experienced roll instability is fairly recently when refurbishing a club trainer. It wouldn't continue to roll over like the 'Spacewalker, but it was not steady. The cause was the trainer was set up with very hot controls and the old servos in it were simply not centering adequately for the setup that because it used L bend and keepers, had aileron linkage fully out on the servo arms to prevent binding of the keepers. Moving the linkages to the inner servo arm hole went a long way to steadying the roll.

If on the OP's plane, the aileron links are full out on the servo arm and the throw then reduced to tame the sensitivity, that would leave the centering variances to have more influence. Better would be to keep throw to around 100% and reduce sensitivity by moving linkage inward on the servo arm.
Old 04-29-2015, 01:00 PM
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cublover
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Originally Posted by speedracerntrixie
CG has nothing to do with this issue. Either the ailerons are not centering, the wing is warped or reverse expo has been adjusted in.
No?..Funny,,,, I had the same problem...oh well....tons of guys know more than I....
Old 04-30-2015, 02:02 AM
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speedracerntrixie
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There is no way that CG can cause a roll instability unless its tail heavy and the snappiness that results is a more a function of pitch instability then anything else. Suggesting the default of " set it up nose heavy " really is not correct. The CG should be placed just forward of the nuetral position. On an Extra platform that would be right at 30%. This is an Extra 300S we are talkings about, the low wing version. Much like the one in my profe only smaller. Been flying Extras in IMAC for 15 years. There is no reason why they cant be set up to fly as.easy
easily as a Super Sportster.
Old 04-30-2015, 04:22 AM
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sensei
 
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In my own experience with many of my own and others aerobatic aircraft including Extra's, improper C/G has only caused pitch instability issues within those airframes and in turn main lined to nasty snap rolls when pulling on the elevator stick just a tad too much. Crjame expressed that his airplane flies like it is sitting on top of a ball; that it falls off to one wing or the other in roll; he did not express any issues in pitch. That said I would be inclined to look at servo centering, a warped wing or tail surface, incidence issues from one wing to the other and I would also check the horizontal stab/elevators incidence for any issues. These types of issues cause pilots to trim for level flight; the problem with trimming for level flight is these issues are all speed sensitive, so no matter what you do for trim you will always have a constantly changing problem. If one wing is heavier than the other then do what it takes to make them weigh the same because again, you will trim to carry the heavy wing at a given speed and then when you fly faster or slower and now you are back on top of that ball. Just saying...

Bob

Last edited by sensei; 04-30-2015 at 04:30 AM.
Old 04-30-2015, 05:42 AM
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The case that I make is that few of us would ever note that a very forward CG could cause roll instability for a variety of reasons. First, few us would intentionally balance a plane that degree of nose heavy. Mine happened because of a procedural error when balancing. Added to that, is that additional dynamics might be needed such as a low wing plane with a top heavy mass. All any of us can say, is that we experienced it or haven't... and just because someone hasn't experienced it, doesn't mean that others haven't.
Old 04-30-2015, 05:58 AM
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speedracerntrixie
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Sorry but your explaination just does not hold water. First I have flown airplanes that were very nose heavy. No roll stability issues. Aerodynamically there is just no possible connection unless he is fighting tip stall the entire time. Next false assumption is weight towards the top of the fuse on a low wing airplane. My Extra has close to 2 lbs of rx battery at the top front of the fuse and then carries a half gallon of fuel above the wing. I have also flown helicopters inverted where the entire aircraft weight is above rotor disk and it handled no different then being right side up.
Old 04-30-2015, 06:36 AM
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AA5BY
 
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With all due respect, I've noted for a long time that you have a great deal of knowledge and experience and it is not my desire to diminish that in any way... I have enjoyed reading your thoughts for a long time and generally regard your thoughts as reasoned and well made.

But... this time you came across rigid that something could not be true because you have never experienced it... that of course is not an adequate argument that it could not happen. I've observed it... I noted it to my flying buddies when it happened because it was so unusual. I know the specific change that was made that relieved it. It is something that I experienced only once in forty years. It is likely that few every experience it, I did so by accident only because of an improper balance procedure.

I don't recall that any definitive explanation was provided explaining the dynamics involved of why it happened nor do I wish to argue the point between those who have and haven't experienced it based solely on observation... the world is full of people who see things differently and bully others to their perspectives... I don't want to do that nor do I wish it done to me. Peace.
Old 04-30-2015, 07:06 AM
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Environmental conditions could be playing a part here.
Flying over or near trees and other objects when it is
windy will subject the plane to eddy currents that
knock it off level. This plane will stay knocked off
until corrected.

Flying in calm over a paved parking lot will give
a similar effect when the plane hits the edge of the
thermal over the lot.

Since the plane falls off on one side then the other
it seems there is more than just the wind. The wind
would cause the first tilt, but after that it would not
immediately fall off the other side. I would first suspect
poor centering. Binding in the linkage would cause
poor centering to have a greater effect.

Bad geometry (using the wrong holes in the servo
arms) would compound the issue. If the linkage is
in the outermost hole of the servo arm, or the
innermost hole of the control horn, the
smallest increment of servo motion will have too
great an effect near the center of the servo throw.

Turning down the endpoints, and using expo instead
of using the correct holes only makes it worse because
the resolution of the signal and the servo resolution doesn't
change.

Finally slop in the linkage will allow the surfaces to
wiggle rather than stop in a specific position. This
is most noticeable in the center when there is less
pressure on the surface holding the linkage tight.
Again, this is magnified by bad geometry.

If correcting these issues doesn't fix the problem,
there could be an issue with the servo pot, or the
transmitter control pot. Both of these pots wear
more at the center that at the extremes. Servo
pots have the additional wear from vibration
(both engine/motor and wind turbulence through the
linkage), so it is the one to check first.

Finally the least likely, but most dramatic
cause of this behavior would be a loose
servo or servo tray. It wouldn't take much
movement of either of these to cause the
effect you are seeing, so it could be easy
to overlook.

Jenny
Old 04-30-2015, 06:06 PM
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combatpigg
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https://youtu.be/K7iYy-WE5Kg WE DUN NEED NO STEENKiNG LATERAL STABEELITEES..!
18 inch span, no steenkeeng diheedrals, no steenkeeng expos...!

If the OP's plane in question is too heavy, it will seem too sensitive if it isn't flying fast enough.
The stiffness of the linkages ought to be checked with moderate hand pressure while the radio system is turned on. Check the linkage while idling and when the surfaces are deflected.
The most slop free aileron torque rods I've ever rigged were greased up good with axle grease, then directly embedded in a mixture of balsa dust and epoxy. Of course, the "trenches" that the torque rods lay in need to be large enough to accomodate the puddles of "epoxy dough". Make sure the torque rod "axles" are greased thoroughly..!
The "Moment of Truth" comes after the "epoxy dough" torque rod bearings cure and you check to see that the torque rods move freely.

Last edited by combatpigg; 04-30-2015 at 10:37 PM.
Old 05-01-2015, 06:05 AM
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Originally Posted by crjame
Hey there,

I originally posted this in the ARF forum but have not been getting much feedback, so I thought I would try it here. I am looking for some input into what I feel is an issue. The aircraft (GP Extra 300SP) is trimmed and cg is in the recommended range. When I fly the aircraft, it does not really want to stay wings level. Its almost like its balancing on a ball always wanting to tip to one side. I make an input back to wings level and shortly thereafter, it falls off the other way. If I was building it I would say it needs a bit of dihedral. Obviously I can not put any in.

Does anyone have any ideas or recommendations? Reflex both ailerons?

Thanks
I have the same airplane with the exact same problem, it was always rolling off one way or the other and never felt locked in. The fix was to change out the aileron servos which were Savox metal gear digital minis. They had a lot of gear train slop. I replaced these with Hitec HS-225's and the problem went away. The airplane flies much better now and stays where I point it.

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