what is aileron flutter?

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The last sound you hear before the THUMP.

Cheers,

Colin

Flutter is exactly that, a "flutter" or oscilation of the aileron during flight that is usually noticed due to the "rippling' sound it makes. It can also happen to the elevator and rudder. There is some debate as to its true cause, and I believe there can be more than one cause, but it can definitely bring your airplane down. The usual suspect is too large a gap in the hinge area between the aileron and the wing, but the construction of the aileron can also be part of the problem. I've only had aileron flutter occur on one plane so far. It was a Stick .40, and it only occurred comming out of high speed dives. I believe it was due to ailerons on this plane having a very large covering surface stretched over a minimal frame structure. This allowed too much flex in the covering and allowed a "billowing" effect in the covering that caused the "flutter".

A sound that can freeze every person at a flying field in their tracks!!!! Trust me, when it's heard everybody will stop immediately and turn towards the sound!!! Why?? Because there's about to be a spectacular crash!!!!!

Ken

Google up "aeroelastic flutter".  The cause is the center of lift ahead or the center of gravity on a control surface.  The solution is to lighten the trailing edge ("back to the drawing board") or add weight at or ahead of the leading edge.  You'll see full-size planes with weights on the end of struts or with small surfaces out on the ailerons called "spades" that have a flutter-reducing effect. 

Many full size aircraft have an internal weight attached to a control surface that goes into the wing or fixed empennange to reduce flutter without being visible (or adding drag) outside of the plane.  That does limit the throw available

I have heard guys say it is the gap that causes it, but to me I question this because I have seen planes with a huge gap in them and never a flutter at all. I believe it may be a combination of things.

We heard a Sig Cavalier flutter like crazy one time and we all looked because we knew it was going to hit the ground with a thud. The pilot slowed down and then landed and we looked it over and could not find one thing wrong with it. Everything on it was tight and well secure with very little aileron gap. We never heard the flutter return again.

The biggest cause of flutter is the ability of the surface to move in the airstream. While a large gap can contribute to flutter it is rarely the cause. As you noted, many plane have large gap and never suffer flutter. The gap is more likely to affect the lift and effectiveness of the surface. For the flutter and type of movement can cause the flutter. Any slop in your control surfaces can lead to it. Control linkages that are too small for the application can lead to it. Weak or damaged servos can lead to it. Worn our servo arms and control arms can lead to it. Wallowed out holes in control arms can lead to it. There are several others that can lead to it as well, but these are the biggest causes that will usually be culprits in flutter. Check your control surface to see if there is any movement with the radio turned on. Any free movement in the control surface is going to put the plane at risk for flutter.

All of the above it good to prevent it, but sometimes a plane can be in perfect shape and suffer from flutter (although it's sometimes rare for this to happen). Flutter can actually start as a harmonic resonance in the airframe that moves to flutter. This can and does happen usually in a matter of seconds and the plane will literally come apart in the air. Sometimes with control surfaces flutter you can cut the throttle and get down immediately before the airframe is destroyed. If you do get your plane back on the ground never ever go back up again until you determine what was causing the flutter. Going back up again without fixing the problem is just inviting flutter to destroy your plane.

Ken</p>

Exactly what happened to this Stick .40, no gap, and nothing else wrong with the aileron, other than large open areas in the frame work. I agree that there is probably not one simple cause. Gap may cause it, but I've never experienced it. In any given case it could be a combination of several things like the aileron construction, balance of lift, and the type of airfoils involved. This Stick .40 had a fully symetrical air foil in the wing, but the ailerons did not have any taper in the chord, just a flat constuction with beveled le and te on the aileron itself. I did not get a chance to experiment with the ailerons as a buddy borrowed and crashed it. But I would have liked to try adding counter weight to the ailerons, and/or building new ailerons with some sheeting over the large open areas.

I saw some surface counterweights weights on here that were made from regular pushrod wire and the round fishing weights with a hole through them. The fellow said that changing the CG of the surface a little would stop most flutter.

Here are some made from 140 grain .270 bullets silver soldered to a 4-40 rod in a drilled center-line hole I did as an experiment. Made no difference I could detect with or without . . . but the ailerons are still attached so it peobably doesn't flutter either way.

.

The are called mass balance and will work every time on any of our airplanes and the speed ranges we operate in, that demonstrate the onset of flutter. Its also easy for any of our surfaces that have aerodynamic balance forward of the hingeline to simply imbed a ball of lead in it.

John

Ken in post number 8 above, gave the best description. Everything will flutter if given the proper stimulus, even an iron bar. The only thing that will not flutter is something with infinite stiffness and zero mass, an impossibility. As Ken said, gaps do not cause flutter. The most common cause in models is sloppy hinges, spongy control rods (to small and they flex under load) or insufficient stiffness in the structure itself.

I didnt know flutter had so many causes. Always thought that

If you had a thick airfoil and flew it too fast, the air flow would seperate over the tapering part of the foil (part after the main spar when the cross-section starts reducing). This would induce turbulence causing sudden high and low pressure pockets to form on the same side of the control surface. This would cause the surface to move exagerating any play in the linkages.

I also thought that aileron hinge gaps would allow air to "leak" from the higher pressure undersurface to the lower pressure upper surface and this crossing of flows would induce the flutter.

Is my understanding of the issue wrong?

Ameyam

Ameyam flutter analysis is an aeronautical engineering displine in itself, careers are devoted to aeronautical flutter analysis.

Therefore the facts of what can happen and what works in its prevention gets lost in the discussion of theory and argument.

In the context of our model airplanes and the range of speeds at which we operate them, yes flutter can happen to any portion of the airplane with the most common being in order: ailerons-elevator then rudder.

I agree gap sealing has little effect in inducing flutter and trying to cure flutter with ever more expensive servos is most often disappointing.

Poor linkage geometry as well as sloppy linkage and strutural elasticity of both the airfame and the control surface are all contributors however you can still flutter after 'fixing' all these things.

But its easily fixable

Its called mass balance. First dicovered in the twentys When during the Pulitzer races and internationally during the Schneider Cups races and simple mass balance stopped the carnage of lost control surfaces and flailing wings.

It will do the same for us if your airplane ever demonstrates the onset in flight. It only takes moments to attach one to any of the three axis's for most smaller airplanes like Charlie P and I posted.

For years frequenting the old SWRA and later PCPro warbird races sometimes up to 13 races a year all over the soutwest where arf's with roughly twice the normal displacements were the norm, flutter incidents were constantly happening both in practice and heats. I took to makeing up pocket fulls of the simple widgit I posted in my first post and gave them away to folks who survived the first incident so they could make their next heat. It never failed and I seldom went home with any.

Its also rather simple to imbed mass balance on any airplane that has aerodynamic balance surface forward of the hingeline. Works just as well except that requires a tiny bit of surgery and adding a pittance of weight during assembly/construction and folks will fight that to the death untill they do suffer a loss to flutter.

John

besides sound is flutter visible from the ground?

That just looks kewl, flutter or not!

I beleve flutter is not always detectible by either sound or visually. This brings to mind the classic cases where a single hinge or more hinges fail continually frequently on ailerons and no once see's or hears it. Thats why the hinge type or hingeing workmanship is blamed when in fact its flutter.

How about this classic the aileron servo out there in its box just keeps mysteriously losing its mounting screws. Worse found just flopping around in the wing but no one heard or saw a thing. Throw a mass balance on it and just as mysteriously the screws stay in place no problem

John

There was a problem in my execution. The Giant Super Sportster apparanty has sheeted ailerons that do not have internal ribs. Ipoked about 30 holes with a needle searching for the internal framing and never found any. Finding none Iwent to the outside edge (I researched several models that put the weights there). They did alright until I lost a wheel collar and threw the wheel - resulting in a ground loop. Tore the starboard weight and strut off. Did not hurt the hinges or aileron, amazingly.By luck it just spun around on the axis instead of pulling straight back.Still, that might not be the ideal location for a low-wing model.

Best solution Ihave seen for flutter is to SLOWDOWN. Most guys who seem to have trouble are playing the throttle like an on/off switch. The other best proactive remedy, as John said, is to eliminate slop in the linkage. The DuBro syle (aka: relatively inexpensive")ball-and-socket connectors have simplified this immensly.

charlie i love it!

mine are usually 9mm.

Ed H. had a flutter problem on one of his planes on the Elevator. this E had spoons so he drilled a hole into the LE of the spoon and screwed in a bolt. problem solved.

Joe

Missed you at the picnic.  Weather held out and it was a good day.

I hate to disagree with Mr. Buckner as he is most often right on when it comes to a solution to a problem; but, mass balancing will not necessarily cure any flutter. Perfectly balanced systems can and do oscillate or flutter if given the impulse or input to generate the frequency at which they are resonant and mass balancing a surface that will flex or maintain oscillation does nothing to cure that. I'm a firm believer in mass balance, especially on elevators and ailerons but not for the reason that it prevents flutter.

Rodney perhaps in the strictest sense you are correct with a couple of caviets . The first is absolutely, we are not curing the flutter but the application of a percentage of mass balance is moving the onset to a higher speed that we will not ever be capable of reaching.

The second caviet is "perfectly balanced" it is not the goal Indeed 100% may even be more likely to induce.

In the early ninetys I did my first expeiments with flutter stired on by the fun fly airplane rage which being the predesesors to the modern 3d were interesting but not as good in many respects. With all the hugh surfaces, fat airfoils extreme lightness adding to flexability and worse fiber glass tube fuse's or aeroshafts. All the worst case senarios and mostly powered by 25, 32's. The were flying rubber bands. You remember and sure you also messed with them also.

Anyway most of these could never use full throttle unless on the vertical line, any other time you were certain to loose an aileron or elevator. It was very easy at the time with the square surfaces to just use wire extension at the outboard ends with a ball of lead. At first I disconnected the pushrods and did set a perfect 100% balance, took more lead than I thought. It was kinda cool that old elevator or aileron would just set there bout neutral and thump it with your finger and it would just wave at you for a while and settle back bout neutral. OK now came the test flights guess what no real detectable change except I lost another elevator.

Back to the drawing board and the next ship (I built bunchs of these things) I balanced only perhaps around 10%, That did the trick all my later fun flys had no problem with loosing surfaces to flutter never happened agine to any of mine.

I don,t know what to say but every ship I have added mass to since were airplanes that survived their first encounter and it never happened agine after adding the mass. That includes a small percentage of my normal airplanes a few other folks here and there and an awfull lot airplanes at the races.

John

I had aileron flutter with a Tiger 3 (40 sized low wing). I could hear it flutter quite clearly but was able to bring the plane down without incident. Others in our club have had the same problem with this particular model and solve it by replacing the single servo (with longish linkages) for the ailerons with a separate servo for each aileron.

An old timer at the field who heard my aileron flutter applied a long strip of cellophane tape along the forward edge of the aileron and the flutter didn't come back. Unfortunately I crashed the plane only a couple of flights later due to a completely different reason.

I have another Tiger 3 now and have fitted dual servos and it has no aileron flutter although I've only flown it a few times.

So, aileron flutter is the moving of the aileron? Not the wings flapping? I have not seen a plane flutter but if I do witness one and inspect the plane, would there be significant play between the servo and aileron (somewhere in the linkage)?