I've sealed the hinge line, got rid of any slop, . . . I rebuilt the wing, and instead of full length, I kept them short 5" from the tip, and kept that solid. Kind hard to explain, here's a picture:


The plane is proped for 140 plus mph. Would wing tip plates help? If I angled the tips back (kinda swept) help? So that the LE is longer than the TE. What will stop the high speed flutter, without causing extra drag? Does AR have anything to do with this? I've had suggestions of chopping the wing at the end of the aileron, and tapering the ailerons back from nothing at the tip, to full width at 5" back.

RS:

That's not what tip plates do.

You might try balancing the ailerons, or getting closer to it. If you can stand the weight get some lead sticks, like that used in helicopter rotor blades, and bury it in the ailerons as close to the hinge line as you can.

And, are you sure you're getting aileron flutter? It seems to me hard to tell at 140 mph.

Surface flutter in your plane,
. often makes your crash count gain.

Bill.

I can hear it, and it makes the plane drop the rt wing. It starts at almost wot, maybe 125 or 130 mph, or in a dive. I thought the plates would reduce tip vortices, as I figured the turbulent air over the ailerons is causinig the flutter. How about sweeping the tips?

RS:

Tip plates do limit tip vortices, but they also increase the tendency to snap. Further, with the ends of the ailerons so far in I don't think the tip vortices are having any effect. Same comment for tip sweep, or Hoerner tips.

You say you have all the slop out of the linkage. Including the gear lash in the servos? You might replace all the pivots with ball joints, another thing that sometimes helps is increasing the mass of the links - either use heavier material, or wrap a layer of solder, stick it with CA, and cover it with shrink tube. If there is any slop at all in the horn pivot this wont do any good. Ball joint pivots are best.

My first choice would still be the weights in the leading edges of the airerons. But the solder wrap is the easiest/fastest.

Aileron separated?
. Plane's not man-rated.

Bill.

Where are your ailerons driven from? If they are strip types driven from the root then that's a long airleron to keep stiff enough to prevent flutter.

Ailerons made of balsa? Here agian lots of flex. Actually I'd say, without seeing it... , that this is the most likely cause. Thin airfoils flying fast are always going to be flutter prone.

Possible cures. Lead like Will is suggesting may help but it's still behind the hinge line. Another fix can often be to stiffen the surface up enough that the new flutter point is beyond the speed teh model can fly at. Off the top of my head I'd say the easiest fix would be new ailerons made from spruce. Or strip the balsa ones and cover them with light glass cloth WITH THE WEAVE ON A 45 DEGREE BIAS. Note the capitols. You are adding a stressed geodetic skin here so the weave must be at 45 for it to work. Cut strips on the bias like this and just overlap by 1/4 where they run short if needed. Use polyester finishing resin or epoxy for adhesive. Or if you're feeling rushed and rich use wax paper and thin CA. Press the surface up against a flat surface over wax paper and wick a little under the bottom surface. Work your way out to the tip and then flip and start over. The cloth should wrap around the leading edge from lower to upper in one piece. We won't bother making you wrap it at the trailing edge.

The spruce surfaces are sounding better aren't they....

There's one last possible fix that doesn't require you cut into the model. Horn balances. Little lead blobs that are soldered onto a wire that is then attached to the surface on top or bottom and extends forward. I had the same problem on a little 1/2 A racer type. Buzzed like crazy in dives. I went the easy way and stuck on the balance horns It was ugly but it did the trick.

The idea here is to get the CG of the surface close to or ahead of the hinge line. That is what Will's weights are trying to do. But the horn is more effective. Put them out at about the 60% mark along the ailerons. To avoid harmonics you don't want them at exactly the halfway point, 1/4 point or the tips. If you want to be really techy put them at the 58 or 63% point. That should solve any harmonic problems. And don't skimp on the weight. My little 1/2A used a 1/4 oz. Your model should probably use at least 1/2.

Bruce:

I was pushing the enclosed weights instead of the horn balancers because of drag considerations. But I definitely like your hardwood replacement.

And you mentioned harmonics. If he has the horn centered on the aileron?

And RS, there is a thread titled "Extreme speed prop planes" that might give you some additional ideas.

Aileron loss in flight -
. Wrong way to make it light.

Bill.

Is the aileron full span with the outboard edge of the aileron 5 inches from the tip or are the ailerons 5 inches long with the inboard edge the cut shown and go to the tip? It makes a difference in the answer.

If the ailerons are torque rod driven the stiffness of the torque rod is extremely important. It might be hard to get the wire big enough to not flex.

The other suggestions are great, stiffness of the system and mass balance are the two of the main things to reduce flutter. I would tend to blame lack of system stiffness first.

Exactly! Ever seen that vid of the Tacoma Narrows Bridge from the 40's? They built the one that fell totally symetrical. On the next one they pulled the suspension cables out of symmetry somehting like 3%. That was enough to solve the problem

It's not a total assurance of non flutter but any mass balance or control horn mounts should never be at any of the points on the surface that may support any even harmonics of lower levels. This means the ends, half and quarter length points should be avoided. Just pick one of those points and move the horn or balance by another 5 % one way or the other.

Here is a test for stiffness. Turn the radio on. Grab the trailing edge of the aileron and try to deflect it. Do it several times grabbing it at different spots along the trailing edge. Apply at least two and preferably three of four pounds of force to deflect the aileron and see what gives. Some servos are low torque around neutral. If the servo gives then replace them with digital servos of sufficient power. If the linkage gives, replace it with a stiffer linkage. If the aileron bends, then stiffen it top and bottom, with a skin of 1/64 ply under the covering. If the hinges give, then replace then with heavier pinned hinges.

Make a counter ballance weignt or two or three. Insert the wire of the counter balance in the leading edge of the aileron so that the weight extends into the inside of the wing ahead of the aileron. Make the weights heavy enough that the aileron does not droop with the linkage disconnected.

I like this idea even better. It's cleaner but it means making holes in the wing. But it's the best method I think as long as you don't mind the work.... and the surface throw doesn't jam the internal weight against the outer skins. Spitfire like fairing bubble?

The balance being heavy enough that the surface doesn't droop is something I forgot to mention. Thanks Ollie.

Actually you shouldn't mass-balance the ailerons completely. You want the aileron to be slightly heavier as the counterweight...not much, just a little.

Dealt with a lot of this over the years so I'll toss my .02 in as well. One thing that I didn't see mentioned was attention to the geometry of the linkage when using control horns. Always try to keep the distance between the servo arm center and linkage attach point as short as possible and the aileron horn as long as possible. You'll have to use maximum throw available on the servo to make this work. This arrangement gives the servo the mechanical advantage rather than the aileron. However, the best way to insure success is the mass balance method which almost always works.

One very simple solution would be to reduce the area of the ailerons (cut them shorter and fill the gap with the piece of aileron you cut off) and increase the throw of the ailerons.
Pay attention to any slop when you do this as the increased slop due to larger throw could actually make things worse. You could then use dual/exponential rates to compensate for the reduced area when landing or flying at slower speeds.

-Q.

Howdy folks,
while struggling one night to get all the stuff inside a much too narrow pylon racer fuselage my overheated brain :bananahea came up with the idea to use spring tension halfway out on the aileron to "crank it up" to about neutral instead of mass balancing at the inner(or outer) end. Would that work at all...??
As you all know mass balancing is not that easy due to space limitations, drag, hinge stiffness and torsional bending when attaching the counterweight only at one end.
Please feel free to laugh at the idea (I´m a chemist anyway) but I could not resist the temptation to ask........Cheers/Harald

PS: Rudeboy, why should the aileron NOT be fully balanced, only partly..?
Bill R., long time no hear - I´m still around..DS

Harald:

That spring balance is such a good idea that it's known as "SAS," or Stability Augmentation System. If you check many light aircraft, they have this sort of spring centering the elevator.

It's not for flutter prevention, though, it keeps gravity from pulling the elevator off center, leading to less effort needed to maintain level flight. In some cases the spring tension can be altered in flight to change elevator sensitivity. I don't know of any full scale installations on rudder or aileron surfaces, give it a try and let us know.

There are many toy r/c models, Wal-Mart $20 sort that do not have proportional control, using magnetic actuators for deflection and a spring to center the surface for level flight. Or straight driving - cheap cars use it too.

I thought you had gone off-line, Harald, no notes, no posts, recently I see you have come back to life. All the best to you.

If you tune your spring.
. Will the ailerons ring?

Bill.

I just wanted to comment on something said earlier in the thread. Tip plates will not increase the tendency to snap.

I have been using tip plates since 1977 and in every case they have reduced the stall speed, lowered the landing speed, made the plane harder to snap and spin.

On several different occasions during the years, friends of mine have tried planes with removeable tips to compare characteristics with and without tip plates. In every case they have verified my findings: Lower stall and landing speeds, harder to enter spins and snaps.

--------- PS: Rudeboy, why should the aileron NOT be fully balanced, only partly..?


If the mass distruibution is such that the center of mass of the aileron is in front of the hinge line it is basically an unstable system. Mass on the hinge line is neutral and mass aft of the hinge line is a stable system.

So you want it a little aft of the hinge line to be stable and ease the mass inertia loads on the servo. Then you are left with the airloads.

!!!!!!! Note, the above is not true unless it is standing on its tail!! I goofed with this statement. It is the total moment of inertia of the surface that effects the frequency that the surface will flutter at. !!!!!!!!!!!!!!

Airload balancing is done the same way. There is a bias because the loads are at the 25% point of the surface (kinda) and make a forward airload tab more effective. You want enough to unload the servo but still want the resultant of the airloads aft of the hinge line to have a stable system.

If you use a set of springs to neutralize the aileron the problem is that it will help make it buzz at some frequencies. If you use a spring of some sort you also need a damper of some sort to stop the spring natural frequency thing from being more of a problem than a solution.

There are all sorts of good equations that will let you determine what happens where but I darn near flunked dynamics and have conviently forgotten what they are.

OK, folks.

Here's one to ponder.

My (kit built) Electrostreak had its first crash when it lost an aileron a few weeks ago.

It had tape hinges, after some five years of service one hinge let go.

In the crash it bent the motor armature, so in repairing it I installed a CEM 0506 cobalt motor, and used monocote to replace the hinges. Standard skin method, fully sealed upper and lower.

It turned out a LOT faster than it was with the 550 can motor, flat out level flight - flutter.

I took the easy way out - put a 550 can back in it. More than sufficient power.

Point is, if there is a flutter that is highly resistant to a cure, just slow down.

That wont work for rsieminski, of course, since his object is to go faster. But for the rest of us...

Ailerons always roar?
. Cut the throttle a bit more.

Bill.

Full-scales are usually designed so the -inevitable- flutter is beyond the speed capability of the aircraft.
They seldom survive flutter. It's over and done with so quickly the part falls off before the pilot can respond.
.
"Always try to keep the distance between the servo arm center and linkage attach point as short as possible and the aileron horn as long as possible. You'll have to use maximum throw available on the
servo to make this work. This arrangement gives the servo the mechanical advantage rather than the aileron."
.
I don't do it this way. I use the outermost hole on the servo and the surface. This reduces the potentail for slop in the holes to contribute to flutter. The further out on the servo arm, the smaller the influence of slop.
.001" at 1" is less proportionately than .001" at .5"

I use long servo arms and control horns too. Servo's these days are virtually free of play in the gears and on the output shaft. So what's left? Play due to worn ball-links and/or kwiklinks. And this play has less effect when you use long arms and horns...

I like the slow down idea. My Cherokee ran into some flutter I couldn't get away from. Simple solution went from a 10x7 to a 10.5x6. This slowed it down enough that I didn't have to worry about it unless doing loops or diving maneuvers. Saved a lot of hassle and I was able to do it at the field.

Point to ponder...
I went back and cured the flutter. It was the rudder - the pull-pull cables were just the least bit loose. I had used aramid cord and it had stretched, I suppose. The rudder servo is a Hitec HS-85BB, no slop in it.

Retried the cobalt motor - plane went like the hammers, no flutter, but the can went back in. Want it for an electric twin I'm hacking.

Can't cure the flutter?
. Headed for the gutter.

Bill.

Ben,

In what sense is it unstable to have the CG ahead of the hinge line? I don't think I buy this. There is a stability issue with the aerodynamic balancing, but not the mass balancing. I think this notion of 'mass balancing' is a bit misunderstood here. When you add or shift mass in an effort to prevent flutter, or other undesireable vibrations, what you are doing is shifting the frequency at which the structure will have the vibration problem, and possibly reducing the magnitude. Incidentally, you also run the risk of increasing the destructive forces that occur if it still flutters. For the most part, adding the mass on the aileron or on a horn at about 90 deg. from the aileron will have the same effect, which is to increase the moment of inertia.\

banktoturn

banktoturn - only in the morasses of my mind. Doh! You are right it is inertias effecting the flutter of the surface, not the location of the mass as I said. The aero horn is a good place to add mass as it will help balance the static loads on the control system.

I was thinking along the lines of an all moving tail with the pivot at 25%. Assume the airplane were pulling positive gs with the mass of the horizontal ahead of the pivot. The aero loads go through the pivot but the mass hanging out in front tends to give a leading edge down moment on the pivot. If there is any slop in the system it will allow the elevator to command more positive gs than you want. The airplane might oscillate about a trim condition as a result but this is definitely not a flutter thing.

Thanks for pointing the flutter thing out. As I mentioned I barely passed dynamics so I wouldn't trust my answers myself!!

Ben,

I think that EVERYONE only barely passes dynamics! But I find that its fun to look back through the fog once in a while, to see if any of it comes back.

banktoturn