Ideas for Mass Balancing Control Surfaces to Prevent Flutter? - RC Groups
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Apr 04, 2005, 01:05 PM

Ideas for Mass Balancing Control Surfaces to Prevent Flutter?

This is actually for a molded composite sailplane used for Dynamic Soaring. You turbine guys are the only ones flying at similar speeds to us. What are you guys doing to prevent flutter? I'm assuming that you guys who are much more in the scale aspect of things and also have a lot more money "hanging in the wind" have actually done this. Surprisingly none of the DS guys are doing this. Instead we are cutting down control surface size and running real stiff linkages and sometimes taping the ends of the control surface and flaps to limit movement. This last week it was blowing 60mph on the front of the slope. I was fluttering bad in the 180-200mph range (on radar) right as we cross into the sheer in which case our airspeed instantly accelerates to 180-200 +60mph or 240-260mph. I also have a small 54" wingspan plane with tiny cut down ailerons and even they were fluttering so bad in the 170's I eventually broke the metal horns on the aileron.

The problem is we buy our planes already molded, painted, and live hinged and the wings trailing edge spar is always very close to the live hinge where the control surfaces attaches. This means for a clean mass balance we'd have to cut out or into the trailing edge spar to make room for the counter weight thus compramising the integrity of the wing. I have researched some mass balance techniques for home built aircraft and I have seen they sometimes run the counterweight hanging under the wing to where they almost touch the wing when the control is in full down deflection. Of course this is the least aerodynamically effecient way to do this. But I'm thinking i could use some servo covers or something and it would be worth the drag. Also does anyone acutally make some nice aerodynamic mass balance counterweights so I don't have to fiddle with cutting/melting lead into an aerodynamic shape?

Thanks for any input guys.
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Apr 04, 2005, 03:14 PM
An Aussie in Canada
Marty C.'s Avatar

Mass balancing

Thats a damn good question... Not to many people really bother with mass balancing, most tend to just use higher torque digital servos I think. The fastest jet I own is an AD F-16 although I have not clocked it I reckon speeds are 220-240 mph easy in a level pass!!! maybe more if pushed and diving pass attempted (no bypass fitted either).
As to avoiding flutter solutions, if room were to permit you could also use ganged servos.... I fly my F-16 on tailerons only and use two ganged high torque digital servos per stab and she is rock solid.... I Hope this my also be another possible solution for you...

Its always hard to capture the higher speed models on video to give an idea.... but here is a link to a friends website-
with a short clip of a 1/2 throttle flyby with my F-16 and this was with its TJT-3000 (30 lb thrust) reduced max thrust output of only 22 lb.
I hope to fit a TJT-4000 (40 lb thrust) turbine later this summer for some extra vertical excitment
Last edited by Marty C.; Apr 04, 2005 at 03:23 PM.
Apr 04, 2005, 04:10 PM
Yea marty I suppose its possible to fit two servos per control surface but you would have to cut another hole in the bottom of the wing skin for them. Do you run two control horns, one for each servo? I think another problem is the rigidity of the control surface its self. The point furthest away from the horn is likely to flutter. Got any pictures of your ganged servo setup? Thanks for the info!
Apr 07, 2005, 03:46 PM
Registered User
bt33's Avatar

Mass balance

I mass balanced my Avonds F15 by drilling a hole in front of the pivot point on the tailerons and pouring lead shot in until it balanced and sealed it with epoxy
Just one way of doing it.

Apr 11, 2005, 01:45 PM
How did you allow the leadshot to move? I assume if you poured in epoxy it would have stuck its self to the stab. Also how did you know when it was balanced? Got any pics?
Apr 13, 2005, 05:14 PM
Registered User
bt33's Avatar

Mass balance

Sorry no pics its all sprayed up now
but i used a very small and light spirit level on the pivot point
poured shot in and sealed it with masking tape to check balance of the tailplane mounted in the pivot tube,after balance achieved removed some lead shot and weighed it then replaced with equivelant weight of epoxy
Last edited by bt33; Apr 13, 2005 at 05:22 PM.
Apr 23, 2005, 01:02 AM
Registered User


Here is how I do it on the fast stuff.
Apr 23, 2005, 07:04 PM
Wow very nice. But it looks like your compramising the wing by cutting into the TE spar. With a control surface that big it sure looks like it needs it. Do you melt your own lead slug and shape it to what you want?
Apr 23, 2005, 08:00 PM
Registered User
Originally Posted by Z06kal
But it looks like your compramising the wing by cutting into the TE spar.
Yes, but the advantage gained with the balance weight has proven to be worth it..

Originally Posted by Z06kal
Do you melt your own lead slug and shape it to what you want?
Apr 24, 2005, 12:32 PM
So where you having flutter, done this and had it all go away?
Apr 26, 2005, 01:04 AM
Registered User
Originally Posted by Z06kal
So where you having flutter, done this and had it all go away?

Yes I blew the rudder off of 2 jets, once I did a mod like this, I could go as fast as I want.

And these are big 20+ pound jets, going faster than I am allowed to say.
May 09, 2005, 12:34 PM
Registered User

F3D pylon flutter

Hi all,
I would just like to give you a little tip from the F3D pylon racers I know.
F3D racers reaches speeds of 350km/h!
Apparently flutter is more easily induced if the leading edge of the control surface is thinner than the adjacent trailing edge of the wing. In order to get rid of an excisting flutter tendency they increase the thickness of the control surface (typical the ailerons) by adding a thick tape on each side.
The wing of a modern F3B racer looks very much like a modern glider.

May 09, 2005, 02:02 PM
Any of the sailplanes we use for DS don't have a thinner control surface then TE but thats a good tip. I mass balanced the Ailerons on one of my planes which was fluttering in the 150's. Did some 150-160mph passes this weekend and all seems good . This stuff really does seem to work. I'll have wait to ring it out closer to 200 to know for sure. To give you guys an idea how bad my plane was fluttering you can hear it here in this video. that last 178mph pass snapped the metal control horns flush off both ailerons which is where you hear it go into full loose ailerons. By a miracle i managed to land it undammaged with no ailerons. Keep in mind the wind was blowing 60mph so the actual airspeed at times was around 240mph.
May 25, 2005, 10:09 PM
Registered User
FYI.. flutter is a neglected subject with full size aircraft as well.. it does not matter what the speed is.. if the control is not staticlly balance along the leading edge chord, it CAN be induced to flutter ( ever wonder why Piper change the elevators and rudder on the J-5 Cruiser and up... ? almost identical to the J-3 but 10 kts faster... thats 90 mph!!!) I whould think that you could use thin lead tape across the leading edge between the hinges... you can load it positive or neutral balance depending on your models control surface area.. ( smaller can be neutrally balanced)... hope this helps

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