I have enjoyed my 4star 60 so much over the last 2 seasons but I found a loud flutter at higher speeds and now need to retire it..... I want to build another and want to soak up everyones experience on how to build to avoid the same flutter again. Yes, my seals were all gapped and I had strengthened the stabbilizer area. I did put flying wires on the aft area....How do I put a 91FX on a 4star and get more than 1 season out of it..... please, don't tell me I have over powered it..... I've tried several engines and this is by far the biggest bang for the buck. All constructive comments gladly appreciated.

1) Add balance weights to the elev &/or ailerons.

2) Make the moveable control surfaces thicker than the fixed surfaces.

3) Ensure that the control surfaces & adjacent fixed surfaces areas are as smooth as possible.

4) Ensure that the trailing edges are either well taperd out to a fine edge, or cut off square -- don't have rounded trailing edges.

5) Seal gaps at hinge-lines

6) Don't over-size control surfaces

Adding my 2cents to the list:

7) Ensure that the push rods are stiff and that the control linkages are not loosely fitted.

make the movable and fixed portions of all surfaces as stiff and strong as possible.

Balancing is about the only thing that will work for sure, but it should be a distributed balance weight if the movable surface it flexible. If you use a big single weight at the wrong mounting location you might just make the situation worse.

Just what is fluttering?
I'm presuming it's the elevator, but ailerons are more prone to flutter.
If elevator, what is driving the elevator? Where is the servo, what's the pushrod, etc...
A long unbraced push rod can create flutter as vibration whips it around.
The -entire- system from the servo to the surface has to be stiff, not just part of it.

The Sig Four-Stars are somewhat prone to flutter, largely, I think, because their wings are not very torsionally stiff. Keeping control surfaces as lightweight as possible helps - note how many WW2 fighters used fabric covered control surfaces.

I have noticed that kit models that have rather dense balsa in their control surfaces fall victim to flutter far more often than those with 4-6 pound per cubic foot stock. The best fix by far is mass balancing the control surface, but putting excess balance weight at the tip of a control surface can make things worse, as Mr. Matt says. If you use a single balance weight at the tip, it should counterbalance only about a third of the unbalanced moment of the control surface; otherwise you run the risk of exciting higher harmonics that can rear their ugly head and tear your model to pieces in a split second, even after many hours of flutter-free flying. A single balance weight, located at 60-65%% of the length of the control surface from its inner end is better, although a bit inconvenient, and it should counterbalance about 80% of the unbalanced moment.

Every thing will flutter given the proper stimulus. We get by by having the natural resonance outside the range of the normal stimulus, i.e. keep the natural frequency very high. We can do this by making very stiff structures, keeping any slop out of the control system (no enlarged holes in control horns or servo arms, supporting any long runs of a push rod to prevent flexing, large cross sectional area of push rods, firm servo mounts etc.). While static and airodynamic balance have their advantages they do little or nothing to prevent flutter and improperly implemented can cause flutter. The ideal surface would weigh nothing and be unbendable and would never flutter. Since these are impossible to obtain, we just try to make our structures natural resonance fall out of the range of the normal stimulus of engine vibration and airodynamic turbulance.

I am not sure I understand the control surface being thicker than the stabilizer.... also, if I don't want to hang weight forward of the leading edge of the elevator, is it better to use balance tabs on the ends? I will keep the servos/pushrods very close to the elevator so there shouldn't be any slop there.... any thoughts on using spruce in place of balsa for the horiz stab or elevator?

Thanks for all the advice..... should help out alot.

I built a Stinger 120 a while ago and overpowered it with a Brison 3.2. Instead of moving the firewall back, I added lead balance weights to the tail. Plus I also sheeted the surfaces. Just a couple ounces back there offset the big motor. They also seemed quite flutter proof. I once had the rudder cables completely slacken up, and I only noticed because the rudder control seemed soft. With no direct control connection, the rudder floated in a straight trail position, even at high throttle.

THe balancing wouldn't have worked well if the tail structures had been flexible. The kit came stock with 3/8 square balsa stick tail surfaces covered by monokote (or whatever). I simply sheeted the stabs with 1/32 ply and the elevator and rudder with 1/16 balsa. The stabs were unexpectedly stiff with the thin ply glued on.

The thicker sheeting on the elevator and rudder created surfaces thicker than the stabs. This helps promote keeping the airflow attached to the surfaces around neutral and eliminates the deadband around neutral. The deadband is a symptom of being flutter prone because the airflow is not stable over the surface.

Almost all full-scale aircraft use mass balanced control surfaces. If mass balancers do not work, someone should tell these people that they have been wrong all these years, and save them a lot of money and effort. Mass balancers are not a cure-all, but they are, by far, the most effective single fix that I have found, and a single counterbalancer at the tip of the control surface will cure at least 90% of flutter problems.

Even with a well balanced aileron, or no aileron at all, wings are quite capable of getting into serious flutter, so unbalanced ailerons or elevators are not the only culprit, although they are frequently the prime suspect. Keeping the structure as stiff as possible in relation to its mass helps a lot, as well as keeping its center of mass as far forward as possible - ideally at the aerodynamic center, although this far forward is usually not very practical, except on helicopter rotor blades.

Navy18, I had this discussion with one of my professors in collage way back when calculus and slide rules ruled! I was flying models even then, mostly control line, but we on occasion experienced flutter and my dynamics instructor at the time put forth this theory which I’ve proven to be correct time and time again. Notice when looking at control surfaces from real airplanes that they all have the pivot point berried within the surface, while we almost with out thinking always hinge to the LE of the surface.

His assumption was that if Force = mass times acceleration (F=MA) and it is the force generated by the control surface that the servo and airframe have to resist until the weakest link breaks causing the failure of the system. What if the control surface had no mass? The force would be = to acceleration times zero or 0. So how is this done? Well going back to the real planes with the pivot point berried within the control surface that is exactly what they are trying to do. The mass of the control surface is its weight times the distance from the centroid of that weight to the pivot point. So if we put the pivot point at the centroid of the weight for the control surface the mass becomes Zero making the Force Zero. Eliminating the flutter!

Putting the pivot on the centroid of the weight works in theory. But in practice it’s a bit more complicated but we can move the centroid as far forward as possible when building the control surface. If that does not stop the flutter mass counter balances can be used to move the move the weight centroid closer to the pivot point until the flutter stops. But stop and think about how your control surface is build and how you can move the weight forward on it and the hinge point back. Some times its just a matter of selecting the wood correctly, you want the punky wood in the back and the harder wood in the front.



Good luck

Joe Felice

Locating the control surface hinge at a considerable distance behind its leading edge is a great way to reduce its aerodynamic hinge moment so that it requires much less servo effort, at the cost of considerable structural complexity. Locating the hinge axis at the center of mass of the control surface is also ideal for resisting flutter, but the resulting gap between the control surface and the rest of the structure is difficult to seal, so that a larger, and heavier, control surface is required, which is more likely to excite flutter even if it is perfectly balanced. Hanging any more than the minimum weight of control surface required to do the job on the after portion of a wing, stab, or fin is definitely not the way to go.

Thanks guys..... I will play with both central control and mass balancing and see which is more practical.... ease over perfection often wins.

Navy18, when using mass balancing look at the WWII warbirds Me109 is a good example. Adding weight (usually in the form of lead) to the section of the elevator that is forward of the hinge line you move the center of mass for the surface forward. Also look for photos of lasers and extras with mass balancing built right into the wingtip.

Good luck

Joe Felice