atsioukl

My Feedback: (1)

Hi,

While flying my airplane this morning, I encountered a horrid flutter. I landed the airplane without damage but as it turns out, the aileron servo (Futaba S3004) ended up with a couple of stripped gears. I was very lucky to have gotten my airplane back down in 1 piece.

My question to the experts is: what type of servo should I replcae this with to prevent it from reoccurring? Does it need to have metal gears or does it just need to be high-torque?

Any help is appreciated.

Thanks,

Anthony

DMcQuinn

My Feedback: (13)

First, check your mechanical setup. Is the linkage slop-free between the surface (aileron) and the servo? Is the control rod connected to the servo arm at a hole near the center? An don the other end (the aileron) is the control rod on a hole near the end of the control arm? In other words, you want a short servo arm and a long control arm.

If all of this is OK, then try a stronger servo. Tell us what the plane is, so we can offer advice on servo size. If it is a .40 size glow plane, standard servos generally should suffice. If it is a 50cc gasoline engine, larger servos are in order. Servo requirements are a function of plane size, surface size, surface throws, and air-speed.

JohnBuckner

My Feedback: (1)

There are many techniques to minimize flutter some fairly effective and others not as. There is one that will work every time on our aircraft at the speeds we fly at. Its called Mass balance and only takes only minutes to do. I,ve done many sport planes and pylon warbird racers that showed systems of flutter with complete success every time including some contemporary 3D types that fluttered notoriously. It will work even with a sloppy servo.

John

Dr1Driver

A better servo will not necessarily prevent flutter. Make sure you have a mechanical advantage in the servo arm/control horn linkage. The servo arm should be equal to, or slightly shorter, than the control horn. Make sure your hinges are not sloppy, and the hinge line gap is sealed. If the wood in the aileron seems soft, you may need to replace it. As a last resort, use counterweights as described in an earlier post.

Dr.1

dirtybird

My Feedback: (5)

Its amazing to me that modelers like to ignor this easy fix. I simply would not fly a large model without mass balance. Its standard practice in full scale.

atsioukl

My Feedback: (1)

Hi gents,


Thanks for the replies. My airplane is a .40 size (glow fuel) 'Rick Stick'. These airplanes were built in my area by a local hoobyiest. Anyway, the airplane is a high-wing stick type airplane. It has a straight, symmetrical wing. The control rods connect to the ailerons in the same fashion as trainers do. In other words, the servo is in the center of the wing and uses 2 control rods which actuate two other control rods that are glued to each aileron.

I checked the linkages and found the following:

-The control rod has no slop where it connects to the aileron control rod.
-I am using a long/large servo arm
-The control rods connect to the outer-most holes in the servo arm

Lastly, how do you perform a mass balance? I've been flying for 15 years and have never heard of this. Any help is appreciated.

Thanks,

Anthony

JohnW

My Feedback: (6)

To mass balance you place mass in front of the hinge line. The idea is to move the static balance point of the control surface forward. This is common practice on full scale designs.

However, it is basically never required on models, and if done improperly, it will actually increase the likelihood of flutter. The main reason why the full scale procedure doesn't apply well to models is due to the difference in control surface rigidity. If anything, you'd be better off using a counter balance tab as opposed to mass balance. I know some mass balance a lot, which is fine for them, but in general I feel it isn't necessary to mass balance the vast majority of model control surfaces.

If you encounter flutter, you can be 99% sure the problem is with one or all of the following: Bad or loose hinges, linkage slop, servos slop and poor mechanical setup. Fix the issue, don't cover it up with a mass balance… or at least that is my humble opinion.

From your description, using the large servo arm outer most hole, that could easily be part of the problem. Without more info, I can't be sure, but you may have a poor mechanical setup. How long are the control horns? Where are your ATVs or endpoints set? Were you in a dive when this happened, or at WOT?

Cheers.

JohnBuckner

My Feedback: (1)

Strip ailerons like you have if made of balsa of insufficient density are quite subseptible to flutter if flown fast with a resonably powerful engine. A classic example of this is the original Bridi kitted 'Killer Kaos', many of these suffered mild to severe flutter problems due to the use of selected aileron material that was too light and suffered very poor torsional stiffness. This very light wood selection is what made the airplane so highly prized otherwise but did present this problem.

Agine the solution is increditably simple at the nominal weight penilty of around a half ounce total. Your airplane is a perfect candidate for mass balance.

Just bend up two 4/40 threaded rods in the shape of the photo I provided and allow the leg with the lead to project about one inch forward of the hinge line. Crimp on a quarter ounce bullet shaped fishing weight (the type with a hole through it) and also drip on a little thin CA.
This is then inserted in a small hole from the bottom side one third the way out on the aileron and a little behind the hinge line so the weight projects forward and the bend is adjusted so the aileron has full travel.

This will in no way overbalance your aileron, not even close and over balancing is the only way mass balance can incite flutter. There is no need to balance the ailerons weight 100% or is it desirable. As little as 10% will do the trick. What this does is raise the speed at which incipient flutter is excited. Now at the risk invoking much wrath I am going to point out that all unbalanced control surfaces will flutter at some speed without fail. Fortunately in most cases our aircraft never reach those speeds but if you mass balance this speed will be raised much higher. And I am not any kind of engineer but am a survivor of a lifetime of making a living flying fullscale as well as aircraft modeling in many disciplines for the same period. I have experienced in flight flutter in a full scale airplane and do not care to repeat the experiance. I certainly don,t need to repeat it in RC aircraft when the fix for any airplane that demonstrates a tendency is so simple.

Just my opinion

John

JohnW

My Feedback: (6)

One item I forgot is hinge gap in my prior post. A leaking hinge line can induce flutter. Seal your hinge gap with covering, tape, etc.

No wrath from me, I totally agree that every surface will experience flutter at some speed unless it is mass balanced. I however feel the issue is minor with models, unless they are large and/or flown at extreme speeds. Most models have enough rigidity in the control system at the Re numbers we fly at to suppress any flutter. There are plenty of things a modeler can do to reduce flutter, and mass balance is probably the most effective as it is the only one that really totally solves the issue. However, it comes at a weight penalty. While this penalty may be small in some cases, it is still dead weight. On something like a 40 sized stick, a little weight isn’t a big deal, but on a 2M pattern ship us crazy pattern folk, like myself, will go to great length and expense to remove a ¼ ounce on a 11lbs ship. This is why I prefer checking linkages, slop, hinges, sealing hinge line etc. as these “fixes” basically weight nothing.

As for an improperly placed mass balance inducing flutter, I am repeating what a major giant scale model aerobatic airplane designer told me in a 2 hour conversation about aerodynamic design on 35%-50% models. They apparently had done testing with mass balancing and found it actually worsened flutter on thier designs. I had this conversation about 4 years ago and I can’t remember all the details. But from what I remember, the theory they came up with was that when the surface started its oscillation, because of the low torsion stiffness of the surface, the mass inertia would get out of phase with the rest of the surface. This in turn would fail the surface by tearing it in half, etc. I believe the problem wasn’t the mass balance per se, but the lack of stiffness on the control surface coupled with the additional inertia load of the mass balance. The solution they selected, instead of stiffing up the control surface which would have added weight, was to use higher torque servo which added basically no weight over the existing servo. I know this isn’t a “real” solution, but it did effectively push the flutter velocity up such that is wasn’t a problem in the planes flight envelope.

Cheers.