I can understand the counter balance theory. I understand like the crankshaft and equal and opposite forces. But what is the counter weight on a full scale CAP lets say doing? Is it counter balancing the aileron to Neutral??? I am a little confused at what forces are being equaled. Thanks

It eases stick force.

Boy, that explains a lot...

Tall Paul is right as far as he goes. I assume you are referring to the little triangular plate mounted on sort of a pylon ahead of and below the aileron. The little triangular surface provides a torque opposite to the resistance of the aileron in order to reduce the control force required to move the aileron out of the faired position. The weight moves the cg of the control surface to a location at or ahead of the hinge line to avoid a tendency to flutter. Many production aircraft have the counterbalance weight concealed within the leading edge of the aileron where it isn’t obvious. But all have some way to keep the control surface balanced near the hinge line to avoid flutter.

There's two types. Mass balances that work like Lou suggested and aerodynamic balances that help ease the stick forces by providing a certain amount of area ahead of the hinge line. The ears on the old Fokker WW1 aircraft come to mind. And then there's this classic example of the Piper Cub with it's aerodynamicly balanced rudder...



As Paul suggested the forward area eases the loads on the controls to the pilot.

I can easily understand the aero balanced rudder. That I can see. And have understood for a while, but this weight thing....I guess I would have to have a mock up dummy with with a weight on it. It still doesnt seem like a 8 lb plate could keep a 70 lb aileron from flutter or ease of stick force. It must, or they wouldnt run them. I am not arguing, just confused...LOL

I doubt seriously if the aileron on the CAP 232 weighs 70 lbs. However it isn’t just the weight but also the moment arm that must be taken into consideration. The aileron is fairly narrow so that the cg is relatively close to the hinge line. The balance weight is extended pretty far forward on a sort of pylon beneath the surface. (There is also the possibility that there is additional weight in the aileron leading edge, which would not be obvious from the pictures.) The little triangular surface on the forward edge of the pylon is what gives the aerodynamic force to lighten the control force. I suspect that the size and shape of the surface was experimentally determined.

The size shape and location of any aerodynamic balance affects the slope and shape of the stick force gradient. For aerobatic aircraft the stick force gradient is tailored so that the pilot can fly the required maneuvers without being tired out by high forces, but the gradient must still be enough so that he has a good feel for the airplane.

I think what is confusing is that there are two independent factors. First the surface must be mass balanced such that the natural frequency of the surface is not in the range of forcing frequency at the speeds at which the aircraft is designed to operate. This doesn’t necessarily mean on the hinge line. Many small aircraft that operate at lower speeds have surfaces without additional mass balance and the cg is well aft of the hinge line. Flutter only occurs when the natural (resonant) frequency of the surface and the forcing frequency at a particular speed coincide.

On the other hand, aerodynamic balance is for the purpose of changing the stick force gradient, usually to lighten control forces. This frequently involves putting some area ahead of the hinge line that will protrude into the air stream and add a force to assist moving the surface.

In the case of the CAP series aircraft the designer has chosen to use the same hardware to accomplish both purposes.

Mass balance also helps greatly (on the larger models particularly) by minimizing the forces on the servo gear trains during a hard landing or crash. Without mass balance, some pretty large forces can be transmitted back thru the pushrod to the servo gears, with mass balance, the forces are concentrated in the aileron hinges. This also applies to the elevator as well. Note: if using airdynamic balance, I have found that if you get more than 10% of the area ahead of the hinge line, you may have trouble near neutral, the surface tends to hunt or gallop.

Rodney's servo stripping forces are what the pilot's arms have to work against in full-scales.

The Fokker Eindecker's (and those similar) comma-shaped rudders and aerodynamically balanced elevators were free to float to whereever the airload would take them, making the planes physically exhausting to fly. These surfaces had to held in position, as there was no way to trim them.
Adding the fixed vertical and horizontal surfaces relieved a large part of the pilot's load, the surfaces streamlining behind the fixed surface restricted their motions with gusts and off-center air loads.

This is the key to the whole mass balancing issue. All surfaces will flutter under the right conditions. Adding mass balancing is just a way to move the overall mass distribution and balance to a point were this flutter resonace is outside the aircraft's normal operating range.

Everyone is saying that mass balancing is intended to move the balance point to the hingeline or in front of it. That would be a perfect case but I seriously doubt that many achieve that.

Years ago I used horn type mass balancing on a little 1/2A speed sport model. In my case the strip ailerons were made from light wood that was just TOO flexible and this made the surfaces flutter like a clarinet reed in a dive. I added a couple of little wire arms and wound some solder onto the arms flared in with epoxy and that fixed the flutter probem. It wasn't enough to totally balance the ailerons but it did move the balance forward enough and change the resonant frequency enough that I could dive vertically from great height without any flutter from that point on.

I agree pretty much with BMatthews. There are a lot of considerations in balancing a control surface. Some are to ease the stick pressure, some are for flutter prevention, some are for both. A claasic example of the counterweighted surface is the P-38 elevator, which was huge, and had a resultant high stick pressure due to the airflow over it. The counterweight in the middle doesn't really "counterbalance" as much as it just shifts the balance point further forward, relieving some of the effort required to move it at higher airspeeds. The other purpose was to shift the center of pressure to help reduce the incidence of flutter. However, it did nothing for the problems of compressibility in high speed dives which placed a shock wave at the hinge point and acted to "freeze" the elevator, This problem on the -38 was only corrected after a dive flap was added to the wing to limit dive speed below the speed where compressibility became a problem.

The triangular plates on an aerobat are called spades and are a variation on the balanced control surface like BMatthews points out on a Cub. When an aileron with spades is deflected, the spade has a force placed on it by the airstream in opposition to the the force on the aileron, lessening the stick force required to move the control surface, which translates to less fatigue to the pilot and quicker , more neutral control response. Spades ideally counteract input pressures to the point that the aileron will stay where it's put; this reduces the "feel" to zero, which some pilots like and some don't, so most of the time spade size varies to the pilot's preference. Most general aviation craft don't require counterbalancing, at least on all their controls, because they either have some power assist or their performance doesn't warrant it (it may be that the airspeed where flutter may occur is beyond the Vne of the aircraft)

I think some of you are still missing the point. Mass balance is solely for the purpose of moving the resonant frequency of the control surface out of the speed range to avoid flutter. It has no significant effect on control forces. Aerodynamic balance is for the purpose of shaping the stick force gradient and has little effect on the resonant frequency.

Such an airplane can’t be certified per FAR 23, which says in part “The stick force must vary with speed so that any substantial speed change results in a stick force clearly perceptible to the pilot”. Although experimental aircraft don’t have to comply with FAR 23, an airplane with such a neutral balanced surface would be quite dangerous to fly.

Hi Lou,

Can you explain the different "modes" that provide energy coupling between the fixed surface and the movable surface?

And is it true that a movable surface could never flutter on an "infinitely stiff" mount? In other words for the aileron to flutter, the wing has to be moving and providing an energy coupling back and forth to the aileron?

Sorry in advance if I butchered the terminolgy, I am almost certain I did!

And Bruce and whomever else, feel free to jump in.

Thanks in advance,

Maybe i misunderstood what I read in an article about the Edge 540, at least i think that was the plane. Got so many dang books around here i can't find the particular article., but it was speaking to the totally neutral controls this plane possessed; it also spoke to the inherent danger of such lack of "feel", but also to the advantages in aerobatic capability to a competitive pilot qualified to fly such a plane. Maybe I also overstated how spades should reduce control feel. Personally I would want some resistance to the stick, otherwise there would always be that thought in my mind whether the controls were hooked up to anything or functioning properly. I've never flown an aerobat, my piloting experience is limited, to 150-172's and a T-34 (I guess it will do aerobatics fairly well, I meant like Extras, Edges, etc.)

Like Lou says mass and aero are totally separate. That big lump of lead on the P38 elevator is striclty a mass balance to help avoid flutter. It did nothing to aid the pilot with moving the control.

I don't think this is true. Certainly a flexible wing can aid the formation of flutter but it's not a requirement in itself. Flutter of control surfaces occurs due to an OSCILLATING energy acting on the surface at a frequency that is the same as the resonant frequency of the control system. If the energy input is not cyclic, as in providing an oscillating force, then the system will not be forced into resonance and the surface will not flutter. Think of a violin string or the blade of grass between your thumbs. Part of the resonance equation is fles in the system through either structural flex (my soft aileron wood in that 1/2A model) or slop in the system so that things can flop around. If the control and activation system was TRULY rigid then flutter could never occur. But all practical construction and systems can flex and have slop inherent in them. These are enough to cause flutter. Consider that strip ailerons are much more prone to flutter than conventional designs are. This is because the surface is so long and slender with the control input at the inside end so the surface is free to twist in the wind and can often flutter. The energy to cause that comes from the airflow which would indicate that there's a "noise" generator at work on the wings that has a cyclic effect on the air flowing over the wing. This "noise" changes frequency with the speed and if the frequency reaches the resonace frequency of the control surface system it's flutter time. Adding weights can raise that frequency by moving the balance point closer to the leading edge of the surface. If it moves the resonance point to where the model cannot fly fast enough to generate the noise frequency to match then there's no flutter any more.

Howwzat?

Extensive testing on the Lockheed AH-56A Cheyenne after a fatal crash showed the pilot's arm was just the right mass to excite an oscillation in the collective control system, leading to the rotor cutting thru the cockpit, and slicing the tail off the airplane.
A man of a different physical construction would not have encountered this.
A Cheyenne mounted in the full-scale tunnel at NASA Ames also cut itself in half during an unmanned test following the accident.
The airplane although extremely advanced in other areas, some of which are just being equalled today was deemed unsafe to continue with and the program (and all the other rigid-rotor programs at Lockheed) abandoned.