3
$\begingroup$

Is it not a good idea to balance all control surfaces, no matter what they weigh?

I am building a small plane, with small control surfaces. There are many of the same type of plane flying. Most have no type of counterweight "balance" on the elevator, or rudder. I am told that is because the surfaces are small and light.

$\endgroup$
  • 2
    $\begingroup$ Welcome to aviation.SE! $\endgroup$ – Pondlife Nov 9 '19 at 21:35
  • $\begingroup$ It depends totally on the size and speed of the aircraft what forces it'll encounter in the first place. IMO It's obviously not always a good idea to increase the load on (especially) control surfaces with additional mechanisms. The is the reason why a 747 has split rudders while a 737 doesn't. $\endgroup$ – PapaMike99 Nov 10 '19 at 9:37
4
$\begingroup$

Flutter is an energy storage and release phenomenon. Surface moves, applies a load to the fixed surface it's attached to, fixed surface bends under the load, stores some of the energy like a spring, then releases it in moving back to its original position, and some of this released energy is transferred to the moving surface attached to it, which is also stored and released back into the fixed surface in a way that adds more total energy than the first time, magnifying the original movement, repeat, and away you go.

For the this to happen the moving surface has to be unbalanced with a fair amount of mass away from the hinge line, the fixed surface has to be flexible (spring like), the whole system has to have a natural frequency that is close to the frequency of the natural oscillation frequency of the moving surface, and there has to be enough energy present in the first place to get the whole thing going.

Generally airplanes that go less than say 80-100 mph don't have the energy to induce flutter if there is reasonable stiffness in fixed surface like the horizontal stab. Light airplanes with fabric covered wire braced tails have stabilizers that are quite stiff due to the wire bracing, and the combination of high natural frequency resulting from the rigid system and the low input energy at the speeds they fly at means they don't really need a balanced surface (look at a Champ's unbalanced elevator; the C of G of the surface is probably at 30% of chord - no big deal because it simply can't excite sympathetic movement in the stabilizer it's attached to at the energy level present at its operating speed range).

If the fixed surface is not so stiff, the moving surface is really heavy, or you want go faster, then you start having to consider mass balancing. To really know for sure what the threshold is, you'd have to have someone that knows what they are doing do the analysis. But generally low speed light airplanes have the inherent rigidity to be immune from flutter in their speed range without balancing control surfaces.

You still have to confirm it in the real world though. When you test fly your creation, you do some rudimentary flutter testing by diving to a speed a bit above your Vne (actually creeping up to it in a series of steps), pull up so you are decelerating, and as you pass the speed of your test point you slap the control stick sharply, trying to induce an oscillation. By doing it while decelerating, the risks of an oscillation starting and building into full fledged flutter are low. But, if you see an oscillation that starts but then goes away, well you've "tickled the tiger's tail" and you'll need to balance the surface or reduce the airplane's speed range. You're wearing a parachute while doing this of course (or whomever you're able to talk into doing your test flying).

$\endgroup$
  • $\begingroup$ It is better to increase speed in steps, excite the motion and test for the time of the oscillation to die down. Graph that time over speed and you can extrapolate at what speed flutter will occur. Diving to an untested speed for flutter testing is poor advice. $\endgroup$ – Peter Kämpf Nov 9 '19 at 21:39
  • $\begingroup$ Yes you are right. I've edited my post thanks Peter. In any case, I think it's safe to assume someone will research the proper procedure and not proceed based on a posting by an anonymous nobody on the internet ;) $\endgroup$ – John K Nov 9 '19 at 21:58
3
$\begingroup$

Not necessarily if the aircraft flies slowly so forces stay small. But it is better to have balanced control surfaces.

Now imagine that you fly through a vertical gust and the aircraft pitches up, then down. Since it rotates around its center of gravity, the tail will make the largest excursions from a straight line. Now imagine the elevator has no mass balance. When the aircraft pitches up, the tail goes down and inertia will cause the elevator to lag behind, so you get a trailing-edge up deflection of the elevator. This increases the pitch motion!

On the way back, when the aircraft pitches down, the tail goes up and the same happens in reverse: Now the inertially induced elevator deflection will push the tail further up.

If you now add a mass balance, the elevator stays at a constant deflection angle while the tail bounces up and down, causing smaller excursions and improving pitch damping. This makes for a smoother ride and lower loads.

However, if the aircraft flies slowly and you have some friction in the control linkage, the elevator will move less and stick forces will be low so the pilot can add the remaining force to keep the elevator from moving. Adding a mass balance would still be beneficial, but not crucial.

By using some tricks, you can even balance your elevator without adding extra mass. Given a T-tail and pushrods, give the pushrod going from the base of the vertical up to the elevator just the right gear ratio so its mass will balance that of the elevator. If the elevator itself is stiff enough in torsion, that is already enough for a proper mass balance.

Mass balancing becomes essential when you move up to speeds where flutter becomes likely. Small, slow planes are normally too slow to experience flutter.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.