Aeronautical Engineering/Control cable failure and critical flutter speed margin
QUESTION: I'm a LSA and PPL piIot, working on my CPL, and I have some questions to those who have knowledge of the aeroelastic effects and flutter phenomenon. I would like to talk a little about aerodynamic flutter onset speed and flight control malfunction. It is known that freeplay, worn-out control rods or slop in flight control cables might induce flutter.
What I'm interested in is how critical flutter speed is affected by those problem. I'm wondering especially about a cable control failure where the surface would be disconnected and freefloating. There are small light sport aircrafts and even some FAR 23 standard certified aircrafts which don't have mass-balanced surfaces, especially ailerons which I guess would be more prone to flutter. How critical flutter speed lowers in a situation like that (and how prone to violently flutter are these ailerons in an emergency disconnected sitation)? Is there a linear drop in flutter speed? Can it reach even lower speeds in the normal operating envelope e.g. lower than Vno or Va? Normally, assuming no malfunction, flutter speed is at least 10% above Vne or Vdf, which is dive test speed used during flight testing. How do you think things might change?
The same about the others control surfaces e.g. a broken trim tab linkage.
I really appreciate your help. Thank you very much!
ANSWER: Steve - It is hard to generalize about flutter because it is so design dependent. A proper aileron, for example, will be designed to have balanced hinge moments so it will float parallel to the wing chord line if it is free. It can still flutter, but the aerodynamic forcing function is smaller than if the aileron floats to some odd angle. Hinge moments can be controlled by many things such as hinge line setback. If no care was taken in the design to minimize float angle, then yes flutter might occur at a low airspeed if something breaks. Of course if the control cable is broken you have other serious problems besides flutter. So, I am not sure where you are going with this. If an aircraft has a catastrophic failure of a system, a catastrophe can happen. A broken trim tab may not be real serious unless it forces the pilot to exert a force beyond his capability to maintain flight.
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QUESTION: "Of course if the control cable is broken you have other serious problems besides flutter."
Not really, for example, if it's aileron control failure, besides flutter, you don't have bigger problems. You can turn with rudder, there is no problem to get the airplane back on the ground. Did it several times as a training maneuver. That's why I'm more concerned about flutter. If it's gonna flutter at lower speeds, let's say below cruise speeds, that cable seems more like a structural failure trigger.
ANSWER: Yes, you're right. And if the control cable was a real part of the control stiffness, which it likely is, then flutter of a free aileron could occur at a lower airspeed than normal. But to predict that would take either a wind tunnel test or some detailed information on the dynamic response of the free aileron. The designer may have that, but I doubt anyone else would have the necessary information.
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QUESTION: In doing some more reading on this, I found wing torsional stiffness is a major player on aileron flutter. If you start with a stiff wing and light ailerons at low speeds, the probobility of flutter is reduced. If you design has some torsional flex in the wing, or the ailerons have a lot of mass and they are not balanced the probobility of flutter increases. As the speed increases, the probability of flutter increases. The control system - the cables merely work to dampen the tendancy to flutter. So a broken cable will not necessarily cause an otherwise stable system to flutter, but a broken cable on a system that was already prone to flutter will result in less damping action, which could lead to flutter at lower speeds or conditions that may not normally experiance flutter. Do you agree?
Yes, I agree. But note that all heavy aircraft are designed with flexible wings so they don't break when encountering a gust. In that case, the aileron design is critical and would probably require more than just control cable force to keep it from fluttering. In a perfect design, the aileron would try to float to zero deflection angle as wing angle changes. But you are correct, the control cable is a stabilizing factor, and if it breaks the chance of flutter goes up.