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Aerospace and Aeronautical Engineering: What are the aeroelastic implications of critical flutter speed margin in the event of full or partial control cable failure in aircrafts?

• I'm  a private pilot, 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  problems. 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. below usual cruise speed? Normally, assuming  no  malfunction, flutter speed is at least 10% above Vne (never exceed  speed) or Vdf (max dive test speed used during certification flight  test). How do you think these 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:

  In a basic form, it all comes down to stiffness. Aerodynamic loads are largely a function of the square of the aircraft's speed. The resisting force is essentially a function of the stiffness of the structure and control system system components. That stiffness tends to vary linearly with deflection. The limiting speed occurs when the resisting force due to the stiffness is no longer greater than the aerodynamic force. Thus the velocity is a function of the square root of the stiffness. You are however interested in the dynamic aeroelastic behaviors, in partiular, flutter and the limiting speed at which the phenomena occurs. The following lecture by Dr. Garth Pearce addresses Aeroelasticity, both Static and Dynamic effects. Flutter is addressed from time index 1:10:25 onward. In the case of Flutter, we are concerned with a change in lift due to the changine in position of the control surface with respect to the wing. This change in lift constitutes the restoring force. It has both an exponential nature and a sinusoidal nature and is a function of the square of the velocity. Play in the control system, such as backlash, cable stretch, etc., can lead to the control surface being free to move out of position and thus generate that change in lift we mentioned above. My comments are presented in a very simplistic manner but basically what I'm trying to convey is that the driving force behind flutter is a change in lift due to a control surface's oscillatory positioning ( if left to move freely between two positions ) and that for a given range of movement, there will be a speed for which the square of such, will impart a force on the structure to which that control surface is mounted, such that the force causes the structural strength of the structure to be exceeded.