Control cable failure and critical flutter speed margin?

[stevejeff]

Hi everyone here!

I'm a LSA and PPL pilot, 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!

 

[eljose79]

Hello,

Thank you for your questions regarding aerodynamic flutter onset speed and flight control malfunction. As a scientist who specializes in aerodynamics, I am happy to share my knowledge on this topic.

Firstly, let me explain what aerodynamic flutter is. It is a phenomenon that occurs when the interaction between aerodynamic forces and structural flexibility causes oscillations in an aircraft's control surfaces. These oscillations can become unstable and lead to catastrophic failure if not addressed.

In regards to your question about how critical flutter speed is affected by control rod freeplay, worn-out control rods, or slop in flight control cables, the answer is that it can significantly decrease the critical flutter speed. This is because these issues can introduce additional flexibility and looseness in the control surfaces, making them more susceptible to flutter. The severity of the decrease in critical flutter speed will depend on the extent of the problem and the specific aircraft design. It is important to regularly inspect and maintain these components to ensure they are in good condition.

In the event of a cable control failure where the surface is disconnected and freefloating, the ailerons would be more prone to flutter due to their lack of mass-balance. In this situation, the critical flutter speed can decrease even further, potentially reaching speeds lower than Vno or Va. It is difficult to predict the exact decrease in critical flutter speed as it will depend on various factors such as the specific aircraft design and the extent of the cable control failure. However, it is important to note that even a small decrease in critical flutter speed can have a significant impact on the safety of the aircraft.

Regarding your question about the effect of a broken trim tab linkage on flutter speed, this would also depend on the specific aircraft design and the extent of the damage. However, in general, any damage or malfunction in the control surfaces can increase the risk of flutter and decrease the critical flutter speed.

In conclusion, any issues with control rods, cables, or control surface components can significantly decrease the critical flutter speed and increase the risk of flutter. It is important to regularly inspect and maintain these components to ensure the safety of the aircraft. I hope this answers your questions and please let me know if you need any further clarification. Best of luck in your studies and safe flying!