| “I had several days with Martin Holman inspecting an airplane, and we discussed the holes added to the IV fuselage for A/C. I suggested that adding reinforcement should take care of this problem, and he disagreed. He said nearly the same thing Brent said, Holes---"will have an effect on the torsional stiffness of the fuselage. This, in turn, effects the natural frequency of the empennage. Making the fuselage less stiff by decoupling the window (holes) reduces the empennage flutter margin."” These two comments appear to contradict each other. Adding reinforcement would increase stiffness thereby raising the natural frequency. That would be a good thing unless it
happened to drive the frequency up to a point that matched some other component, in which case the interaction could result in a lower flutter speed. In the case of the A/C holes you might want to add reinforcement to bring the natural frequency back up to that of the unmodified plane. I was not arguing for additional reinforcement, just making the cover attachments very secure to provide a continuous load path through the fuselage.
Sent: Friday, April 25, 2008 10:28 AM Subject: [LML] Window Strength
The hole that has always concerned me is the inspection hole at the smallest diameter of the IV fuselage, just ahead of the horizontal stab. Next time your near a IV hold the tip of the horizontal stab between your thumb and index finger and apply 3 pounds of force alternating in an up and down direction. Increase the frequency until you hit the resonant frequency. Continue applying the 3 pounds of force at that frequency and watch the amplitude increase. Try it with and without the cover attached.
If I had a IV I would attach that cover with many screws and keep them tight.
This does not work on the small tail 360, the resonant frequency is too high.
Regards, Bill Hannahan
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Regards, Bill Hannahan
Be a better friend, newshound, and
know-it-all with Yahoo! Mobile. Try it now. |