Locating Airframe Resonances
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Home > Articles & Tips Index > Flying > Locating Airframe Resonances

[Courtesy of Simon Van Leeuwen lomcovak "at" telusplanet.net, July 2000 - excerpted from a discussion on the IMAC email list]  

Resonances that seem to be propeller-based may actually originate somewhere else.  Don't make the mistake of thinking it may be the propeller (only). Vibration from, say, an improperly balanced prop can set up vibration elsewhere on the airframe. 

At very specific rpm's these vibrations will double up and create quite a racket. Actually, sometimes there is more than one at which it will occur. If you look carefully, at some point higher in the rpm range, one can observe another vibration, usually at half the amplitude and twice the frequency. This occurs more often in the air, where everything has the opportunity to develop their own sympathetic vibrations. 

I believe every aircraft exhibit this, just some worse than others. A really interesting way to locate troublesome vibrations, is to use a strobe light that you can adjust the cycle rate. With your aircraft running at night, with the strobe on, it is utterly amazing to see different parts of your aircraft waving at you at different frequencies! You name it, it is moving to the beat of many different drummers! 

Second or third order harmonics which combine in one part of the aircraft can cause a wing or other surface to resonate like a drum. This is not to say it is actually the wing causing the vibration and making the noise! It could be transmitted from the gear to another part of the aircraft for example, and cause the wing to transmit the vibration. The wing can act as a very efficient speaker and given it's relative position and shape, transmits sound easily to your ear. 

This brings up my next point in deciphering whether your airframe, or the prop is the suspect. If possible to set the throttle while airborne to the point it creates the offending noise, roll the aircraft to see if the sound attenuates as the wing is parallel to the ground (therefore, not transmitting in your direction). 

If it does not change, the prop may be suspect, and this should be visible on the ground. 

Another idea might be to set the elevated aircraft on soft foam under the fuse at the CG with some one holding the tail. 

Look for the offending rpm (it's range is usually no more than 30-50rpm. The holder will definitely know where it occurs ;^) 

torquerolls


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