keizo/weblog

Here’s a video (Chinook Ground Resonance Test) that shows a chopper getting torn apart from a slow and steady spin. This occurs because the natural frequency of the blades and the frequency of the internal vibrations (engine, rpm of blades) coincide causing the stresses to amplify by huge amounts. There is nothing wrong with this helicopter and something like this could probably happen to any helicopter. It turns out that the natural frequency of helicopter blades turns out to be rather low since they are so long and flexible. Typically when designing something you want to avoid these frequencies altogether, but with the blades this is pretty much not possible. What happens instead is that when the engine starts up, it gets going fast enough so the internal (forced) vibrations pass through the lower mode natural frequencies as fast as possible. In the video it appears that the speed is kept at one of the natural frequencies for a long enough time period that the stresses grow exponentially until the thing just explodes. I guess the thing to learn from the video is not to let your chopper idle at the wrong speed. At least that’s my current understanding, correct me if you know otherwise.

This entry was posted on Friday, April 21st, 2006 at 7:06 pm and is filed under , , . You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.