Frames and frame resonances and vibrations

I have a few quad frames of various sizes. These are not build but I assembled them naked frames. There is a 3.5” whoop, a 5” dead cat frame and one of the super cheap (and flimsy) F450 knockoff frames with the plastic arms. The F450 frame was selected specifically for its very flexible pcb top and bottom plates, and the way those arms attach.

I am no engineer nor aerodynamasist but to my eye, most frames look kinda not very aerodynamic, especially those arms.

So what is the point about naked frames with no electronics, you ask? Simply to analyse the vibrations. With no mems sensor, how? Strapping a sensor on the frame is not viable as it will likely change the resonances. So we can use sound and fft the signal.

In this way we can get data, it may not be the best possible data, but in a relative comparison, it is way good enough, would be my thinking. We need not be absolutely accurate and precise, just repeatable in measurement. I have rested two apps, both on Android and iPad. I do have a 40MHz signal gen that is referenced to a gps driven and locked tcxo, so fairly accurate. Compared its output to what was measured by the mobile devices and they were within ±1Hz or so. Good enough for who is using it.

Suspending the frame and hitting various parts with a screwdriver handle should generate enough of a signal.

This is one picture with the average in blue, and peak in red for the sound a frame made. We can clearly observe the frequencies above 100Hz with the resonant peaks. See jut above 250 and below 500Hz.