8/24/02, a friend asked me to look at a crack he had discovered in the vertical stabilizer of his turbo charged A36 Bonanza.
The crack is in the skin just behind the aft spar. This skin area overhangs the spar by roughly 2 inches and functions as cover for the rudder hinge slot. The aft edge is formed to a "J" section with a radius of roughly 1/8 inch. There is a blade antenna mounted just forward of the crack.
I suspected the crack was caused by a resonant vibration of the overhang, possibly aided by a sharp, sheared, edge on the underlying spar.
I formed a piece of 0.025 in the "J" shape and clamped it in a vise to determine the resonant frequency. A high Q resonance occurs at 130 Hz.
A three blade prop spinning at 2600 RPM will produce 130 Hz pressure and velocity oscillations at the empennage.
The images below show the crack and the test setup to determine the resonant frequency of the overhang.
LOUDSPEAKER DRIVEN AT 100 Hz
LOUDSPEAKER DRIVEN AT 130 Hz