Another consideration is mounting location. You stated you wanted to put it on the bottom; I'd recommend putting it in a pod about half-span out on the wing on the opposite side of your exhaust, and place a counterbalance pod on the other side. This would keep the camera out of the exhaust stream and prevent a build-up of oil on the lens, and out of the propeller arc (if you point it forward) so you don't get a strobe effect from the difference in engine speed and shutter speed. You could also put it inside the plane with a clear window of lexan or some other low-distortion material, aim it in whatever direction you want.