Geistware, I calculated the same rpm. I would be really surprised that the DA50 is revving up to nearly 11,000 rpm when it unloads, and at partial throttle, to boot. With a 22" prop, it would be really ripping the air and sound very quick.

BTW, APC says the max rpm for their props is 190,000 divided by prop diameter. Thats around 8600 for a 22".

I suspect that a radar gun would be a more direct, and therefore more accurate method to measure the speed.

I did some experiments a while back with a PC/RC Digitach. It's an inexpensive (about $60) onboard tach that can be set to record peak rpm or constant rpm. The sensor is mounted to the backplate and senses the steel pin in the crank going around. It has a digital display that mounts to the plane. I found that on a Fox 46 in a sleek pattern plane, I was gaining roughly 1800-2000 rpm, increasing from 12,000 to 13,800 or 14,000 rpm in the air. This explained why that plane just SCREAMED in the air as it went by at full throttle. That's a little bit over a 16% rpm increase in a small, high revving glow engine.

I would love to see more speed data collected by another method, so see if you are truly getting what is probably a 30% increase in rpm. That would be amazing.