Cox engine rpm runaway on a CL plane

While running some speed test on my COX powered speed plane the crankshaft breaks right at the spline connecting the propeller washer. the prop goes flying, and the engine runs unloaded, causing an rpm runaway. I have no idea how fast the engine ran, but it sounded like 40.000 rpm. By the time i got to it and shut it down the cylinder and head had reached a very high temperature.
The piston got a yellow tint, and a burnt looking glow head.
The engine a modified 0.049 COX, with a max rpm measured to 21.500 rpm with a 4,5x4 prop

[@jetpack5]

That is the highest RPM I have ever heard out of a Cox 049. I cannot believe it did not lose a piston

[@lacassed6189]

50.000 rpm sure craze

[@gertnood]

Sounds like the pitch of the audio went up about a tritone or a 5th, so the rpm jumped by about 50%, according to my ears anyways.

[@ronitsingh85]

defiantly the crankshaft was a faulty one, Cox made virtually unbreakable crank shafts, there is no way it should have broken. Get a vintage used cox 0.049 engine, it looks so much better interns of looks and built quality. I can tell the crank broke in mid air, that is RARE.

[@christopherpoucher483]

Holy shit!

[@jens8111]

MaxwellForce Det er jeg ked af :(
Hvor flyver i henne sådan ca? Jeg flyver selv RC, men kunne godt tænke mig at se noget line styret en gang :)

[@davidlavergnelacasse2356]

One of m'y old black widow hapen the same on a rc plane lol 😆😅

[@ronitsingh85]

oh i am sorry, its NOT the crankshaft that broke, its the SCREW!!!!! It might have been tightened too tight, it sheared off! But yeah, I would believe the engine would die cause there is no inertia driving it, you need a 'flywheel' in the form of a prop here.