Yep, that's it. Just try the 17x12W. I think you will be pleasantly surprised. Much better pulling power and still good downlines. The engine likes the lighter weight also, and spools up and down faster.
Put the two pieces of your broken crank together, and you will see the sharp edge that has to be radiused.
Jon
ORIGINAL: Jeff Boyd 2
ORIGINAL: jonlowe
The stock crank can and will break at the rear of the intake port window from a sharp machining edge. I radiused the sharp corner with a Dremel and polished it. I had three break in one year using an APC 18.1x10W. All three had exactly the same failure, and the pieces were so matched they would almost intechange with each other. I started radiusing the port after that, and stopped using that prop at the same time, and had no further failures. The 18.1x10W is a heavy prop, which I think exacerbated the problem. The 17x12W turns out to be a better match for the engine anyway.
Like this Jon ?? . . and guess which prop !
I really like the 18.1 x 10W . . it flies slower with better throttle control. It also has slower downlines. I am running an Aeroslave pipe though . . so may be the difference.
Cheers, JB