There have been two failures of 8" long prop extensions that we know of! Neither of them occurred on a RAF design but both were on pusher aircraft. Both prop extensions were purchased from Sport Flight in Florida. We understand that his company manufactures their prop extensions form 6061-T6 aluminum. All RAF-designed (Brock manufactured) prop extensions are machined from 2024-T3 aluminum which is approximately 20% stronger.
The sizes of the radii between the flange and barrel are critical. At least one of these failures probably was due to a resonant vibration at the natural frequency of the prop/prop extension. This could be a serious mater and RAF is currently working with experts in this field on just what magnitude the problem is, or isn't. Burt ran his Defiant with 8" Brock prop extensions for over 900 hours with no sign of a problem. We should have a lot more info on this subject in the next CP. If you are currently running an 8" Sport Flight prop extension, our recommendation would be to remove it and replace it with a Brock 2024-T3 prop extension before next flight.