"3rd party hollow rotor shafts have been failing when used on certain helicopters, allowing rotor heads to detach in flight."
RECENT ACCIDENTS INVOLVING MODEL HELICOPTERS
There have been a couple of incidents of rotor heads becoming detached from model helicopters either in flight, or in a heavy landing. Whilst no injuries have been caused by these incidents, the safety hazard posed by such a failure is obvious.
In both incidents the detachment was caused by the failure of the main rotor shaft at the point where the head-retaining bolt passes through. The rotor shafts in question were NOT the helicopter manufacturers original parts, but "3rd party" parts. These rotor shafts are hollow, to save weight, and appear to have been hardened as part of the manufacturing process.
Whilst as a general rule there is no problem with such a design, there does appear to be a problem when they are used on Robbe Futura and possibly Millennium helicopters. On the Robbe Futura, the bolt retaining the head to the shaft is very close to the bottom of the head block. As a result, the leverage on the bolt is quite substantial. The loads imposed on the bolt, and the shaft that it passes through, can be severe, especially during "3D" type flying or in a heavy landing.
In both the incidents recorded, the shafts had fractured cleanly across the boltholes, presumably as a result of the loads imposed on them. The problem has not occurred so far on other machines, where the head-retaining bolt is further up the head block. Here, the leverage is a lot less, and there is more clamping action to spread the load. Nonetheless, anyone using a non-original, hardened hollow main-shaft is advised to inspect it regularly paying particular attention for any sign of fatigue close to the boltholes.
Anyone who has fitted a non-original, hardened hollow main shaft to a Robbe Futura or Millennium helicopter is advised to replace it immediately with an original manufacturers part.