Main Shaft

One of the largest problems with the main shaft is it's susceptibility towards failure. Carbon Fiber is a strong medium when trying to stretch or bend it - however, like any fibrous material, doesn't offer as much resistance to being twisted. Thus, the torque placed on the shaft between the input (main gear) and output resistance (rotor head) causes a twisting action on the shaft. Over time, this torque can cause hairline fractures to develop in the shaft. These obviously will get worse with time. In an "incident" where the rotor blades may strike the ground (or other object), the force of this impact will place similar stress on the shaft and cause fracture or complete breakage of the shaft.

A fractured shaft, even with tiny hairline cracks that are not readily visible, will allow the shaft to bend or warp, particularly while under load. This can result in slight problems which both cause other problems, and get worse over time:

The most evident sign of breakage in the main shaft is wobbling. This can occur in the main rotor head, but is easiest observed in the main gear. This should be done with the helicopter firmly clamped down. Run the rotor speed up and observe the main gear from the side of the heli. (Don't hit your head on the spinning rotor!) If there is any movement in the gear (wobbling,) other than pure rotation, you may have main shaft damage.

There are other symptoms which can occur due to this. One is a vibration in the entire heli - this is caused by the rotor head no longer being centered on the rotational axis, and will cause the entire heli to shake and vibrate in flight. Another big symptom is gear noise. This can come from the fact that the teeth in a wobbling main gear are not meshing properly (and consistently) with the motor and tail shaft. Note, that this can also cause damage to the teeth on these gears.

Hairline fractures cannot be easily seen on the main shaft. To view hairline fractures, do the following:

  1. Remove the main shaft from the Heli
  2. Remove the Rotor Head from the Main shaft
  3. Insert 1mm pins in the pin holes at the top and bottom of the shaft.
  4. Holding the pins, gently twist the upper and lower halfs of the mast in opposite directions.
  5. While applying twisting force, inspect shaft for cracks.

Needless to say - too much force can result in you cracking or breaking the shaft, so be careful!

This can also be accomplished while leaving the shaft in the heli, and holding the main gear while rotating the main shaft. 

An in-tact shaft should allow about 5 degrees of deflection. A damaged shaft may allow 90.

The obvious remedy is to replace a cracked or broken shaft. Several people have been experimenting with steel, aluminum and brass shafts. So far, many people have reported high levels of success. Note that a metal shaft will increase the weight of the heli slightly, but will not affect the Center of Gravity - because the Heli's CG is supposed to be right in-line with the shaft axis. As for the added extra weight, the increased reliability and stability well outweigh the downside - (no pun intended...)