A shaft driven helicopter has excellent handling property. The response is instantaneous because it does not need to spin up like a motor driven tail rotor. However, there is one weakess. The main and the tail rotor is connected to rotate together. That works great as long as you don't crash.
The main rotor of the helicopter has a lot of inertia. The tail rotor has little mass, and will stop rotating as soon as it come into contact with anything either a wall, ground or tall grass. So the main rotor continue to rotate while the tail rotor stops rotating. Something has to give in the drivetrain. Typically, is the gears that gets stripped. Solution to stripped gear is to install a over-running clutch into the drivetrain. This modification will install an over-running clutch into the drivetrain to prevent breakage and stress to the drivetrain. No more stripped gears. No more extreme strain to the rest of the drivetrain.
More background info.
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See diagram for parts location:
Dimensions: Dia: .72 +/-.02 mm Length 2.5 +/-.03mm (Make your own using .029" music wire)
Tail Shaft $12.00
Keyed Plate $3.00
M2X2 Setscrew and Hex Wrench $2.50
Gear Shim $1.50