How It All Works – This now results in two opposing forces acting on the tail rotor system. The aerodynamic force wants to streamline the blades to zero pitch while the centrifugal force wants to rotate the blades to their maximum pitch. These tail rotor weights produce a centrifugal restoring moment (CRM) because they act to counter the aerodynamic forces on the tail rotor blades. And this is the purpose of the four weights shown in the full scale tail rotor. If the aerodynamic force is greater the blades will tend to return to zero pitch, and if the centrifugal force is greater the blades will tend to increase in pitch. If the two opposing forces are identical, the tail rotor pitch can be changed, and held in position, with very little, if any, force at all.
This neutral force position may seem to be ideal, but is not practical because slight changes in tail rotor pitch will result in flutter and an unstable tail rotor. It would be best to have the centrifugal force counter most, but not all, of the aerodynamic force, thereby requiring very little thrust from the tail rotor servo. This would mean the tail rotor servo can work faster and use less power.
A Practical Application – The amount of CRM is determined by the combination of the amount of weight used and the length of the tail rotor mounting bolt. A shorter mounting bolt can be used with larger weights, or a longer bolt with smaller weights. A little experimenting should be able to get you very close to a good combination. In my next blog I’ll give you a few experiments you can try with your helicopter to see if this can be helpful to your flying.