Consider the physical characteristics of typical tail rotors on production helicopters, as listed in the appendix. A comparison of the size of the tail rotor to the main rotor shows that it is roughly one sixth the diameter of the main rotor. Because the ratio decreases with gross weight, the size of the tail rotor grows more rapidly than the size of the main rotor with increasing gross weight, as shown in Fig. 6.29. Note that for machines with fenestrons, the size of the fenestron can be much smaller than that of a conventional tail rotor. Again, the trends (for conventional tail rotors) can be explained with the aid of the square-cube law. It has been shown previously that the main rotor power (or torque) required is oc W3/2. Also, because the size of the helicopter (and length of the tail moment arm) grows with W1/3 so the anti-torque force requirement should be a W7/6. Because tail rotor disk loading remains relatively independent of helicopter gross weight (for good efficiency), the size of the tail rotor should be oc W~7^12. This trend is in general agreement with the data shown in Fig. 6.29. Note also that the tail rotor tip speed is approximately the same as the main rotor tip speed. This means that the rotational speeds of tail rotors are roughly six times the main rotor frequency, and this has particularly important consequences on rotor noise levels. While the noise energy produced by the tail rotor is only a fraction of that produced by the main rotor, the higher frequencies generated by the tail rotor can be more discernible to the human ear (see Section 8.19).