Solidity and Airfoil Section

With the diameter and tip speed chosen, the next task is to choose the tail rotor solidity. The criterion that most often dictates this is the ability of the tail rotor to balance the torque of the main rotor in a full-power vertical climb at a specified density altitude with at least a 10% thrust margin remaining for maneuvering before stalling. During the analysis of lateral-directional trim in hover in Chapter 8, it was shown that the required value of tail rotor thrust just to balance main rotor torque was:


It ~ їм

From this, the minimum tail rotor solidity can be obtained as:

CT/oTmxTRj{ilR)j^lT — lM)

The critical condition may be either at low altitudes, where the power available is a maximum, or at high altitudes, where the density is low. The minimum solidity, of course, should be increased for design purposes to account for the 10% maneuvering margin and for any blockage effects of the fin, as discussed in Chapter 4. A further check should be made for right sideward flight (assuming "American” rotation of the main rotor) since the tail boom drag may be high as a result of main rotor wake impingement, as discussed in reference 10.11. The required blade area depends not only on the maximum thrust but also on the maximum usuable value of Cr/ox, which in turn depends on the airfoil section. It is generally true that compressibility effects on tail rotors are limited to relatively small power penalties, and so it is permissible to choose a thicker airfoil for the tail rotor than would be desirable for the main rotor, in order to take advantage of the higher maximum lift coefficient.

The charts of Chapter 1 give the maximum predicted values of CT/o for rotors with the NACA 0012 airfoil. The charts can be used directly if the actual airfoil has similar maximum lift characteristics, or they can be modified up or down to account for the difference between airfoils as measured in two-dimensional wind tunnel tests.