TAIL ROTOR-FIN INTERFERENCE IN HOVER

Another form of interference similar to vertical drag is the mutual interference of the tail rotor and the fin. The interference manifests itself in two ways: as a force on the fin that decreases the effective antitorque force generated by the tail rotor; and as a change in the flow conditions at the rotor that may either increase or decrease the tail rotor power required. Figure 4.9, based on the test data in references 4.7 and 4.8, shows the nondimensionalized interference force on the fin as a function of the blockage area ratio and the separation distance for both pusher and tractor arrangements. The gross thrust required of the tail rotor is:

Source: Cassarino, “Effect of Rotor Blade Root Cutout on Vertical Drag,” AAVLABS TR 70-59, 1970.

Source: Fradenburgh, “Aerodynamic Factors Influencing Overall Hover Performance,” AGARD CP 1111, 1972.

where TT is the net thrust required to balance the main rotor torque.

For a tractor installation (i. e., with the wake blowing on the fin), the tail rotor benefits from a pseudo ground effect just as the main rotor does in the vertical drag situation. For a pusher installation, the fin slows the inflow, thus producing a beneficial pseudo ceiling effect. The fin also puts discontinuities into the flow, which might result in local stall. Figure 4.10 shows one set of experimental results for a tail rotor with and without a fin. For this configuration, the value of F/T from Figure 4.9 is 0.13. The effect of the fin on power may be found by

Source: Lynn, Robinson, Batra, & Duhon, “Tail Rotor Design,” Part I: “Aerodynamics,” JAHS 15- 4, 1970; Morris, “A Wind-Tunnel Investigation of Fin Force for Several Tail-Rotor and Fin Configurations,” NASA LWP-995, 1971.

comparing the power required for the rotor with fin off at a CT/(3 13% higher with the measured power with the fin on. Such a comparison at CT/a = 0.08 shows that the measured power is approximately 94% of what would have been predicted from the fin off data. Based on this one set of test data, it is suggested that the tail rotor hover performance be based on the empirical equation:

The example helicopter has an area ratio, S/A, of 0.25, and a separation ratio, x/R, of 0.3. Thus from Figure 4.9 the interference ratio, F/T, is 0.125. The corresponding equations for the gross thrust and the power are:

Tr = 1.125 Tn

h. p.r = .94 (h. p. for TT(J

Source: Internal Lockheed document.