Passive Venting for Alleviating Helicopter Tail-Boom Loads (DRC98-96)

The tail boom of a single-rotor helicopter is subjected to a complex flow field that includes the wakes of the main and tail rotors, the freestream, and the wake from the forward fuselage. Hovering and sideward flight present the operational regimes that are most critical with respect to adverse sideward and downward loads on the tail boom. These adverse loads necessitate additional engine power, thereby reducing payload, performance, and available yaw-control margins. In addition, non-linear side-force gradients near conditions of boom stall can make precise yaw control very difficult for the pilot. The addition of strakes to the tail boom is one method that has been used to modify the flow field and reduce these adverse loads. In recent research on fixed-wing aircraft, porosity of surfaces has been used to alter distributions of pressure (and thus loads) on the surfaces. It was postulated that, by passively venting portions of a helicopter tail boom (generally represented as a blunt body), the pressure distribution and therefore the loads on the boom could be modified in a favorable manner. Further, it may be possible to capture a portion of the relatively high-speed downwash from the main rotor as it impinges on the upper surface of the boom and channel that flow to another area on the boom. The results indicate that passively venting a helicopter tail boom can alleviate some of the adverse side forces that are generated in hover and in sideward flight. There was found to be a mild penalty in increased down load that is partly attributed to the increased skin friction associated with the non- optimal porous surface (a commercially available material) that was used in the experiments. It was found that for the same loading, it takes more power to overcome a side load than to overcome a down load because the tail rotor is much more heavily loaded and less efficient than is the main rotor.

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