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, nonlinear 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.

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Figure 1. The UH-60 Helicopter was used as the baseline helicopter in a study of the effect of venting the tail boom.

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. Various venting schemes are possible, and vents could be used in conjunction with other devices, including strakes. The venting schemes include, but are not limited to, the use of porous material on all or parts of the boom connected to a specific plenum or open to the boom cavity, and the use of doors, grilles, slots, or other such openings. 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.

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Figure 2. These Preliminary Results of Experiments and Calculations indicate that porosity could be useful for reducing adverse helicopter tail-boom loads.

A wind-tunnel investigation was conducted to obtain some preliminary indications of the effectiveness of venting the boom for reducing adverse forces and moments. This study was conducted with two-dimensional tail-boom shapes in the 14-by-22-ft (4.3-by-6.7-m) subsonic wind tunnel at NASA Langley Research Center. The shape that was tested most extensively was representative of the tail boom of a UH-60 helicopter (see Figure 1) with the tail-rotor drive-shaft cover on. The aerodynamic forces, including the down load and side force, were measured at freestream dynamic pressures up to 30 psf (1.4 kPa), at full-scale Reynolds numbers, and at angles of incidence (wind azimuth angles) from –90° to +90° to simulate left and right cross winds. Calculations were performed, using the normal-force and side-force coefficients, to determine the trends of the aerodynamic forces on the boom of a full-scale UH-60 helicopter and to estimate the main-rotor and tail-rotor power needed to trim those forces in the presence and absence of venting.

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 nonoptimal 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. Therefore, the penalty in down load was found to be less than the beneficial reduction in side load. The overall result was found to be reductions in the adverse forces and the calculated power demand, similar to the reductions afforded by the application of strakes.

Other venting arrangements or more optimal porous material could reduce the down-load penalty while also alleviating the side force. Plots of the side-load coefficient (Cy) and the down-load coefficient (Cz) are shown in the upper part of Figure 2. Calculated trends in the power demand are shown in the lower part of Figure 2. Although the calculated trends in the power demand were found to vary widely among different configurations, the net result was found to be a reduction for the porous configuration, relative to the baseline solid configuration. Also, for the porous configuration, the character of the plots of Cy versus the angle of incidence was smoother, without the discontinuous nature and a less abrupt boom stall, as compared with that of the baseline solid configuration. The smoother characteristics of the porous configuration would result in fewer yaw-control disturbances during flight in gusty or turbulent air.

This work was done by Daniel W. Banks of Dryden Flight Research Center and Henry L. Kelley of the U.S. Army Aeromechanics Laboratory at Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Dryden Flight Research Center,
Attn: Yvonne Kellogg, D-4839A,
P.O. Box 273,
Edwards, CA 93523-0273.

Refer to DRC-98-96.