Aerodynamics of Helicopter Flight Near the Ground

Nap-of-the-earth (NOE) tactical doctrine will require sustained flight close to the ground at low speeds. All helicopters in the battle zone frequently will be forced to seek cover and fly for long periods of time at very low heights which were formerly encountered only in the takeoff run and during the landing flare or avoided entirely. Such transitory operations within the low-slow flight regime until recently have placed a low priority on aerodynamic research on ground-proximity effects. There is now a compelling need, however, to understand the unsteady and complex flow environment resulting from the interactions of rotor downwash, the ground boundary, and the velocity of translation. Recent studies have begun to provide the required insights for this regime of flight, especially the role of the ground vortex. This anomaly is the dominant feature of the aerodynamic flow field from approximately 10 to 30 knots. The flow associated with the vortex and the loads it produces on the helicopter can be a key factor in many problem areas of flying qualities, component design, performance, and operations. This paper presents the results of powered-model wind-tunnel investigations conducted by Boeing Vertol in the 1971–76 period. The ground vortex is the focal point for this discussion in terms of its occurrence, structure, and interactions with the rotor and airframe through extensive flow visualizations and model measurements. The flows induced by the vortex are shown to be causative factors in aircraft-trim distortions, exhaust-gas-ingestion potential, unexpectedly large power-required increases, some very non-linear fuselage loadings, and high steady and fluctuating wind velocities near the ground. The impact of sustained exposure to the vortex flow on the potential problem areas is reviewed. Finally, further research is recommended to expand the data basis, to foster theory development, and to develop meaningful design and operational guidelines.