Predicting the behavior of the helicopter when near the ground is practically very important because when flying at very low levels the pilot has very little tolerance for pathological or unexpected behavior on the part of the aircraft. Aerodynamically, as a hovering rotor is moved from free air or “out-of-ground effect” (OGE) toward the ground, the wake impinges on the ground surface – see Fig. 5.35 previously. Operating “in ground effect” (IGE) results in a change in the inflow through the rotor, and this usually manifests as an increase in rotor thrust for a constant power or, alternatively, as a decrease in power required for a given rotor thrust – see Section 5.8 for details. Forward flight further complicates the aerodynamics of the rotor problem IGE. For example, experimental results (Fig. 5.38) have shown that during transition into forward flight, ground effect can result in an increase in power required for a constant thrust (or a decrease in thrust for constant power), in contrast to the characteristics found for operations in hovering flight IGE. From a handling qualities perspective, large control movements can be required from the pilot during acceleration into forward flight to compensate for rather sudden transitions in the geometry of the flow surrounding the helicopter when near to the ground – see also Fig. 5.37.
Modeling the effects of the ground on helicopter performance has remained a severe challenge for all computational aerodynamic models because of the extremely complicated behavior of the rotor wake when it interacts with the ground. While the rotor IGE problem has been examined using Navier—Stokes approaches [see, for example, Kang & Sun (1997, 2000)] the problems posed by vorticity diffusion prove a significant impediment to accurate calculations, this time because the dynamics of the rotor wake when near to the ground appear to be controlled by the development and dynamics of unstable structures in the wake that take a very long time (when measured in rotor revolutions) to grow to appreciable amplitudes – see Brown & Whitehouse (2004). In this type of flow situation, vorticity conserving methods such as those described in Section 14.3 come into their own.
For example, Fig. 14.17 shows a calculation of the wake generated by a helicopter in low-speed quartering flight while very close to the ground. An interesting feature at this flight speed is that the wake ahead of the main rotor rolls up to form a crescent-shaped “bow vortex” along the forward edge of the footprint of the wake on the ground – see also Fig. 5.37 for experimental observations of this phenomenon for an isolated rotor. This bow vortex interacts with the ground and with the fuselage of the helicopter, buffeting the aircraft and inducing, potentially, an unacceptable increase in pilot workload in this critical flight condition. At higher flight speeds, the bow vortex is swept away into the flow downstream of the rotor, the wake lifts off the ground and the helicopter essentially behaves as it would out of ground effect.