Chapter Review

The content of this chapter has described some of the important aerodynamic interactions that can take place between a helicopter rotor and its airframe. Because there are often (but not always) deleterious effects associated with these interactions that can have

a significant impact on the performance of the helicopter as a whole, the understanding and accurate prediction of aerodynamic interactions have become an important issue in helicopter design. Problems of interest include the need for a better understanding of the physics of rotor wake distortion caused by the fuselage and empennage, the effects of the fuselage on the rotor airloads, and the consequences of main rotor and tail rotor wake interactions on unsteady airloads and acoustics. The need to mitigate these effects at the design stage rather than after first flight has never been more imperative to the success of the modern helicopter.

There have been many wind tunnel experiments with subscale helicopter models to heir» quantify the various aerodynamic interactions between a rotor and an airframe. Some of these tests have been performed with simplified airframe topologies, which are ideal for understanding the physical phenomena, whereas other tests with scaled airframes are better for the validation of comprehensive flow models. Airloads on the fuselage and on the rotor have been measured in an attempt to assess the magnitude of the effects and to better understand the significance of aerodynamic interactions. At low flight speeds the helicopter fuselage is often immersed in the wake from the main rotor and so the downwash from the rotor will influence significantly the fuselage aerodynamics. The presence of the rotor wake creates a significant download on the fuselage that increases with increasing rotor thrust and decreases with advance ratio, but only to a point. An upload can be produced on the airframe by the rotor wake when the helicopter is in high-speed flight. The corresponding effect of the fuselage on the rotor can increase its mean thrust at low advance ratio. The rotor wake has been shown to produce substantial effects on the time-averaged fuselage pressures, especially at the boundaries of the rotor wake where the induced velocities are highest. These mean pressures are sensitive to variations of rotor thrust and/or advance ratio. The induced pressures on the fuselage are generally asymmetric and will result in a net side force on the fuselage. Overall, the results from several wind tunnel experiments and flight tests have shown the relative complexity of the interactional aerodynamic effects that exist between a thrusting rotor and its airframe.

Unsteady effects play an important, and perhaps underestimated, role in the understand­ing of interactional aerodynamic phenomena on helicopters and other types of rotorcraft. The unsteady pressure fluctuations induced on the fuselage by the rotor and its wake are very large; in fact, in many cases, the magnitude of these unsteady fluctuations exceeds the mean pressure values. However, the unsteady pressure fluctuations are not necessarily the greatest in the regions of highest mean pressure. Blade passage effects on the fuselage increase in proportion to the blade thrust (disk and blade loading) and may be an important factor in the design of new helicopters with high disk loadings and smaller rotor-fuselage spacings designed for low parasitic drag and high-speed flight. The dominant frequency of the unsteady pressure fluctuations corresponds to the blade passing frequency. Wake vortex impingement on the fuselage results in more complex transient pressure loadings. These loadings, however, are not the severest form of induced pressures obtained on the fuselage because close wake vortex-surface interactions appear to produce much stronger effects. While the complexity of the problems involved in rotor-airframe interactions pro­vides ample scope for both the theoretician and experimentalist, and indeed offer many new research opportunities, it provides a particularly good challenge for the more advanced CFD methods. These methods, and their limitations, are discussed further in Chapter 14.