Rotor-Fuselage Interactions

The effects of the rotor wake on the fuselage airloads are a senous cause for conce* n on all modem helicopters. This is especially the case in hovering and low-speed forward flight where the rotor wake will envelop a good part of the fuselage (and perhaps also the empennage). While there have been earlier studies of this problem, the first systematic wind tunnel study into rotor-airframe problems was conducted by Sheridan (1978) and Sheridan & Smith (1979). In more recent years, a significant amount of detailed experimental and theoretical research has been accomplished to provide a more thorough understanding of the aerodynamic interactions between rotors and (nominally) nonlifting bodies of relatively simple geometric shape, such as bodies of revolution [e. g., Komerath et al. (1985), Smith & Betzina (1986), and Leishman & Bi (1990b)], although more complex body shapes and scaled helicopter airframes have also been studied [e. g., Freeman & Wilson (1980), Trept (1984), Berry (1988), and Le Pape et al. (2004)]. More recent efforts to study rotor – airframe interactions have been specifically aimed at gaining a more complete understanding of the underlying fluid mechanics. There has been particular experimental and theoretical emphasis on the unsteady airloads caused by the interaction of the rotor blade tip vortices

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airframe, the intense unsteady airloads produced can contribute significantly to overall helicopter vibration levels. Other unsteady airloads are produced each time a blade passes over the fuselage, which creates an abrupt pressure pulse over a substantial part of the airframe. These pressure pulses depend on rotor blade loading and occur in phase with each blade passage; they can lead to low-frequency airframe and rotor vibrations that have considerable intensity. Several prototype helicopters have been designed with minimal rotor-fuselage spacing and have encountered insurmountable vibration problems that could only be solved (ultimately) by increasing the rotor-fuselage spacing. Currently, it is not clear if such vibration problems can be solved by other means. An unfortunate byproduct of increased spacing, however, is an increase in parasitic drag.

A reciprocal effect of the fuselage may occur on the rotor, where the blade airloads and rotor performance are changed – see Wilby et al. (1979), Rand (1989), and Crouse & Leishman (1992). The presence of the fuselage distorts the inflow through the rotor disk and this affects the blade airloads, the rotor performance, and the blade pitch control angles required for trimmed flight. Wilby et al. (1979) have shown that in forward flight the fuselage-induced upwash velocities can provide a perturbation to the aerodynamic angles of attack over the front of the rotor disk, which can significantly affect the blade airloads and net rotor response. Smith (1987) has shown that the wake distortion associated with fuselage interactions can generate enhanced rotor wake interactions that may trigger a torsional aeroelastic response of the rotor and could lead to a premature retreating-blade stall. Such effects are recognized based on flight testing experience, but are not well understood or predictable because they are sensitive to the flight conditions and to the ability to predict the detailed aerodynamics at the rotor. This also makes the underlying aerodynamic issues hard to study in more controlled wind tunnel experiments because not all flight conditions can be replicated. The effects and consequences of fuselage-on-rotor aerodynamics will require relatively sophisticated and fully coupled rotor wake and airframe models if their effects are to be predicted accurately. Other authors, including Johnson & Yamauchi (1984), have also emphasized the importance of and difficulties attributed to rotor-airframe aerodynamic interactions when predicting the airloads and aeroelastic response of rotors.