This is the HTML version of a paper presented at:
The American Helicopter Society International Specialists' Meeting, Stratford, Connecticut, October 11-13, 1995.
Introduction
The interactional aerodynamic effects between a helicopter rotor and its airframe have been recognized as highly significant, but extremely complex and difficult to model by means of existing computational tools. This is especially true for lifting surfaces on the airframe such as the empennage, since unsteady viscous effects can dominate the flow field. During changes in forward flight speed, the rotor wake boundary changes position relative to the empennage. This means that the angle of attack at the empennage can undergo large excursions. Also, the empennage may be wholly or partially immersed inside the high total pressure of the rotor wake. These effects can result in substantial changes to fuselage forces and moments, and can produce an aircraft with undesirable handling qualities. The difficulties with the prototypes of the AH-64 Apache and the EH-101 helicopters are well documented examples of this type of problem (see Ref. 1 and 2).
Despite their significance, experimental studies of rotor/empennage/lifting surface interactions are quite rare. Lynn (Ref. 3) discussed specific problems associated with helicopter rotor/wing interactions, showing the effects of an auxiliary wing on the performance, stability and control, structural loads, and maneuver characteristics. However, the details of the interactions between the rotor wake and the wing were not studied. Sheridan and Smith (Ref. 4) examined some of the issues associated with rotor/tail airloads in their seminal work on the rotor/airframe interactions. More recently, Leishman and Bi (Ref. 5) studied the interactions between a rotor and an isolated lifting surface, confirming the complexity of the problem. Torok and Ream (Ref. 6) obtained flight test data of interactions between the rotor wake and the empennage of a S-76B Fantail helicopter. Frederickson and Lamb (Ref. 7) presented results from the same test, and included data from wind tunnel tests involving a model of the RAH-66 Comanche. Funk and Komerath (Ref. 8) and Foley et al. (Ref. 9) also performed a series of experiments involving the interaction between a lifting surface and a rotor, particularly in regard to flow separation effects.
The theoretical prediction of the effects of rotors and the associated vortical wakes on the airloads induced on bodies and wings has received somewhat more attention. Bramwell (Ref. 10) was one of the first to use potential flow methods to examine this problem, and the significance of unsteady effects was clearly demonstrated. However, the development of more general numerical methods has been hindered by the lack of detailed experimental data, both for guidance in developing the models and particularly, for validation purposes. Gangwani (Ref. 11 and 12) used a prescribed wake model coupled with a doublet-lattice model of a fixed wing to predict the unsteady bending moments measured on a helicopter tail. Reasonable correlation was obtained with CH-53A flight test data. Mello and Rand (Ref. 13) used a similar model, and showed that the predicted unsteady loads on the empennage were very sensitive to the rotor wake geometry. Curtiss and Quackenbush (Ref. 14) considered the effects of the rotor wake induced velocity field at the empennage location on helicopter stability derivatives. The limited correlations obtained with flight test data reiterated the complexity of the wake/empennage interaction problem. Recently, Weinstock (Ref. 15) has examined a simplified model of the same problem for use in rotor flight mechanics.
An improved understanding of the rotor/empennage interaction problem presents several experimental and theoretical challenges. In addition to the high rotor downwash at the empennage, there are several possible constituent sources of unsteady loading that may be produced on a lifting surface located near a rotor or its wake. It is known that for the non-lifting (body) case, unsteady effects induced by the rotor and its wake can be extremely large (Ref. 16,), and in some cases can be larger than the time-averaged loads. One unsteady effect is due to blade passage, which produces an impulsive type of loading on the surface. Another is due to the presence of the rotor tip vortices, which induce rapid changes in downwash and also produce large unsteady aerodynamic effects. In addition, there may be time-dependent aerodynamic effects produced by the wake of the lifting surface itself. The relative magnitude of all these effects will depend on the size of the lifting surface, its proximity to the rotor, the wake slipstream velocity, the tip vortex strengths, and the proximity of the wake to the empennage. The latter, of course, will be sensitive to the flight condition, i.e., the rotor thrust and advance ratio. In view of the present limited understanding of these interrelated effects, the purpose of this work was to conduct a systematic experimental investigation into the problem of rotor/empennage/lifting surface interactions. Particular emphasis was placed on documenting the rotor wake geometry and unsteady effects induced on the empennage. The overall objective was to obtain a better understanding of the aerodynamic environment encountered by lifting surfaces located near a rotor and/or immersed in the rotor wake, and to provide specific measurements for ongoing validation studies with analytical models.
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