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Modeling of Rotor—Fuselage Interactions
Effective analytical tools that are capable of treating interactional aerodynamic problems are still undergoing development and will continue to do so until wake modeling is improved further (see Chapter 10). Some of the earliest theoretical work on the rotor – fuselage aerodynamics problem was performed by Bramwell (1966) using potential flow assumptions and conformal mapping techniques. The results compared well with the few experimental data that were available at that time. Landgrebe et al. (1977) made one of the first studies of the effects of an airframe on rotor airloads and performance. The task was divided into three levels of complexity: 1. The calculation of velocity perturbations induced by the body in the rotor plane and the calculation of changes in the rotor airloads based on these perturbations, 2. The inclusion of velocity perturbations in the trim analysis, with the rotor loading being adjusted for the velocity perturbation, and 3. The inclusion of the effects of the body on the rotor wake geometry. The results clearly demonstrated the overall importance of accurately predicting interactional effects on helicopters.
The basic approach of linear superimposition of flow fields about the airframe and the rotor, but without considering the mutual interactions, has been pursued by several other researchers. Wilby et al. (1979), Ryan et al. (1988), and Dehondt (1989) have all used this type of superposition methodology. Although these models give an indication of the types
of interactional effects that can be expected, there are clearly many questions about the quantitative validity of the results, particularly in flight regimes where it is known that there are strong interactions between the rotor wake and the fuselage (i. e., in hover or low-speed forward flight). Under these conditions the interactional aerodynamics problem cannot be treated by linear-superposition techniques. It has already been shown how the rotor wake heavily influences the fuselage flow field, and likewise, the fuselage influences and distorts the rotor wake development. A somewhat better approximation was first introduced by Freeman (1980), who included a time-averaged vortex tube wake model during the calculation of the body aerodynamics. Clark & Maskew (1985,1988) later developed a more sophisticated time-averaged wake model using vortex theory. A major deficiency of all of these models, however, was the omission of the distortion of the wake induced by the body.
More sophisticated surface singularity (panel) methods have been developed that allow a more complete computation of interactional effects, including the interaction of the wake and the airframe. These panel methods are discussed in Section 14.7. Some of these methods, however, have shown less predictive success in the low-speed forward flight regime, and this is a serious deficiency because the various flow interactions are often more significant here. Landgrebe et al. (1977) and Lorber & Egolf (1990) recognized the significance of the rotor wake distortions induced by the body, and developed an analysis using a prescribed rotor wake with geometric displacement rules to distort the wake about the body. More sophisticated analysis have coupled free-vortex wake models and surface panel models, including the work of Clark & Maskew (1988, 1991), Mavris et al. (1989), Berry (1988), Berry & Althoff (1990), Quackenbush et al. (1990), Crouse & Leishman (1992), Boyd et al.
(2000) , and Wachspress (2003). All these methods are potential flow models, so naturally are limited to essentially inviscid flows. Furthermore, for most flight situations there is a need to retain unsteady terms in the governing equations to give acceptable levels of predictive success, a point alluded to previously, and this leads to a greater computational cost.
All of these types of panel models inevitably include some empirical features (including viscous tip vortex models) and so require detailed experimental measurements for validation purposes. Unfortunately, there are still only limited measurements available that are directly relevant to the somewhat more idealized interactional problems currently under theoretical study (i. e., vortex-surface impingement). More experimental data are available for complex geometrical configurations, but it is difficult to isolate the effects of individual flow phenomena from these results. This paucity of specific types of experimental data hinders the development and validation of more effective analytical models for airframe and rotor airloads. Validation predictions against existing experiments has been fairly successful, however, allowing a clear delineation of the primary effects of the body on rotor airloads and performance and vice versa. For example, predictions using the free-vortex wake and surface singularity method of Crouse & Leishman (1992) has been shown in several prior figures, including Figs. 11.3, 11.8, and 11.15. These predictions are in general agreement, with measured results, which gives overall confidence in these types of essentially potential flow models for predicting a priori the possibilities of various rotor-airframe interaction problems. Predictive capabilities with panel methods are always reasonably good in airframe regions unaffected by flow separation. It is possible to represent 3-D separated flows by extending the singularity panels off the airframe surface as free shear layers, however, the points of 3-D flow separation on the airframe are hard to predict.
As previously alluded to, it is the interaction of blade tip vortices with the airframe surface that has proved more difficult to model, and this problem has perhaps received the most recent attention from a theoretical perspective. As previously described, these interactions can range from almost benign interactions where the tip vortex trajectories just
glance the airframe or empennage surfaces, to direct impingement and associated large – scale reorganization of the flow topology. Even though the encounter may vary in terms of its severity, in almost all cases the tip vortices induce large unsteady airloads on the airframe surface. Significant progress has been made in modeling this behavior numerically by means of vortex methods – see Conlisk & Affes (1993), Conlisk et al. (1993), and Quackenbush et al. (1994). However, rotor wake-surface interaction predictions are challenged by their inability to calculate the vortex strengths and wake distortions with good accuracy, and also because the local distortion of the wake by the body must be accounted for.
Because rotor-mrframe interactional problems often involve 3-D viscous effects and flow separation, these problems a particularly good challenge for advanced computational fluid dynamic (CFD) models based on Navier-Stokes (NS) or Reynolds-averaged Navier – Stokes (RANS) equations (see Section 14.2.1). Fundamental studies of the problems has been conducted by Stremel (1987, 1990). More ambitious attempts with entire rotor and airframe configurations [e. g., Park et al. (2003)] have been recently attempted using millions of grid points in the flow (and many hours on a supercomputer), but these methods seem to have provided only limited new insight to the numerous types of interactional aerodynamics problems; most of the understanding still comes from more parsimonious models and from experimental observations. See Section 14.10.4 for further details of CFD approaches to rotor-airframe interaction problems.
