Abstract
AbstractThe rotorcraft is a complex dynamical system that demands specialist modelling skills, and a high level of understanding of the aeromechanics arising from the main rotor wake and aerodynamic couplings. One such example is the difficulty predicting off-axis responses, particularly in hover and low-speed flight, associated with induced velocity variation through the rotor disk resulting from the rotor wake distortions. Various approaches have been developed to deal with this phenomenon but usually demand prerequisites of high levels of expertise and profound aerodynamic knowledge. This paper presents a new and practical approach to capturing this wake distortion through an augmented rotor inflow model. The proposed model is coupled with a nonlinear simulation using the FLIGHTLAB environment, and comparisons are made between the simulation results and flight test data from the National Research Council of Canada’s Advanced System Research Aircraft in hover and low speed. Results show good predictability of the proposed nonlinear model structure, demonstrated by its capability to closely match the time responses to multi-step control inputs from flight test. The results reported are part of ongoing research at Liverpool and Cranfield University into rotorcraft simulation fidelity.
Highlights
Achieving realistic pilot and aircraft flight behaviour are key ingredients for, and outcomes from, quality flight simulation, in which pilots can be trained to operate the aircraft safely
Populating the FLIGHTLAB L matrix is no exception, requiring numerical wake solutions for each new configuration. We obviate this complexity somewhat and present an approach to approximating the wake distortion effects through what we describe as an augmented inflow model, derived from the first principles of momentum theory, where the finite-state inflow (FSI) components are augmented using the fuselage roll and pitch rates; the disc tilt rates are excluded due to their very short duration in large pitch and roll manoeuvres compared with the fuselage motions, e.g. typically following a lateral cyclic step input, the lateral flapping rate has reduced to zero when the roll rate reaches a maximum
These results suggest that wake skew has very little impact on the correction of inflows, and it is mainly the wake curvature and its coupling with the wake skew angle, which can correct the off-axis response in simulations to be comparable to flight test (FT) data
Summary
Achieving realistic pilot and aircraft flight behaviour are key ingredients for, and outcomes from, quality flight simulation, in which pilots can be trained to operate the aircraft safely This requires simulation models with sufficient fidelity to predict the flight responses within reasonable limits, defined, for example, by the guidelines in the Certification Specifications for Helicopter Flight Simulation Training Devices (CS-FSTD(H)) [1], which details the acceptable match between flight and simulation responses to control inputs. The rolling or pitching rotor tip-path-plane leads to the wake effectively being compressed on one side and expanded on the other, so a differential change in the induced velocity distribution is produced and subsequently changes the downwash on the blades ( lift) over the rotor disk. Several researchers have demonstrated the effectiveness of including these wake distortion effects due to the pitch and roll motion in the dynamic inflow equations to improve the off-axis response predictions as discussed
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