Proposed flying quality metrics and simulation studies for elastic vehicles

Abstract

Large, high speed vehicles are expected to exhibit interactions between rigid and aeroelastic dynamics. Current flying quality design guidelines are not directly applicable to this class of vehicles. To correct this problem, critical issues in elastic vehicle open-loop and closed-loop design are presented. With this background, key aeroelastic pole-zero features, which influence the vehicle motions and flying qualities, are identified. These features are quantified and cast as proposed handling quality metrics. An additional key factor is shown to involve control derivatives which describe the relative amount of control power entering the rigid-body and aeroelastic modes, as well as a characteristic deflection shape parameter. Two schemes are proposed to vary the basic vehicle parameters to achieve desired model features which will be flown by research pilots. Correlation of pilot ratings with model features will generate the desired design guidelines. Introduction Large supersonic and/or hypersonic transports, which employ high strength/moderate stiffness materials with minimal overall structural weight, are expected to exhibit significant interaction between the rigid-body and aeroelastic dynamics. I' This interaction will be further accentuated by high bandwidth flight control systems (PCS) which are necessary to stabilize and augment the airframe dynamics to the pilot's acceptance. Guidelines, specifications, and requirements for PCS design like MIL-STD-1797A, which are directly applicable to elastic vehicles, are almost nonexistent, but badly needed. References 4-12 are several notable exceptions. International competition/co-operation has already begun on a second generation supersonic transport.^ ^ Without criteria in place to clearly define boundaries between excellent, marginal, and poor handling and ride qualities, important questions raised during preliminary design activities will remain unanswered. The purpose of this study is to identify key aeroelastic vehicle dynamic features that can serve as possible metrics to predict pilot ratings during preliminary vehicle design activities. Further, procedures to systematically vary these features with stability & control derivative parameterization schemes during ground-based piloted simulation tests are given. Elastic Vehicle Dynamics and Control To capture the full aspects of simultaneous rigidbody and vibrational motion, and their interplay, an integrated modeling approach is required. Ref. 16 provides a modeling framework which is consistent with conventional aircraft dynamics. Application of this procedure leads to a nonlinear model, from which linear models can be extracted for dynamic analysis or flight control synthesis. Ref. 17 contains a model of this flavor, for a large, high speed, elastic vehicle. Configuration geometry consists of a low-aspect ratio swept wing, conventional aft tail, and small canard. A fifth order polynomial matrix realization for this linear model is given in Eq. (1). This model involves the small perturbation longitudinal dynamics of the closely spaced, effective short period and first aeroelastic modes. Rigid-body angle of attack and pitch rate are denoted as a and q = s9, while T| corresponds to the generalized aeroelastic mode coordinate. Further, pitch rate sensed by the pilot at the cockpit q is the response of interest. Control input consists of the elevator deflection 8g. The reference flight condition is level flight at Mach 0.6 and altitude 5,000 ft. Other parameters of interest in Eq. (1) are rigid and aeroelastic stability and control derivatives Z,,, M,,, F,,, total flight velocity Vj, and structural vibration frequency (0, damping £, and mode slope at the cockpit ())'. P(s) 0 -R(s) I z(s) y(s) QOO W(s) u(s) (1) z(s) = cc(s) 9(s) r,(s) , u(s) = 8E(s) , y(s) = q'(s)