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Meeting	ACGS Committee Meeting 99 - Boulder - March 2007
Agenda Location	9 SUBCOMMITTEE D – DYNAMICS, COMPUTATIONS AND ANALYSIS 9.1 Modeling and Control Challenges for Air-Breathing Hypersonic Vehicles
Title	Modeling and Control Challenges for Air-Breathing Hypersonic Vehicles
Presenter	Dave Doman
Affiliation	AFRL
Available Downloads*	presentation
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Abstract	Ongoing research is exploring the challenges of controlling air-breathing hypersonic vehicles. Subscale flight vehicles such as the X-43 have flown and demonstrated the ability to develop positive net thrust at hypersonic speeds. In spite of the success of these subscale tests, the development of full scale vehicles is dependent upon advances in materials and the ability to control an integrated airframe propulsion system. Full scale vehicles will be more difficult to control due to the significant interactions between the aerodynamics, propulsion system, and structural dynamics. The presentation will expose some modeling and control challenges associated with air-breathing hypersonic vehicles. We have developed a model, based on first principles, that captures many of the salient features of this class of vehicle. Not unlike earlier control oriented models, our model predicts that full scale scramjet powered aircraft will be characteristically unstable in pitch and will exhibit non-minimum phase behavior between many important inputs and outputs. Our model; however, relaxes a number of simplifying assumptions of past control oriented models in order to address several unanswered questions regarding the controllability of this class of vehicle. The model of described in the presentation accounts for mass-flow spillage effects that occur as a result of the engine operating in off-design conditions when the bow shock does not impinge on the inlet lip. Off-design conditions will commonly be encountered as the aircraft structure oscillates because the bow shock angle will change as the structure deforms. Operation under off-design conditions affects the aerodynamic forces and moments, as well as the thrust. The nonlinear vehicle model makes use of oblique shock theory and Prandtl-Meyer flow theory (i.e., gas dynamics) to calculate the aerodynamic forces. Reflected shocks are modeled in the scramjet diffuser, while the nozzle is assumed to be isentropic. A fuel flow model is used in the combustor section, which is modeled using Rayleigh flow (1D compressible flow with heat addition.) Because the aircraft structure represents a relatively small amount of the total vehicle weight when it is fully loaded with fuel, the structural dynamics for this class of vehicle play an important role. Typically, the frequency of the fuselage first bending mode, when the aircraft is fully loaded, is low enough to interact with the flight control system. As the aircraft flies its mission, the reduction of the aircraft's weight will cause the frequencies to increase; however, this is offset a small amount (2-3\%) by the aerodynamic heating that propagates into the load bearing vehicle structure. One of the most significant issues associated with the vehicle structural dynamics is that the oscillating forward fuselage changes the pressure distribution over the forebody of the aircraft, which causes a number of undesirable natural loop closures to occur. The fuselage deflection is dependent upon the flight condition (Mach number, altitude, and angle-of-attack) since this determines the pressure distribution on the vehicle. Furthermore, the deflection of the forward fuselage changes the apparent turn angle of the flow. Therefore, during unsteady flight, the resulting changes in the pressure distribution over the aircraft are realized downstream as perturbations in the thrust, lift, drag, and pitching moment. In addition, unsteady aerodynamic effects resulting from the oscillating structure influence the forces and moments on the vehicle as a result of local pointwise pressure variations. Methods to enhance quasi-steady aerodynamic models by using nonlinear piston theory to account for these unsteady aerodynamic effects without incurring a large computational burden are currently being incorporated into control oriented models for study. Recent work in the control of scramjet powered hypersonic vehicles has proceeded along two fronts: control synthesis and vehicle configuration design for control. The first approach involves determining an acceptable control methodology that can extract a reasonable level of maneuvering performance from an unstable non-minimum phase aircraft configuration. A pseudo dynamic inversion controller was developed that decouples the system but leaves the right-half plane zeros intact in order to avoid cancellation of unstable zeros with unstable poles. The second approach involves determining how to modify the vehicle configuration to make it more amenable to the application of control technology. One of the principal problems with tail controlled hypersonic vehicles is the presence of low frequency, non-minimum phase transmission zeros that limits the performance of any feedback control method. The primary cause of this phenomenon arises because when the vehicle is trimmed, a substantial portion of the total lift comes from the elevons. When a change in lift is desired, the elevons must dump lift in order to generate a nose-up pitching moment that, over time, leads to an increase in lift as a result of increased forebody angle of attack. This phenomenon is directly responsible for the low frequency unstable zeros and also puts the vehicle center of rotation several feet in front of the nose of the vehicle. By coupling the elevator and canard via a simple interconnect gain, it was found that substantial increases in the unstable zero frequencies could be achieved. The result is that with a simple configuration change, one may substantially improve the ability of any control technique to produce an acceptable flight path speed of response. Other factors such as low excess power and limited control power are also being considered in the model and control design. These issues ultimately limit speed of response; however, a major fundamental control limitation has effectively been eliminated by a simple configuration change. This case study is an example of how the work of flight control engineers can interject control requirements to vehicle designers early in the design process. Because of the multi-disciplinary nature of scramjet vehicle design, this type of interaction will be critical to achieving successful full scale vehicle designs.