Robust lateral control of hypersonic vehicles

The unique configuration of hypersonic vehicles in terms of aerodynamics and propulsion systems causes specific flight mechanics effects which are not existent with conventional airplanes. In particular, problems related to stability and control deficiencies at high Mach numbers are concerned. These include instabilities, coupling effects as well as a reduction in control power. The proposed paper focuses on aspects of lateral dynamics and control up to Mach numbers of seven, characterized by an unstable dutch roll, a very high roll-yaw coupling and a weak roll damping. For certain configurations another oscillatory, partially unstable mode, the socalled lateral phugoid, appears. A criterion to determine its existence is presented. Multi-Model Eigenstructure Assignment is utilized to design a robust fixed-gain controller that can guarantee robust stability and the desired flying qualities along the complete reference trajectory. Requirements concerning the presence of satisfactory control power to ensure robust stabilization are also taken into account. The robustness of the designed controller against multiple model uncertainties is proven by μ-Analysis. Finally, the control system is evaluated by nonlinear closed-loop simulation, taking time delays, deflection and rate limitations of the control surfaces, external disturbances and model deviations into account.