Envelope protection system using flap hinge moment measurements

The purpose of this investigation was to explore the feasibility of utilizing flap and control surface hinge moments to provide stall warning for flight vehicles. The ultimate goal would be to provide a robust flight envelope protection system. Hinge moment data were acquired for a flapped NACA 3415 airfoil model in the 3-ft × 4-ft wind tunnel at the University of Illinois at Urbana-Champaign. Hinge moments were acquired for the clean model, as well as for six contaminated airfoil configurations. These contamination configurations included both leading-edge glaze and rime ice configurations, two levels of leading-edge roughness, and both 3D leading-edge damage and 3D upper-surface damage cases. Aerodynamic testing was performed at Reynolds numbers of 1.8 million and 1 million for five different flap deflections. Additional data were acquired for the NACA 3415 model with a trim tab for the clean and glaze-ice configurations. The resulting steady and unsteady hinge moment measurements were sensitive to flow separation over the surface of the flap as the airfoil approached the stall angle of attack. Using the acquired hinge moment measurements, a series of three detector functions were developed that operated on the hinge moment signal to predict airfoil stall under the clean and contaminated configurations. The detection algorithms used a threshold-based approach to provide estimated flight envelope boundaries. The three detector functions provided redundancy in envelope prediction and were averaged to eliminate potential outliers within one of the detector functions. The stall warning boundary could be set 1° to 4° prior to stall, and stall warnings were usually provided within ±0.7° angle of attack the prescribed boundary.