The objective of this effort was to show that real-time aircraft control-surface hinge-moment information could be used to provide a robust and reliable prediction of vehicle performance and control authority degradation. For a given airfoil section with a control surface — be it a wing with an aileron, rudder, or elevator — the control-surface hinge moment is sensitive to the aerodynamic characteristics of the section. As a result, changes in the aerodynamics of the section due to angle-of-attack or environmental effects such as icing, heavy rain, surface contaminants, bird strikes, or battle damage will affect the control surface hinge moment. These changes include both the magnitude of the hinge moment and its sign in a time-averaged sense, and the variation of the hinge moment with time. The current program attempts to take the real-time hinge moment information from the aircraft control surfaces and develop a system to predict aircraft envelope boundaries across a range of conditions, alerting the flight crew to reductions in aircraft controllability and flight boundaries.

The concept was tested across a wide range of conditions and observed in-flight contamination, and a system and methodology of using the hinge-moment information to predict sectional airfoil stall in the presence of these contaminants was developed. An experimental test program was designed to provide the broadest test of the hinge moment monitoring concept. A NACA 3415 airfoil section with a 25-percent chord flap was tested with a series of simulated aerodynamic contaminants. These contaminants were designed to provide a range of simulated environmental and structural hazards, which would produce varying degrees of performance degradation, primarily in the form of premature stall and loss of maximum lift. These simulated cases included both leading-edge glaze and rime ice, both moderate and severe leading-edge roughness, and both a simulated 3D leading-edge and a simulated upper surface damage case.

Data from the experimental tests were used to develop a stall prediction methodology and detection algorithm based on the unsteady hinge moment results. The stall detection algorithm provided a warning of stall several degrees prior to actual stall. In this way, the envelope monitoring system can alert the flight crew to the current aircraft envelope boundaries for both longitudinal and lateral control.

The system uses a combination of three separate detection algorithms based on the unsteady hinge moment signal to provide a warning at a preset number of degrees prior to stall. Results from the three algorithms are averaged to provide a single warning prediction. The averaging of the three separate algorithms provides a level of redundancy in the calculation and can also be used as a measure of the confidence of the stall boundary warning prediction. For the majority of the cases, the detection algorithm produced a warning within ±0.7° of the set boundary value. There appears to be sufficient signal to provide a stall warning boundary out to approximately 4° prior to stall. Output from the detector function for the range of shown contaminations collapses onto a single curve, as a function of the angle-of-attack prior to stall. By collapsing onto a single curve, the developed detector function-based system can use a simple threshold approach to set a variable warning boundary, up to several degrees prior to stall.

This work was done by Michael Kerho of Rolling Hills Research Corp. and Michael B. Bragg and Phillip J. Ansell of the University of Illinois at Urbana-Champaign for Dryden Flight Research Center. DRC-010-014