Envelope Protection System Using Flap Hinge Moment Measurements
Phillip J. Ansell and Michael B. Bragg
University of Illinois, Urbana, Illinois, 61801
and
Michael F. Kerho
Rolling Hills Research Corporation, El Segundo, California 90245
ABSTRACT
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 deg. prior to stall, and stall warnings
were usually provided within ±0.7 deg. angle of attack the prescribed boundary.