Expert: Paul Soderman - 10/6/2011

Question
Hello Soderman. I am a student of aeronautical engineering in Argentina. My question is how to measure in the wind tunnel the Chdelta and Chalpha parameters to plot the curves for NACA WRL 663. Sorry for my english. Thanks. Regards

Answer
Sebastian
Chdelta is the change of hinge moment with flap angle.  Chalpha is the change of hinge moment with angle of attack.  These parameters are determined from plots of hinge moment versus angle. Hinge moments are typically measured using torque tube sensors.  The sensors support the control surface such as a flap and allow it to rotate while measuring moments or torque induced by the aerodynamic loads.  The sensor is basically a strain gauge balance that measures a change in electrical signal caused by torque.  See W. Rae and A. Pope: Low-Speed Wind Tunnel Testing for details. If torque tubes are not available, researchers have also found hinge moments by integrating surface pressures measured chordwise on the flap upper and lower surfaces, but that is probably not as accurate.
Paul