USAAVLABS tandem rotor airloads measurement program.

An Army CH-47A helicopter has been extensively instrumented and flight-tested to measure the steady and dynamic loads on the rotors, controls, and airframe. The test conditions included high blade loadings and a range of flight speeds from hovering through transition and up to the rotor stall-limited speed. Measurements of a comprehensive array of blade airload pressures and the resulting blade bending, rotor hub motions, rotor shaft loads, control loads, and fuselage response have been obtained. Automatic analog-to-digital data processing techniques were utilized. Initial results of this program are presented which show significant airload pressure variations due to rotor tip vortex interference. Blade section pressures can differ considerably from those of a two-dimensional airfoil over significant areas of the rotor disk, apparently due to the combined effects of compressibility and the disturbances from tip vortex interference. Integrated airload pressure data show that the azimuthal average loading on the forward rotor of a tandem is nearly constant with radius, apparently due to the upwash caused by the aft rotor. A suggested approach to reducing rotor blade airloads data to synthesized airfoil section data is presented.

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