A discussion of measured static and dynamic rotor loads during testing of the ERICA tilt-wing rotorcraft configuration in DNW-LLF wind tunnel

Abstract

A heavily instrumented 1:5 scaled model of the ERICA tilt-wing configuration has been tested in the DNW-LLF wind tunnel as part of the EU co-funded NICETRIP project. Tests were made for a variety of conditions ranging from pure helicopter and conversion corridor cases up to a low speed aircraft mode, with appropriate changes in tilt rotor and outer wing pitching angles. In total over 400 test conditions were measured. Rotor forces and mo-ments have been measured with rotor balances. Blade bending moment and torsion as well as rotor shaft bend-ing and torque were measured with calibrated strain gauge sensors. Measurements were made for fully trimmed and for non-trimmed conditions. The four bladed rotors are counter-rotating, are rigid in plane and allow cyclic blade pitch control through a remote controlled swash plate. Since cyclic pitch control caused damage to the blade pitch bearings, most of the test conditions had to be done without cyclic blade pitch control, leading to a large in-plane moment in the rotor plane and high bending moments in the rotor shaft. Some test cases were both measured with and without cyclic pitch control. The present paper analysis the rotor induced forces, moments and nacelle vibrations for the trimmed conditions. Both steady and dynamic contents of the signals are analyzed. It is found that with cyclic blade pitch control the time mean rotor in-plane forces and moments and the 1/rev rotor shaft and blade bending moments are effec-tively suppressed, but the dynamic components of the rotor in-plane forces and moments become larger than without cyclic blade pitch control. Without cyclic blade pitch control the 1/rev blade and rotor shaft bending mo-ments become quite large and, rather surprisingly, for some of these test conditions also a large 2/rev blade bending moment is observed. Careful analysis of the data suggests that the 2/rev flap bending is caused by a 1/rev excitation (in the non-rotating system) of both nacelles, caused by the proximity of the nominal rotor rpm frequency to symmetric and anti-symmetric nacelle torsion eigen-frequency and probably also in combination with other nacelle related eigen-modes. The measured data are well suited to validate or verify existing semi-empirical or CFD methods to predict the steady and unsteady loads of the rotors. A correlation between rotor dynamics and (outer) wing dynamic loading and the influence of the outer wing and/or flaperon setting on the rotor forces and moments is left for future anal-ysis.