A study of the aeroelastic behaviour of helicopter rotor blades featuring swept tips

Abstract

The design of a model helicopter rotor blade incorporating a swept tip is described. The swept tip is chosen to provide a coupling between the lift at the tip and the blade twist thus achieving a variation of blade tvist both with azimuth and with forward speed. The design is the first stop in an investigation of aeroelastic tailoring as a means of reducing helicopter vibration and increasing rotor performance. The first prototype blade encountered stability problems and further designs were evaluated using a new modes/stability computer program developed within RAE. Comparisons are made between the stresses measured on a second stable swept tip blade and a dynamically similar rectangular blade. The results show that a beneficial twisting of the swept tip blade is achieved which enhances rotor perforawtnee and reduces the flatvise bending and torsional moments. Comparisons are made between the experimental results and those predicted by the RAE/VOL coupled modes analysis. The predictions are. reasonably accurate for the flatwise moments but a more representative model of the hub is needed to produce acceptable predictions of the torsional moments. The design of a further set of blades is discussed, the aim being to investigate the effects of introducing a strong coupling between the flap and torsion notions of the blade by sweeping back the shear centre. An analysis of-the results shows that there are large gains in blade stability with no severe adverse effects on blade loads. A theoretical investigation has been undertaken to observe the effects of tip sweep on the performance of a full size rotor. The results show that aft tip sweep can reduce control loads and rotor power for a rotor with a cambered aerofoil, section.