Experimental Investigation ofthe Hovering Performance of aTwin-Rotor Test Model

Hover performance of a twin-rotor test model in terms of rotor overlap sweep, blade collective pitch, and blade tip speedwasexaminedexperimentally.The experimental setup consisted of two three-bladed rotors (tandem rotor configuration) with a diameter of1,220 mm and constant chord of 38 mm, giving a blade aspect ratio of 16.05. The blades were of a rectangular planform with NACA 0012 cross-sections with no twist or taper. In this model, the front rotor was fixed on the fuselage and the rearrotor could move longitudinally for tests up to about40% rotor overlap sweep.To accurately examine the hover performance, thrust, power required, power loading (PL), and figure of merit (FM) responses were measured usinga central compositetest plan. Furthermore, four quadratic polynomialswere fitted to all responses, necessary for a better understanding of the main effects and interactions. The results clearlyshowed that significant interactions between variables are evident and therefore overlapping at constant collective pitch reduces thrust much more than reducingthe power required. Moreover, the results showed that, for the twin-rotor system,the maximumefficiency in hover (i.e., maximum power loadingof 14.6kg/kW) is obtainedfor no overlapped rotors at low values of disc loading and blade tip speed.Experimental measurements of thetwin-rotor hover performance based on a central composite plan andinteraction analysis were the main contributionsstated in the current work.Results for the twin-rotor test model can be generalized to actual tandem helicopters through the Reynolds number transformation technique and some modifications.