AN INVESTIGATION OF THE AERODYNAMICS OF AN RAE SWEPT TIP USING A MODEL ROTOR

The interest at the RAE in swept tips for helicopter rotor blades started in 1972 following the work of Caradonna on a method for predicting the pressure distribution over a blade tip in supercritical flow. At the RAE, Grant began to develop a similar method which was valid for general planform shapes and a range of azimuth angles. Although time dependent methods appeared elsewhere, the RAE method did not include the time dependent terms because of the associated large increase in computing time. At ONERA, an interest in blade tip aerodynamics also began about 10 years ago and led to experimental investigations of pressure distributions on non-lifting rotors with both straight and swept tips. These fundamental studies have resulted in the design by ONERA of a sweptback parabolic tip. In parallel with these activities, the techniques for detailed pressure plotting of rotor blades in flight were being developed at the RAE. With this common interest and the complementary activities in mind, a collaborative programme of work was set up between France and the UK which was to involve model rotor tests at ONERA on a swept tip of RAE design and flight experiments at the RAE, using a Puma helicopter, with a further tip design. The two tip planforms are shown in Fig 1. For the Puma blade tip it was essential to keep the centre of pressure on the torsional axis of the blade, which accounts for the leading-edge extension, but this was not necessary for the very stiff model rotor blades. The flight tests with the Puma are just beginning and it is hoped to report on that work at next years Forum. This paper will describe the model rotor tests carried out in the ONERA S2 wind-tunnel at Chalais Meudon.