BERP rotor

The BERP rotor blade design was developed under the British Experimental Rotor Programme. The initial BERP rotor blades were developed in the late 1970s to mid-1980s as a joint venture programme between Westland Helicopters and the Royal Aircraft Establishment, with Professor Martin Lowson as a co-patentee. The goal was to increase the helicopters lifting-capability and maximum speed using new designs and materials.

If we wish to reduce compressibility effects in forward flight, we can use sweep on the tip of a rotor blade. Many modern helicopters use some form of simple sweepback on the blade tip. Examples are the UH-60 Blackhawk and the AH-64 Apache.

However, so we don't get centre of gravity or aerodynamic centre movements aft of the blade elastic axis (which can introduce undesirable aerodynamic and inertial couplings), then the tip must be configured with an area shift forward. This can be kept to a minimum by recognizing that the Mach number is varying along the blade so we do not have to use a constant sweep angle, thereby minimizing the amount of forward area shift.

The methodology used in the design of the BERP blade ensures that the effective Mach number normal to the blade remains nominally constant over the swept region. The maximum sweep employed on the large part of the BERP blade is 30 degrees and the tip starts at a non-dimensional radius r/R=cos 30 = 86% radius. The area distribution of this tip region is configured to ensure that the mean tip centre of pressure is located on the elastic axis of the blade. This is done by offsetting the location of the local 1/4-chord axis forward at 86% radius.

This offset also produces a discontinuity in the leading edge (referred to as a notch), which results in other interesting effects. For example, recent calculations using a CFD code based on the Navier-Stokes equations, has shown that this "notch" actually helps to further reduce the strength of shock waves on the blade. Thus, an unexpected by-product of the notch over and above the basic effect of sweep is to help to reduce compressibility effects even further.

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