BERP rotor

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 (RAE), with Professor Martin Lowson as a co-patentee.

This requires changes in the angle of attack of the blades to ensure the lift is similar on both sides, in spite of the great differences in relative airflow.

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.

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.

We must also recognize that a swept tip geometry of this sort will not necessarily improve the performance of the blade at high angle of attack corresponding to the retreating side of the disk.

This latter attribute was obtained by radically increasing the sweep of the outermost part of the tip (the outer 2% approximately) to a value (70 degrees) where any significant angle of attack will cause leading edge flow separation.

As the angle of attack is increased, then this vortex begins to develop from a point further and further forward along the leading edge, following the planform geometry into the more moderately swept region.

At a sufficiently high angle of attack, the vortex will initiate close to the forward most part of the leading edge near the "notch" region.

For a conventional tip planform, a similar gross flow breakdown would be expected to occur at about 12 degrees local angle of attack.

[3] In 1986, a Lynx specially modified registered G-LYNX set an absolute speed record for helicopters over a 15 and 25 km course by reaching 400.87 km/h (249.09 mph).

After 29 hours of testing it has been found to, "improve rotor flight-envelope performance, reduce power needs in hover and forward flight, ... decrease airframe and engine vibration for a range of take-off weights.