When local airflow exceeds Mach 1,
shock waves form on curved surfaces—such as rotor leading edges—causing significant wave drag. This normally occurs on curved areas, like cockpit windows, leading edges of the wing, and similar areas where
Bernoulli's principle accelerates the air. These shock waves radiate away a great amount of energy that has to be supplied by the engines, which appears to the aircraft as a whole as a large amount of additional drag, known as
wave drag. It was the onset of wave drag that gives rise to the idea of a
sound barrier. Helicopters have the additional problem that their rotors move in relation to the
fuselage as they rotate. Even when hovering, the rotor tips may be travelling at a significant fraction of the speed of sound. As the helicopter accelerates, its overall speed is added to that of the tips, meaning that the blades on the forward-moving side of the rotor sees significantly higher airspeed than the rearward-moving side, causing a
dissymmetry of lift. 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. One solution to the problem of wave drag is the same that was seen on 1950s jet fighters, the use of
wing sweep. This reduces the effect of wave drag without significant negative effects except at very low speeds. In the case of fighters, this was a concern, especially at landing, but in the case of helicopters, this is less of an issue because the rotor tips do not slow significantly, even during landing. Such swept-tips can be seen a number of helicopter types from the 1970s and 80s, notably the
UH-60 Blackhawk and the
AH-64 Apache. To prevent undesirable aerodynamic and inertial couplings caused by rearward shifts of the centre of gravity or
aerodynamic centre relative to the blade’s elastic axis, the blade tip is designed with a 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 forward offset creates a discontinuity, or ‘notch,’ in the blade’s leading edge, which further attenuates shock-wave strength on the swept tip. 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. 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. In fact, experience has shown that a swept tip blade can have an inferior stalling characteristic compared to the standard blade tip. == Programmes ==