ENAE 632 --The British Experimental Rotor Program (BERP) Blade
What is the BERP blade?
- The BERP blade was the result of ten-years (1976-1986) of aerodynamic research collaboration between Westland Helicopters and the Royal Aircraft Establishment.
- This research paid off on 11th. August 1986 when the Westland company demonstrator Lynx (G-LYNX) attained the world absolute speed record for any helicopter, which remains in place more than ten years later. The Lynx achieved an average speed over a 15km course of 249.1 mph (400.87 kph), which broke the previous record of 228mph (367 kph). This high speed could not have been achieved without the use of the BERP blade.
- The confidence that has since been established with the BERP blade means that this technology is now being applied to other helicopters. For example, the BERP blade will be standard equipment on the Lynx III battlefield helicopter and the new EH-101, the latter which is being developed jointly by Westland and Agusta.
Photograph of the BERP blade on a Westland Lynx (G-LYNX). (Photograph courtesy of GKN/Westland).
How does it work?
- 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 center of gravity or aerodynamic center 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 center 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 diskontinuity 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.
- We must also recognize that a swept tip geometry of this sort will not necessarily improve the performance of the blade at high angles 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.
- The BERP blade employs a final geometry that performs as a swept tip at high Mach numbers and low angles of attack, yet also enables the tip to operate at very high angles of attack without stalling. 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.
- Because the leading edge is so highly swept, this leading edge separation develops into a stable vortex structure which rolls around the leading edge and eventually sits over the upper surface (as on a delta wing aircraft). This mechanism is enhanced by making the leading edge of the airfoil in this region relatively sharp.
- 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.
- Evidence has shown that a strong "notch" vortex is also formed, which is trailed streamwise across the blade. This vortex acts like an aerodynamic fence and retards the flow separation region from encroaching into the tip region. Further increases in angle of attack make little change to the flow structure until a very high angle of attack is reached (in the vicinity of 22 degrees!) when the flow will grossly separate. For a conventional tip planform, a similar gross flow breakdown would be expected to occur at about 12 degrees local angle of attack.
- Therefore, the BERP blade manages to make the best of both worlds by reducing compressibility effects on the advancing blade and delaying the onset of retreating blade stall. The net result is a significant increase in the operational flight envelope.
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