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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. == How it works == 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. 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. 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 aerofoil 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「BERP rotor」の詳細全文を読む スポンサード リンク
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