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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 (RAE), 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 worksEdit
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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 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.
The initial programme, BERP I, studied the design, manufacture and qualification of composite rotor blades. This resulted in producing new main rotor and tail rotor blades for the Westland Sea King. Following on from the first, the second programme, BERP II, analysed advanced aerofoil sections for future rotor blades. This fed into the BERP III programme.
BERP III designs have a notch toward the outer end of the rotor blade, with a greater amount of sweepback from the notch to the end of the blade compared to inboard of the notch. BERP III culminated in a technology demonstration on a Westland Lynx helicopter. In 1986, a Lynx specially modified registered G-LYNX and piloted by Trevor Egginton set an absolute speed record for helicopters over a 15 and 25 km course by reaching 400.87 km/h (249.09 mph). Following the successful technology demonstration, the BERP III blade went into production.
BERP IV uses: a new aerofoil, revised blade tip shape, and increased blade twist. 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." Additionally "Rotor hub loading has been found to be the same or less than with the BERP III blade now fitted to the EH101" helicopter. To prevent leading edge erosion the blade will use a rubber-based tape rather than the polyurethane used on UK navy Sea Kings. Under test it was found to last five times longer, 195 minutes vs 39 min. The programme ended in August 2007
Applications of BERP technologyEdit
Current applications are:
- BERP III:
- BERP IV:
- "Martin Lowson (Obituary)" (Preview only). London: The Times. 12 August 2013. Retrieved 27 November 2015.
- J. Gordon Leishman "ENAE 632 - The British Experimental Rotor Program (BERP) Blade Archived 2007-08-21 at the Wayback Machine", University of Maryland, College Park, Retrieved 11 April 2010
- Harrison, Stacey, Hansford "BERP IV The Design, Development, and Testing of an Advanced Rotor Blade" American Helicopter Society 64th Annual Forum, April 29 – May 1, 2008
- Coppinger, Rob (22 May 2007). "BERP IV gives Merlins more payload". flightglobal.com. Retrieved 27 November 2015.
- Brocklehurst, Alan. AIAA-1990-3008, "Experimental and numerical study of the British Experimental Rotor Programme blade". AIAA, 1990.
- The British Experimental Rotor Program (BERP) Blade
- Air Vectors: The Westland Scout, Wasp, & Lynx
- "Fastest Blades in the World", 27 December 1986 Flight International article on the Lynx's BERP rotor