P MAHENDRANATH

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DESGINE FEATURES OF BERP ROTOR BLADE

Introduction What is the BERP blade Earlier blade How does it work and useful Geometric description of blade Structural optimization of blade Aerodynamic Features with BERP rotor blade Aerodynamic Characteristic Feature Developments Conclusion

Overview

Introduction Helicopter performance in terms of lift and maneuverability is basically

depends on the design of the rotor blades such as length of the blade.

And it also depends on the selection of airfoils and blade material. A suitable and successful design configured blade gives you an effective

aerodynamic parameter as follows. High lift coefficient Low drag (all along the blade) High drag divergence Mach number at the tip Camber to give a high CLmax Good lift to drag ratio

The BERP blade employs a unique geometry that performs high-speed forward flight by achieving all above parameters effectively.

BERP blade stands for the British Experimental Rotor Program (BERP) Blade

which is also known as paddle-type tip blade where the blade having

sweepback shape at tip portion

The intension of inventing BERP blade was to increase the helicopters lifting-

capability and maximum speed using new designs and materials.

What is the BERP blade?

Earlier BERP The blade design development had been made in several stages as

BERP I (1975-1978 composites 5% reduction in burn) BERP II (1978-1980 Changes in tip shape and airfoil selection)

BERP III (1982-1985 swept blades)BERP IV(From 1997 sweepback tip with suitable airfoil section)

Finally the BERP IV program launched with an objective of providing wide ranging benefits across all aspects of aircraft performance and cost.

Sweepback tip can reduce compressibility effects in forward flight. 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.

How does it work ?

As per Navier-Stokes equations the "notch" actually helps to 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.

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.

How does it work ?

A normal shock wave produced at rotor tips due to high speed become weaker by sweep geometry with notch portion on the blade.

Thus it reduces the wave drag even in high speeds. It also reduces the tip losses due to down wash which causes to consume the helicopter power.

BERP 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 as shown.

The following picture shows the geometric shape of the BERP blade and alignment of RAE airfoils along span.

Geometric description

The optimization process approximates a generic blade structure using a small number of design variables to define the geometry of key structural components .

A modal analysis provides the blade dynamic properties and a comprehensive aero elastic analysis provides the rotor dynamic response to aerodynamic and structural loads.

Structural Optimization of the Blade

An earlier exercise in development of blade characterizing a number of new fiber/resin combinations against the existing material suite used for the Lynx and AW101 rotor blades.

This process sought to adopt a new material system that provided both technical and commercial advantages than a convention helicopter.

An initial testing confirmed adequate adhesion properties and acceptable rain endurance for the initial trials flying planned, but also indicated the criticality of adherence to the surface preparation procedures.

Material used for BERP blade

The following figure shows a standard design model of the blade as it contain a number of features that may be undesirable in a future production blade.

A standard design model of BERP blade

Hovering: Work carried out during BERP study, which included numerical analysis and model rotor wind tunnel testing of various blade designs with overall twist values ranging from 8° to 18° and result the following hovering performance graph.

Aerodynamic Features with BERP rotor blade

Forward Flight: The BERP specifically designed to meet the conflicting aerodynamic requirements of the advancing and retreating blade conditions, either of which can limit the CL /CD of the blades and performances of the rotor in high-speed forward flight.

Taking into account the aerodynamic and dynamic design changes between conventional rotor blade and BERP IV as shown.

NACA0012 is well known airfoil series for high lift and less drag at higher Mach number values.

The lift and drag coefficient curves shown in the following figure at specific boundary conditions.

Lift and Drag curve over NACA 0012 airfoil:

To produce high lift coefficients, we require very negative pressures on the upper surface of the airfoil.

A key feature of the BERP blade was that it also allowed the use of high lift aerofoil’s over the outer regions of the blade because of its unique shape.

And also the observation from NACA and BERP blade drag coefficient curve concluded that BERP or sweepback tip blade produces less drag values of 0.04 for specified boundary conditions were NACA0012 and NACA0015 airfoils encounters higher drag coefficient values as angle of attack varies.

Lift and Drag curve over RAE 2228 airfoil

As compared to BERP rotor blade configured helicopters, none of the existing conventional models were exceeded the BERP IV rotor features, nor was the flight trial limited in any way by these other constraints.

This is due to the high degree of consistency in aerodynamic and dynamic characteristics between blades.

Rotorcraft had contributed efficient aerodynamic characteristics by delaying the compressibility effect.

Effect of BERP IV on Rotorcraft

The successful implementation of BERP blade concluded an attention towards the development of more advanced airfoil sections in RAE series so that they could make the blade to achieve enhanced aerodynamic characteristics.

And also the intension in development towards improving product economically and life cycle cost, including reduced design complexity, increased design robustness and improved production quality.

Feature Developments

Numerical aerodynamic characteristics shows that stated design objectives are completely fulfilled.

Analytical study BERP rotor blade indicate that the aerodynamic performances of RAE Series airfoil are on the same level or even better than earlier generation advanced rotor blade airfoils like NACA series and also design model confirmed achievement and exceed of performance goals, which are summarized as: When compared with the conventional rotor, the BERP-type rotor offers

no performance improvements in either hover or forward flight. Thus Reduction of hover power by approximately 5%

Reduction of cruise power, with approx 10-15% saving An improved blade stall envelope of a least 10 knots

Conclusion

Questions ?