Linearly variable chord-extension morphing for helicopter rotor blades
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ORIGINAL PAPER
Linearly variable chord‑extension morphing for helicopter rotor blades Rohin K. Majeti1 · Berend G. van der Wall1 · Christoph G. Balzarek2 Received: 26 June 2019 / Revised: 28 August 2020 / Accepted: 25 September 2020 © The Author(s) 2020
Abstract A new morphing concept called linearly variable chord extension was studied for its effectiveness in improving the efficiency of a helicopter rotor. Apart from chord extension itself, an additional feature which is deflection of the extended part of the chord resulting in an effective camber and additional twist to the airfoil, is also studied for its effect on rotor efficiency improvement. Trim analyses were carried out for various chord-extended rotors for hover as well as various forward flight velocities using DLR’s in-house comprehensive analysis code S4. Chord extension of up to 100% and chord-extension–deflection of up to 15° were considered. Results show that the linearly variable chord-extension concept is effective in reducing power requirement in both hover and forward flight. Deflection of the extended chord also helps reduce power requirement in hover, especially at higher blade loadings. However, the root torsional moments and hence, the pitch-link loads are seen to increase substantially for the morphed rotors. Keywords SABRE · Rotor blade morphing · Helicopter rotor efficiency · Variable chord extension morphing List of symbols BL Baseline cg Center of gravity FM Rotor figure of merit c Rotor blade chord length, m Cm Airfoil moment coefficient Cn Airfoil normal force coefficient Cx Airfoil chord force coefficient CP Rotor power coefficient CT Rotor thrust coefficient EIη, Flap stiffness, Nm2 EIζ Lag stiffness, Nm2 f Rotor natural frequency, Hz FPL Pitch-link load, N GJ Torsional stiffness, Nm2 Mx Component of Mach number along the chord line Mz Component of Mach number normal to chord line P Required rotor power, kW r Rotor blade radius variable, m * Rohin K. Majeti [email protected] 1
Institute of Flight Systems, German Aerospace Center (DLR), Brunswick, Germany
Institute of Composite Structures and Adaptive Systems, German Aerospace Center (DLR), Brunswick, Germany
2
r̄ Non-dimensional rotor radius variable rhinge Radial location of chord-extension hinge, m rθ Polar radius of gyration, m R Radius of rotor, m XCG Chordwise location, center of gravity of Airfoil, m XSC Chordwise location, shear center of Airfoil, m α Angle of attack, ° 𝜆 Non-dimensional inflow 𝜇 Advance ratio 𝜎 Rotor solidity 𝜃0 Rotor collective angle, ° 𝜔 Rotor natural frequency, rad/s 𝛺 Rotor rotational speed, rad/s 𝛺ref Reference rotor rotational speed, 44.4 rad/s
1 Introduction The main rotor is the most important sub-component in helicopters, because it provides lift, propulsion and control. It is designed to operate in different flight regimes under complex aerodynamic conditions. It is a challenge to design the rotor to operate efficiently in all flight regimes from hover to fast forward flight and high-g maneuvers
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