Effect of elastic rotor modelling on helicopter rotor performance and control

Abstract

Aim of this study is to investigate the effect on rotor blade elasticity on rotor performance and control angles at hover and forward flight conditions by comparing elsatic rotor solutions to rigid rotor solutions at same flight conditions. Helicopters are air vehicles that can fly at zero airspeed(hover), forward, sideward and even backwards, unlike fixed wing aircrafts which can only fly forward with a minimum flight speed that is defined as stall speed. Helicopters ability to fly like this comes from its main rotor, which is essentially consists of number of wings rotating along a shaft axis By this way, helicopter rotor blades can produce lift, regardless of helicopter flight speeds at any direction. Unlike a fixed wing aircraft, both lift, control and propulsion is happening at main rotor, whereas fixed wing aircrafts produce lift via their wings and produce thrust via their jet engines/propellers. Unique abilities of helicopter rotor come from its ability to control individual blade pitch through azimuth and flapping ability of rotor blades. However, availability of these functionalities requires many parts, which adds cost, design complexity and maintenance problems compared to fixed wing aircraft. Also, rotation of main rotor and complex aerodynamic interactions causes higher vibrations than fixed wing counterparts. Analysis of helicopter rotor is a complex topic due to many moving parts, rotor aerodynamic interactions and blade elasticity. Rotor blades rotate at shaft axis and flaps at flap hinge and has lead-lag motion at lead-lag hinges and also rotor blade pitch varies with azimuth with applied cyclic controls. Since all of this motions changes lift of rotor blade and flapping motion is caused by lift, problem cannot be separated into two different parts and all of rotor lift and rotor dynamics must be solved together. Rotor aerodynamic solution is a complex topic, especially in forward flight where rotor blade tips can approach speed of sound and experiences compressibility related problems at advancing side, and same blade can experience blade stall at retreating side, all in one revolution, periodically. Elastic deformation of rotor blade is dependent all of the mentioned parameters above, and has an effect on both rotor aerodynamics and blade motion. To solve these problems and compare rigid and elastic rotor blades, a blade element momentum theory code in MATLAB is developed and isolated rotor results is validated with FLIGHTLAB rotorcraft analysis tool at same fidelity and at same flight conditions. Then elasticity is implemented into written code and result of rigid and elastic rotor is compared for different forward flight conditions. In the end, results show that effect of elastic rotor is minimal on power requirement for same thrust, however, achieving same lift and propulsive forces requires different control inputs.