Comprehensive investigation of rotor stall onset for future helicopters

Abstract

Helicopter main rotor operates in different aerodynamic environment in hover and forward flight conditions. In hover, downwash is almost steady and there is not significant amount of variation in aerodynamic loads. In forward flight, periodic aerodynamic fluctuations occur in local Mach number (the ratio of section speed to the speed of sound), flapping, lagging and feathering dynamics. The complex aerodynamic environment of a rotor in high-speed flight is tightly coupled with the aeroelastic and rotor dynamics characteristics. Since the aerodynamic loads are affected by local Mach number changes, cyclic pitch and cyclic flapping effects, it becomes harder to predict the stall boundaries. As the local blade sections observe high pitch fluctuations during forward flight, separations and loss of lift may occur. Unlike airfoil stall, where thrust typically decreases, in rotor stall, thrust may continue to rise post-stall, but a substantial increase in power demand is seen. Oscillatory loads increase on the rotor hub and pitch links. These force oscillations create undesired vibrations on the airframe affecting the passenger and pilot comfort as well as control effectiveness, handling qualities, fatigue life of critical components, limiting the lift and propulsion capability of the rotor. This boundary is called "Stall Onset". Determining this boundary at the preliminary design phase of a helicopter is crucial since it serves as a key indicator of rotor performance. This thesis investigates helicopter main rotor stall onset and the factors influencing the onset boundary such as fuselage parasite drag and lift. Comprehensive modeling is employed to accurately determine rotor stall onset. CAMRAD II (Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics) tool is utilized to structurally and aerodynamically model UH-60A Black Hawk main rotor. CAMRAD II, widely recognized in the aerospace industry, integrates multibody dynamics, nonlinear finite elements, structural dynamics and aerodynamics to perform nonlinear dynamic and static analyses. The UH-60A isolated main rotor model generated in CAMRAD II has 21 aerodynamic panels for each blade, 20 stations to define structural parameters and 40 stations to define blade mass properties. The geometry of rotor pitch control system (pitch links), rotor control system and non-linear lag damper are also integrated into the rotor model. The model is verified through comparisons with wind tunnel test results conducted within the scope of UH-60A Airloads Survey Program. The lift – propulsion plots along with the power – lift plots are compared with the wind tunnel test results for different shaft tilt angles and collective values. The comparison is made for 0.100 and 0.175 advance ratio values and both showed a satisfying correlation. Following the verification of the model, rotor maps are generated for different forward flight velocities, altitude and temperature values. The maps give information about the rotor lift and propulsion capacity since they include collective sweeps for different shaft tilt angles. Once the maps are generated for different advance ratios, oscillatory pitch link loads are probed from the maps, at the point where the helicopter weight (blade loading C_T/σ) and parasite drag (C_X/σ) intersects. The oscillatory loads at these intersection points are nondimensionalized with flight density, blade tip speed and rotor disk area to obtain C_PP. This nondimensional oscillatory load coefficient, C_PP is plotted against forward flight velocity and the divergence of the coefficient value as the forward speed increases, is observed. A linear trend for low speed is generated and the onset of stall is defined as four times of the linear trend. Another aspect to determine the onset boundary which is proposed by Lau for Lynx helicopter from its performance tests is utilized. Two methods to determine stall onset points show similar results. Finally, stall boundaries for the two methods are plotted on a blade loading (C_T/σ) vs advance ratio graph along with a comparison from literature data which includes McHugh rectangular blade stall and S70A main rotor stall. The boundary obtained with comprehensive modelling shows are good correlation with the data from literature. Looking forward, rotorcraft technology aims to achieve higher airspeeds by reducing drag and unloading the rotor by using lift-sharing wings on the rotorcrafts. Furthermore, future designs (FLRAA, S97, Bell 360 Invictus, Advanced AH-64 Compound) employ reduced drag fuselage designs together with propeller and wing to produce propulsive force and extra lift respectively. Lift-sharing wing geometries reduce the main rotor's load, C_T/σ, and improve overall aerodynamic efficiency. The fuselages with lower parasite drag shifts the rotor stall onset boundary outward. The effects of these design choices on the stall curve are thoroughly investigated, highlighting potential improvements in rotorcraft performance through aerodynamic refinements.