AN INVESTIGATION OF PERFORMANCE BENEFITS AND TRIM REQUIREMENTS OF A VARIABLE SPEED HELICOPTER ROTOR

Open Access
Author:
Steiner, Jason Henry
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Farhan Gandhi, Thesis Advisor
Keywords:
  • helicopter performance
  • Variable rotor speed
  • trim requirements
Abstract:
This study primarily examines the main rotor power reductions possible through variation in rotor RPM. Simulations were based on the UH-60 Blackhawk helicopter, and emphasis was placed on possible improvements when RPM variations were limited to ± 15% of the nominal rotor speed, as such limited variations are realizable through engine speed control. The studies were performed over the complete airspeed range, from sea level up to altitudes up to 12,000 ft, and for low, moderate and high vehicle gross weight. For low altitude and low to moderate gross weight, 17-18% reductions in main rotor power were observed through rotor RPM reduction. Reducing the RPM increases rotor collective and decreases the stall margin. Thus, at higher airspeeds and altitudes, the optimal reductions in rotor speed are smaller, as are the power savings. The primary method for power reduction with decreasing rotor speed is a decrease in profile power. When rotor speed is optimized based on both power and rotor torque, the optimal rotor speed increases at low speeds to decrease rotor torque without power penalties. Optimization at high flight speeds shows that both power- and torque-weighted optimums occur at the same rotor speeds. This trend is independent of vehicle gross weight. For very high gross weight and high altitude operations, the flight envelope can be expanded by increasing the rotor speed, preventing rotor stall. Shaft torque was not seen to increase appreciably for moderate and high speed flight, with only a small increase in low speed flight. Main rotor collective pitch was seen to increase as rotor speed decreased to maintain thrust. In addition to main rotor collective pitch, main rotor longitudinal cyclic pitch was seen to increase at moderate to high cruise speeds when the rotor RPM was reduced. Blade coning also increased as RPM decreased due to reduction in centrifugal force, though first harmonic flapping angles remained nearly the same.