National Aeronautics and Space Administration

Glenn Research Center

Implementing Variable Rotor Speed Control for Subsonic Rotary Wing Aircraft

By: Sabrina Thompson

Principle Investigator: Mr. Jonathan Litt

Objective and Overview

For the duration of the summer, I will be building upon a research study for variable rotor speed (VRS) control under NASA’s Fundamental Aeronautic Subsonic Rotary Wing project. Variable rotor speed has been identified as a major contributor to improve maneuverability and reduce noise levels for next generation rotorcraft. However, VRS is difficult to implement because of the narrow speed band of turboshaft engines. Conventionally, the propulsion system of a helicopter includes a fixed-ratio gearbox linking two turboshaft engines to the rotors.

Turboshaft engines and the gearbox are designed to operate at constant shaft and rotational speed, respectively. Deviations from the nominal design speed must remain below 15% due to stall margin restrictions and fuel efficiency. Therefore, in order to implement VRS control, the powertrain must be re-configured. In addition, a comprehensive modeling and control analysis and design must be performed. Hence, in order to satisfy the engine speed constraint while achieving a 50% variation in rotor speed, 50% variability in gear ratio must be introduced through a gear shifting mechanism. In a dual-engine configuration, gear shifting is subject to three operational constraints:

1. The engines must be decoupled from the transmission when gear shifting takes place.
2. At least one engine must be coupled to the transmission at all times.
3. The number of gear switches and time spent with one engine decoupled must be minimized during a speed change operation.

My Role as a Research Associate

The current research activities conducted in support of the VRS project have been left incomplete. Ideally, I will be assisting my Principal Investigator, Jonathan Litt, with the implementation and evaluation of control algorithms for VRS control for enhanced performance of rotorcraft. I will get exposure to the area of intelligent control by assisting in the implementation of an optimal supervisory control.

For the completion of the tasks set before me, I have laid out specific goals for myself:

1. Read the appropriate documentation for full comprehension of the problem.
2. Better familiarize myself with Matlab/Simulink.
3. Analyze the VRS control algorithms in Matlab.
4. Evaluate the VRS control algorithms using Matlab/Simulink.
5. Gain experience in rotorcraft and system controls.