Acoustic Analysis of Compound Helicopters with Trim Variations

Open Access
Barron, Heather Marie
Graduate Program:
Aerospace Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
Committee Members:
  • Kenneth Steven Brentner, Thesis Advisor
  • Helicopter
  • rotorcraft
  • rotor
  • craft
  • acoustic
  • aerospace
  • compound
  • analysis
This thesis presents a prediction method that analyzes the acoustic radiation of compound helicopters when trim is varied. The goal of this thesis was to develop a method that uses added features such as a wing and an auxiliary propeller (described as added ‘control degrees of freedom’) on a compound rotorcraft to reduce the noise generated when flight conditions were changed. Noise radiation is an important factor for both military and civilian helicopters. There has been limited research on compound rotorcraft acoustics. Prior research focus has been primarily on tiltrotor configuration and large civil helicopter acoustics. For this thesis, a notional vehicle with a main rotor, tail rotor, wing, and auxiliary propeller was used. A flight simulation model and a noise prediction tool were coupled together to produce all of the analytic results. Predictions indicated that with the additional ‘control degrees of freedom’, the noise from a compound helicopter can be changed drastically. Research showed that reducing the rotor advancing tip Mach number resulted in a large noise reduction in high-speed flight; this can be useful when finding an optimal noise flight condition. Results also showed that changing the thrust produced by an auxiliary propeller located aft of the aircraft does not have a dramatic effect on the overall sound due to the dominance of thickness noise in all areas around the aircraft. Even though noise does not change drastically with the change of thrust produced, the change is still crucial to the performance of the aircraft. Studies revealed that combining both advancing-tip Mach number changes along with thrust changes produced the best way to alter the noise produced from the aircraft. The dominant noise source in the cases run were from the auxiliary propeller. Independently changing the auxiliary propeller’s tip Mach number drastically changed the noise produced by the rotorcraft and is the most effective way to reduce overall noise.