The document summarizes research into reducing noise from a coaxial helicopter design. Key points: - Original dual-swept blade tip design weakened acoustic signals by altering constructive interference and widening signals. Blade sweep also delayed and weakened shock formation. - Reducing rotor RPM from 332 to 276 RPM greatly decreased in-plane thickness noise and yielded a 17% reduction in total power and 30 dB decrease in peak noise, though out-of-plane loading noise increased. - Changing the blade crossover location altered thickness noise interference patterns and mitigated large impulsive out-of-plane loading pulses, with a 30 degree crossover providing benefits.

1. Dual-Swept Tip Blades
Flight Condition: V∞= 250 kts., RPM = 332, MAT = 0.93
Results:
• Original dual-swept tip blade design
alters constructive interference.
• Anti-Symmetric blade design
widens and weakens overall
acoustic signal.
• Blade sweep near the tip delays the
formation of and weakens shocks.
An Acoustic Investigation of a Coaxial
Helicopter in High-Speed Flight
Gregory Walsh1, Kenneth S. Brentner1, George Jacobellis2, Farhan Gandhi2
Vertical Lift Research Center of Excellence
Aerospace Engineering
Motivation
 Compound lift-offset coaxial helicopter designs
have demonstrated the ability to achieve high
forward flight speeds, but the rotor noise of these
configurations at high speeds is a problem that
still needs to be addressed.
 The presence of a second main rotor adds
unknown acoustic complexity to this problem.
Objectives
 Computationally investigate noise characteristics
of a coaxial helicopter in high-speed forward flight.
 Compare the noise of a coaxial helicopter to that
of a similar single main rotor helicopter.
 Explore different design variables and trim
settings to reduce rotor noise.
Session: Acoustics I, May 18 10:15-10:45am Room 1BC
RPM Reduction Study
Flight Condition: V∞= 230 kts., RPM = 276, 207, 179, MAT = 0.84, 0.72, 0.67
Blade Crossover Comparison
Flight Condition: V∞= 230 kts., RPM = 179, MAT = 0.64
Simulation Model
 An XH-59 rotorcraft model was generated using
RCAS.
 A prescribed wake solver calculates blade loads
for the upper and lower rotors.
 The blade loading and the XH-59 geometry are
input into PSU-WOPWOP for acoustic prediction.
Concluding Remarks
The noise from a coaxial helicopter in high-speed flight is both loud and largely impulsive at certain observers. However, design
variables (e.g. blade phasing and blade geometry) and trim settings (e.g. RPM) can be adjusted to mitigate the noise.
1The Pennsylvania State University, 2Rensselaer Polytechnic Institute
Coaxial Noise Characteristics
Flight Condition: V∞= 160 kts., RPM = 335, MAT = 0.8
Original dual-swept tip blade configuration “Anti-Symmetric” dual-swept tip blade configuration
ψ = 180 deg. observer. XH-59 Original Dual Swept Tip,
Anti-Symmetric Anti-Symmetric with ψ = 30 deg. blade crossover
In-Plane Thickness (Top), Out-of-Plane Loading (Bottom)
276, 207, 179
Results:
• Reduction in RPM greatly decreases in-plane
thickness noise at all observer locations.
• RPM reduction for out-of-plane loading noise results
in an increase of noise at all observer locations.
• 27% reduction in RPM yields a 17% reduction in total
power and a 30 dB decrease in peak OASPL.
Conventional
Coaxial
ψ = 180 deg. observer,
thickness noise
ψ = 270 deg. observer,
loading noise
Coaxial
Results:
• Constructive
interference of
thickness signals at
observers on
symmetry plane.
• Impulsive BVI noise-
like loading pulses far
from tip-path plane.
ψ = 0 deg. blade crossover
ψ = 30 deg. blade crossover
In-Plane Thickness (Top), Out-of-Plane Loading (Bottom)
Results:
• Locations of thickness
noise constructive
interference are
altered by changing
the blade crossover
locations.
• Large impulsive BVI
noise-like loading
pulses at out-of-plane
observer locations are
mitigated by the new
blade crossover
locations.
• Ψ = 30 deg. blade
crossover
configuration
provides both a
thickness and loading
noise benefit.
SB>1 Defiant. Boeing Photo