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Abstract and Figures

This study presents the integration of a flight simulation code (PSUHeloSim), a high fidelity rotor aeromechanics model with free wake (CHARM Rotor Module), and an industry standard noise prediction tool (PSU-WOPWOP) into a comprehensive noise prediction system. The flight simulation uses an autonomous controller to follow a prescribed trajectory for both steady and maneuvering flight conditions. The aeromechanical model calculates blade loads and blade motion that couple to the vehicle flight dynamics with suitable resolution to allow high fidelity acoustics analysis (including prediction of blade-vortex interaction (BVI) noise). The blade loads and motion data is sent to PSU-WOPWOP in a post-processing step to predict external noise. The coupled analysis is being used to evaluate the influence of flight path on aircraft noise certification metrics like EPNL and SEL for various rotorcraft in work for the FAA. The software was used to analyze the acoustic properties of a blade planform similar to the "Blue Edge" rotor blades developed by DLR and Airbus Helicopters-predicting BVI noise reduction as compared to more conventional blade geometries on the same order as that reported for the "Blue Edge" rotor.
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1
Rotorcraft Simulations with Coupled Flight Dynamics, Free Wake, and Acoustics
Umberto Saetti
uxs116@psu.edu
M.S. Candidate
Joseph F. Horn
joehorn@psu.edu
Professor
Kenneth S. Brentner
ksbrentner@psu.edu
Professor
Willca Villafana
wrv105@psu.edu
M.S. Candidate
Dan Wachspress
dan@continuum-
dynamics.com
Senior Associate
Department of Aerospace Engineering
The Pennsylvania State University
University Park, PA, USA
Continuum Dynamics
Ewing Township, NJ,
USA
ABSTRACT
This study presents the integration of a flight simulation code (PSUHeloSim), a high fidelity rotor aeromechanics model
with free wake (CHARM Rotor Module), and an industry standard noise prediction tool (PSU-WOPWOP) into a
comprehensive noise prediction system. The flight simulation uses an autonomous controller to follow a prescribed
trajectory for both steady and maneuvering flight conditions. The aeromechanical model calculates blade loads and blade
motion that couple to the vehicle flight dynamics with suitable resolution to allow high fidelity acoustics analysis (including
prediction of blade-vortex interaction (BVI) noise). The blade loads and motion data is sent to PSU-WOPWOP in a post-
processing step to predict external noise. The coupled analysis is being used to evaluate the influence of flight path on
aircraft noise certification metrics like EPNL and SEL for various rotorcraft in work for the FAA. The software was used
to analyze the acoustic properties of a blade planform similar to the Blue Edge” rotor blades developed by DLR and Airbus
Helicopters predicting BVI noise reduction as compared to more conventional blade geometries on the same order as that
reported for the “Blue Edge” rotor.
INTRODUCTION
The integration of rotorcraft simulation software with
complex aeromechanical models can provide increased
fidelity and functionality of the system as compared to any
of the individual tools. Continued advancement in
computational resources allows coupled codes to be
executed efficiently and even in real-time. In 2006, the
GENHEL-PSU simulation code was integrated with the
CHARM free wake module and the coupled code was
shown to provide improved fidelity in flight dynamics.1
Real-time operation required limitations in the rotor wake
geometry, but with use of parallel computing and the steady
improvement in CPU performance these limitations can be
relaxed. Coupling of GENHEL-PSU with Navier-Stokes
CFD solutions has also been performed, with specific
application to simulation of ship airwake interactions with
the helicopter2. These simulations are still far slower than
real-time, but scaling studies have shown that with
massively parallel processing and reduced order CFD
models, real-time simulation and CFD coupling might be
possible in the near future.
GENHEL-PSU was also coupled with the acoustics
prediction software PSU-WOPWOP.3 This coupling was a
serial “one-way” coupling in that GENHEL-PSU
simulations first calculated the helicopter motion and blade
loads and then PSU-WOPWOP used this information to
predict the external acoustics. One-way coupled simulations
were reasonable since the acoustics have no impact on
aircraft dynamics. The simulations allowed predictions of
rotorcraft noise in maneuvers, whereas historically such
calculations were only performed in steady-state trimmed
flight. The limited fidelity of the blade loads predicted by
GENHEL-PSU meant that the acoustics prediction could
not account for Blade-Vortex-Interaction (BVI) noise.
However, subsequent work used a free-wake model to re-
construct more detailed blade loads for the prediction of
BVI4. The wake model was coupled “one-way” in that the
flight dynamics simulation was based on a simple blade
element rotor with finite-state inflow and was not affected
by the free wake. The free wake was used to re-construct
more detailed blade loads for use only in the acoustics
prediction.
The prediction of noise in generalized maneuvering flight is
relevant in that it can be used to determine flight procedures
that minimize noise and impact on communities. This is of
particular interest to the Federal Aviation Administration,
who through the Aviation Sustainability Center of
Excellence (ASCENT), is seeking to develop noise
abatement procedures. Physics-based models are
particularly useful for noise prediction when no measured
data is available, such as for new rotorcraft designs and
configurations. To achieve these goals, the noise prediction
should be coupled with flight simulation codes that generate
realistic trajectories and pilot control input histories for
typical rotorcraft maneuvers. This could be done through
either real-time piloted simulations or through batch
simulations using an autonomous controller (that models a
pilot compensation to track a desired trajectory). In
addition, such simulations should be coupled with high
fidelity aeromechanical models that provide suitable blade
load and blade motion predictions for acoustics analysis
(including BVI), and these aeromechanical models should
be consistent with the total forces and moments acting on
2
the rotorcraft during the flight simulation. In this study, we
continue development of a comprehensive noise prediction
system5, that couples a flight simulation code
(PSUHeloSim), a high fidelity rotor aeromechanics model
with free wake (CHARM Rotor Module6), and an industry
standard noise prediction tool (PSU-WOPWOP7-9). All of
these tools are physics-based models that can be adapted to
predict flight dynamics, rotor loads, and noise on a variety
of rotorcraft configurations. In this paper, we present the
coupling of these codes and preliminary results showing
vehicle motion and noise prediction for steady flight
conditions.
SIMULATION ARCHITECTURE
Helicopter Flight Dynamics Model
The flight dynamics simulations were performed using the
PSUHeloSim code. This is a basic simulation tool
developed at PSU to provide a generic rotorcraft flight
dynamics model for research and education. PSUHeloSim
is developed in the MATLAB/Simulink environment for
ease of development and adaptation to different rotorcraft
configurations. The simulation model is constructed in first
order state space form    , which makes it well
suited for numerical integration, trim, and linearization
calculations. It includes the 6 DoF non-linear equations of
motion of the fuselage, 2nd order rotor flapping dynamics,
and a 3-state Pitt-Peters inflow model, resulting in a 21-state
non-linear model. It uses a simple aerodynamic model of the
fuselage and empennage based on given lift and drag
properties. A static Bailey model is used for the tail rotor,
and while the main rotor includes flapping dynamics, it uses
linearized blade equations of motion and simplified analytic
integrations of the aero lift and drag forces along the blade.
The limitations in rotor model fidelity are not significant for
the current application, as the simple rotor model is replaced
with the high-fidelity CHARM (Comprehensive
Hierarchical Aeromechanics Rotorcraft Model) rotor
module in the final results. The simple rotor model is only
used in the trim calculation for initializing the simulations
and in the controller design process. A general schematic of
the PSUHeloSim flight dynamics model (not including the
controller described below) is shown in Figure 1.
The simulation is integrated with a non-linear dynamic
inversion control law.10 This control law has been
developed for rotorcraft application on a number of research
programs at PSU, and has recently been used for non-real-
time simulations with complex aeromechanical models.2
The controller achieves high precision closed-loop control
of the simulated helicopter and tracks a commanded
velocity vector and heading in NED frame. Note that an
evolution of this controller, designed to follow a specific
trajectory (x,y,z coordinates), will be used in future coupled
simulations. Engineering simulations require a “pilot
model” to regulate the helicopter (which may have unstable
dynamics) and keep it on a specific flight path during
maneuvers. The NLDI controller serves this purpose.
High-Fidelity Rotor Module
PSUHeloSim is integrated with a high-fidelity rotor module
for fully-coupled or one-way-coupled simulations. The
CHARM Rotor Module uses a Constant Vorticity Contour
(CVC) full-span free-vortex wake model, combined with a
vortex lattice, lifting surface blade model11. The module
calculates blade motion including structural modes in the
blade dynamics. This module runs as a separate code
obtaining the state, state derivatives and controls from
PSUHeloSim at each time step of the simulation and
returning the forces, moments, and flapping coefficients of
the rotor systems. In the one-way coupled mode, the blade
loads are stored for use in acoustic prediction, but are not
used by the PSUHeloSim flight dynamics model. In the
fully-coupled mode, the forces and moments calculated by
CHARM are used as inputs for the PSUHeloSim code. Thus
in the fully coupled mode, CHARM acts as the main rotor
module and/or tail rotor module of the simulation (it
replaces the simple built-in rotor models in PSUHeloSim).
In either mode, CHARM is able to produce loading files that
are then used by PSU-WOPWOP to determine the
aerodynamically induced noise. One can choose to couple
the main rotor, the tail rotor or both. The acoustic prediction
is able to operate with more than one rotor at a time. The
only present limitation is that the loading output for acoustic
prediction is limited to a single rotor revolution which is
assumed to be periodic. This means that acoustic analysis
can be performed just for steady or quasi-steady flight
conditions. (It is planned to relax this limitation to enable
fully coupled transient maneuver simulations.)
Noise Prediction Model
The noise prediction model used in this work, PSU-
WOPWOP7-9, is a numerical implementation of Farassat’s
Formulation 1A11 of the Ffowcs Williams Hawkings (FW-
H) equation.12 Formulation 1A is used to predict the discrete
frequency noise prediction (thickness, loading, BVI, etc.)
from first principles when provided with the aircraft and
rotor blades position, motions, and blade loading. PSU-
WOPWOP predicts the acoustic pressure time history for
either stationary or moving observers and the code is also
able to convert the output signals into acoustic spectra, such
as 1/3rd octave bands and multiple types of noise metrics
relevant to noise certification and community annoyance
(PNL, PNLT, SEL, EPNL, and OASPL, etc.). The
broadband noise is computed in PSU-WOPWOP by
implementing an empirical prediction developed by Pegg13
that predicts the broadband noise in 1/3rd octave bands.
This is then combined with the discrete frequency noise for
a total noise prediction. In a recent research effort, it has
been demonstrated that a flight simulation coupled with
3
CHARM and PSU-WOPWOP can predict the noise in “real
time”. The system developed in this work is somewhat
different, but it is still reasonably fast.
Schematic of the Simulation Process
The simulation process consists of three main steps: 1)
solving trim for the prescribed flight condition, 2) running a
PSUHeloSim /CHARM coupled simulation, and 3)
performing an acoustic prediction with PSU-WOPWOP
based on the results of the simulation. A Newton-Raphson
based trimming algorithm is used to find an equilibrium
condition for the state and the controls. Note that this trim
solution is based only on the base PSUHeloSim model.
Once trim is achieved, the trim state and control solution is
used as initial conditions of the coupled simulation (both for
PSUHeloSim and the CHARM rotor modules).
During the simulation, the time history of velocity and
heading commands are fed to the dynamic inverse controller
in the PSUHeloSim code. The controller calculates the
control input based on the tracking error and feedforward
signals as defined by the control law. The sim code updates
the state, state derivatives, and swashplate inputs, which are
then used as inputs for the CHARM rotor module. The
resulting main rotor forces and moments calculated by
CHARM are either saved as output (in one way coupled
mode) or fed back into the simulation model in fully-
coupled mode. When performing fully-coupled simulations,
the full coupling is not initiated until three seconds of
simulation have passed. This allows the free-wake model
time to develop and initialize. After the simulation is
completed, the PSU-WOPWOP acoustics analysis is
performed using the aircraft state and loading files
generated by CHARM. Figure 2 shows the flowchart of the
simulation process.
Figure 1. Schematic of the PSUHeloSim/CHARM/PSU-WOPWOP simulation model.
4
SIMULATION RESULTS
Flight Simulation Results
The helicopter used for the current simulation results is a
Bell 430; a summary of its characteristics is presented in
Table 1.
A number of basic maneuvers were simulated using the
standard PSUHeloSim model and the fully-coupled
simulation with the CHARM rotor module. The simulations
were used to verify that the fully-coupled simulations
followed the expected behavior and that the NLDI
controller can adequately stabilize and control the coupled
model.
When using the one-way coupled simulation or the stand-
alone PSUHeloSim model, the CHARM rotor module is not
used in the flight dynamics solution. This means that the
dynamic simulation involves just the base PSUHeloSim and
the Dynamic Inversion based controller. The DI controller
is designed around linearized models of the PSUHeloSim,
which leads to very accurate tracking of controller
commands. The trim solver is also based on the
PSUHeloSim model and results in near perfect
initialization. This is seen in the red results of Figure 3,
which show the attitude response when the commanded
trajectory simply holds a 100 kts level flight trim condition.
It can be seen that there is no deviation from trim.
Mass and Inertia Properties
W











Figure 2. Flowchart of the simulation process.
5
In the fully-coupled case, the main rotor forces, moments
calculated by the CHARM rotor module are fed back into
the dynamic simulation, changing the nature of the
nonlinear model. So when the coupling is turned on, after
three seconds of simulation, the helicopter goes through a
transient due to the differences of forces and moments
between CHARM and the PSUHeloSim model. The
controller is robust enough to restore the trim, causing the
aircraft to converge to a steady state after a period of time.
The new equilibrium is usually slightly different from the
initial trim. This is partly due to differences in trim of the
CHARM rotor and the simple rotor model in PSUHeloSim.
In addition, a helicopter can trim with different
combinations of roll attitude and sideslip angle. When
trimming PSUHeloSim the yaw attitude / sideslip are set to
zero, but after coupling is initiated the system settles into a
slightly different steady state.
A decelerating descent maneuver was simulated with and
without coupling. The results are shown in Figure 4. With
coupling, the simulation is initially flown in steady 100 knot
level flight for a period of time to allow the helicopter to
return to trim after the transient at initialization. Figure 4
shows the response after the initial wait period. The
maneuver consists of a 6 decelerated descent from 100 to
60 kts at 0.1 g of deceleration. Figure 4 compares responses
of the “de-coupled” baseline PSUHeloSim and the fully
coupled model with CHARM. In both cases, the vehicle
response follows the command after the initial transient, and
the responses are similar for both models. The velocity and
altitude profiles are essentially identical, which is expected
since these are tracked by the controller. There are slight
differences in attitudes due to differences in the two rotor
models. Note that there is some deviation of the lateral cross
track (y-position), but the lateral drift is only about 4 ft after
900 ft down range motion.
Main Rotor





e


Tail Rotor




2



Table 1. Bell 430 key characteristics.
a) Position
b) Absolute velocity and acceleration
Figure 3. Coupling transient: dashed line marks the
start of the coupling.
6
Figure 5 shows a 90 turn maneuver at 100 kts forward
airspeed. Once again, time was allotted to allow the
controller to stabilize the aircraft after the coupling
transient. Once again we see very similar flight path with
both the coupled and de-coupled PSUHeloSim. Note that
the velocity fluctuations are quite small. The accelerations
seen are largely in the lateral axis due to Dutch Roll
oscillations. This mode appears to be less damped with the
coupled model.
When simulating main rotor and tail rotor physics with the
CHARM rotor module, the simulation time step is driven by
the largest allowable tail rotor blade sweep per time step
(since the tail rotor has a larger RPM). For example, 15°
blade sweep per time step is usually considered the largest
acceptable time step for blade element rotor simulations in
flight dynamics. Consequently the main rotor (which turns
slower) will have a smaller blade sweep.
One of the unique features of this system is the ability to
capture relevant physics for the acoustics. In particular, the
free wake needs a sufficient number of elements for
accurate blade loading, which in BVI conditions should be
as fine as 1 azimuthal resolution. With such high temporal
(azimuthal) and corresponding spatial resolution
requirements, it is impossible to perform real-time analysis
with the free wake and could take substantial computational
power to be useful in the computation of realistic maneuvers
tens of hours on a single processor. Fortunately, this issue
has been addressed in the CHARM rotor module through
“reconstruction” of the rotor wake in post-processing. In
this approach, a low resolution wake and larger time step is
used in the flight simulation step, which is acceptable for
flight dynamics modeling. Then, for the regions of the
maneuver where acoustics are of interest, a higher
resolution wake and blade loading is reconstructed in the
CHARM rotor module14. These high resolution blade loads
are then used by PSU-WOPWOP to predict BVI-dominated
noise. In recent work, this method was used to perform real-
time, BVI-noise predictions15. Currently reconstruction can
only be applied to one rotor.
c) Attitude
Figure 4. Results from a 6 decelerated descent at 0.1 g
of deceleration.
a) Trajectory
b) Absolute velocity and acceleration
c) Attitude
Figure 5. Results from a 90 turn at 100 knots.
7
Acoustic Results
Helicopter rotor noise consists of several noise sources
including discrete frequency noise (thickness, loading, and
blade-vortex-interaction (BVI) noise), broadband noise, and
high-speed-impulsive (HSI) noise (HSI noise only occurs in
high speed forward flight). Each of these noise sources has
a unique directivity, as shown in Figure 6. Thickness noise
is dominant in the plane of the rotor, so it is the primary
noise heard as a distant helicopter approaches. Only the
motion of the rotor blades and the aircraft, along with the
geometry of the rotor blades, is needed to compute the
thickness noise; hence, the flight simulation code is readily
able to provide this information (at very low computational
cost). High-speed impulsive noise has the same directivity
as thickness noise, but it only occurs in high-speed flight
(and will not be addressed here).
Loading noise is another important source of rotor noise,
which is typically directed below the rotor so loading
noise is important as the aircraft is overhead. There are two
important types of loading noise that are generally dealt
with separately: BVI noise and broadband noise. BVI noise
is the dominant noise source when it occurs. It has a very
impulsive nature and originates from a nearly parallel close
interaction between a blade and the tip vortex of a previous
blade. BVI noise is highly directional and depends strongly
on the vortex strength, miss distance, and interaction angle.
This is the reason that a high-fidelity rotor and wake model
are needed in this system to predict BVI noise accurately.
Broadband noise is another type of loading noise that is a
result of stochastic loading due to various airfoil self-noise
sources, turbulence ingested into the rotor, or turbulence
entrained by tip vortices (when they are not quite close
enough to cause significant BVI noise). The empirical
model derived by Pegg13 are used in PSU-WOPWOP to
predict the broadband noise and the input data is relatively
easy to obtain from the flight simulation system.
Figure 7 shows the contribution to each of the noise
components to the Overall Sound Pressure Level (OASPL)
as function of the uprange/downrange distance for the Bell
430 helicopter flying at 100kts at an altitude of 150m. At
  , the helicopter is directly overhead. OASPL is not
weighted by frequency and hence tends to reflect the large
amplitude of the low frequency components of the rotor
noise. Notice that as the aircraft approaches (negative
distances) the thickness noise is the dominant source of
noise. This is because the thickness noise directivity is in
the plane of rotor; hence, the observer hears it first. The
loading noise becomes begins takes over as the dominant
noise source as the aircraft passes overhead and continues
downrange (positive distances). The broadband noise
makes only a contribution to OASPL, so it is not shown in
the figure.
Figure 8 shows a similar plot of the noise components, but
in this case the tone corrected, perceived noise level (PNLT)
is plotted as a function of the uprange/downrange distance
from the target observer location and the aircraft is directly
overhead at  . PNLT uses a frequency weighting that
is intended to be representative of human annoyance; hence,
higher frequencies are more important. The relative
importance of the various noise sources is quite different in
this case. The thickness noise is still the dominant noise as
the aircraft is approaching (larger negative distances), but
the broadband noise is significant as the aircraft approaches
the overhead condition and dominant for all downrange
positions (positive distances). The loading noise also
increases overhead and downrange, but is always lower in
this flight condition than broadband noise. This is because
the loading noise in level flight has fairly low frequency
content as this is a level flight condition. For level flight
BVI noise is not expected, but if there had been BVI noise
Figure 6. Typical direction of primary radiation for
various rotor noise sources.
Figure 7. Noise components and their contribution to
the OASPL predictions for a 100 kts flight case flown
at 150 m altitude.
8
the loading noise levels would have been substantially
higher.
Demonstration Calculation Prediction of BVI Noise
Reduction using the Blue Edge-like Blades
An important attribute of this work is that the coupled
system can accurately predict the acoustic characteristics of
dominant noise sources without reliance on test data. During
approach and landing, blade-vortex interaction (BVI) noise
is a dominant out-of-plane noise source responsible for
much of the ground noise. In order to provide a tool for
evaluating the impact of modifications to flight path and
rotor design on ground noise exposure during landing, it is
necessary to demonstrate that the model can accurately
predict BVI noise for BVI-dominant flight conditions. The
ability of the CHARM/WOPWOP and subsequently
CHARM/PSU-WOPWOP solutions to predict main rotor
BVI noise in these flight conditions for conventional rotors
(and tiltrotors) was demonstrated in prior work.14,15 In the
current work, this demonstration was extended to an
advanced blade design known to reduce BVI noise (Figure
9)16, Airbus Helicopter’s “Blue Edge” blade. The concept
behind this design is described in Ref. 17 as:
With a standard blade, air coming off the end of the blade
causes a vortex around the tip. Under certain flight
conditions the advancing blade then hits the vortex of the
preceding blade. This causes a sudden change in the
relative angle of attack and thus a change in pressure on
the surface of the blade. This BVI causes the slapping
sound ubiquitous to helicopter operations. With Blue Edge
technology, the blade tip is swept forward, then aft. This
causes the advancing blade tip to hit the previous blade’s
vortex at an oblique angle, reducing the noise level by 3 to
4 EPNdB.17
Calculations were performed to demonstrate the ability of
the new analysis system to predict the reduction in BVI
noise obtained using a Blue Edge-like planform. Three
blade planforms were compared operating on a Bell 430
rotor/aircraft configuration: 1) conventional rectangular
blades nominally the current Bell 430 blade; 2) tapered
blades, 3) Blue Edge-like planform with taper and
forward/aft sweep. The planform characteristics of each of
these three blade sets are provided in Table 2 below. Figure
10 compares the tapered and Blue Edge-like planforms. No
optimization of the tapered and Blue Edge-like planforms
was performed to minimize noise. The Blue Edge-like
planform forward/aft sweep schedule is roughly comparable
to photographs of the Airbus Blue Edge blade, capturing the
key feature of reducing the “parallel” nature of the BVI.
Figure 8. Noise components and their contribution to
the PNLT predictions for a 100 kts flight case flown at
150 m altitude.
Figure 9. Blue Edge blade concept from Eurocopter
(now Airbus Helicopters).16
Tip Speed

Rotor Cutout

Rotor Chord
 - rectangular
linear taper -
tapered and Blue Edge
Anhedral Tip
None
Swept tip
none - rectangular and tapered
forward -12 at r/R=0.6
aft 34.4 at 0.85 (Blue Edge)
Root Airfoil
NACA 0012
Tip Airfoil
NACA 0012 - rectangular
NACA 0009 - tapered and Blue
Edge
Air Density

Speed of Sound

Thrust
Coefficient

Hub Type
articulated
Lock Number
8
Table 2. Characteristics of the three blade planforms.
9
The flight condition studied was a descent at low speed
(=0.15) with the rotor tilted back 6 relative to the flight
path. The sound pressure level was determined in a plane
one rotor radius beneath the rotor plane. The CHARM
solution was performed with an azimuthal resolution of
=15 and then reconstructed to a resolution of =1
using the method described in14. The blade aerodynamics
and acoustic solution at 187 observer points was completed
in 3 minutes on a single core of an off-the-shelf CPU.
Figure 11 shows predictions of both the overall sound
pressure level (OASPL) and the BVI sound pressure level
(BVISPL) (harmonics 6-40 in blade passage frequency) for
this configuration. The magnitude and directionality
predicted is characteristic of the results seen for BVI-noise
dominated descent flight conditions. The analysis predicts
that the taper reduces the peak BVISPL by roughly 2dB and
the Blue Edge-like planform further reduces the peak
BVISPL by another 3dB for a total reduction of peak
BVISPL of 5dB, capturing the documented benefit of the
Blue Edge planform.
Figure 10. Tapered and Blue Edge-like planforms.
OASPL (harmonics 0 50 BPF)
BVISPL (harmonics 6 40 BPF)
(a) RECTANGULAR (b) TAPERED (c) BLUE EDGE
Figure 11. CHARM/PSU-WOPWOP main rotor OASPL and BVISPL predictions one rotor radius beneath the nominal
Bell 430 rotor for the three blade geometries; s=6 (back), =0.15 and CT=.00143. The black circle represents the rotor tip
advancing side on the right.
10
Figure 12 shows the advancing-side BVI for the Blue Edge
planform compared with a rectangular blade as predicted by
the CHARM code. Notice in the figure that the tip vortex
(the red curved line) is nearly parallel to the entire length of
the blade for the rectangular blade (left), while the shape of
the Blue Edge planform (right) results in an interaction that
occurs over a wider range of rotor azimuth angles; hence, it
is a much less impulsive interaction.
The measurement plane shown in Figure 11 reveals that the
main rotor BVI noise is significantly reduced by the Blue
Edge-like rotor planform, but a more typical noise
prediction for a complete rotorcraft is made either on a
hemisphere or a ground plane. To demonstrate the fully-
coupled system this BVI noise was predicted for the full
helicopter configuration. Here the aircraft flight condition
is a forward speed of 68 kts and a 6° descent flight profile
providing the same main rotor operating condition as shown
in Figures 11 and 12. Figure 13 shows the OASPL of the
Bell 430 helicopter (with rectangular main rotor blades and
the tail rotor included). Notice in the figure that the focused
region of BVI noise is still clearly evident on the hemisphere
surface.
Figure 14 shows the acoustic pressure time histories for
each of the main rotor planforms at a point located on the
hemisphere at an azimuth of 125° and down 45° from the
main rotor tip-path plane (indicated by a small black dot in
Figure 13). Notice in the figure, for each blade geometry
there are 4 very narrow and high amplitude pressure spikes
(or group of spikes). These are the BVI from each of the
four blades on the main rotor. The thickness and loading
noise of the main rotor also occur approximate the same
time, so they are difficult to see. These pulses are the tail
rotor thickness noise. Comparison of the three different
rotor blade geometries shows how the BVI acoustic
pressure spikes amplitude is greatly reduced for the case of
the Blue Edge-like rotor. The tapered blade also has a small
reduction in BVI spike amplitude, primarily seen on the
positive part of the pressure spike. The other features, i.e.,
the tail rotor noise, is essentially unchanged.
(b) BLUE EDGE
(a) RECTANGULAR
Figure 12. BVI event as predicted by CHARM for the baseline
RECTANGULAR blade and the BLUE EDGE blade.
Figure 13. Contours of OASPL on a 30.48m radius
hemisphere, centered at the Bell 430 c.g. location. The
hemisphere follows the aircraft. OASPL contours shown are
for the standard rectangular blades.
Direction of flight
11
The noise comparisons shown in this section demonstrate
the utility of the flight simulation, high-fidelity wake, noise
prediction system that has been developed here.
Furthermore, design changes to reduce BVI noise one of
the more challenging components of the noise to predict
show the expected noise reduction trends.
CONCLUSIONS
A simulation tool has been developed that couples: 1) A
flight dynamics simulation with closed loop control
(PSUHeloSim), 2) A high-fidelity aeromechanics model of
the rotor with a free wake (CHARM rotor module), and 3)
an industry-standard noise prediction code (PSU-
WOPWOP). Both uncoupled and fully coupled simulations
were first tested for some basic maneuvers, and the results
showed that the rotorcraft (with controller) was able to fly
the prescribed trajectory when using the CHARM rotor
module for rotor force and moment calculations, and that
the control and attitude response was reasonable. Coupling
with PSU-WOPWOP acoustics analysis was then tested,
with a focus in this paper on a BVI-dominated descent
condition. Then three different blade geometries were
evaluated on the Bell 430 helicopter, including a Blue Edge-
like blade planform that is expected to provide significant
BVI noise reduction. The acoustic analysis predicted the
expected noise reductions with approximately the same
level of noise reduction as reported by Airbus Helicopters.
Some specific conclusions from this effort:
1. It has been demonstrated that the integrated simulation
was capable of predicting realistic maneuvers when
coupling the CHARM rotor module and PSUHeloSim
simulation. It is crucial that the simulation include a
robust flight controller, to handle the transients and
change in aeromechanics upon coupling with the higher
fidelity main rotor and tail rotor models.
2. The time step and wake resolution requirements for
accurate acoustic predictions of the main rotor and tail
rotor would be much slower than real-time execution,
but the use of wake reconstruction to get higher
resolution of the blade loading in post-processing
(especially in BVI dominated conditions) was found to
be a critical tool for improving efficiency of the tool
and approaches real-time prediction speeds while still
providing highly accurate, high-fidelity blade loading
for noise prediction.
3. The use of the CHARM rotor module significantly
enhances the fidelity level of the simulation, by adding
free wake and nonlinear dynamics of flexible blades (as
opposed to 3-state inflow, and a rotor disk model with
linearized flapping dynamics). While this level of
fidelity is not necessarily required for flight simulation,
the CHARM rotor module captures higher resolution
blade loading needed for acoustics calculations. One of
the motivations for full coupling (feedback of CHARM
rotor forces to the vehicle dynamics) is to ensure
consistency of the rotor force output with the flight
trajectory flown.
4. The CHARM rotor module successfully captured the
behavior of the “Blue Edge” blade in terms of blade
vortex interaction thus underlining its strength in
comparison to other more classic blade geometries.
This case also demonstrates the predictive capability of
the entire system.
5. The coupling of the simulation and CHARM rotor
module results in a coupling transient. The transient is
a simulation artifact and not relevant to the physics of
interest. Some additional processing time is required to
allow the controller to stabilize and re-trim the aircraft
before performing the maneuver of interest. We are
currently working to reduce this transient to improve
efficiency of the tool.
ACKNOWLEDGEMENTS
This work was funded by the U. S. Federal Aviation
Administration (FAA) Office of Environment and
Energy as a part of ASCENT Project 6 under FAA
Award Number: 13-C_AJFE-PSU-006. Any opinions,
findings, and conclusions or recommendations
Figure 14. Acoustic pressure time history at azimuth angle   , elevation angle    below the rotor plane,
and radius of 30.48m from the helicopter c.g. (i.e., the location of the black dot in Figure 13).
12
expressed in this material are those of the authors and
do not necessarily reflect the views of the FAA or other
ASCENT Sponsors.
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