RotCFD Analysis of the AH-56 Cheyenne Hub Drag

55 Pages·2016·5.26 MB·English

NASA/CR—2016–219395

RotCFD Analysis of the AH-56 Cheyenne
Hub Drag

Eduardo Solis
Monterey Technologies, Inc.
Ames Research Center, Moffett Field, California

Tal A. Bass
Princeton University
Ames Research Center, Moffett Field, California

Matthew D. Keith
University of Alaska Fairbanks
Ames Research Center, Moffett Field, California

Rebecca T. Oppenheim
Mississippi State University
Ames Research Center, Moffett Field, California

Bryan T. Runyon
University of Minnesota – Duluth
Ames Research Center, Moffett Field, California

Belen Veras-Alba
The Pennsylvania State University, University Park
Ames Research Center, Moffett Field, California

October 2016
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NASA/CR—2016–219395

RotCFD Analysis of the AH-56 Cheyenne
Hub Drag

Eduardo Solis
Monterey Technologies, Inc.
Ames Research Center, Moffett Field, California

Tal A. Bass
Princeton University
Ames Research Center, Moffett Field, California

Matthew D. Keith
University of Alaska Fairbanks
Ames Research Center, Moffett Field, California

Rebecca T. Oppenheim
Mississippi State University
Ames Research Center, Moffett Field, California

Bryan T. Runyon
University of Minnesota – Duluth
Ames Research Center, Moffett Field, California

Belen Veras-Alba
The Pennsylvania State University, University Park
Ames Research Center, Moffett Field, California

National Aeronautics and
Space Administration

Ames Research Center
Moffett Field, CA 94035-1000
October 2016
ACKNOWLEDGMENTS

A sincere thank you to our mentor, Eduardo Solis. Much of this work could not have been done
without your guidance and encouragement.

Another thank you to Dr. William Warmbrodt for your enthusiasm and motivation. The Taste of
California History presentations were an integral part of our work.

Thank you to Robert Vocke for always being available to answer our questions and providing us
with your report and code.

Additional thanks to Adam Ewert, Alexander Grima, Ganesh Rajagopalan, Gloria Yamauchi, and
Luke Novak for all of your advice and for entertaining our questions. Much of the insight we gained
stemmed from your suggestions.

A final thanks to the entire Summer 2016 Simulation Group at the NASA Ames Research Center
Aeromechanics Branch for making our office the best in the whole building.

Available from:

NASA STI Support Services National Technical Information Service
Mail Stop 148 5301 Shawnee Road
NASA Langley Research Center Alexandria, VA 22312
Hampton, VA 23681-2199 [email protected]
757-864-9658 703-605-6000

This report is also available in electronic form at
http://ntrs.nasa.gov
TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................................iv
LIST OF TABLES .................................................................................................................................. v
NOMENCLATURE .................................................................................................................................vi
ABBREVIATIONS ..................................................................................................................................vi
SUMMARY ............................................................................................................................................ 1
INTRODUCTION ................................................................................................................................... 1
ROTORCRAFT COMPUTATIONAL FLUID DYNAMICS ...................................................................... 2
HUB CONFIGURATIONS ...................................................................................................................... 3
SIMULATION CONDITIONS ................................................................................................................. 7
Free-Stream Simulation Conditions ................................................................................................... 8
Wind Tunnel Simulation Conditions ................................................................................................... 9
RESULTS ............................................................................................................................................ 10
DISCUSSION ...................................................................................................................................... 13
Epsilon Residual Solution With Time ............................................................................................... 20
CONCLUSIONS .................................................................................................................................. 21

APPENDIX A—CONFIGURATION INFORMATION ........................................................................... 23
APPENDIX B—AERODYNAMIC TARE COEFFICIENTS .................................................................... 31
APPENDIX C—UNCORRECTED SIMULATION GRID INFORMATION ............................................. 33
APPENDIX D—CORRECTED SIMULATION GRID INFORMATION .................................................. 39

REFERENCES ..................................................................................................................................... 47

iii
LIST OF FIGURES

Figure 1: 3D CAD representation of the AH-56 rotor hub [3]. .............................................................. 2
Figure 2: A comparison of the initial geometry modeled in Creo Parametric (top) and simplified
geometry modeled in Rhinoceros 5 (bottom). ....................................................................... 3
Figure 3: Configuration 4—Fixed hub and blade stubs ........................................................................ 4
Figure 4: Configuration 6—Configuration 4 with pitch horns and control gyro (blue). .......................... 4
Figure 5: Configuration 8—Configuration 4 with aerodynamic fairings (red). ....................................... 5
Figure 6: 3D CAD representation of Configuration 0............................................................................ 5
Figure 7: 3D CAD representation of the hub in the 7- by 10-foot wind tunnel with labeled
angles and dimensions [3]. ................................................................................................... 6
Figure 8: Computational domain as shown in RotCFD, including the rotor hub and
refinement boxes. ................................................................................................................. 8
Figure 9: RotCFD display of the grid surrounding one of the arms. ..................................................... 9
Figure 10: Force and moment history of Configuration 4 at (cid:2032) = 0° and (cid:2009) = 0° modeled in
(cid:3046)
free-stream conditions. ....................................................................................................... 10
Figure 11: Plot of D/q vs. azimuth angle for wind tunnel data [3]. ........................................................ 13
Figure 12: Plot of D/q vs. azimuth angle with RotCFD results overlaid on wind tunnel data. ............... 14
Figure 13: Example of RotCFD grid generation method with a 2D geometry [4]. ................................ 15
Figure 14: Close-up of small and irregular cell-shape that could lead to diverging solutions........ ....... 16
Figure 15: Side-by-side grid comparison of Configuration 4 rotated and nonrotated. .......................... 17
Figure 16: Comparison of the original geometry to the body generated by RotCFD. .......................... 18
Figure 17: Picture of the floor cavities beside the test stand in the 7- by 10-foot wind tunnel [3]. ........ 19
Figure 18: Force and moment history for Configuration 6 at (cid:2032) = 45° and (cid:2009) = 6°, modeled in
(cid:3046)
free-stream conditions. ....................................................................................................... 20

iv
LIST OF TABLES

Table 1: Test case matrix for the RotCFD simulations studied. ............................................................. 7
Table 2: Simulation flow properties. ....................................................................................................... 7
Table 3: RotCFD results compared to 7- by 10-foot wind tunnel test data. ......................................... 11
Table 4: Data with aero tares applied (used for comparison with RotCFD cases under
wind tunnel conditions). ......................................................................................................... 12
Table 5: Original calculated time grid. .................................................................................................. 14

v
NOMENCLATURE

A = Grid boundary cell size
a = Smallest estimated cell size in Cartesian simulation grid
D = Aerodynamic drag force
L = Aerodynamic lift force
PM = Aerodynamic pitching moment
q = Corrected free-stream dynamic pressure for wind tunnel conditions
c
q = Uncorrected free-stream dynamic pressure
u
v = Free-stream air velocity
(cid:2009) = Corrected angle of attack
(cid:3030)
(cid:2009) = Hub shaft angle of attack
s
(cid:2025) = Free-stream air density
(cid:2032) = Rotor hub azimuth angle

ABBREVIATIONS

ADD = Aviation Development Directorate
CAD = Computer-Aided Design
CFD = Computational Fluid Dynamics
CPU = Central Processing Unit
GPU = Graphics Processing Unit
RotCFD = Rotorcraft Computational Fluid Dynamics
RotUNS = Rotor Unstructured Flow Solver Application
ShapeGen = Shape Generator Software
OpenCL = Open Computing Language
CFL = Courant–Friedrichs–Lewy

vi
ROTCFD ANALYSIS OF THE AH-56 CHEYENNE HUB DRAG

Eduardo Solis,1 Tal A. Bass,2 Matthew D. Keith,3 Rebecca T. Oppenheim,4 Bryan T. Runyon,5
and Belen Veras-Alba6

Ames Research Center

SUMMARY

In 2016, the U.S. Army Aviation Development Directorate (ADD) conducted tests in the U.S. Army
7- by 10- Foot Wind Tunnel at NASA Ames Research Center of a nonrotating 2/5th-scale AH-56
rotor hub. The objective of the tests was to determine how removing the mechanical control gyro
affected the drag. Data for the lift, drag, and pitching moment were recorded for the 4-bladed rotor
hub in various hardware configurations, azimuth angles, and angles of attack. Numerical
simulations of a selection of the configurations and orientations were then performed, and the
results were compared with the test data. To generate the simulation results, the hardware
configurations were modeled using Creo and Rhinoceros 5, three-dimensional surface modeling
computer-aided design (CAD) programs. The CAD model was imported into Rotorcraft
Computational Fluid Dynamics (RotCFD), a computational fluid dynamics (CFD) tool used for
analyzing rotor flow fields. RotCFD simulation results were compared with the experimental results
of three hardware configurations at two azimuth angles, two angles of attack, and with and without
wind tunnel walls. The results help validate RotCFD as a tool for analyzing low-drag rotor hub
designs for advanced high-speed rotorcraft concepts. Future work will involve simulating additional
hub geometries to reduce drag or tailor to other desired performance levels.

INTRODUCTION

In the late 1960s, the US Army developed strong interest in a high-speed military helicopter that
was both lightweight and heavily armed. In response, Lockheed Corporation developed the AH-56
Cheyenne, which featured a novel “door hinge” for feathering of the blades and an externally
mounted gyro for hub control [1]. In recent years, the Cheyenne’s unique hub design has been
noted for its potential drag savings by removing the gyro and replacing it with a modern controller,
which could reduce the hub drag by roughly 60 percent [2]. The U.S. Army Aviation Development
Directorate (ADD) renewed interest in the improved aerodynamics of this design, and in 2016,
tests were conducted in the U.S. Army 7- by 10-Foot Wind Tunnel at NASA Ames Research
Center, on a 2/5th-scale AH-56 main rotor hub, to quantify the drag with and without the gyro and
pitch arms [1]. Data for the lift, drag, and pitching moment were recorded for 9 different hardware
configurations at varying azimuth angles and shaft angles of attack [3]. The gyro and various other
parts of the hub are labeled in Figure 1.

1 Monterey Technologies, Inc., 24600 Silver Cloud Ct., Monterey, California 93940.
2 Princeton University, 1 Nassau Hall, Princeton, New Jersey 08544.
3 University of Alaska Fairbanks, 505 South Chandalar Drive, Fairbanks, Alaska 99775.
4 Mississippi State University, Lee Boulevard, Starkville, Mississippi 39762.
5 University of Minnesota Duluth, 1049 University Drive, Duluth, Minnesota 55812.
6 The Pennsylvania State University—University Park, 201 Old Main, University Park, Pennsylvania 16802.
1
Figure 1: 3D CAD representation of the AH-56 rotor hub [3].

The goal of this work is to compare the force and moment results from the Rotorcraft
Computational Fluid Dynamics (RotCFD) software package to the experimental wind tunnel data.
RotCFD was used to simulate the rotor hub in both wind tunnel and free-stream conditions to
predict lift, drag, and pitching moment. Drag was of greatest interest as the hub is a low-drag
design. The simulations conducted in free-stream conditions were compared with wind tunnel data
that was corrected to quantify hub performance in such conditions. In addition, simulations
conducted in wind tunnel conditions were compared with uncorrected experimental data with the
aim of validating the CFD software as an accurate tool in predicting hub performance. This would
allow further configurations to be evaluated using RotCFD instead of wind tunnel testing.

ROTORCRAFT COMPUTATIONAL FLUID DYNAMICS

RotCFD is a software package for the simulation of rotorcraft flows developed by Sukra Helitek, in
collaboration with NASA and the U.S. Army. It is a mid-fidelity CFD software that features an
integrated design environment with tools to create simple geometries, generate grids, and simulate
fluid flow using multiple flow solver applications. The use of RotCFD for this project contributed to
generating feedback for the developer and debugging the code.

Rotor Unstructured Flow Solver Application (RotUNS) is an incompressible, unsteady, and
unstructured Navier–Stokes flow solver in RotCFD. RotUNS was chosen to run all of the
simulations for its ability to capture geometries with a higher fidelity than the other structured flow
solvers available. While unstructured grids generally take more computational time than structured
ones, the increase in geometry fidelity was necessary to capture the complex hub configurations to
a certain accuracy.

2