1. Use of HPC in Advanced Rotorcraft Systems
Matt Floros
US Army Research Laboratory
April 15, 2008

2. Rotorcraft Aeromechanics—Never a Dull Moment
Mach number range 0—1
Steady state is unsteady
Large induced inflow
Flexible blades
Intermeshing rotors
3-D acoustic field
360-degree angle of attack range
Flaps & multi-element airfoils
Rotor operates in its own wake
Fuselage and tail are in rotor wake
Image from Bhagwat, Dimanlig, et al, CFD/CSD Coupled Trim Solution for the Dual-Rotor CH-47 Helicopter Including Fuselage Modeling, AHS Specialists’ Conference on Aeromechanics, Jan, 2008

3. All Lift, Propulsion, and Control From Main Rotor
Fixed Wing
Rotorcraft
Lift
Wing
Main Rotor
Propulsion
Jet Engine/Prop
Pitch Control
Elevator
Roll Control
Ailerons
Yaw Control
Rudder
Tail Rotor
Yaw control on multi-rotor aircraft?
- Main rotors

4. Local Velocities from Static to Transonic Speeds
Advancing Tip:
a ≈ 0, M~0.85-1
Retreating Tip:
a near stall, Low speed subsonic, dynamic stall
Retreating Root:
a ≈ 180, Low speed subsonic

5. Local Velocities from Static to Transonic Speeds
Tangential velocity V = Wr + m sin(y), for traditional helicopter 0 < m < 0.4 (0 < m < 0.2 for tilt rotor), some new concepts much higher
Flow approaching airfoil from trailing edge for 0 < r < m on retreating side
Near-body grid for blade has to encompass
Transonic flow/shocks
Separated flow
Dynamic stall
Reverse flow, radial flow
All in one revolution!

6. Near-Body Grids for Blades Deform at Every Time Step
Moment balance, propulsion, and control come from blade flapping
Most helicopters have hinge offset rotors
Rigid body rotation at flap hinge
Elastic deformation with time
Grid deforms at every
Time step
Image from Bhagwat, Dimanlig, et al, CFD/CSD Coupled Trim Solution for the Dual-Rotor CH-47 Helicopter Including Fuselage Modeling, AHS Specialists’ Conference on Aeromechanics, Jan, 2008

7. What About “Advanced” Rotors
Apache, Blackhawk, Chinook, etc. ~30 years old, V-22 20
Active rotor technologies being researched:
Active Flaps – Vibration reduction
Active Slats – Lift augmentation
Active Blowing – Stall alleviation
Active Twist – Vibration or performance improvements
Near-body grids must account for these in rotating frame
Passive technologies also actively researched
Advanced tips
Advanced airfoils

8. Flapping Wing the “Buzz” in Vertical Lift
Nascent research area for UAV, MAV applications
Small scale, low Reynolds number critical for flapping wing lift
Physical features of flow dramatically different than traditional rotorcraft aerodynamics
Ample work to be done in development and validation

9. Boundary Conditions Are Not Straightforward
Induced inflow large in hover, diminishes with forward speed
~ 50 ft/sec for 20,000 lb helicopter, depending on rotor radius
Cat 4 hurricane for “SoloTrek” ducted fan exoskeleton
Grid must either be large enough that inflow is zero or must account for inflow at boundaries

10. Rotor Wake Critical for Hover Performance
Calculation of downwash and swirl affect figure of merit/power required
Wing download critical for tilt rotor hover performance
Special topics:
Coaxial rotors
Vortex ring state
Intermeshing rotors
Tandem rotors

11. Rotorcraft Wake Modeling
Several approaches being studied:
Grid refinement
Vortex transport method
Particle vortex transport method
Traditional free wake methods highly empirical, sensitive to parameter changes in model

12. Velocity Gradient in Tip Vortex Important for Vibration, Noise
Would like to keep tip vortex organized for multiple revs
Fine mesh required to resolve velocity gradient in trailed vortices
Traditional RANS CFD numerically diffuses vortex within several chord lengths, does not model rigid body rotation

13. Wake Impinges on Fuselage and Tail Even in Benign Conditions
Interaction between main rotor wake and tail rotor important for noise, control, and vibration
Download can adversely affect performance
How to measure extremely
Complex flowfield for validation

14. Rotor Dynamics 101
Natural frequency of rotating blade hinged at the root: 1/rev (Hinge offset rotor < 1.05/rev)
Cyclic pitch used to balance moment, control helicopter
Cyclic pitch inputs applied at 1/rev => rotor being forced near or at resonance
Do not get infinite response because of large flap damping ~ 50% critical
Controls highly coupled

15. Don’t Forget Structural Dynamics
“A helicopter is a fatigue testing machine that also flies”
Severe vibration in rotor system
In theory, only multiples of N/rev transmitted down shaft
In reality, largest vibration comes from 1/rev
High-fidelity, nonlinear structural dynamics models would be useful but don’t exist
Multi-body dynamics models more common, often require extremely small time step, difficult to parallelize

16. Coupled CFD/CSD Analysis—Loose Coupling
Couple comprehensive analysis with CFD airloads
“CSD” is stick model—beam theory for blades, simple fuselage if at all, mutlibody dynamics
Exchange data once per revolution for loose coupling
Calculating periodic response—“trim solution”
Can’t put airloads on right hand side
Elastic + inertial = CFD blows up—damping wrong
Elastic + inertial + simple aero = CFD – simple aero

17. Coupled CFD-CSD Analysis—Tight Coupling
Couple comprehensive analysis with CFD airloads
“CSD” is stick model—beam theory for blades, simple fuselage if at all, multibody dynamics
Exchange data once per time step for tight coupling
Integrating equations in time—“transient solution”
Airloads go on right hand side at every time step
Elastic + inertial = CFD ok for tight coupling

18. What Would We Do In CSD If We Did CSD Research?
Detailed 3-D model of blades
Current technology is beam theory
Reality is complex composite structures with tuning weights, large variation in sectional properties, actuators?
Rotor dynamics and fuselage dynamics run independently
Rotor dynamics code has elastic modes for fuselage
Fuselage code has forcing function to simulate rotor
Coupled rotor/airframe analysis not on the radar
Fuselage nonlinear from windows, doors, fasteners, etc.

19. Acoustics—CFD for Noise Sources
Noise calculated, then propagated to observer
Either calculated at source or on “permeable sphere”
Noise often dissipated in CFD solution because it’s “in the noise”
Large grid required for permeable sphere around entire helicopter.