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The
Defence Aeronautics Programme at the CSIR in South
Africa has been a key technology solution provider
to the South African Air Force (SAAF) for the past
five decades. Considerable resources have been dedicated
to Helicopter Technology in particular. The Programme’s
CFD, modeling and simulation, aeroelasticity, advanced
structures and wind tunnel facilities are being used
to advance aeronautical technology relevant to helicopters
in support of helicopter acquisition, operations,
aerodynamic analysis and optimisation, flight simulator
development, hardware development and improvement,
etc.
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The CSIR’s Defence Aeronautics
CFD group is directing substantial effort in simulating
the flow field around helicopters in hover and forward
flight. At this stage CFD models of two well-known
helicopters, viz. Rooivalk Combat Helicopter and the
Oryx Transport and Medium Lift Helicopter, have been
developed and simulation results have already been
used in the design and countermeasure environments.
STAR-CD, and in particular pro-am, have been invaluable
tools in the development of this technology.
If one considers the nature of the
flow field around a helicopter in hover or trimmed
forward flight and the complex geometry of the entire
aircraft, one can understand the extreme demands placed
on the CFD software and hardware. While it is possible
to simulate a hovering helicopter using steady state
simulation techniques, a transient simulation is required
for the helicopter in trimmed forward flight as each
rotor blade pitches as it rotates around the rotor
shaft through the helicopter azimuth. Thus the motion
of each blade has to be specified and controlled individually
as the simulation proceeds. Moreover, the tail rotor
movement must be specified independently from that
of the main rotor. STAR-CD’s advanced mesh movement
capability enables one to model such a case.
The fuselages are complex, and multi-block
mesh generation techniques proved time-consuming and
inefficient. Automatic meshing comes to the rescue!
First, solid models were generated in SolidWorks and
surfaces were wrapped around the solid fuselages using
a STAR-CD plug-in. These surfaces were conditioned
and modified to an acceptable level using pro-am’s
surface tools. Thereafter, the original surface was
expanded by an offset distance to create a subsurface.
Using alternating cell classification and local refinement
in pro-am, an efficient trimmed-cell mesh, external
to the subsurface, was created. The sub-layer, a layer
of cells normal to the helicopter surface, fills the
space between the original surface and the subsurface,
allowing more control over the cell quality adjacent
to the helicopter surface and improving turbulence
modeling and viscous drag prediction.
Rotor collective, lateral and longitudinal
rotor tilt, lateral and longitudinal cyclic settings,
and rotor geometry are among the various parameters
required to model the main and tail rotors. These parameters,
together with the fuselage attitude, are extracted
from dynamic flight simulations using CAMRAD J/A and
entered into a FORTRAN code that automatically generates
the independent rotor grids using block structures,
and performs the integration with the fuselage grid.
A mesh movement script, with the lateral and longitudinal
cyclic pilot inputs, controls the pitch attitude of
each main rotor blade while being rotated around the
rotor centre through the helicopter azimuth. The same
script rotates the tail rotor.
The modeling of the fuselage and
the main rotor is being validated with results from
tests conducted in the Low Speed Wind Tunnel and 7
metre wind tunnel at the Defence Aeronautics Programme.
This intense validation exercise is planned to take
place over the next two years with funding from the
SAAF and Armscor. However, this CFD Groups’ efforts
to date have provided invaluable information and insight
for the South African Air Force.
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Velocity Magnitude
Contours for Oryx in Hover
Velocity Magnitude
Contours for Oryx in Hover with superimposed pressure
contours on surface
Surface of trimmed
cell mesh for
Rooivalk
Attack Helicopter
Pressure contours
on helicopter
surface in forward flight
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