26-06-2012, 01:52 PM
Aerodynamic Perturbations Encountered by a Helicopter
Landing on a Ship - Effects on the Helicopter Flight Dynamics
Aerodynamic Perturbations Encountered.pdf (Size: 2.65 MB / Downloads: 46)
INTRODUCTION
Flying a helicopter above or around a frigate deck,
landing on it or taking off from, are considered by
pilots as highly risked operations. Indeed, not only the
frigate moves, but also in the neighbourhood of the
deck, the helicopter has to face with the changing
aerodynamic wake of the ship superstructure. This
unsteady flow provides high mean speed gradients to
which aerodynamic turbulence (fluctuations) is added.
These conditions have important effects on helicopter
global performance, and behaviour.
Under SPAC funding, ONERA has realised wind tunnel
tests on a l/50* scaled La Fayette frigate model, and
has developed an aerodynamic wake model of the ship
landing area. This model was then used in order to
study the effects on flight mechanics.
WIND TUNNEL TESTS
Test equipment description
Wind tunnel measurements were carried out in the
ONERA-IMFL low speed wind tunnel (SH). This wind
tunnel has a closed circuit and a test section of 2.4 m in
diameter. The first 50 meters of the marine atmospheric
boundary layer was also simulated.
Measurements were performed on a l/50* La Fayette
frigate model, configured with its Crotale Missiles
(figure 1). Because of the model and the test section
sizes, the relative wind side-slip angles were limited to
+15” around the frigate longitudinal axis.
3D unsteady velocities were measured using crossed
hot film anemometer. Two velocity components were
simultaneously measured (u, v), and then (u, w) after a
90” rotation of the sensor. Thus, 2 redundant
longitudinal velocity measurements (Uv, Uw) were
provided. Speed measurements error is estimated as
2.6% of the infinite upstream wind.
Model realisation
The La Fayette air-wake model includes a mean wake
model and a model of velocity fluctuations
(turbulence).
a- Mean air-wake model
The test area above and around the ship deck is actually
a grid according to the test points definition. At any
point (H) of this area the 3 mean air-wake components
are interpolated, using the mean air-wake
measurements of neighbouring points.
The approach consists in locating the helicopter centre
of gravity in the test area elementary parallelepiped
(figure 23). The mean air-wake on this point is defined
via its components in the frigate axes, using a linear
combination of measured mean velocities on the
elementary parallelepiped tops, in respect with their
distance to the considered point.
EFFECTS ON HELICOPTER LOADS AND FLIGHT DYNAMICS
The frigate air-wake model has been connected to the
Eurocopter simulation code HOST (Helicopter Overall
Simulation Tool) [5]. The connection was done
assuming the helicopter as a mass point.
The air-wake model uses some general data coming
from HOST (wind velocity and direction), and specific
ship state data (velocity and heading). The ship
trajectory is then calculated in the air-wake model,
assuming a constant speed. Figure 26 illustrates this
implementation.
CONCLUSION
This paper presents an ONERA activity on helicopter
ship landing operations simulation improvement.
The first phase of this activity started with wind tunnel
tests in ONERA-IMFL, on a 1/50th model of the French
frigate La Fayette. A detailed database was provided at
50 kts for 3 wind side-slip configurations (O’,
15”,180°). A partial database was also generated at 25
kts of wind with 0” side-slip.