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to 90° of blade rotation.)
Experiment
Classical wake
Distorted-wake analysis
Fig. 6.27 Displacement of wake boundary
0
Rotor aerodynamics in forward flight 219
The pioneering studies of Piziali, Landgrebe and associates, together with those of
Clark and Leiper, for hovering flight, and others of that era laid the foundations for
much subsequent wake modelling research. The main difficulties that had to be
overcome, once computational power became sufficient, were largely numerical,
such as instability and/or poor convergence. In order to avoid such problems, Bagai
and Leishman14 used a relaxation method, as opposed to a time-stepping approach,
which enforces periodicity as a boundary condition and specifies that the trailing
vortex filaments be attached to the blades as an initial condition. The basic model
was similar to that in Fig. 6.23. An improvement over an existing free wake modelling
code was demonstrated and comparisons made with existing experimental data.
Perspective views of the computed rotor wake in the hover and in forward flight
shown in Fig. 6.28; the rolling up of the wake boundaries downstream at the faster
forward flight speeds is clearly seen.
The advent of computational fluid dynamics, or CFD, has brought with it a steady
advance in the complexity of flows that can be analysed. However, the computation
of the complete flow field around a helicopter in forward flight, which may include
interaction with the fuselage and tail surfaces, remains an outstanding problem with
current computing capabilities. A brief account of the difficulties of using the various
CFD approaches is given by Coppens et al.15 in the introduction to an account of a
time-marching method of computing the rotor wake.
Figure 6.28 for the forward flight cases shows the rotor blades passing close to tip
vortices shed by preceding blades; this proximity, which is termed blade–vortex
interaction, or BVI, leads to impulsive loading and noise problems. It is particularly
severe in descending forward flight, such as during an approach to landing, when the
rotor tends to fly through its own wake. The plan view of a typical pattern of BVI is
shown in Fig. 6.29 (from Tangler16) for a two-bladed rotor at an advance ratio of
0.145. Four interactions are shown on the advancing side and three on the retreating
Hover μ = 0.05
μ = 0.075 μ = 0.1
Fig. 6.28 Perspective views of computed rotor wake (Bagai and Leishman14)
220 Bramwell’s Helicopter Dynamics
side. The strength of each interaction is dependent on the strength of the tip vortex,
the vortex core size, the instantaneous angle between the blade and the vortex at the
interaction, and the normal separation between the two, or the ‘miss’ distance. When
the rotor is in descending flight, the miss distance can become quite small.
On the advancing side, the approach of a blade section to a preceding vortex is as
shown in Fig. 6.30, with a typical resulting lift coefficient history indicated (adapted
from Leishman17). On the retreating side, the rotation direction of the vortex with
respect to the blade is reversed, and the lift coefficient history is similarly affected.
Since the lift change occurs over a very small period of time, it is practically impulsive,
leading to two predominant effects. One is that of blade loading, which possesses a
high frequency content, and the other is that of acoustic radiation, or noise.
The introduction of the improved wake modelling described earlier has allowed
the problems of BVI to be defined more accurately and solutions proposed with a
greater degree of confidence. The impulsive blade loading problem is a problem
because it leads to undesired structural vibration, but a possible solution can be
provided by use of higher harmonic control, or HHC. In this approach, higher than
180°
V
4
90°
6
7
5
2
3
1
0°
Fig. 6.29 Plan view of blade-vortex interactions (from Tangler16)
Rotor aerodynamics in forward flight 221
once-per-revolution input is introduced to the pitch control by means of subsidiary
signals to the actuators below the swash plate. An alternative scheme allows for the
signals to be fed to supplementary actuators positioned between the swash plate and
the blades, this being termed individual blade control, or IBC. Practical tests on both
arrangements applied to a four-bladed wind tunnel rotor model are described by
Kube and Schultz18, and these cover both vibration and noise aspects.
Considerable effort has been expended in studying noise propagation from
helicopters, since commercial viability is strongly affected by environmental
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Bramwell’s Helicopter Dynamics(111)
