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in use. A comprehensive study by Desopper et al.24 was based on a series of
computational and experimental studies at ONERA. Preliminary work (including
that of other researchers) indicated that the intensity of the transonic flow was reduced
for a large azimuthal sector of the advancing blade side by using a constant 30 degree
sweptback tip, and hence the power required to drive the rotor was also decreased.
However, a strong expansion observed on the outboard part of this tip limited the
total benefit to be gained. A sweptback tip having a progressively increasing angle of
sweep, i.e. a parabolic leading edge, was found to eliminate, or at least delay, this
expansion, hence providing a further gain.
A systematic study of blade tip shapes was then undertaken covering analytical
and experimental studies, which provided ample verification. The tip shapes considered
in the former are shown in Fig. 6.34 and the pressure contours computed using a
three-dimensional unsteady transonic small perturbation method are shown in Fig.
6.35. The parabolic swept tip (PF2) indicates a significant decrease in supercritical
flow intensity, this being borne out by experiment as indicated in Fig. 6.36 (the FL5
tip is similar to PF2 except that there is no discontinuity in sweepback angle at the
start of the parabolic tip section).
On the retreating side of the rotor disc, the main performance limiting factor is that
of stall, or to use a more appropriate term, dynamic stall. The stalling characteristics of
an aerofoil subject to rapidly changing or dynamic conditions is markedly different
from that operating in steady conditions, as will be described later. The maximum
value of CL under dynamic conditions that can be attained determines the stall boundary,
and blade design aimed at increasing this value can lead to a significant improvement
in performance.
Research conducted by the UK Royal Aircraft Establishment (RAE), now the Defence,
1.4
1.2
1.0
ψ = 112°
0 0.5 1.0
ML
x/c
1.4
1.2
1.0
ψ = 237°
0 0.5 1.0
ML
x/c
Fig. 6.33 Chordwise pressure distribution (showing local Mach number)
224 Bramwell’s Helicopter Dynamics
Rectangular F30
0.1R
0.05R
F-30
0.1R
0.1R
Anhedral effect
δ = 10°
PF2
Fig. 6.34 Different tip shapes, ONERA non-lifting unsteady calculations24
ISO-Mach Lines
μ = 0.5
ψ = 90°
NACA 00.11
F-30
MωR = 0.64
Fig. 6.35 Study of different tip shapes, non-lifting unsteady calculations, pressure contours24
RECT F30 PF2
Rotor aerodynamics in forward flight 225
V0 = 91 m/s
r/R = 0.9
Rect.
FL5
0.5
0
0.25 0.5 0.75
– Cp Thrust level
CT/σ = 0.0665
r/R = 0.95
0.5
0
0.25 0.5 0.75
– Cp
Rect.
FL5
x x
Fig. 6.36 Measured pressure distributions on the upper side of rectangular and parabolic swept (FL5) blade tips24
Evaluation and Research Agency (DERA), with GKN Westland Helicopters led to the
BERP (British Experimental Rotor Programme) rotor design25. The blade has a sweptback
paddle-shaped tip, as shown in Fig. 6.37, which is its most obvious observable
characteristic. However, the design of the blade as a whole and the aerofoil sections
selected are also vital features. Flight tests on rectangular blades had shown that the
high incidence performance required on the retreating side was not needed over the whole
span, but the requirements could not be relaxed between 65 and 95 per cent radius.
Thus, if a thin section were used near the tip to avoid advancing blade Mach number
constraints, this would have an adverse effect on the retreating blade stall performance,
even though a more appropriate, i.e. thicker, aerofoil section were used inboard.
As will be seen in a later paragraph, the BERP tip confers a high incidence
capability independent of these considerations, and allows the conflicting requirements
on the advancing and retreating side of the rotor to be met independently. The BERP
blade uses an aerofoil (RAE 9645) between 65 and 85 per cent of blade radius that
produces a high dynamic CLmax for good stall performance. However, it also gives
rise to a high nose down pitching moment which would be undesirable were it not
counteracted by using a different section, RAE 9648, inboard of 65 per cent radius.
This has a reflexed trailing edge and a nose up pitching moment characteristic; being
inboard, its lower stalling angle is not so important. The BERP tip is swept and its
increased chord provides it with a lower thickness to chord ratio than just inboard
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Bramwell’s Helicopter Dynamics(113)
