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degree of mass and stiffness taper along the blade span. This improvement is
characterised by the separation of the second and third flapping mode frequencies
from the 3Ω and 5Ω aerodynamic forcing frequencies respectively. Calculations
indicate that the 4Ω moment applied to the fuselage is reduced by 47 per cent for the
improved design.
The ratio of material properties that strongly influences the dynamic characteristics
of a blade is the modulus of the material divided by the density, i.e. E/ρ and Es/ρ. As
the values of these are essentially constant for all normally used metals, the scope for
the dynamic tuning of metal blades is severely limited.
However, for fibre reinforced composite materials, the values of E/ρ can vary
from 50 per cent to 250 per cent of the typical metal values, and the value of Es/ρ can
vary from 20 per cent to 200 per cent of the metal values, dependent upon fibre type
and orientation.
Modern composite blades can use a mixture of glass and carbon fibre, and the
Blade weight distribution Blade flapwise stiffness
10Ω 9Ω
L3 8Ω
F4
7Ω
5Ω
6Ω
L2
4Ω
F3
3Ω
F2
2Ω
F1 1Ω
L1
R.P.M
Rotor speed
Frequency
L3
F4
F3
F2
F1
L1
L2
Blade radius
Blade chordwise stiffness
50 100 150 200 250 300
Fig. 8.5 Frequency spectrum for blade with tapered radial properties
→
Rotor induced vibration 297
required values of flapwise, lagwise and torsional stiffnesses can be achieved almost
independently.
Figure 8.6 indicates a possible method of achieving satisfactory dynamic
characteristics.
Flapwise stiffness can be adjusted without affecting lagwise or torsional stiffness
by introducing discrete layers of carbon fibre on the upper and lower surfaces of the
glass fibre D-spar. The lagwise stiffness can be adjusted by introducing carbon fibre
in a spanwise sense at the extreme trailing edge of the blade without affecting flapwise
or torsional stiffness. Control of torsional stiffness is best achieved by selecting the
appropriate fibre type and orientation for the trailing edge skins.
The desirable characteristics of spanwise taper of mass and stiffness can be readily
obtained by the selective introduction of unidirectional carbon fibre in the spanwise
sense on the upper and lower walls of the main spar section.
8.4 Main rotor gearbox mounting systems
In order to minimise the vibratory forces fed into the fuselage, various methods of
mounting the main rotor gearbox have been used.
The following descriptions form a representative selection of the main passive
systems which have been used with varying degrees of success.
(i) Simple soft mounting of the rotor/gearbox/engine system (Fig. 8.7). Soft
suspension systems derive from the principle illustrated by the simple single
degree of freedom sprung mass undergoing forced vibration that may be found
in student textbooks on vibrations, e.g. Thomson1. The force transmitted through
the spring to the support is seen to be reduced the lower the ratio of natural
frequency to excitation frequency becomes.
Since the quasi-static deflections of the suspended system would be
unacceptably large if conventional soft mountings are placed at the gearbox
Blade section
Trailing edge skins
Glass cloth, uni. glass (±45°)
or uni carbon (±45°) to
control torsional stiffness
Uni. glass or carbon
to control lag stiffness
Discrete layers
of carbon to
control flap
stiffness
‘D’-SPAR
Uni. glass to
give basic strength
to blade
Metal erosion shield
Fig. 8.6 Composite blade construction
Chord line
1/4 chord
298 Bramwell’s Helicopter Dynamics
feet, it becomes necessary to isolate as large a mass as possible. Hence it is
attractive to mount the gearbox and engines on a raft, and then attach the raft
to the fuselage using the chosen mountings. By this means, an adequately low
natural frequency of the isolated system can be achieved without unacceptably
large displacements of the rotor, gearbox, engines and controls under trim and
manoeuvre loads. However, attenuation of the vibratory forces by this means
is only modest and certain loadings may be transmitted without any reduction.
(ii) A rotor/gearbox/engine mounting system designed to respond to the bΩ forcing
frequency in such a manner that the spring and inertia forces generated by the
response cancel, as far as possible, the effects of the rotor generated forcing
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Bramwell’s Helicopter Dynamics(149)
