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(n – 1) is equal to b.
The type of hub influences the vibratory loading systems applied to the fuselage.
The vibratory pitch and roll moments generated by an articulated rotor are significantly
smaller than those produced by a semi-rigid or hingeless rotor.
The choice of the number of blades is also important. This is primarily due to two
effects. First, the level of aerodynamic oscillatory loading tends to decrease as the
harmonic order increases, and since the bΩ forcing is the prime cause of fuselage
vibration, the larger the number of blades, the smaller will be the basic aerodynamic
vibratory input.
Figure 8.3 shows the predicted rotor head loadings for a typical medium weight
helicopter with a semi-rigid hub and four or five blades through the forward speed
range. It indicates a very significant reduction in hub vibratory moments when five
blades are used, which is primarily due to the fact that the 3Ω aerodynamic loading
is not transmitted to the fuselage with a five bladed rotor.
294 Bramwell’s Helicopter Dynamics
100 120 140 160
Forward speed, knots
600
400
200
100 120 140 160
Forward speed, knots
600
400
200
100 120 140 160
Forward speed, knots
600
400
200
4 Blades
5 Blades
80000
60000
40000
20000
80000
60000
40000
20000
100 120 140 160
Forward speed, knots
100 120 140 160
Forward speed, knots
Pitch moment
lbs ins
Roll moment
lbs ins
Longitudinal
shear, lbs
Vertical
shear, lbs
Lateral
shear, lbs
Fig. 8.3 Comparison of four and five blade hub loads
8.3 The dynamic design of rotor blades
The successful design of a helicopter rotor system is concerned with meeting the
exacting dynamic requirements which it is necessary to satisfy if acceptable behaviour
is to be achieved in the areas of handling qualities, fuselage vibration levels, acceptable
airframe and rotor blade fatigue lives, and rotor system aeroelastic stability. The
trend towards increasing mechanical simplification combined with increased
aerodynamic and structural efficiency has intensified the difficulties experienced in
achieving the optimum dynamic characteristics. However, the particular properties
of composite materials, often used in a hybrid glass and carbon form of construction,
have more readily enabled the design aims to be met.
The correct dynamic design of rotor blades is essential for two main reasons:
(i) the minimisation of the amplification of the rotor blade aerodynamic vibratory
loading which is transmitted to the fuselage; and
(ii) the minimisation of the total vibratory loading of the blade to provide an
acceptable fatigue life.
For many years, helicopters utilised rotor blades of essentially constant radial
Rotor induced vibration 295
distributions of mass and stiffness, and their dynamic characteristics, when associated
with a rotor hub having a low offset of flapping hinge, and a modest helicopter cruise
speed, led to a not very satisfactory but acceptable situation. However, the advent of
rotor systems with higher actual or effective offsets of flapping hinge, together with
helicopter cruise speeds in excess of 150 knots, has led to the necessity for refining
the structural design of the rotor blade to offset the increase in vibratory loading due
to the above effects.
Figure 8.4 indicates the unsatisfactory positioning of natural frequencies which
occurs if rotor blades of constant radial structural properties are associated with a
rotor hub having a significant effective offset of flapping hinge.
The proximity of the second and third flapping mode frequencies to the 3Ω and
5Ω aerodynamic forcing loads will lead to high vibratory fatigue loading of the
blades and, for the case of a four-bladed rotor, large 4Ω pitch and roll moments
transmitted to the airframe.
Figure 8.5 indicates the situation that could be achieved utilising the design flexibility
10Ω L3
2Ω
F4
F3
L2
9Ω
8Ω
7Ω
6Ω
5Ω
4Ω
3Ω
F2
F1 1Ω
R.P.M
L1
L3
L2
F2
F4
F3
F1
L1
Frequency
Rotor speed
Blade radius
Blade mass distribution Blade flapwise stiffness Blade chordwise stiffness
50 100 150 200 250 300
Fig. 8.4 Frequency spectrum for blade with constant radial properties
→
296 Bramwell’s Helicopter Dynamics
possible using a hybrid combination of composite materials to produce a significant
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Bramwell’s Helicopter Dynamics(148)
