The Art of the Helicopter John Watkinson - Karatunov.net

108 The Art of the Helicopter
incorporate electronic systems and sensors used either to control or navigate the
machine or as part of its mission equipment. Electronic equipment does not take
kindly to vibration, or if specially adapted, must be more expensive.
Vibrating masses are a common origin of sound, so it follows that a vibrating helicopter
will be noisy both inside and out. External noise causes disturbance in civil
operations and may compromise military missions. A significant part of the design process
of the modern helicopter is taken up in minimizing noise and vibration. There are a
number of approaches that will be combined in various ways. Study of the aerodynamic
origins of vibration may allow a reduction at the source. Once the forcing frequencies
are known, the design of the structure should ensure that resonant responses to those
frequencies are minimized. This is known as detuning. Vibration may be isolated or
decoupled in some way, damped using lossy materials to convert flexing into heat, or
opposed or absorbed through various cancelling techniques, both passive and active.
The reduction of the vibration at source has to be the technically superior solution.
The disadvantage of decoupling the vibration from the passengers is that although their
comfort is improved, the vibration experienced by the parts that are not decoupled
may actually increase. It is easy to see why. For a given exciting force, the amplitude
of a vibration is inversely proportional to the mass being vibrated. In a conventional
helicopter the dominant masses are those of the transmission, hull and payload. If the
hull and payload are decoupled, the mass seen by the vibration source is now only that
of the transmission and the amplitude of the vibration there must be greater and not
necessarily beneficial.
An obvious way of reducing vibration at source is to use non-sinusoidal cyclic pitch
control that allows the lift moment to be more nearly constant as the blade turns.
The means to do this are non-trivial and are described in section 3.28. Another way
of reducing vibration is to use more blades. This has several advantages. First, the
load on each blade is reduced and with it the magnitude of the forcing function of
each blade. Second, the more blades, the higher the frequency of the vibration will be
and the lower will be the amplitude for a given mass. Figure 3.37 shows that a threebladed
rotor eliminates the 2P hop of the two-bladed rotor, whereas a four-bladed rotor
reintroduces some 2P lateral vibration. It will be seen from Figure 3.38(b) that an even
number of blades gives lower rocking frequencies, making an odd number preferable.
Consequently the change from four to five blades gives a significant improvement. Large
transport helicopters inevitably have a large and generally odd number of blades – seven
or nine – simply to contain the large vibrations that must result from generating high
rotor thrust. Using many blades will be expensive and result in a large rotor head having
high drag.
It is common to design the blades so that their natural frequencies of vibration are
not in the spectrum of the excitation. If any of the excitation frequencies coincide
with the resonant frequencies there could be a significant response at that frequency.
Any coincidence can be seen from an interference diagram as shown in Figure 3.39.
This interference diagram is for the Westland Lynx and shows the operating speed
(318 RPM) as a vertical line. The excitation frequency may be at any integer multiple
of the rotor frequency. Figure 3.39 shows that, as rotor speed increases, the excitation
harmonics (thin lines starting at the origin) fan out. The resonant frequencies of the
blades also change with rotor frequency because of the effect of centrifugal stiffening.
Figure 3.39 shows how flapping, ωf , and lagging, ωl, resonant frequencies may change
with rotor speed.
The only coincidence between excitation and response is the close correspondence
between the fundamental flapping resonant frequency and the rotor fundamental frequency.
This is the characteristic that results in rotor precession. Note that as the Lynx