The Art of the Helicopter John Watkinson - Karatunov.net

158 The Art of the Helicopter
At (d) a variant of the teetering rotor is one in which the blades have individual
flapping hinges so that no bending moments occur due to coning. This allows the blades
to be more lightly built than in a conventional teetering system, and also reduces 2P
hop. The effect of the offset between the flapping hinges is eliminated by the action of
a third underslung bearing. This approach is known as a tri-hinge head and is used in
the Robinson R-22.
At (e) is the so-called door-hinge hub. This hub is relatively thin in section to reduce
drag. The thin section also introduces some flexibility to reduce the effect of two-per
hop. This type of head was used on the Bell AH-1G.
At (f) is the semi-teetering head of the Bell 222. This contains elastomeric feathering
bearings in compression, a thin flexural section for accommodating 2P hop and a
centring spring on the teeter bearing to give stability in low-g manoeuvres.
Zero-offset heads are relatively intolerant of the CM location of the helicopter and
cause handling difficulties in low-g conditions. The teetering hinge also allows blade
sailing and starting the rotors may be impossible above a certain wind velocity because
of the danger of a boom strike. This was acceptable in early machines, and possibly
remains acceptable in today’s light machines, given the weight advantage due to the
elimination of dampers. However, it is not acceptable in helicopters intended for military
or emergency service. Other types of head can overcome these difficulties.
Although most zero-offset heads have two blades, more blades can be fitted.
Figure 4.10 showed the zero-offset head of the Sikorsky R-5 that has three blades. As
the flapping hinge axes pass through the mast axis no flapping moments can be communicated
to the mast. However, the blades are individually hinged and no underslinging
is possible. This means that the CM of the coned rotor is above the flapping bearings.
When the disc tilts relative to the mast this arrangement cannot allow the blades to conserve
momentum and dragging hinges must be fitted. This then requires drag dampers
and landing gear damping. Such a head is a poor combination because it needs the drag
hinge/damper complexity of the articulated head but does not offer the advantages.
In the classical articulated head the flapping bearings are offset from the mast axis
so that a couple is exerted on the mast when the disc tilts, tending to make the hull
follow the disc attitude. This couple may also be used to allow a non-ideal hull CM
position. Any number of blades can be fitted in this way, usually between three and nine.
When coned, the disc does not flap about its CM and so dragging hinges are needed.
It was realized early that making the flapping and dragging hinges intersect allowed a
conventional Hooke joint to act as flapping and dragging hinge. The feathering bearing
was then placed outboard. This arrangement became relatively standard for a long
time and appeared on rotor heads from numerous manufacturers including Sikorsky,
Westland and Enstrom. The larger Mil machines place the dragging axis outboard of
the flapping axis, probably to allow space for dragging dampers when a large number
of blades are required.
The flapping blades need droop and sailing stops and sometimes these are centrifugally
operated so that they only operate as the rotor slows down.
The articulated head results in a rotor that can start in higher winds than can a
teetering rotor, and which can retain some control at zero g. On the other hand the
articulated rotor is extremely complicated and needs a lot of maintenance. As materials
have improved, it has become possible to build structures that will withstand
stress without excessive weight or that will flex without fatigue. Thus instead of using
a mechanical hinge to relieve the stress, the designer has the choice of withstanding
the stress with a stiff structure or relieving it by flexing. With modern materials such
as Kevlar and carbon fibre it is possible to make a rotor in which flexing is minimal.
However, this is not desirable as a great deal of vibration would be transmitted to the