Rotors in practice
The loads on the rotor blades are large and time variant and the articulated head was developed to minimize them. Articulation also reduces the rate at which the aircraft responds to the controls. In this chapter the dynamics and control of the articulated rotor will be introduced, along with later developments such as hingeless rotors that became possible with the development of modern materials.
The essential functions of the rotor head are to transfer shaft power to the blades, to transfer the resulting rotor thrust to the mast, to resist the blade tension and yet to allow the blades to move on their feathering axes so that cyclic and collective pitch control is possible. This simple and elegant arrangement was initially not possible because it requires extremely strong materials. Thus practical rotor heads are often much more complex.
Figure 4.1 demonstrates that the rotor blade of a typical helicopter pulls outwards with a force roughly equal to the weight of a bus. It’s no wonder those floppy blades straighten out. It requires little imagination to predict what would happen if a blade attachment failed. Clearly the rotor head has to be extremely strong and reliable and as a result it is one of the heaviest individual components in the machine.
Fig. 4.1 Given the coning angle and the weight of the helicopter, the blade root tension follows. Here it will be seen that the same coning angle could be obtained by suspending the helicopter on cables with a bus on the other end. Blade attachments have to withstand tremendous loads yet allow the blades to feather.
The variation in lift, lift distribution and drag as the blades turn, especially in forward flight, produces alternating stresses which can fatigue materials. The adoption of articulation was first suggested by Renard in 1904 and was essential in early rotor – craft to reduce the stresses involved, particularly the alternating stresses in the blades and the moments applied to the mast when rolling and pitching manoeuvres are performed. Some texts claim that articulation is necessary to handle the lift asymmetry in translational flight, but this is quite incorrect.
In addition to the feathering bearing, the fully articulated rotor head carries each blade on freely turning bearings which allow flapping, or movement above or below the plane of the rotor head, and dragging, a swinging movement in the plane of the blades which is also called lead/lag. The presence of the flapping and dragging bearings means that moments about their axes cannot be transferred from the rotor head to the blades, and so bending stresses in the blade roots are dramatically reduced.
Figure 4.2(a) shows one arrangement which has been widely used by, for example, Sikorsky and Enstrom. The feathering bearing is outboard of the flapping bearing. This arrangement has the advantage that the loads fed into the control system when the blades flap and drag are minimized. The flapping and dragging hinges can be displaced or coincident in various designs and this is considered in section 4.7.
If the axes of the flapping hinges pass through the shaft axis, the result is called a zero-offset rotor head and these will be considered in section 4.10. In a conventional articulated head, the flapping bearings are horizontally displaced, or offset, typically by a few per cent of the rotor diameter, and it will be seen from Figure 4.2(b) that blade tension can produce a control moment on the rotor head if the shaft is not at right angles to the tip path plane. Consequently the hull tends to follow the disc better when offset is employed and so the machine becomes more manoeuvrable, although a stronger mast is needed to withstand the moments.
Whether by flexing or by movement of a bearing, each blade can flap and feather as it rotates. A consequence of these degrees of freedom is that three rotational axes need to be considered when studying the behaviour of a turning rotor. The way in which these axes interrelate is fascinating and allows an insight into the behaviour of a blade in flight.
Figure 4.3(a) shows a helicopter with an articulated rotor in forward flight. The cyclic stick will need to be trimmed forward to maintain airspeed, since this will reduce the angle of attack of the advancing blade and increase that of the retreating blade, so that they generate equal lift moments. The stick will also need to be trimmed towards the retreating side to oppose the inflow roll. The tip path plane is tilted forward to obtain a forward component of rotor thrust to balance drag.