The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 373
For easy loading, the Chinook is designed around a constant cross-section cabin
with a rear ramp. The fuel and undercarriage are housed in two long side sponsons
that provide stability on water and cause no intrusion into the load space. For similar
reasons the engines are placed above the cabin, and the synchronizing shaft and control
pushrods run in a hump or spine above the cabin. The immense rear pylon/fin is
used to house the engine combining transmission, the oil cooler, the rear gearbox and
swashplate actuators, the APU and almost anything else that needs a home with the
result that the hull is kept free for payload.
The control system of the tandem rotor helicopter is more complex than in a tail
rotor-type machine. The rotor heads need to be articulated for reasons that will be
seen. Figure 9.26(a) shows that the swashplates tilt simultaneously for roll control and
tilt differentially for yaw control and to oppose any residual torque imbalance. They
move up and down together for thrust control and differentially for fore-and-aft (pitch)
control and to handle CM shifts. All of this is achieved in a Chinook in a diabolical
mechanical mixer located in the control closet behind the port pilot’s seat which takes
the inputs from the pedals, cyclic stick and thrust lever (tandemspeak for collective)
in the cockpit floor and adds and subtracts in various ways to obtain four pushrod
outputs which go into the roof, one to control each side of each swashplate. The conceptual
mechanism is shown in Figure 9.26(b). The autopilot is incorporated in an
unusual fashion. Electrical autopilot signals and mechanical pilot inputs are supplied
to small duplicated hydraulic actuators in the control closet located before the control
mixer. These are called integrated lower control actuators (ILCA) and they operate
the control mixer. The mixer pushrod outputs are then led to duplicated high pressure
hydraulic actuators at the swashplates. These actuators have mechanical inputs. This
approach allows a conventional autopilot to output yaw, pitch, roll and thrust commands
to the airframe. If the autopilot had to control the swashplate actuators directly,
an autopilot mixer would have been needed.
When the cyclic stick is pushed forward, differential collective pitch increases the
thrust of the rear rotor relative to the front rotor (Figure 9.27(a)). The hull is tilted
forward about the pitch axis (b) and this makes both rotor shafts tilt relative to the
tip path axes. This hub tilt causes a cyclic feathering effect such that both rotors pitch
forward to follow the hull (c). This is the control mechanism used in hovering.
In order to move into forward flight, forward differential collective is maintained
so the machine gathers speed. The lift asymmetry due to translational flight in both
rotors will cause the tip path axes to flap back with respect to the two control axes as
shown in Figure 9.28(a). At a given airspeed, this results in a larger change in cabin
floor angle, reduced blade clearance at the rear of the front rotor, more hull parasite
drag and a large amplitude of flapping and dragging at the rotor heads because of the
large differences between the tip path and shaft axes.
Thus in practice a tandem helicopter requires means to apply forward cyclic control
to both swashplates in translational flight as a function of airspeed. Figure 9.28(b)
shows that this results in closer alignment between the tip path and shaft axes, reducing
the amount of flapping and dragging and improving the clearance of the front rotor.
In the Chinook the fore-and-aft cyclic controls are not connected to the cyclic stick.
Instead they are driven by an automatic system which is airspeed sensitive and starts
to apply forward cyclic at speeds above about 70 knots. The application of forward
cyclic to reduce flapping will also tend to increase the airspeed. To compensate for this,
a series actuator, known as the DASH actuator, in the fore-and-aft cyclic pushrod is
also operated by the automatic system in such a way that the pilot sees a fairly constant
positive gradient between cyclic stick position and airspeed which is not impaired by
the automatic forward cyclic control of the swashplates. The DASH actuator can be
seen in Figure 9.26(b).