The Art of the Helicopter John Watkinson - Karatunov.net

376 The Art of the Helicopter
The yaw control is by differential tilt of the rotor thrust and clearly if there is no rotor
thrust there will be no yaw couple. If rotor thrust reverses the yaw control will reverse.
Whilst this is not a problem in most flight conditions, a zero-g pushover could result in
loss of yaw control. A crop-dusting turn or return-to-target manoeuvre is also difficult
for a tandem because when the hull is pointing straight up after the first quarter loop
the rotors need to be set to flat pitch and this removes the yaw authority. Consequently
the tandem will perform a variation on the manoeuvre in which the yaw is commenced
during the quarter loop. Momentum carries the yaw on during the flat pitch phase.
The tandem is generally designed to have a level cabin in translational flight and
when at rest on the ground. This means that in the hover the cabin will slope down to
the rear and the machine must touch down with the rear wheels first. This allows an
interesting taxi mode where the rear wheels are carrying a significant weight but the
front wheels are clear of the ground. Forward speed can be controlled by rear wheel
brake and steering is possible by applying the brakes differentially.
The waterborne characteristics of a tandem are worth a few comments. The contrarotation
means that the rotors can be started and stopped whilst afloat without the
machine rotating as a single main rotor machine would. The built-in tip path plane tilt
causes a tendency to forward motion when the hull is floating level. When afloat the
acrylic cockpit chin windows are partially submerged, and whilst this is not a problem
whilst stationary, water taxiing at speed will put them under tremendous stress. To
avoid this, pilots will lift the nose in a water taxi. The floor of the cabin is sealed to
act as a raft for use on water and the ramp can be lowered whilst afloat to deploy or
recover small boats or swimmers.
In the tandem rotor helicopter the rotor heads are designed to meet different requirements.
The yaw control is obtained by tilting the two rotor discs laterally in opposite
directions. Effective yaw requires that the tip path axes actually make a significant tilt
and the flapping hinges are needed to allow this to happen. If conventional heads with
significant flapping hinge offset are used, the result of an application of the yaw control
is a serious twisting couple applied to the fuselage. Thus for a tandem rotor helicopter,
the flapping hinge offset needs to be significantly reduced. This need not reduce the roll
response, because a roll is obtained by applying lateral cyclic to both rotors in the same
sense. As there are two rotors, each rotor only needs to apply half the roll moment.
In order to prevent heavy underslung loads reducing the lateral cyclic authority the
main load hook is on rollers and can traverse a curved lateral beam whose centre of
curvature is near a line joining the rotorheads.
In the Chinook, the flapping hinges are provided with a relatively small offset for the
above reasons. If the dragging hinges were coincident with the flapping hinges there
would be a large drag angle change between the power-on and autorotation states and
there would be little space for drag dampers. Instead the dragging hinges are placed outboard
of the feathering hinges. This arrangement works very well and the only drawback
is that when the blades are stationary gravity causes them to swing about the dragging
bearing. These are not necessarily vertical, being outboard of the feathering bearings.
9.10 Remotely piloted and radio-controlled
helicopters
In some cases putting a pilot in a helicopter is not the right thing to do. Without a pilot
the helicopter can be much smaller, or it can lift more. The pilotless helicopter can also