The Art of the Helicopter John Watkinson - Karatunov.net

242 The Art of the Helicopter
tank forms part of the engine air intake so that de-icing is available with no additional
power required. The gearbox usually has a temperature probe which drives a gauge in
the cockpit. It is absolutely vital that the main gearbox is filled with the right kind of
oil. EP oil generally has a sickly smell of almonds.
The gearbox will be fitted with a sight glass so that the oil level can be seen from the
outside. Some machines have a chip detector. This consists of a pair of electric contacts
that are also magnetic. These are situated at the bottom of the gearbox. If a gear loses
part of a tooth, or a bearing starts to break up, the debris will be attracted by the
magnet and will bridge the contacts, turning on a warning light in the cockpit. Some
machines may also be designed so that the transmission can continue to function for a
short period even if all of the oil is lost.
As a helicopter rotor gets larger, it will turn at lower RPM as the tip speed tends to
remain fixed. The larger rotor will need more power but at a lower RPM. Thus the
torque delivered by the gearbox increases disproportionately with size, and may limit
the maximum size of conventional helicopters. The twin-rotor helicopter has an advantage
which is that for a given disc area the rotor radii will be smaller and the RRPM
higher. This reduces the torque needed and the numerical gear ratio, both of which
lighten the gearboxes to the extent that the two gearboxes may be lighter than the
single gearbox in a conventional machine of the same weight.
Rotor forces must be carried into the hull by a suitably reliable thrust bearing. In
many helicopters, the main rotor is fitted directly to the transmission output shaft so
that the rotor thrust is carried by the transmission bearings and the gear case. As an
alternative, the rotor is carried on a bearing attached to the hull structure. In this case
the transmission only provides drive torque and the drive shaft is freed from flight
loads. It is advantageous to transfer rotor forces into the hull as directly as possible. An
offset CM causes alternating bending loads in the shaft but constant loads in the hull.
A further advantage of the use of a separate thrust bearing is that the transmission
can be replaced without removing the rotor. Figure 6.34(a) shows the arrangement of
Fig. 6.34 In the AH-64 at (a)the main rotor bearing is attached to the hull and the rotor shaft does not carry
the weight of the machine. In the rear rotor of the Chinook, (b), a thrust bearing at the top of the fin carries
flight loads.