The Art of the Helicopter John Watkinson - Karatunov.net

188 The Art of the Helicopter
down the tail boom, cooling the engine in the process. The exhaust gases were mixed in
to provide further energy. At the rear of the hollow boom the gases were ejected from
a sideways-facing nozzle at about 150 feet/sec (45 m/sec). The Weir was also unusual in
having a constant velocity joint in its zero-offset rotor head. The Weir proved that the
tail rotor could be eliminated, but it also proved that a simple fan/nozzle arrangement
is very inefficient: around 25% of the engine power was consumed in the anti-torque
system.
Stanley Hiller built a similar machine that flew in 1946. At around the same time
Antoine Gazda built the Helicospeeder. These had piston engines. In France in 1954,
Nord flew a machine called the Norelfe that used the exhaust from its turbine engine
to oppose torque. All were abandoned. Today it is clear that these results follow from
momentum theory. It is less efficient to derive thrust by ejecting a small cross-section
of air at high speed than it is to eject a large cross-section slowly.
These experiments were enough to convince the industry that the tail rotor would not
quickly be replaced; not in fact for about 30 years. In the 1980s, McDonnell Douglas
developed a viable system which they called NOTAR and which does not require any
more power than the conventional tail rotor. The NOTAR system has two components,
a unidirectional system that provides about 60% of the anti-torque requirement, and
a bi-directional system that provides the remainder of the thrust as well as providing
yaw control.
The unidirectional system uses the tail boom as a wing operating in the rotor
downwash. The tail boom is cylindrical and boundary layer control is used. Figure 5.18
shows that on one side of the boom are two downward facing slots fed with air from the
interior and pressurized to about 1 psi by a fan. The air emerging from the slots energizes
the boundary layer and delays separation significantly so that the downwash flows
around the boom and exits with a sideways component. Effectively the downwash has
been accelerated to the side and the reaction to this is a thrust on the boom. The thrust
has a lateral component and a downward component corresponding to the induced
Fig. 5.18 In the NOTAR system, the boom is pressurized by fan and fitted with boundary layer control slots
that energize the flow of the downwash to turn the tail boom into an airfoil.