The Art of the Helicopter John Watkinson - Karatunov.net

In the case of a rotor having offset hinges, a teetering spring or a hingeless head, the
rotor head can apply a couple to the fuselage trying to roll the shaft axis in line with the
tip path axis. The angle at which the fuselage settles in the hover will now be a function
of the rotor head flapping stiffness, the vertical position of the hull CM and the tail
rotor roll.
Finally, the designer may mount the main transmission at a suitably slight angle so
that the hull stays more or less level with the disc and shaft tilted by the same amount.
This means there will be little or no flapping in the hover. The Sikorsky Skycrane is an
example of this approach. In the Mi-24 Crocodile (NATO code name Hind), the hull
and transmission are tilted with respect to the undercarriage so that disc is tilted at
approximately the correct angle. To compensate, the hull is twisted ahead of the main
rotor so the cockpit remains level.
In forward flight the hull is moving through the air and so can develop a side thrust
if it is set to a suitable angle of attack. If the machine is flown with no sideslip, the
side thrust must come from the rotor tilt, whereas if the rotor disc is level, the side
thrust must come from side slipping the hull. Clearly the machine can be flown with
any combination of these two effects.
The least drag will be suffered if the hull is aligned with the direction of travel, and
this may be significant under marginal power conditions. Zero-slip trim has the further
advantage that the compass or direction indicator is actually displaying the helicopter’s
heading, making navigation easier. Flying at zero slip is aided by an airflow-sensing
device showing the direction from which the air is approaching the hull.
Unfortunately most helicopters are fitted with an instrument inherited from fixedwing
aviation, where it is more useful. It is called a slip indicator because in fixed-wing
aircraft that is what it does. It is not commonly appreciated that in helicopters the same
instrument does not indicate true slip. This will be discussed in detail in Chapter 7.
5.3 The conventional tail rotor
The conventional tail rotor is mechanically a small main rotor. The term small being
relative because, for example, the tail rotor of the Mi-26 is about the same size as the
main rotor of an MD-500 and produces a similar thrust. The tail rotor needs no cyclic
pitch control, only a collective mechanism actuated by the pedals. Aerodynamically
it is also a scaled down main rotor, having much the same physics in the hover, and
suffering the same indignity of being thrust through the air edge-on in translational
flight.
As the tail rotor is expected to produce thrust in either direction, the blade section
will generally be symmetrical and blade twist is only occasionally used. Blade taper
can still be usefully employed to make the inflow more even, and this has been seen in
practice, although it is not common because constant chord blades are cheaper to make
from metal. When blades were made of wood, taper was relatively easy to adopt and as
the use of moulded composites grows there is a possibility that taper will make a return.
The teetering rotor has many advantages for use at the tail, and the disadvantages
it has as a main rotor are not relevant. As a teetering tail rotor is supercritical (see
section 4.15) it cannot suffer from ground or air resonance. The two-bladed teetering
tail rotor is simple and therefore light and has become extremely common. Teetering
can still be used with four-blade rotors. Two independently teetering rotors can be
fitted to a common shaft with a small offset between the disc planes. It is not necessary
to mount the two rotors 90 ◦ apart; in fact it is advantageous not to do so. Mounting
the blades in an X or scissors configuration produces less noise because there is no
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