The Art of the Helicopter John Watkinson - Karatunov.net

198 The Art of the Helicopter
the amount of fuelling. This does not happen in a free turbine engine where only the
power turbine is geared to the rotor. Instead, greater power will be supplied by an
increase in the gas generator speed (N1) that delivers greater torque at the free turbine.
N1 cannot increase immediately. It takes time for the spool to accelerate. As a result
there will be a temporary loss of RRPM during the time taken to spool up. This is
known as transient droop. Figure 6.4(c) shows transient droop following a step change
in power demand. Turbocharged piston engines will also display transient droop as the
turbocharger accelerates.
Transient droop may be reduced through sensing the collective lever velocity. During
a landing, collective will be lowered and this will cause the gas generator to spool down.
If the landing is to be rejected, the pilot will pull the collective lever up. A system sensing
collective velocity will be able to start spooling up sooner than one which simply waits
for the lever to come up to provide power demand.
If the collective lever is raised too far on a turbine machine, the result will be an
overtorque condition as the governor maintains RRPM. However, on a piston-engine
helicopter once the throttle is fully open, further application of collective will cause
overpitching. Induced drag causes a reduction in RRPM. In a machine with a manual
throttle the correct recovery from overpitching is to maintain full throttle and to reduce
collective pitch momentarily to regain rotor speed. If such a machine were fitted with a
correlator the act of lowering the collective pitch lever could reduce the throttle setting
and delay the RRPM recovery. The operation of a governor would not be impaired
because a reduction in RRPM would result in a large speed error that would fully
open the throttle. In the optional governor system of the Robinson R-22 the throttle
is controlled by a governor mechanism responding to RRPM. However, an additional
system is fitted which prevents overpitching. If the throttle is wide open to recover
RRPM but the rotor speed fails to respond, an additional servo motor reduces the
collective pitch setting until the speed error has been reduced. The collective servo
linkage is fitted with a slipping clutch so that in the case of a failure the pilot can
override it. The first use of a mechanism to maintain rotor speed by lowering collective
was by Flettner in the 282 Kolibri.
In practice a free turbine engine needs two governors, one for the gas generator spool
and one for the power turbine. These will be part of the fuel control system. Either of
them can limit the turbine fuelling to prevent overspeeding. The gas generator governor
is used for starting, idling and shutdown, and is controlled by the throttle lever in the
cockpit. For flight, the throttle lever is set to maximum so that gas generator RPM is
no longer able to restrict fuelling. This results in acceleration of the gas generator spool
and increased gas flow that brings the rotors up to flight RPM. The rotor speed is then
controlled using the power turbine governor which limits fuelling as correct RRPM is
reached.
6.6 The gasoline engine
The piston engine is now over 100 years old but modern designs still work in the same
way as the machine built by von Otto.
Figure 6.5 shows a section through a typical horizontally opposed piston engine.
The pistons slide to and fro inside the cylinders and they are joined to the rotating
crankshaft by connecting rods. The connecting rods, generally abbreviated to con rods,
must be able to swivel in the pistons, and this is the function of the gudgeon pin, which
passes through the little end of the con rod. The other end of the con rod must have a
larger bearing in it to fit around the strong crankpin. In the boxer engine the crankshaft