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The Fuselage
The tail section is a full monocoque
assembly, made of aluminium, whilst
the intermediate section, where the
engine lives, is semi-monocoque.
They are bolted together through
large castings that are checked every
hundred hours. The forward section
(i.e. the passenger compartment) is
an aluminium honeycomb structure.
The skids are designed to absorb
energy in a hard landing.
The hook can normally carry 1200
lbs, but there is a 1500 lbs version.
In the LongRanger, the horizontal
stabilizer assembly has a movable
rear elevator connected to the cyclic
to provide greater authority. The
side fins are offset, opposite to the
vertical fin angle (the horizontal
stabilizer is also offset in relation to
the tailboom, in that it protrudes
more one side than the other). These
are designed to improve the
efficiency of the vertical stabilizer.
An increased tail rotor pitch travel is
provided by the rigging procedure
for greater authority.
The Engine
Somebody once said that this looks
like a dustbin with the garbage on
the outside!
The Bell 206 143
The III model has an Allison 250-
C20B, which weighs 158 lbs.
Although it can produce 420 shafthorsepower,
the transmission limits
it to 317. Inside its compartment,
there is enough space between the
bottom of the engine and the
titanium deck underneath to allow
the servicing of accessories without
removing the engine.
Essentially, a six-stage axial flow,
one-stage centrifugal compressor is
bolted to a two-stage turbine, which
itself consists of 2 two-stage free
turbines (the first of which drives the
compressor), feeding into the
second, which drives the power
output shaft. Air is initially
compressed to over 6 atmospheres
then transferred to the combustion
chamber at the rear, through two air
transfer tubes that are plainly visible
on each side.
After passing forward through the
turbines, it is exhausted upwards
through twin exhaust ducts. All this
means the engine can occupy much
less space.
Inside, the air ducts are shaped so
the flame does not come into
contact with the casing, or any other
part of the engine, but is contained
within a cocoon of cooler air called a
toroidal vortex, which is why the intake
air is ducted along both sides of the
engine, through 180° and up
through the hot section in the
direction of travel, for better control.
In fact, 60% of the air sucked in the
front is for cooling. There have been
several types of engine over the
years, and the original 250
designation was because it was
intended to push out that amount of
horsepower, but actually was rated at
300 up till the 400 or so of the
present day.
Lubrication uses a circulating dry
sump system which has an external
reservoir and heat exchanger.
Various pressure and scavenge
pumps in the engine are driven by
the accessory gear box. Only the
pressure and scavenge lines to the
front and rear bearings are external.
The fuel control unit is controlled
with centrifugal bobweights. The
throttle has three settings, namely
off, idle and open. The idle speed is
somewhere between 63-65% of
engine RPM (or gas producer, as the
Americans call it).
The engine starter is either a Bendix
or a Ceco, the former giving you no
control over the start process other
than by opening or closing the
throttle. The only way you can tell is
look at the label on it in the engine
compartment. The Ceco allows you
some measure of control over the
TOT during the start.
LongRanger
The C28 is larger and heavier, using
up 135 litres per hour. The
centrifugal compressor is probably
the most obvious visual difference.
Being wider but shorter, it gives the
impression the engine is ‘shoved’
forward against the firewall. The
144 The Helicopter Pilot’s Handbook
accessory gearbox is squarer but the
turbine is similar. There is also a
bipod rear mount on the turbine to
combustion case split-line.
The C28 does not need a particle
separator, as the inlet to the
compressor has a bulbous duct
which houses an in-built extraction
system. It was replaced on the C30,
and such airframes can have a
standard separator fitted. The
exhaust goes out through a single
rectangular duct.
The accessories are also different.
The EDFP and FCU, for example,
are ‘piggy-backed’ on each other and
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(95)