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anyway. Springs or compressed
nitrogen may assist the feathering.
The primary governor uses a valve
controlled by the prop levers that
uses flyweights to control oil going
to the hub. If RPM decreases, the
flyweights slow down and are drawn
inwards, since they are normally held
out by centrifugal force against
springs. This allows more oil in to
reduce the pitch.
The overspeed governor increases blade
angle automatically if the primary
governor fails, to control RPM. It is
usually a relief valve. The fuel-topping
governor also prevents overspeed, but
only to within a small margin of the
lever settings. It adjusts the fuel flow
instead.
The Beta range (or ground range) is the
range of the power lever aft of the
centre of its quadrant (the prop lever
remains forward as set for landing),
used to put the props nearer flat
pitch for ground manoeuvres. This is
because a lot of thrust is generated
even at idle, unlike piston-engined
planes, which produce very little.
The beta valve bypasses the primary
governor to send more oil to the
hub, and allows the power lever to
control only prop pitch in beta
range, as opposed to its normal
torque. Low pitch stops or
hydromechanical locking devices prevent
the propeller entering Beta range or
even reverse pitch in the cruise.
In reverse range, the power lever
controls pitch and torque.
The Alpha range (in the forward part
of the quadrant) has the opposite
function.
High Speed Flight
Principles of Flight 167
Jets are typically flown at high
altitudes, which not only means thin
air that provides less drag, but also
that your cruising speed is much
closer to your stalling speed,
sometimes as little as 10% away. In
addition, at transonic speeds (that is,
between sonic and supersonic, when
only part of the airflow is above the
speed of sound), things can be
unpredictable, as with a
phenomenon called "jet upset"
which has caused jets to flip into a
steep dive with the controls
becoming immovable, which is why
you have to use Mach numbers
(pronounced like Mark) rather than
airspeed so you know when you’re
approaching it. Mach Tuck occurs
when the Centre of Pressure moves
so far back as speed is increased that
the nose pitches down, to increase
speed and move the C of P further
back, and so on. Slowing down is the
only way out. The Pitch Trim
Compensator continually readjusts the
horizontal stabiliser according to the
Mach Number, compensating for
longitudinal instability.
Subsonic flight means up to about .75
Mach, or the maximum airspeed at
which no airflow reaches the speed
of sound. Transonic flight is
somewhere between .75 and 1.2,
where there is a mix. Supersonic flight
is above the speed of sound.
The Mach number represents the
TAS compared to the local speed of
sound, and is discussed under
Machmeter in the Instruments chapter.
The Critical Mach Number (MCRIT) is
the highest speed you can get
without supersonic flow (about M
0.72) or, to put it another way, the
speed at which any part of the
airflow over the upper wing
becomes supersonic in level flight
(that is, it first reaches, but does not
exceed it). At this speed, a small
shock wave forms on the upper
forward surface of the wing, which
increases with speed - it is caused in
the first place from the
compressibility characteristics of air.
If a "shock stall" occurs on both
wing roots at the same time, there is
a loss of lift and a loss of downwash
on the tail, with separation of the
boundary layer at about M 0.82. At
this point (MMO), there is a pressure
and temperature increase behind the
shock wave, causing a sudden
increase in drag (called Drag Rise or
Drag Diversion). In swept wing
aircraft, the centre of pressure
moves aft as well, contributing to the
tail lifting tendency. Fuel
consumption increases as well.
MMO is the Maximum Mach Operations
Speed. It is displayed on the ASI by a
self-adjusting red-and-white needle
which looks like a barber pole. The
speed of sound, however, decreases
with altitude, while the stall speed
increases.
Mach Buffet arises out of turbulent air
from the wings hitting the tail
surface, caused by shock waves on
top of the wing. The low speed buffet,
which can be violent, means you are
near the conventional stall - it occurs
　
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