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at higher speeds with altitude.
The high speed buffet (which may
involve a complete loss of elevator
control) occurs at lower speeds with
altitude, so at some stage they will
merge, called the aerodynamic ceiling.
The margin between them (called
coffin corner) is reduced by G forces
caused by turning and turbulence,
168 Canadian Professional Pilot Studies
and you will want to choose a level
that gives you enough breathing
space - watch out for a chart in the
Flight Manual that shows the upper
and lower boundaries against gross
weight, altitude and bank.
The Limiting Mach Number is the
highest speed at which the aircraft
becomes noticeably uncontrollable.
A sweptback wing is angled forward or
aft from a right angle to the fuselage
(usually aft). A 30° sweep will
increase MCRIT up to about M 0.75.
Although they suffer from less drag,
because they produce less lift,
aircraft with swept back wings must
fly faster to compensate, a problem
when taking off and landing, which
is why high lift devices are used (their
purpose is to increase lift at low
airspeeds and delay stall until a
higher angle of attack). They are,
however, less affected by turbulence.
There is also a tendency towards
Dutch Roll, (see below).
Subsonic machines should not use
rudder to correct this. A Yaw Damper
(often essential equipment in terms
of an MEL) uses gyros to sense and
correct changes in yaw. An
alternative to sweepback is the use of
vortex generators and boundary
layer energisers, as in the Learjet.
The Helicopter
This is just a flying machine whose
wings go round instead of remaining
still, cynically referred to by some as
50,000 rivets in loose formation –
this means that the lift-producing
surfaces (rotors) are separate from
the body, unlike an aeroplane.
Another difference is that an
aeroplane engine is directly related to
the forward movement of the
aircraft, whereas the engine on a
helicopter isn't – its function is to
drive the rotor system which is really
what makes the machine move
through the air.
In fact, the rotors provide lift, thrust
and directional control in one go –
all three are separate functions on an
aeroplane. In some ways, this is
good, since loss of an engine on a
twin does not need the strength of a
gorilla to keep it straight, but it
makes flying more demanding.
In a helicopter, lift and thrust
together (or the sum of the lift of all
blades) are often referred to as Total
Rotor Thrust (see above).
Lift is a vertical component, and
thrust is horizontal:
Principles of Flight 169
The Tip Path Plane is the path
described by the tips of the rotor
blades, viewed horizontally. The
Coning Angle is the angle between the
blade and the Tip Path Plane. The
Rotor Disc is the area formed by the
rotation of the blades, inside the
blade tips. As the blades start coning,
it will change its size slightly (the
word rotor includes the blades, the
hub and the shaft). The smaller the
disk gets, the less area there is to
generate lift – a situation that would
arise if the RPM were too low,
causing the coning angle to increase
and centrifugal force to decrease.
Coning is the resultant of centrifugal
and lift forces – as the blades
produce lift, the coning angle increases,
but it decreases as RPM increases -
the blades need centrifugal force for
stiffness, and their ability to support
the machine in flight. It is measured
in tons, against pounds for lift,
thrust weight or drag, which will give
you an idea of its importance.
Main rotor blades obey the same
rules as any aerofoil, with special
shaping on modern machines to suit
peculiar requirements (such as the
Bell 407). Otherwise, they are
generally symmetrical in cross
section, to restrict movement of the
Centre of Pressure.
The pitch angle is that between the
blade's chord line and the spin axis
of the main blades, and the plane of
rotation, which is parallel to it (usually
above). The plane of rotation
contains the centres of mass of the
rotating blades (the axis of rotation is
a vertical element, which the blades
rotate around – it is at right angles to
the plane of rotation, and not
necessarily in line with the rotor
mast. The difference between them
is the flapping angle). The pitch angle
is varied with the collective and
　
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