曝光台 注意防骗
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D = CD½ rV2S
Drag will also increase with speed.
Load Factor
The total lift divided by the total
weight, the ratio being 1:1 in level
unaccelerated flight. In other words,
the weight carried by a wing
expressed in terms of "G".
When you turn, the aircraft tries to
continue in a straight line, and a
force is needed to point it towards
the centre of the turn – this would
be Centripetal Force, which must be
generated by extra lift from the
wings. In effect, the (upwards) lift
vector is reduced by the same
amount, which needs to be
compensated for. In a 60° banked
turn, therefore, the amount of lift
you need is doubled, so the load
factor becomes 2. This can also be
increased temporarily by sharp
manoeuvres or gusts and turbulence
(a gust with a speed of 66 feet per
second will change the angle of
attack at 200 kts by as much as 11°,
which will either lift you very quickly
or cause a stall).
Here is a chart expressing angles of
bank against load factors:
Angle Factor
0° 1
15° 1.04
45° 1.41
60° 2
75° 4
The point to watch is that the load
factor may increase so much that the
maximum weight of the aircraft is
exceeded, and the wings won't be
able to do their job properly.
Propellers
These are just aerofoils with a twist
in them (washout) to spread the lift
evenly over the whole length, as the
tips run faster than the center and
need less angle of attack (the word
pitch is sometimes used loosely to
describe this). The basic propeller is
averaged to cope with many flight
conditions, so is not perfect for
them all, particularly the takeoff. The
real problem is that you have to
make the engine run faster for more
performance from the prop, and
engines work best within a certain
speed range.
Officially, a propeller's function is to
convert the crankshaft's rotary
movement into thrust, by moving a
large column of air backwards, to
propel the aircraft forward. As a
propeller is an aerofoil, the thrust it
creates is the same as the lift from a
wing – it's just used differently.
A tractor (at the nose) will propel fast,
turbulent air over the lifting surfaces,
whereas a pusher (somewhere behind
the fuselage) provides better high
speed performance because it
doesn't produce so much drag. On
the other hand, the tractor bites into
clean air, while a pusher spins in air
that is already disturbed.
A rotating propeller creates various
forces which may be allowed for in
the design stages, including gyroscopic
precession (see Instruments), where
lifting the tail tends to make the nose
yaw to the left. Torque results from
26 JAR Private Pilot Studies
the airframe going the opposite way
to the direction of rotation:
The effect is to produce a roll, which
is countered by washout on the
upgoing wing.
The blade going down pulls more at
high angles of attack, resulting in
asymmetric thrust (also known as the P
factor). Where the propellers rotate
the same way on a multi-engined
aircraft (as with the PA 23), the
failure of one engine may cause
more problems than the other
because of this – in the case of the
Aztec, the downgoing blade is on the
right side, since the blades rotate
clockwise, so there is more of a
turning moment if the left engine
fails, as the thrust from the
downgoing blade of the right
propeller is further away from the
longitudinal axis.
The left engine in this case is called
the critical engine, because its loss
creates the most adverse conditions.
Later aircraft (such as the Navajo
Chieftain) have contra-rotating
propellers to deal with this (that is,
the right engine rotates to the left).
Watch for the designation of parts
for particular engines with L or R so
they go in the right place. The other
thing to note is that the slipstream
going back over the flight controls
will also be asymmetric, and parts of
some controls will be more
responsive than others.
Slipstream results from rotating air
going round and round the fuselage
until it eventually hits the tail fin,
forcing it one way or the other (thus
causing yaw), depending on which
way round the propeller is going:
It can be reduced by offsetting the
fin, as most of an aeroplane's life is
spent in the cruise. Outside of that,
simply use rudder.
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