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underside of the wing, to be forced
downwards, forcing the wing up.
This is a bit of a brute force solution,
so the wing will also be shaped to
help things along with the venturi
effect, discovered by Bernoulli, a
principle made use of in
carburettors, described in the
Airframes, Engines & Systems chapter,
and air-driven instruments (see
Instruments). Bernoulli found that the
pressure of a fluid decreases where
its speed increases or, in other
words, in the streamline flow of an
ideal fluid, the quantity of energy
remains constant - there is a given
amount of energy involving speed
and pressure, and each affects the
other directly.
Principles of Flight 7
Decreased Pressure
Increased Velocity
If you take a tube with a smaller
diameter at its centre than at either
end, and blow air through it, the
pressure in the centre is less because
speed and pressure interact with
each other, in that, if you increase
one, the other decreases:
In this case, the air being forced
around the obstructions in the
middle has to increase speed to keep
up with the rest because it is taking a
longer path. Of course, the same
molecules of air don't meet up at the
other end – those taking the longer
route may be up to 30% of the
distance away, depending on the
angle of attack. Since speed is
increased, pressure is reduced. This
system also pulls fuel into a
carburettor, and it's also the reason
why a door closes by itself if left
slightly ajar – there is less pressure in
the gap between it and the door
frame as the air moves through it.
If you take the top half of the tube
away, the phenomenon still works
on the remaining half, which looks
like the top surface of an aerofoil.
You can see this yourself by taking a
large piece of paper and folding it
back over the top of your hand,
keeping hold of it with your fingers.
If you blow across the top, you will
see the paper rise. Used sideways,
this is how yachts make use of the
wind to get along.
The aerofoil will therefore have a
natural tendency to go up or, looked
at another way, to pull air down, to
the low pressure area on the top and
help the brute force effect. Around
two thirds of the total lift comes
from the reduced pressure effect
across the top, not forgetting the
higher pressure underneath.
The wing loading is the average weight
lifted by each square foot of the
wing. It works in a similar way to
loadspreading, in that a larger area
has a lesser loading.
Aeroplanes
Aeroplanes are either low wing, like
most Piper aircraft (see overleaf), or
high wing, like the average Cessna:
High wing planes may have struts to
help keep the wings up, as well as
internal bracing. Both types are also
called cantilever, or semi-cantilever,
respectively.
Aeroplanes are single- or multi-engined,
with fixed or retractable
undercarriages, if they are
landplanes, skis if they land on snow,
and floats if they operate on water:
A monoplane has one pair of wings,
while a biplane has two (the Red
Baron's triplane had three. I forget
8 JAR Private Pilot Studies
what Snoopy had). The shape of a
wing as viewed from above is known
as a planform, and could be rectangular,
tapered (from root to tip), elliptical,
delta or sweptback. Large, wide ones,
for example, are good for large
transport aircraft, and short, stubby
ones will be found on fast sports
aircraft. The aspect ratio of a wing is
the relationship between its length
and width, or span and chord. You
could have two wings of equal
surface area but different aspect
ratios, depending on what they were
designed for. The higher the ratio
(i.e. the longer the wing relative to its
width), the more lift you get, with
less induced drag and downwash (as
with gliders):
However, it does stall at a lower
angle.
An aeroplane's rated strength is a
measure of the load the wings can
carry without being damaged. Light
aircraft can take total loads in three
categories:
· Normal, 3.8 x the gross weight
· Utility, 4.4 x gross weight
· Acrobatic, 6 x gross weight
Naturally, there is a safety factor
involved, but the above should not
be exceeded.
Normal or utility categories do not
allow manoeuvres with high positive
and negative load factors. Bank
angles would normally be inside 60°.
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