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If you've ever been through a car
wash, and your car is still wet, you
may have noticed droplets of water
remaining quite still on the bonnet
no matter how fast you drive. This
happens on aeroplane wings as well
– large specks of dust will remain on
them even through a Transatlantic
flight. On the other hand, there is a
point at which the air will flow
smoothly over any surface. The area
between the two is called the
boundary layer, which ideally should
have a laminar flow (i.e. smoothly
layered) but, in practice, this doesn't
happen much farther back than the
thickest part of the aerofoil. In most
small aircraft, the boundary layer
ranges from about half an inch up to
the transition point (where the air
becomes turbulent), to around three
inches afterwards, where it becomes
the turbulent layer.
Anyhow, back to drag, which is a
force that tends to slow an aircraft
down, acting in the opposite
direction to thrust, parallel to the
relative airflow. In order of priority,
it can be split up into various
components:
· Induced Drag comes from the
air's reaction to the aerofoil, or
is induced from the creation of
lift, so it comes from liftproducing
surfaces and varies
with the angle of attack, so the
slower the aircraft the more you
get (more lift, more drag). It
may come from wingtip
vortices, for example, and is
inversely proportional to the
square of the velocity, that is to
say, halving velocity increases
24 JAR Private Pilot Studies
induced drag by four times. It
also increases as an aircraft pulls
out of ground effect on takeoff,
as the ground will interfere with
vortex formation, and can be
affected by the aspect ratio of
the wing.
· Parasite Drag comes from
anything moving through the air
not actually creating lift, like the
fuselage, undercarriage, etc. It
consists of:
· Interference Drag, or the
result of the interaction
between various
components, say the wings
and the fuselage. In other
words, if you added the
various types of drag
together, you would find
the result to be less than
the actual total.
Interference drag is the
difference.
· Profile Drag is made up of:
· Form Drag, from the
shape of any body
moving through the
air. It is minimised by
streamlining
· Skin friction, mentioned
above. It's the result of
the smoothness or
otherwise of surfaces.
Profile Drag is proportional
to, and increases as, the
square of the speed.
Aileron Drag comes from downgoing
ailerons, causing a yaw in the
opposite direction of the bank.
Maximum Range Speed
This gives you the most lift for the
least drag, for the most economy,
and the most distance for altitude
lost. Flying either side of that speed
will decrease the range when gliding.
The Lift/Drag Ratio comes from
dividing lift by drag. The angle of
attack for the best ratio varies with
the design of the wing, but is around
a third to a quarter of the size of the
stalling angle. It never changes, but
without an angle of attack indicator,
you need an indirect method of
guesstimating it, such as speed,
which may have to be increased
slightly with aircraft weight, reducing
your range, as you will be using more
power, and hence fuel, to attain it.
The biggest factor concerning your
range will be the wind, which will
reduce your groundspeed when on
the nose, causing you to use more
fuel. However, a slight increase in
airspeed, say 5-10%, will get you
there sooner with only a slight effect
on fuel consumption.
Maximum Endurance Speed
This gives you the most time in the
air for least amount of fuel, which is
useful when waiting for the weather
to clear, or when asked to hold clear
of a control zone, but, in practice, it
gives you little or no controllability,
so there will be a recommended
endurance speed in the flight manual,
which is a few knots above.
The endurance is longer the lower
you can fly (allowing for safety, of
course), and turbulence and flaps
will affect the speed considerably.
There is more drag with endurance
speed than there is with range speed,
which is higher.
Principles of Flight 25
Formula
Guess what? There's a formula for
drag, too, which, luckily is very
similar to that for lift:
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