User Guide

65
Flight Instruction
64
Flight Instruction
On the other hand, if it’s pointed
even slightly downward, as it would
be during a descent, its aero force
points slightly forward, pulling the
airplane along (fig. 4). This is
how gliders (sailplanes) can keep
moving, even though they don’t
have engines: they’re always
descending through the air. How
do they stay up all day? By finding
areas where the air is rising faster than
the glider descends... just like when you
used to get yelled at for playing on the
escalators at the mall.
The amount of lift a wing can produce depends on four
things. One is more or less constant: the design of the wing and its
airfoil. Generally, a thick, highly curved wing produces lots of lift
at low speeds, making it ideal for slow, light aircraft. A thin wing
produces less lift, but is more efficient at high speeds; you’ll find it
on jets. (How do jets manage to take off and land at reasonably
low speeds? By changing the shape of their wings with various
flaps, slats, and similar movable bits and pieces.)
WHAT’S YOUR ANGLE?
Two more variables can change the amount of lift a wing
produces: the speed at which the wing is moving through the air,
and its angle of attack - the angle between the wing’s chord line
and the oncoming air (also called the relative wind). At high
speed, it only takes a little angle of attack to generate enough lift
to support the airplane. The slower we fly, the more angle of at-
tack is necessary to generate the same amount of lift. Next time
you’re near an airport, watch the airliners coming in. Even though
they’re descending as they near the runway, they’re flying along
slightly nose-high - at their low approach speeds, it takes a lot of
angle of attack to provide enough lift. As they descend the last few
feet, their noses rise even more. This maneuver is called the land-
ing flare. The pilot is trying to make the touchdown as soft as pos-
sible. As speed bleeds off over the runway, it takes even more
angle of attack to reduce the rate of descent and avoid one of those
“take that, La Guardia” arrivals.
Figure 4
There’s another reason the
curve is important as well. Look at
these two pictures. The first (fig.
1) shows a flat surface angled to
the air, as tried by the first (un-
successful) experimenters.
You’ll see that it produces a
very limited amount of lift from
the “push” on its bottom sur-
face...but the airflow over the top
“trips,” or separates, as soon as it gets
past the sharp leading edge, and rather
than speeding up over the top it just
swirls in useless turbulence. Not only does it not create any lift, it
also causes a great deal of drag.
In the second picture (fig. 2), we’re
looking at a cross-section of a typical
wing, or airfoil. Because of its
curved surface, the air can flow
smoothly over the top surface. This
is where most of the lift is pro-
duced. Notice, too, that we’ve
drawn a line from the center of the
leading edge to the trailing edge.
Aeronautical engineers call this the
chord line of the wing...and what’s im-
portant about it is that any aerodynamic
force the wing produces
will always act exactly at right angles to
the chord line. This means that if the
wing is tilted up (as it is even in
level flight, if only by a small
amount), its lift points very slightly
backward. If it weren’t for the
thrust of the engine, the airplane
would slow down (fig. 3).
Figure 1
Figure 3
Figure 2