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enough to enable them to launch without external assistance.
The engines also may be used to sustain flight if
2-4
Figure 2-7. Self-launch gliders are different in performance, as well as appearance.
Figure 2-6. Towhook locations.
2-5
Figure 2-8. Some gliders have fixed main wheels, others have retractable main wheels. Nose skids, or nose
wheels, tail wheels and wing tip wheels are found on many gliders.
2-6
3-1
To understand what makes a glider fly, you need to
have an understanding of aerodynamics. This chapter
discusses the fundamentals of the aerodynamics of
flight.
AIRFOIL
Airfoil is the term used for surfaces on a glider that
produce lift. Although many different airfoil designs
exist, all airfoils produce lift in a similar manner.
Some airfoils are designed with an equal amount of
curvature on the top and bottom surface. These are
called symmetrical airfoils. Airfoils that have a
different curvature on the bottom of the wing when
compared to the top surface are asymmetrical.
The term camber refers to the curvature of a wing
when looking at a cross section. A wing possesses
upper camber on its top surface and lower camber on
its bottom surface. The term leading edge is used to
describe the forward edge of the airfoil. The rear edge
of the airfoil is called the trailing edge. The chord line
is an imaginary straight line drawn from the leading
edge to the trailing edge.
Relative wind is created by the motion of an airfoil
through the air. Relative wind may be affected by
movement of the glider through the air, as well as wind
sheer. When a glider is flying through undisturbed air,
the relative wind is represented by its forward velocity
and is parallel to and opposite of the direction of flight.
ANGLE OF ATTACK
The angle of attack is the angle formed between the
relative wind and the chord line of the wing. You have
direct control over angle of attack. By changing the
pitch attitude of the glider in flight through the use of
the elevator/stabilator, you are changing the angle of
attack of the wings. [Figure 3-1]
ANGLE OF INCIDENCE
The wings are usually mounted to the fuselage with
the chord line inclined upward at a slight angle. This
angle, called the angle of incidence, is built into the
glider by the manufacturer and cannot be adjusted by
the pilot’s movements of the controls. It is represented
by the angle between the chord line of the wing and
the longitudinal axis of the glider.
CENTER OF PRESSURE
The point along the wing chord line where lift is
considered to be concentrated is called the center of
pressure. For this reason, the center of pressure is
sometimes referred to as the center of lift. On a typical
asymmetrical airfoil, this point along the chord line
changes position with different flight attitudes. It
moves forward as the angle of attack increases and aft
as the angle of attack decreases.
FORCES OF FLIGHT
Three forces act on an unpowered glider while in
flight—lift, drag, and weight. Thrust is another force of
flight that enables self-launch gliders to launch on their
own and stay aloft when soaring conditions subside.
The theories that explain how these forces work include
Figure 3-1. Aerodynamic terms of an airfoil.
3-2
Magnus Effect, Bernoulli’s Principle, and Newton’s
laws of motion. [Figure 3-2]
LIFT
Lift opposes the downward force of weight and is produced
by the dynamic effects of the surrounding
airstream acting on the wing. Lift acts perpendicular to
the flight path through the wing’s center of lift. There is
a mathematical relationship between lift, angle of
attack, airspeed, altitude, and the size of the wing. In the
lift equation, these factors correspond to the terms coefficient
of lift, velocity, air density, and wing surface
area. The relationship is expressed in Figure 3-3.
This shows that for lift to increase, one or more of the
factors on the other side of the equation must increase.
Lift is proportional to the square of the velocity, or airspeed,
therefore, doubling airspeed quadruples the
amount of lift if everything else remains the same.
Likewise, if other factors remain the same while the
coefficient of lift increases, lift also will increase. The
coefficient of lift goes up as the angle of attack is
increased. As air density increases, lift increases.
However, you will usually be more concerned with
how lift is diminished by reductions in air density on a
hot day, or as you climb higher.
MAGNUS EFFECT
The explanation of lift can best be explained by looking
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Glider Flying Handbook(23)
