There are four forces that act on an
aircraft in flight: lift, weight, thrust, and drag.
having both a magnitude and a direction.
of the aircraft through the air depends on the
relative magnitude and direction of the various
of an airplane is determined by the size
and materials used in the airplane's construction and on the payload
and fuel that the airplane carries.
The weight is always directed towards the center of the earth.
by the size and type of propulsion system used
on the airplane and on the throttle setting selected by the pilot.
Thrust is normally directed forward along the center-line of the aircraft.
are aerodynamic forces that depend on the
shape and size of the aircraft, air conditions, and the flight
Lift is directed perpendicular to the flight path and drag is
directed along the flight path.
Because lift and drag are both
aerodynamic forces, the
of lift to drag is an indication of the aerodynamic efficiency of the
airplane. Aerodynamicists call the lift to drag ratio the L/D
ratio, pronounced "L over D ratio." An airplane has a high L/D
ratio if it produces a large amount of lift or a small amount of
drag. Under cruise conditions lift
is equal to weight. A high lift aircraft can carry a large payload.
Under cruise conditions thrust is equal to drag. A low
drag aircraft requires low thrust. Thrust is produced by
burning a fuel and a low thrust aircraft
amounts of fuel be burned.
As discussed on the maximum flight
time page, low fuel usage allows an aircraft to stay aloft for a
long time, and that means the aircraft can fly long
So an aircraft with a high L/D ratio can carry a large payload,
for a long time, over a long distance.
For glider aircraft
with no engines, a high L/D ratio again produces a long range
aircraft by reducing the steady state glide
angle at which the glider descends.
As shown in the middle of the slide, the L/D ratio is also equal
to the ratio of the lift and drag coefficients. The
indicates that the lift L is equal to one half
the air density r times the square of the velocity V
times the wing area A times the
lift coefficient Cl:
L = .5 * Cl * r * V^2 * A
relates the aircraft drag D to a
drag coefficient Cd:
D = .5 * Cd * r * V^2 * A
Dividing these two equations give:
L/D = Cl/ Cd
Lift and drag coefficients are normally
determined experimentally using a wind
tunnel. But for some simple geometries, they can be determined
You can investigate the various factors that affect the lift to drag ratio by using
the FoilSim III Java Applet. Have fun! You
can use the browser "Back" button to return to this page. If your
want your own copy of
FoilSim to play with, you can download
it at no charge.
Forces on an Airplane:
Forces on a Glider:
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