The amount of lift generated by an object
depends on the size of the object. Lift is
an aerodynamic force and therefore depends
on the
pressure variation
of the air around the body as it moves
through the air. The total aerodynamic force is equal to the pressure
times the surface area around the body. Lift is the
component
of this
force perpendicular to the flight direction.
Like the other aerodynamic force, drag, the
lift is directly proportional to the area of the object. Doubling the area
doubles the lift.
There are several different areas from which
to choose when developing the reference area used in the
lift equation.
Since most of the lift is generated by the wings,
and lift is the force perpendicular to the flight direction,
the logical choice is the
wing planform area. The planform area is the
area of the wing as viewed from above the wing, looking along the
"lift" direction. It is a flat plane, and is NOT the total surface area
(top and bottom) of the entire wing, although it is almost half that
number for most wings.
We could, in theory, use the total surface area as the reference
area. The total surface area is proportional to the wing planform area.
Since the
lift coefficient
is determined experimentally, by measuring the lift
and measuring the area and performing the necessary math to produce
the coefficient,
we are free to use any area which can be
easily measured.
If we choose the total surface area, the computed coefficient
has a different value than if we choose the wing planform area,
but the lift is the same, and the coefficients are related by
the ratio of the areas.
This slide shows the projected surface area for two different
aircraft. The airplane on the left is shown in a
cruise
condition
while the airplane on the right is shown in a
takeoff
or landing
condition. Takeoff and landing are times of relatively
low velocity,
so to keep the lift high (to avoid the ground!) designers try to
increase the wing area. This is done by sliding the flaps
backwards along metal tracks and shifting the slats forward to
increase the wing area. The next time you fly in an airliner, watch
the wings during takeoff and landing to see the change in wing
area.
Since aircraft come in many shapes and sizes, an aerodynamicist
has to be able to compute the wing area for
many different shapes. Most of these skills are learned in high
school.
Let's investigate the dependence of lift on wing area by using a Java
simulator.
You can vary the wing area by changing the wing
span or the chord
length by using the sliders below the view
window or by backspacing over the input box, typing in your new value and
hitting the Enter key on the keyboard.
The wing area is the product of the span and the chord for the red rectangular
wing shown in the view window.
On the right is a graph of the lift
versus wing area. The red dot shows your conditions. Below the graph is the
numerical value of the lift and the computed wing area. You can display these
values in either
English or Metric units by using the choice
button next to the lift output box. Click on the choice button and select
from the drop-menu.
There is an additional three dimensional effect on wing lift caused by the
downwash
from the wing tips. The amount of the effect depends on the
Aspect Ratio of the wing, which is the span divided by
the chord for a rectangular wing. You can see the magnitude of the effect by
turning the correction ON and OFF, using the choice button to the right of the
output box for Aspect Ratio.
As an experiment, set the chord to 5.0 feet and and the span to
50 ft. Note the amount of lift. Now increase the span to 100 ft.
What is the new value of lift? How does this value compare to the previous value?
Now set the span to 50 ft and the chord to 10 ft.
What is the new value of lift? How does this value compare to the previous value?
You can also download your own copy of the program to run off-line by clicking
on this button:
You can further investigate the effect of wing size and the other
factors affecting lift by using the
FoilSim II Java Applet.
You can also
download
your own copy of FoilSim to play with
for free.
