The amount of
lift
generated by a wing depends on
how much the flow is turned, which depends on the shape of the
object. The lift is, in general, a very complex function of the
shape. Aerodynamicists model the effect by a lift
coefficient which is normally determined through
wind tunnel testing.
The Wright brothers built a
wind tunnel
in 1901 to determine
the best
shape
for their wings.
One simple result of their testing showed
that the greater the flow turning, the greater the lift.
You can investigate the effects of shape on lift by using a computer simulation
of the Wright's
1901 tunnel.
This slide shows the flow fields for two different
modern airfoils.
The airfoil on the left is a symmetric airfoil; the shapes above and
below the white centerline are the same. The airfoil on the right is
curved near the trailing edge. The yellow lines on each figure show
the streamlines of flow from left to right.
The left figure shows no net turning of the flow and produces no
lift; the right figure shows a large amount of turning and generates
a large amount of lift. The front portions
of both airfoils are nearly identical. The aft portion of the right
airfoil creates the higher turning.
This explains why the aft portion of modern wings have
hinged sections, and why the Wrights used
wing warping
to control and maneuver their aircraft. Deflecting the aft
section down will produce a geometry similar to the figure on the
right producing more lift. Similarly, if the aft section is deflected
up, it will create less lift (or even negative lift). The ability to
vary the amount of lift over a portion of the wing gives the pilot
the ability to maneuver an aircraft. The following pages show the
deflection of the control surfaces and the resulting motion of the
Wright
1902 aircraft,
the first aircraft to be controlled about all three axes.
Let's investigate the dependence of lift on airfoil shape by using a Java
simulator.
You can download your own copy of this applet by pushing the following button:
The program is downloaded in .zip format. You must save the file to disk and
then "Extract" the files. Click on
"Shape.html" to run the program off-line.
You can vary the shape of the foil by using the slider below the view
window or by backspacing over the input box, typing in your new value and
hitting the Enter key on the keyboard. On the right is a graph of the lift
versus
camber.
The red dot shows your conditions. Below the graph is the
numerical value of the lift. You can display either the lift value (in
English or Metric units) or the lift coefficient by using the choice
buttons surrounding the output box. Click on the choice button and select
from the drop-menu.
As an experiment, set the camber to 0.0 per cent chord and
note the amount of lift. Now increase the camber to 5 per cent.
Did the lift increase or decrease?
Set the camber to -5 per cent. What is the value of lift?
Which way would this airfoil move?
