The amount of lift generated
by an object depends on how much the flow is turned,
which depends on the shape of the object.
In general, the lift is a very complex function of the
shape. Aerodynamicists model the shape effect by a lift
coefficient which is normally determined through wind
tunnel testing. For some simple shapes, we can
develop mathematical
equations
to determine the lift coefficient.
The simplest model,
the two dimensional Kutta-Joukowski
airfoil, is studied by undergraduate students.
The FoilSim
computer program provides the results of this analysis in a form
readily usable by students. A result of the analysis shows
that the greater the flow turning, the greater the lift
generated by an airfoil.
This slide shows the flow fields for two different 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.
The example shown above explains why the aft portion of
wings have hinged
sections to control and maneuver an aircraft. Deflecting the aft
section down produces a geometry similar to the figure on the
right producing more lift. Similarly, if the aft section is deflected
up, it creates 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 slides show the
deflection of the control surfaces and the resulting motion of the
aircraft:
Let's investigate the dependence of lift on airfoil shape by using a Java
simulator.
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.
Camber is a measure of the amount of airfoil curvature.
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?
You can download your own copy of the program to run off-line by clicking on this button:
You can further investigate the effect of airfoil shape 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.
