Lift is created by deflecting a flow of
air and drag is generated on a body in a
wide variety of ways. From Newton's second law
of motion, the aerodynamic forces on the body (lift and drag) are
directly related to the change in momentum
of the fluid with time. The fluid momentum is equal to the mass times
the velocity of the fluid.
Since the air moves, defining the mass gets a little tricky and
aerodynamicists usually relate the effect of mass on lift and drag to
the air density. The mathematical
derivation for this conversion is given on another slide dealing with
momentum effects on lift. As a result of
this derivation, we find that lift and drag depend on the square of
the velocity.
The velocity used in the lift and drag equations is the relative
velocity between an object and the flow. Since the aerodynamic
force depends on the square of the velocity, doubling the velocity
will quadruple the lift and drag.
Let's investigate the dependence of lift on velocity using a Java
simulator.
As an experiment, set the velocity to 50 mph and note the amount of lift.
Now double the velocity to 100 mph. What is the value of the lift? How
does it compare to the previous measurement?
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 velocity 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.
