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.
Modern aerodynamicists relate the effect of mass on lift and drag to
the air density.
At the time of the Wright brothers, the effect of mass was included in the
Smeaton pressure coefficient.
In both systems,
the 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.
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
"velocity.html" to run the program off-line.
You can change the value of the velocity by using the
slider below the airfoil graphic, or by backspacing, typing in your value,
and hitting "Return" inside the input box next to the slider.
You can perform the calculations in
either English or metric units by using the drop menu labeled "Select Units".
The red dot on the graph shows the current set of conditions.
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?
