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 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.

Navigation...

- Beginner's Guide to Aerodynamics
- Beginner's Guide to Propulsion
- Beginner's Guide to Model Rockets
- Beginner's Guide to Kites
- Beginner's Guide to Aeronautics

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- Beginner's Guide Home Page

*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *