Lift is a force. From Newton's second
law of motion, a force F is produced when a mass m is
F = m * a
An acceleration is a change in velocity V with a change
in time t.
F = m * (V1 - V0) / (t1 - t0)
We have written this relationship as a difference equation, but it
is recognized that the relation is actually a differential from
F = m * dV/dt
The important fact is that a force causes a change in
velocity; and, likewise, a change in velocity generates a force.
The equation works both ways. A velocity has both a magnitude
called the speed and a direction associated with it. Scientists and
mathematicians call this a
So, to change either the speed or the direction of a
flow, you must impose a force. And if either the speed or the
direction of a flow is changed, a force is generated.
Lift Generated in a Moving Fluid
For a body immersed in a moving fluid, the fluid remains
with the surface of the body. If the body is shaped, moved,
or inclined in such a way as to produce a net deflection or turning
of the flow, the local velocity is changed in magnitude, direction,
or both. Changing the velocity creates a net force on the body. It is
very important to note that the turning of the fluid occurs because
the molecules of the fluid stay in contact with the solid body
since the molecules are free to move. Any part of the solid body
can deflect a flow. Parts facing the oncoming flow are said to be
windward, and parts facing away from the flow are said to be
leeward. Both windward and leeward parts deflect a flow.
Ignoring the leeward deflection leads to a popular
theory of lift.
Let's investigate how lift is generated by flow turning by using a Java
Here we see a yellow flat plate immersed in a flow of air. The air appears as
small blue and white particle traces which move from left to right. The plate
is inclined at an angle and notice that both the flow above and below the plate
are turned along the plate. The white lines are the
which intersect the plate and are called stagnation streamlines.
You can vary the angle of the plate 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
side of the simulator is a gage with some buttons
and some sliders. The gage tells you the value of the velocity
or pressure at the location of the probe (little purple dot)
in the left view window. You can change the location from side
to side by using the slider located below the gage, and you
can change the location up and down by using the slider
to the left of the gage. You select which variable to display
by using the white buttons labeled Velocity, Pressure,
or Smoke. Smoke causes green particles to be released from the probe.
The blue buttons control the type of display shown in the left
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 III Java Applet.
You can also
your own copy of FoilSim to play with
Changes in Speed or Direction
Lift is a force generated by turning a
flow. Since a force is a vector quantity (like the velocity), it has
both a magnitude and a direction. The direction of the lift force is
defined to be perpendicular to the initial flow direction. (The
drag is defined to be along the flow
direction.) The magnitude depends on several
factors concerning the object and the flow.
Lift and drag are mechanical forces generated on the surface of an
object as it interacts with a fluid. The net fluid
force is generated by the pressure acting over the entire surface
of a closed body. The pressure varies around a body in a moving fluid
because it is related to the fluid momentum (mass times
velocity). The velocity varies around the body because of the flow
deflection described above.