When a solid body is moved through a fluid (gas or liquid), the
fluid resists the motion. The object is subjected to an aerodynamic
force in a direction opposed to the motion
which we call drag.
As with aircraft lift, there are many
factors that affect drag. We can group these factors into (a) those
associated with the object, (b) those associated with the motion of
the object through the air, and (c) those associated with the air
Geometry has a large effect on the amount of drag
generated by an object.
As with lift, the drag depends linearly on the
of the object moving through the air.
of an object determines the form drag
created by the
around the object.
The three dimensional planform shape affects the
of a lifting wing.
If we think of drag as aerodynamic friction, the amount of
drag depends on the surface roughness of the object; a smooth, waxed
surface produces less drag than a roughened surface. This
effect is called skin friction and is usually included in the
of the object.
Motion of the Air
Drag is associated with the movement of the
aircraft through the air, so drag depends on the
velocity of the air.
Like lift, drag actually varies with the square of the
between the object and the air. The
inclination of the object to the flow
also affects the
amount of drag generated by a given shaped object.
If the object moves through the air at speeds near the
speed of sound,
are formed on the object which create an additional drag component called
The motion of the object through the air also causes
to form on the object. A boundary layer is a region of very low speed flow
near the surface which contributes to the skin friction.
Properties of the Air
Drag depends directly on the mass
of the flow going past the aircraft. The drag also depends in a
complex way on two other properties of
the air: its viscosity and its compressibility.
These factors affect the wave drag and skin friction which
are described above.
We can gather all of this information on the factors that affect
drag into a single mathematical equation called the Drag
Equation. With the drag equation we can predict how much drag
force is generated by a given body moving at a given speed through a
You can investigate the various factors that affect drag by using
the FoilSim III Java Applet. Have fun! You
can use the browser "Back" button to return to this page. If your
want your own copy of
FoilSim to play with, you can download
it at no charge.
Factors that Affect Drag:
- Beginner's Guide Home Page