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.
There are many factors that affect aerodynamic forces.
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
itself
The Object
Geometry has a large effect on the aerodynamic forces
generated by an object.
Lift and drag depend linearly on the
size
of the object moving through the air.
The cross-sectional
shape
of an object determines the form drag
created by the
pressure variation
around the object.
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
measured
drag coefficient
of the object.
Motion of the Air
Lift and drag are associated with the movement of the
rocket through the air, so lift and drag depend on the
velocity of the air.
Lift and drag actually vary with the square of the
relative velocity
between the object and the air. The
inclination of the object to the flow
also affects the
amount of lift and drag generated by a given shaped object.
If the object moves through the air at speeds near the
speed of sound,
shock waves
are formed on the object which create an additional drag component called
wave drag.
The motion of the object through the air also causes
boundary layers
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
Lift and drag depend directly on the mass
of the flow going past the rocket. 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 lift and
drag into two mathematical equations called the
Lift Equation and the
Drag Equation.
With the these equations we can predict how much aerodynamic
force is generated by a given body moving at a given speed through a
given fluid.
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