Aerodynamic forces are generated and act on a
as it flies through the air.
having both a magnitude and a direction.
The magnitude of the aerodynamic forces
on the shape, size and
of the rocket and some
of the air through which it flies.
By convention, the single
into two components: the
force which is opposed to the direction of motion, and the
force which acts perpendicular to the direction of motion.
The lift and drag act through the
center of pressure
which is the average
location of the aerodynamic forces on an object.
Aerodynamic forces are mechanical forces.
They are generated by the interaction and
contact of a solid body with a fluid, a
liquid or a gas.
Aerodynamic forces are not generated by a force field,
in the sense of the gravitational
field,or an electromagnetic field.
For lift and drag to
be generated, the rocket must be in contact with the air. So
there is no lift and no drag.
Aerodynamic forces are generated by the difference in velocity
between the rocket
and the air. There must be motion between the rocket and the
air. If there is no relative motion, there is no lift and no drag.
Aerodynamic forces are
for a model rocket than for a
full scale rocket
because the entire
of the model rocket takes place in the atmosphere. A full scale rocket
above the atmosphere very quickly.
Aerodynamic forces are used differently on a rocket than on
On an airplane, lift is
used to overcome the weight of the aircraft,
but on a rocket,
is used in opposition to
Because the center of pressure is not normally
located at the
center of gravity
of the rocket, aerodynamic forces can
cause the rocket to rotate in flight.
The lift of a rocket is a side force
used to stabilize
and control the direction of flight.
Lift occurs when a flow of gas is turned
by a solid object. The flow is turned in one direction, and the lift
is generated in the opposite direction, according to
Newton's third law
of action and reaction.
For a model rocket, the nose cone,
body tube, and fins can turn the flow and become a source of lift
if the rocket is inclined to the flight direction
While most aircraft have a high lift to drag ratio,
the drag of a rocket is usually much greater than the lift.
We can think of drag as aerodynamic friction, and one of
the sources of drag is the skin friction between the molecules
of the air and the solid surface of the moving rocket. Because the skin
friction is an interaction between a solid and a gas, the magnitude
of the skin friction depends on properties of both solid and gas. For
the solid, a smooth, waxed surface produces less skin friction than a
roughened surface. For the gas, the magnitude depends on the
viscosity of the air and the relative
magnitude of the viscous forces to the motion of the flow, expressed
as the Reynolds number. Along the surface, a
of low energy flow is generated and the magnitude of the
skin friction depends on the state of this flow.
We can also think of drag as aerodynamic resistance to the
motion of the object through the fluid. This source of drag depends
on the shape of the rocket and is called form drag. As air
flows around a body, the local velocity and pressure
are changed. Since pressure is a measure of the momentum of the gas
molecules and a change in momentum produces a force,
a varying pressure distribution will produce a force on the body. We
can determine the magnitude of the force by
integrating, or adding up the local pressure times the surface area
around the entire body. The base area of a model rocket
produces form drag.
Accurately determining the size of the drag force is very difficult in practice.
The size of the drag changes depending on the thrust setting and the flow of gases at the
base of the rocket. When the engine is operating, the drag
is usually low. But when the engine is off during the coast phase of a model rocket,
the entire base of the rocket produces a large drag.
Drag is most often determined by
wind tunnel testing
a model of the rocket.
Forces on a Rocket:
Fundamental Terminology: Grade 10-12
Wind Tunnel Index
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