Following the liftoff of a model rocket,
it often turns into the wind. This maneuver is called
weather cocking and it is caused by aerodynamic
forces on the rocket. The term weather cocking is derived from
the action of a weather vane which is shown in black at the top of the figure.
A weather vane is often found on the roof of a barn.
It pivots about the vertical
bar and always points into the wind.
Older, more artistic weather
vanes used the figure of a rooster with large flaring tail feathers
instead of the wing shown on the figure. This type of weather vane
was called a weather cock.
Why does weather cocking occur?
As the rocket accelerates away from the
launch pad, the velocity increases and the aerodynamic forces on the
rocket increase. Aerodynamic forces depend on the square of the
velocity of the air passing the vehicle. If no
wind were present, the flight path would be vertical as shown at the
left of the figure, and the relative air
velocity would also be vertical and in a direction opposite to
the flight path. If you were on the rocket, the air would appear to
move past you toward the rear of the rocket.
The velocity of an object is a
vector quantity
having both a magnitude and a direction and when discussing
velocities we must
account
for both magnitude and direction.
The wind introduces an additional velocity component
perpendicular to the flight path, as shown in the middle of the figure. The
addition
of this component
produces an effective flow direction shown in red on the figure.
The effective flow direction is inclined to the horizontal at an angle
which we shall call angle b.
The size of angle b depends on the relative magnitude of the wind and the
rocket velocity.
Since the effective flow is inclined
to the rocket axis, an aerodynamic lift
force is generated by the rocket body and fins. The lift force acts
through the center of pressure cp of the
rocket. For
stability reasons,
the cp is located below the
center of gravity cg.
The lift force generates a
torque
about the cg which causes the rocket to rotate.
The rotation of the rocket produces a new flight path into the wind,
as shown on the right of the figure. When the new flight path is aligned with the
effective flow direction, there is no longer any lift force generated and the
rocket continues to fly in the new flight direction. The flight path is
inclined to the horizontal at angle b. We can determine this angle by
considering the middle of the figure. If the wind velocity is w and the
flight velocity is V, then:
