A flying “weather vane”

The aerodynamic stability of a rocket (or any object moving through a fluid medium such as air or water) depends upon its ability resist changes in its direction of flight.

There are two parameters which affect the stability of a rocket in flight. These are the Centre of Aerodynamic Pressure and the Centre of Mass and their relationship to each other.

The concept of the Centre of Aerodynamic Pressure is easy to understand. Simply take a large cardboard cutout of a bottle rocket (sillouette).

Support the cardboard from below, and on its edge by inserting a thin metal rod (such as a straightened coat hanger) into the cardboard.

Move the cardboard rapidly through the air, or alternatively, take it outside on a breezey day as shwon in the photos
which follow.

Observer the behavior of the cutout as you insert the rod at various positions along its edge (see the diagram below)

You will note that for almost all positions of the rod, the cardboard will attempt to rotate on the pin. The point at which no rotation occurs is called the Centre of Aerodynamic Pressure

The response of an object to its Aerodynamic Centre of Pressure a very important concept.

Key Idea

In a weather vane, or any object which is free to rotate about a fixed axis,the Aerodynamic Centre of Pressure is always located on the downwind side of the axis of rotation.

Designers of machines , from race cars to rockets, which depend upon aerodynamics to provide in-flight stability, make use of this importance concept.

As suhggested above,
the following discussion illustrates a simple experiment whereby one can acquire an understanding of the concepts related
to aerodynamics. In this case we investigate the Aerodynamic Centre of Pressure on a side of a rocket.

A cardboard cutout held into the wind
(the basic concept)

Case 1


Pivot point lies “ahead” of the Centre of Aerodynamic pressure

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Case 2

Pivot point passes through the Centre of Aerodynamic pressure

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Case 3

Pivot point lies “behind” of the Centre of Aerodynamic pressure

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Rotates clockwise

No rotation

Rotates counterclockwise
The rocket will align itself so that it points its nose into the airflow. It is stable in this configuration.
When the rocket’s natural pivot point is near the front of the rocket, well ahead of its Aerodynamic Centre of Pressure, it will fly well.

In this instance the rocket has no preferred direction of rotation.

It may begin to rotate in either direction or
remain fixed. This is an unstable configuration.

When the pivot point is behind the CAP the rocket will try to rotate so that it points away from the direction of the airflow. This is a highly unstable condition for a rocket. It tries to flip around so that it continually reverses its direction causing it to spiral.

A cardboard cutout held into the wind
(the experiment)

Case 1


Pivot point lies “ahead” of the Centre of Aerodynamic pressure

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Case 2


Pivot point passes through the Centre of Aerodynamic pressure


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Case 3

Pivot point lies “behind” of the Centre of Aerodynamic pressure

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Locating the Centre of Aerodynamic Pressure

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The centre of aerodynamic pressure for a symmetrical object, such as a rocket, lies on a plane section through the object.

The geometry of the rocket is such that the effect of air pressure on the surface of the rocket above this plane (area A) is exactly balanced by the effects of pressure on the rocket below this plane above (area B).

[NOTE: The areas (labeled A & B in the diagram above) which are on each side of the centre of pressure, are not usually equal.

The location of the centre of aerodynamic pressure is determined by the position on the rocket about which equal, and opposite, torques are
created.

It is possible , in certain designs, that the Centre of Aerodynamic Pressure can lie “outside” of the
rocket body, as shown below.

The Concept of Torque

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Consider the diagram to the left.

It is not the position defined by equal areas which determine the centre of pressure but rather it is the position defined by equal torques which determine the centre of pressure.

It is the sum of all of torques generated by air pressure acting on these areas that
must add up to zero; i.e. the centre of pressure is defined as the point about which the rockets’s tendency to turn in one direction is exactly balanced by the rocket’s tendency to turn in the opposite direction]