Things to Emphasize

1. For any object to be aerodynamically stable when moving through any fluid, its centre of mass must be ahead of the object's centre of pressure.
2. A object's centre of mass can be moved by adding or removing mass from the object or by redistributing the mass within the object.
3. An object's centre of pressure can be moved by adding or removing fins to the object.
4. In general, centre of mass depends upon the composition and structure of an object, whereas, centre of pressure depends only on the shape of the object.

Conditions required for Aerodynamic Stability

Understanding Stability in the horizontal (YAW) plane

If the centre of mass of a body is well ahead of its centre of pressure , the body will be dynamically stable when moving through a fluid (gas or liquid). That is, it will automatically align itself in the forward direction of motion through the fluid. This is a highly desirable state, especially in rockets. However excessive stability makes it difficult to change direction.	Transparency Master
If the centre of mass of a body is at or near its centre of pressure, the body will be in a metastable state when moving through a fluid (gas or liquid). In this state the object is prone to sudden and violent changes in direction. Cars, trailers, trucks, boats, and airplanes designed or loaded so that this condition exists are very dangerous since their behavior is unpredictable.	Transparency Master
If the centre of mass of a body is behind its centre of pressure , the body will be in an unstable state when moving through a fluid (gas or liquid). In this condition the object will spontaneously try to flip around and face in the opposite direction as it moves.	Transparency Master

Designing for Aerodynamic Stability

Rockets

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Rockets In general it is easy to ensure that rockets are aerodynamics stable. From an aerodynamic point of view, a rocket is nothing more than a self-propelled arrow. Large fins at the back ensure that the centre of pressure is well behind the centre of mass. Since a rocket's primary direction is essentially up it is designed so that the centre of pressure is well its centre of mass. Since the major mass component of a rocket is its fuel, the centre of mass will change position a the rocket flies. For solid fuel rockets in which the fuel burns from the back of the rocket forward towards the nose cone, the rocket's centre of mass moves forward .The effect is to gradually improve the rocket's stability as it flies.

and Airplanes

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Airplanes Ensuring the aerodynamic stability of airplanes is slightly more complex than it is for rockets. For one thing, we would like an airplane to have the ability to move its nose upwards or downwards with respect to the tail of the airplane (called changing the pitch angle). Secondly, we need the ability to turn left and right (called yaw) and finally an airplane should have the ability to bank (called roll). Too much stability makes it difficult to maneuver an airplane, too little stability makes it prone to crashing.

Transparency Master	Yaw stability requires that the centre of pressure fall well behind the centre of mass as seen from the sides of the airplane. Transparency Master
Transparency Master	Pitch stability requires that the centre of vertical pressure (lift) fall approximately at the centre of mass, as seen from above (or below) the airplane. Transparency Master
Transparency Master Summary Transparency Master	Roll stability requires that longitudinal roll axis of the airplane (lift) lie above at the centre of mass, as seen from the front (or tail) the airplane. Transparency Master

Transparency Master	Yaw stability requires that the centre of pressure fall well behind the centre of mass as seen from the sides of the airplane. The centre of pressure is defined geometrically as the point along the fuselage at which the force due to air pressure acting on area A, which tends to turn the aircraft clockwise (into the page in this diagram) is exactly balanced by the force due to air pressure acting on area B which tends to turn the aircraft counterclockwise (also into the page.) We can say that "at the centre of pressure, the sum of all rotational torques is zero" in the horizontal plane. In this condition the aircraft flies straight ahead.
Transparency Master	Pitch stability requires that the centre of vertical pressure (lift) fall approximately at the centre of mass, as seen from above (or below) the airplane. The centre of pressure lies on a line, usually through the wing along which the force due to air pressure acting on area C, which tends to lift the aircraft's nose upwards is exactly balanced by the force due to air pressure acting on area D which tends to lift the aircraft's tail upwards. Once again, we can say that "at the centre of pressure, the sum of all rotational torques is zero" in the vertical plane. In this condition the aircraft maintains level flight.
Transparency Master	Roll stability requires that longitudinal roll axis of the airplane (lift) lie above at the centre of mass, as seen from the front (or tail) the airplane in straight level flight. The aircraft is most stable when the roll axis passes below the aircraft's centre of lift

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