Static margin

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Static margin is a concept used to characterize the static stability and controllability of aircraft and missiles.

  • In aircraft analysis, static margin is defined as the distance between the center of gravity and the neutral point of the aircraft.
  • In missile analysis, static margin is defined as the distance between the center of gravity and the center of pressure.

The differences in the two fields arise largely from the use of cambered wings in aircraft. Missiles are symmetric vehicles and if they have airfoils they too are symmetric. With cambered wings the location of the center of pressure on the wing is a strong function of angle of attack - see movement of the center of pressure.

The response of an aircraft or missile to an angular disturbance such as a pitch disturbance is determined by its static margin.

With the center of gravity forward of the neutral point, an aircraft has positive longitudinal static stability.[1] (For an aircraft this may be described as positive static margin.) With the center of gravity aft of the neutral point, an aircraft is statically unstable, and requires some form of augmentation to be flown with an acceptable workload. (For an aircraft this may be described as negative static margin.)

For missiles, positive static margin implies that the complete vehicle makes a restoring moment for any angle of attack from the trim position. If the center of pressure is behind the center of gravity then the moment will be restoring. For missiles with symmetric airfoils, the neutral point and the center of pressure are coincident and the term neutral point is not used.

If an aircraft in flight suffers a disturbance in pitch that causes an increase (or decrease) in angle of attack, it is desirable that the aerodynamic forces on the aircraft cause a decrease (or increase) in angle of attack so that the disturbance does not cause a continuous increase (or decrease) in angle of attack. This is longitudinal static stability.

Relationship to aircraft and missile stability and control[edit]

  • If the center of gravity (CG) of an aircraft is forward of the neutral point, or the CG of a missile is forward of the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that returns the angle of attack of the vehicle towards the angle that existed prior to the disturbance.
  • If the CG of an aircraft is behind the neutral point, or the CG of a missile is behind the center of pressure, the vehicle will respond to a disturbance by producing an aerodynamic moment that continues to drive the angle of attack of the vehicle further away from the starting position.

The first condition above is positive static stability. In missile analysis this is described as positive static margin. (In aircraft analysis it may be described as negative static margin.)
The second condition above is negative static stability. In missile analysis this is defined as negative static margin. (In aircraft analysis it may be described as positive static margin.)

Depending on the static margin, humans may not be able to use control inputs to the elevators to control the pitch of the vehicle. Typically, computer based autopilots are required to control the vehicle when it has negative static stability - usually described as negative static margin.

The purpose of the reduced stability (low static margin) is to make an aircraft more responsive to pilot inputs. An aircraft with a large static margin will be very stable and slow to respond to the pilot inputs. The amount of static margin is an important factor in determining the handling qualities of an aircraft. For an unguided rocket, the vehicle must have a large positive static margin so the rocket shows minimum tendency to diverge from the direction of flight given to it at launch. In contrast, guided missiles usually have a negative static margin for increased maneuverability.

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