Week 1 illustrated states, types, dynamics and degrees of stability. Present week continues the same illustrating further different conditions of stability.
LONGITUDINAL STABILITY
An aircraft is longitudinally stable if it returns to the equilibrium condition after a pitching disturbance. Let us consider an aircraft with a fuselage and wings, but with no tail plane, which suffers a disturbance causing the nose to push up for the time being under a disturbing force. (Remember that we consider the centre of gravity to continue to move in a straight line.)
The effects are: (a) The angle of attack increases, (b) The centre of pressure moves forward, (c) The lift force now provides a clockwise moment about the centre of gravity.
This causes the nose to keep on rising so that it will not return to the equilibrium position. The aircraft is unstable longitudinally.
Similarly, if the pitching disturbance causes a nose-down attitude on such a longitudinally unstable aircraft, the centre of pressure moves to the rear and the aircraft is again unstable.
For the above aircraft to be longitudinally stable, there must be two requirements:
• A nose-down pitching disturbance must cause aerodynamic forces which give a nose-up moment (restoring moment)
• The restoring moment must be large enough to restore the aircraft to a straight and level flight position.
For any aircraft, in order to obtain stability in pitching, we must ensure that if the angle of attack is temporarily increased under the influence any unlikely disturbing influence, forces will act in such a way as to depress the nose and thus decrease the angle of attack once again. In general it can be said that this is dependent on four factors:
1. The position of the centre of gravity, which must not be too far back; this is probably the most important consideration.
2. The pitching moment on the main planes; this usually tends towards instability, though it can be modified by the means mentioned.
3. The pitching moment on the fuselage or body of the aeroplane; this too is apt to tend towards instability.
4. The tail plane -its area, the angle at which it is set, its aspect ratio, and its distance from the centre of gravity. This is nearly always a stabilizing influence.
LONGITUDINAL STABILITY
An aircraft is longitudinally stable if it returns to the equilibrium condition after a pitching disturbance. Let us consider an aircraft with a fuselage and wings, but with no tail plane, which suffers a disturbance causing the nose to push up for the time being under a disturbing force. (Remember that we consider the centre of gravity to continue to move in a straight line.)
The effects are: (a) The angle of attack increases, (b) The centre of pressure moves forward, (c) The lift force now provides a clockwise moment about the centre of gravity.
This causes the nose to keep on rising so that it will not return to the equilibrium position. The aircraft is unstable longitudinally.
Similarly, if the pitching disturbance causes a nose-down attitude on such a longitudinally unstable aircraft, the centre of pressure moves to the rear and the aircraft is again unstable.
For the above aircraft to be longitudinally stable, there must be two requirements:
• A nose-down pitching disturbance must cause aerodynamic forces which give a nose-up moment (restoring moment)
• The restoring moment must be large enough to restore the aircraft to a straight and level flight position.
For any aircraft, in order to obtain stability in pitching, we must ensure that if the angle of attack is temporarily increased under the influence any unlikely disturbing influence, forces will act in such a way as to depress the nose and thus decrease the angle of attack once again. In general it can be said that this is dependent on four factors:
1. The position of the centre of gravity, which must not be too far back; this is probably the most important consideration.
2. The pitching moment on the main planes; this usually tends towards instability, though it can be modified by the means mentioned.
3. The pitching moment on the fuselage or body of the aeroplane; this too is apt to tend towards instability.
4. The tail plane -its area, the angle at which it is set, its aspect ratio, and its distance from the centre of gravity. This is nearly always a stabilizing influence.
No comments:
Post a Comment