HOW DIRECTIONAL STABILITY IS ACHIEVED
To be directionally stable, an aircraft should return to the equilibrium position after a yaw disturbance. It will do this if the centre of pressure of the aerodynamic forces exerted on the side surface of the aircraft is behind the centre of gravity. This can be ensured by the attachment of a fin at the rear of the fuselage.
If the centre of gravity is well forward in the aircraft the fin may be small, but if it is a long way back the fin may need to be very large
Many modern aircraft have long fuselages with the centre of gravity well back and therefore require a large fin area. On fast aircraft the fin becomes less efficient at high speed and this means that extra size is required.
Note: Although the diagrams show a fuselage surface only, forces which affect directional stability, are also exerted on wings, external engine nacelles etc.- It has already been mentioned that fin area and position affect lateral stability, as well as directional stability.
LATERAL STABILITY
From what has been said above an aircraft has lateral stability if, following a roll displacement, a restoring moment is produced which opposes the roll and returns the aircraft to a wings level position. In that, aerodynamic coupling produces rolling moments that can set up side slip or yawing motion. It is therefore necessary to consider these interactions when designing an aircraft to be inherently statically stable in roll.
To secure lateral stability we must so arrange things that when a slight roll takes place the forces acting on the aeroplane tend to restore it to an even keel. In all aeroplanes, when flying at a small angle of attack, there is a resistance to roll because the angle of attack, and so the lift, will increase on the down going wing, and decrease on the up-going wing. But this righting effect will only last while the aeroplane is actually rolling. It must also be emphasized that this only happens while the angle of attack is small; if the angle of attack is near the stalling angle, then the increased angle on the falling wing may cause a decrease in lift, and the decreased angle on the other side an increase; thus the new forces will tend to roll the aeroplane still further, this being the cause of auto-rotation.
To be directionally stable, an aircraft should return to the equilibrium position after a yaw disturbance. It will do this if the centre of pressure of the aerodynamic forces exerted on the side surface of the aircraft is behind the centre of gravity. This can be ensured by the attachment of a fin at the rear of the fuselage.
If the centre of gravity is well forward in the aircraft the fin may be small, but if it is a long way back the fin may need to be very large
Many modern aircraft have long fuselages with the centre of gravity well back and therefore require a large fin area. On fast aircraft the fin becomes less efficient at high speed and this means that extra size is required.
Note: Although the diagrams show a fuselage surface only, forces which affect directional stability, are also exerted on wings, external engine nacelles etc.- It has already been mentioned that fin area and position affect lateral stability, as well as directional stability.
LATERAL STABILITY
From what has been said above an aircraft has lateral stability if, following a roll displacement, a restoring moment is produced which opposes the roll and returns the aircraft to a wings level position. In that, aerodynamic coupling produces rolling moments that can set up side slip or yawing motion. It is therefore necessary to consider these interactions when designing an aircraft to be inherently statically stable in roll.
To secure lateral stability we must so arrange things that when a slight roll takes place the forces acting on the aeroplane tend to restore it to an even keel. In all aeroplanes, when flying at a small angle of attack, there is a resistance to roll because the angle of attack, and so the lift, will increase on the down going wing, and decrease on the up-going wing. But this righting effect will only last while the aeroplane is actually rolling. It must also be emphasized that this only happens while the angle of attack is small; if the angle of attack is near the stalling angle, then the increased angle on the falling wing may cause a decrease in lift, and the decreased angle on the other side an increase; thus the new forces will tend to roll the aeroplane still further, this being the cause of auto-rotation.
No comments:
Post a Comment