METHODS OF LATERAL STABILITY
Lateral and directional stability are so closely interconnected that it is impossible to consider one without the other, and they are therefore often grouped together under the single term of lateral stability. But, for simplicity, we will first consider them separately; then we, shall try to see how they affect each other.
Methods: From what has been said earlier, 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. The main contributors to lateral static stability are:
• Wing dihedral,
• Sweepback,
• High wing position,
• Keel surface.
A design feature that has the opposite effect to those given above, i.e. that reduces stability is anhedral. The need to reduce lateral stability may seem strange, but combat aircraft and much high speed automatically controlled aircraft, use anhedral to provide more maneuverability.
3.3.3. Wing dihedral and lateral stability: Dihedral angle is defined as the upward inclination of the wings from the horizontal. The amount of dihedral angle being dependent on aircraft type and wing configuration i.e. whether the wings are positioned high or low with respect to the fuselage and whether or not they are straight or swept back.
The righting effect from a roll using wing dihedral angle may be considered as a two stage process; where the rolling motion is first stopped and then the down-going wing is returned to the horizontal position.
So we first stop the roll. In Figure 3.6(a) we see that for an aircraft in a roll, one wing will move down and the other will move up, as a result of the rolling motion. The vector diagrams (Figure 3.6(b)) show the velocity resultants for the up-going and down-going wings. The direction of the free stream airflow approaching the wing is changed and the AOA on the down going wing is increased, while the AOA on the up-going wing is decreased (Figure 3.6(c)). This causes a larger CL and lift force to be produced on the lower wing and a smaller lift forces on the upper wing, so the roll is stopped. When the roll stops the lift forces equalize again and the restoring effect is lost. In order to return the aircraft to the equilibrium position, dihedral angle is necessary. A natural consequence of banking the aircraft is to produce a component
In Figure 3.7(a) the component of lift resulting from the angle of bank can clearly be seen. It is this force that is responsible for sideslip. Now if the wings were straight the aircraft would continue to sideslip, but if dihedral angle is built-in the sideways air stream will create a greater lift force on the down-going wing (Figure 3.7(b)). This difference in lift force will restore the aircraft until it is no longer banked over and side-slipping stops.
Lateral and directional stability are so closely interconnected that it is impossible to consider one without the other, and they are therefore often grouped together under the single term of lateral stability. But, for simplicity, we will first consider them separately; then we, shall try to see how they affect each other.
Methods: From what has been said earlier, 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. The main contributors to lateral static stability are:
• Wing dihedral,
• Sweepback,
• High wing position,
• Keel surface.
A design feature that has the opposite effect to those given above, i.e. that reduces stability is anhedral. The need to reduce lateral stability may seem strange, but combat aircraft and much high speed automatically controlled aircraft, use anhedral to provide more maneuverability.
3.3.3. Wing dihedral and lateral stability: Dihedral angle is defined as the upward inclination of the wings from the horizontal. The amount of dihedral angle being dependent on aircraft type and wing configuration i.e. whether the wings are positioned high or low with respect to the fuselage and whether or not they are straight or swept back.
The righting effect from a roll using wing dihedral angle may be considered as a two stage process; where the rolling motion is first stopped and then the down-going wing is returned to the horizontal position.
So we first stop the roll. In Figure 3.6(a) we see that for an aircraft in a roll, one wing will move down and the other will move up, as a result of the rolling motion. The vector diagrams (Figure 3.6(b)) show the velocity resultants for the up-going and down-going wings. The direction of the free stream airflow approaching the wing is changed and the AOA on the down going wing is increased, while the AOA on the up-going wing is decreased (Figure 3.6(c)). This causes a larger CL and lift force to be produced on the lower wing and a smaller lift forces on the upper wing, so the roll is stopped. When the roll stops the lift forces equalize again and the restoring effect is lost. In order to return the aircraft to the equilibrium position, dihedral angle is necessary. A natural consequence of banking the aircraft is to produce a component
In Figure 3.7(a) the component of lift resulting from the angle of bank can clearly be seen. It is this force that is responsible for sideslip. Now if the wings were straight the aircraft would continue to sideslip, but if dihedral angle is built-in the sideways air stream will create a greater lift force on the down-going wing (Figure 3.7(b)). This difference in lift force will restore the aircraft until it is no longer banked over and side-slipping stops.
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