Stability And Manoeuvrability
Let us start by making the distinction between the stability and the manoeuvrability of the plane:
its manoeuvrability (its control) is ensured by the movements of moving parts of the plane making it possible to change its altitude, speed and direction.
its stability is its property to be maintained at its altitude and to resist a displacement (due to a gust of wind for example) and in the event of perturbation to develop a force restoring the initial flying conditions. Until now we only considered the case of the airplane in cruising flight at constant speed and altitude. In this case we saw that the plane is subjected to 4 balanced forces (Lift-Weight, Thrust-Drag) applied in its centre of gravity.
its manoeuvrability (its control) is ensured by the movements of moving parts of the plane making it possible to change its altitude, speed and direction.
its stability is its property to be maintained at its altitude and to resist a displacement (due to a gust of wind for example) and in the event of perturbation to develop a force restoring the initial flying conditions. Until now we only considered the case of the airplane in cruising flight at constant speed and altitude. In this case we saw that the plane is subjected to 4 balanced forces (Lift-Weight, Thrust-Drag) applied in its centre of gravity.
Aircraft Manoeuvrability
Three Axes of Rotation
Three Axes of Rotation
In addition to its displacement in horizontal direction, it is possible to consider side movements (rare for the planes) and rotation movements around the three axes. The movements (represented on the opposite figure) around the three axes are respectively called roll, pitch and yaw. These movements are characterized by rotations. Contrary to a car, a boat or a train, the planes are able to be driven according to the 3 axes. This movement occurs around the centre of gravity of the plane. The centre of gravity of the plane is the average position where the weight of the plane is applied. On the traditional planes, three types of devices are at the base of the control of the flight: the ailerons, the rudder and the horizontal stabilizers.
Pitch
The pitching of the plane is the movement where the plane turns around its centre of gravity and where the nose of the plane moves in a vertical plan (from top to bottom or upwards). Pitching is controlled thanks to horizontal stabilizers also located in the tail of the airplane. These stabilizers have hinged sections called elevators. The pilot can change the position of the elevator to raise or lower the nose of the airplane. The ailerons work in opposition: if one goes up, the other goes down.
The pitching of the plane is the movement where the plane turns around its centre of gravity and where the nose of the plane moves in a vertical plan (from top to bottom or upwards). Pitching is controlled thanks to horizontal stabilizers also located in the tail of the airplane. These stabilizers have hinged sections called elevators. The pilot can change the position of the elevator to raise or lower the nose of the airplane. The ailerons work in opposition: if one goes up, the other goes down.
Roll
Roll, the second axis of motion, is the rolling of an airplane from side to side, which causes the wings to go up or down. This movement of roll is produced owing to the fact that on the side where the aileron is lowered, the lift of the wing will increase, whereas on the side where the aileron is raised the lift decreases. This creates an imbalance of the forces on the right and on the left of the wing and the plane is inclining on the right or left-hand side. The hinged sections at the rear of each wing, called ailerons, help control the roll.
Roll, the second axis of motion, is the rolling of an airplane from side to side, which causes the wings to go up or down. This movement of roll is produced owing to the fact that on the side where the aileron is lowered, the lift of the wing will increase, whereas on the side where the aileron is raised the lift decreases. This creates an imbalance of the forces on the right and on the left of the wing and the plane is inclining on the right or left-hand side. The hinged sections at the rear of each wing, called ailerons, help control the roll.
Yaw
The third axis of motion, yaw, is the motion of an airplane's nose from side to side (or left to right), and the plane turns around its center of gravity in a horizontal plane. This movement of yaw is controlled by the rudder generally located in the tail of the plane. This device functions on the same principle as the rudder of a boat.
The third axis of motion, yaw, is the motion of an airplane's nose from side to side (or left to right), and the plane turns around its center of gravity in a horizontal plane. This movement of yaw is controlled by the rudder generally located in the tail of the plane. This device functions on the same principle as the rudder of a boat.
Aircraft Stability
- Ability to return to original flight path
- Allows aircraft to maintain uniform flight conditions, recover from disturbances, and minimize pilot workload
- Aircraft are designed with positive dynamic, which implies positive static as well
- More stable in right turns due to left turning tendencies
- Aircraft axis are imaginary lines passing through the aircraft; thought of as pivot points
- Longitudinal Axis: extends from the nose to the tail, through the fuselage
- Lateral Axis: runs from wingtip to wing tip
- Vertical Axis: passes through the center of the fuselage, from the top to the bottom
- An aircraft is considered stable when there is no rotational motion or tendency about any of the aircraft axis
Static Stability
Figure 3: Neutral Static Stability
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Figure 1: Positive Static Stability
Figure 2: Negative Static Stability
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Dynamic Stability
- Dynamic stability is the tendency of the aircraft over time
- An aircraft must have positive static to have dynamic stability [Figure 4]
- Positive Dynamic:
- Positive dynamic stability is the tendency of an aircraft to dampen toward original position once disturbed
- Neutral Dynamic:
- Neutral dynamic stability is the tendency of an aircraft to dampen back to its original position once disturbed to new position
- Negative Dynamic:
- Negative dynamic stability is the tendency of an aircraft to trend away from original position once disturbed
- Positive Dynamic:
Figure 4: Dynamic Stability
Longitudinal stability
- The longitudinal axis is an imaginary line running from the nose to the tail of the aircraft, motion about this axis is called roll, and it is controlled by the ailerons
- Longitudinal stability is the tendency of an aircraft to return to the trimmed angle of attack
- Accomplished through elevators and rudders
- Contributors:
- Straight wings (negative)
- Wing Sweep (positive)
- Fuselage (negative)
- Horizontal stabilizer (largest positive)
- Aerodynamic center aft of C.G. is a stabilizing moment
- Aerodynamic center forward of C.G. is a de-stabilizing moment
Vertical Stability
Dutch Roll
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Figure 8: Dutch Roll
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Lateral stability
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Figure 5: Dihedral Effect
Figure 6: Swept Wing Effect
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Directional stability
- Stability around the vertical axis
- Vertical tail accomplishes this
- You must have more surface area behind the CG than in front of it
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