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.

Aircraft Manoeuvrability
​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.

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.

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

• Static stability is the initial tendency of the aircraft
• Stability can be described as either positive, negative or neutral
  • Positive Static:
    • Tendency to return to original position [Figure 1]
    • If an airplane yaws or skids, the sudden rush of air against the fuselage and control surfaces quickly forces the airplane back to its original direction
      
  • Neutral Static:
    • Tendency to remain at new position [Figure 2]
    • If an airplane is put into a turn and the pilot lets go of the controls and the aircraft remains in that turn but neither rolls out or gets steeper
  • Negative Static:
    • Tendency to continue away from original position [Figure 3]
    • If an aircraft is rolled to a high bank angle, letting go of the controls results in the aircraft continuing to roll further

​Figure 3: Neutral Static Stability

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

Figure 4: Dynamic Stability

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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

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Vertical Stability

• The vertical axis is an imaginary line running from the top of the plane to the bottom of the plane, rotation about this axis is called "yaw" and is controlled by the rudder

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• Tendency to resist yawing


• Yawing moment

• Accomplished through rudders

Figure 7: Rudder Effect

Dutch Roll

• Coupling of the lateral and directional axes causes Dutch roll
• Dutch roll is a combined yawing-rolling motion of the aircraft and may be considered only a nuisance unless allowed to progress to large bank angles
• Large rolling and yawing motions can become dangerous unless properly damped
• Side-slip disturbance will cause the aircraft to roll
• The bank angle, in turn, causes side-slip in the opposite direction
• While not unstable, this continual trade-off of side-slip and angle of bank is uncomfortable
• Dutch roll may be excited by rough air or by lateral-directional over controlling
• Once induced, it is damped by normal aircraft stability
• Poor Dutch roll characteristics may make the aircraft susceptible to pilot induced oscillations (PIO)
• Lateral-directional PIO is most common when the pilot chases line-up in the landing configuration

Figure 8: Dutch Roll

Lateral stability

• The lateral axis is an imaginary line running from wing tip to wing tip, movement about this axis causes the nose of the aircraft to raise or lower, and is caused by moving the elevators
• Lateral stability is the tendency of an aircraft to resist roll
• Dihedral Effect:
  
  Figure 6: Swept Wing Effect
  • Dihedral is evident when an aircraft rolls, creating a side-slip (assume no rudder)
  • One of the wings is lower than the other and this creates a difference in the angle of attack experienced by each wing
  • The lower wing has an increase in angle of attack which causes it to create more lift and therefore rise while the opposite is true for the higher wing [Figure 5]
    • The net result is the aircraft rolling away from the side-slip, thus resisting roll and attempting to bring the wings back to level
  • Use of the rudder will smoothen the turn and overcome these forces as well as others, such as adverse yaw
• Swept Wing Effect:
  • Side-slips create more direct relative wind to the upwind swept wing which creates a roll back toward wings level [Figure 6]

Figure 5: Dihedral Effect

Figure 6: Swept Wing Effect

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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