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Refer to figure.
Which constructive feature is shown in the figure (L: Lift)?

  • A

    Longitudinal stability by wing dihedral.

  • B

    Lateral stability by wing dihedral.

  • C

    Differential aileron deflection.

  • D

    Directional stability by lift generation.

Refer to figures.
Longitudinal Stability (pitching) is the quality that makes an aircraft stable about its lateral axis. It involves the pitching motion as the aircraft’s nose moves up and down in flight. A longitudinally unstable aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall.

Differential aileron deflection

Ailerons control the aeroplane around the longitudinal axis. They increase the lift at one wing and at the same time they decrease the lift at the opposite. That creates a roll motion. Because lift increases at the up going wing, induced drag is also increased and that creates an “Adverse Yaw” tendency.

For this question, the left wing has an increased lift and drag also. So the nose of the aeroplane will turn initially to the left. That will apply as long as we keep the ailerons in that position and we have to oppose the rudder.

Directional Stability (Yawing)

Stability about the aircraft’s vertical axis (the sideways moment) is called yawing or Directional Stability. The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors that make the aircraft act like the well known weather vane or arrow, pointing its nose into the relative wind.

Wing Dihedral

Some aircraft are designed so that the outer tips of the wings are higher than the wing roots. The upward angle thus formed by the wings is called dihedral.

When a gust causes a roll, a sideslip will result. This sideslip causes the relative wind affecting the entire aeroplane to be from the direction of the slip. When the relative wind comes from the side, the wing slipping into the wind is subject to an increase in Angle of Attack and develops an increase in lift. The wing away from the wind is subject to a decrease in angle of attack, and develops a decrease in lift. The changes in lift effect a rolling moment tending to raise the windward wing, hence dihedral contributes to a stable roll due to sideslip.

Stability about the aircraft’s longitudinal axis, which extends from the nose of the aircraft to its tail, is called Lateral Stability. Positive Lateral Stability helps to stabilize the lateral or “rolling effect” when one wing gets lower than the wing on the opposite side of the aircraft.

There are four main design factors that make an aircraft laterally stable:

  • Dihedral.
  • Sweepback.
  • Keel effect.
  • Weight distribution.

Lateral Stability by wing dihedral is the constructive feature, which is shown in attached figure.

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This question has appeared on the real examination, you can find the related countries below.

  • Austro Control