Stability Flashcards
Define Static Stability
Static Stability is the initial tendency of an aircraft to return to its original state after being disturbed.
Define Dynamic Stability
Dynamic Stability refers to the subsequent motion of the aircraft once the effect of static stability has taken effect.
An aircraft can only be dynamically stable if it is first statically stable.
What is the difference between controllability and stability.
Stability is the inherent tendency of the aircraft to return to its original position after being displaced without pilot input.
Controllability is the ease that the pilot may manoeuvre the aircraft using flight controls.
What are the 3 types of stability affecting an aircraft?
Longitudinal (around the lateral axis), pitch.
Directional (around the vertical axis), yawing moments.
Lateral (around the longitudinal axis), roll.
What is the main criteria for the longitudinal stability of an aircraft?
For longitudinal stability to be positive, the restoring force of the tailplane must be stronger than the unstable pitching moment from the wings.
What are the factors affecting longitudinal stability?
Factors Affecting Longitudinal Stability:
Tailplane Design and Size (Larger size for improved restoring force)
COG Position - Increase arm between COG and Tailplane
Longitudinal Dihedral - Reduced angle of incidence at tailplane for downwards restoring moment during cruise.
What are the factors affecting directional stability?
Keel/Surface Area behind COG. The more surface area, the more restoring force is produced.
Tail Fin design - Dorsal Fin, or Ventral Fin to increase surface area and arm. Sweepback fin.
Arm between COG postion and tail fin.
What are the factors affecting lateral stability?
Dihedral - Lower wing has higher AOA, producing more lift and thus a restoring moment.
Shielding - Lee side of aircraft shielded, providing drag during the slip as the aircraft is disturbed in roll. This drag force creates a restoring roll.
Wing Position - High wing is more laterally stable, as Lift acts slightly off centre from weight, towards the don-going wing. This provides a restoring moment.
Sweepback - The down going wing will have a greater effective span as the aircraft slips towards the airflow during the roll.
What exactly is Lateral Stability?
Lateral Stability is the tendency of the aircraft to resume its original state after being disturbed in roll around the longitudinal axis.
What exactly is Directional Stability?
Directional Stability is the tendency of the aircraft to resume its original position after being disturbed by yawing moments around the vertical axis.
What is Dutch Roll?
Dutch Roll occurs when the aircraft is overly laterally stable and less directionally stable.
Self perpetuating wallowing motion once disturbed in yaw.
What is Spiral Instability?
Spiral Instability occurs when the aircraft is more directionally stable than lateral stable.
once disturbed in roll, the aircraft will enter a spiral dive unless counteracted by pilot input.
