22.26.8
Define directional stability and explain the factors affecting it.
Directional stability is in the yawing plane, about the normal axis. It can be considered as the inherent ability of the aircraft to weathercock, so the nose points into the oncoming wind.
Directional stability relies on having a greater amount of keel (or side) surface behind the CG than in front of it. The fin (rudder) can be considered to produce the majority of the directional stability.
22.26.6
Define longitudinal stability and explain:
(a) the action of the tailplane in maintaining longitudinal stability;
(b) wing pitching moments;
(c) the effect of CG position.
Longitudinal stability is the stability of an aircraft in the longitudinal, or pitching plane about the lateral axis.
To be longitudinally stable, the aircraft must have an inherent tendency to return to the same pitch attitude after a disturbance.
(a) The tailplane is the primary means of restoring longitudinal stability by providing a nose-down restoring moment because of an increase of its AoA when disturbed.
Image – Fig 13-3 Page13-3 Waypoints
(b) Pitching moment on an aerofoil is the moment (or torque) produced by the aerodynamic force on the aerofoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the aerofoil. In most aircraft this aerodynamic center is located aft of the CG. This creates a stable restoring moment. The pitching moment on the wing of an airplane is part of the total moment that must be balanced using the lift on the horizontal stabilizer
(c) Longitudinal stability is largely determined by the CG position, which can be controlled by loading.
A forward CG position, gives the tailplane a longer arm, therefore creating a larger restoring moment, making the aircraft more stable in pitch. With an aft CG position, the shorter the tailplane moment arm, the less stable the aircraft becomes in pitch.
22.26.4
Differentiate between stability and controllability.
Stability is the inherent ability of the aircraft to return to its original attitude after being disturbed.
Controllability refers to the ease with which a pilot can maneuver the aircraft and change its attitude using the control surfaces.
22.26.2
Explain static stability and dynamic stability.
Static stability refers to the initial reaction of a body after being disturbed or displaced from a position of equilibrium. If it initially tends to return to its original position, it has positive static stability.
Dynamic stability relates to the subsequent motion of the disturbed body, once the static stability reaction has taken place. An aircraft will have dynamic stability if it eventually returns of its own accord to its original position of equilibrium.
22.26
Stability
Propeller solidity is an important design parameter for the axial flow impeller and is defined as the ratio of blade chord length to pitch.
22.24.18
Describe asymmetric blade effect.
22.24.16
Explain propeller centrifugal and aerodynamic twisting moments.
Play Anination
22.24.14
Describe the forces acting on a propeller when:
(a) windmilling;
(b) feathered;
(c) in reverse thrust.
22.24.12
Describe the correct procedure for handling manifold pressure and propeller controls.
The manifold control is the throttle control. In an aircraft with a constant speed propeller the throttle controls manifold pressure. The propeller control controls the aircraft rpm.
So when flying the aircraft the throttle is opened for takeoff and the propeller control is fully fine.
Once the aircraft is established in flight, the throttle is closed to achieve the required manifold pressure, and then the propeller control is reduced to achieve the desired rpm.
Anhedral aircraft with downward wings are typically used on large high-wing aircraft. It has a destabilizing effect to prevent the natural lateral stability becoming too strong.
Shielding:
Once the aircraft begins to side-slip, the trailing (up-going) wing, becomes shielded by the fuselage, which contributes to dihedral effect.
Keel surface:
Where the keel surface/fin area, is above the CG, the side slip force exerted on the fin, tends to right the aircraft back to level. The bigger the fin/tail, coupled with a low CG, the better the lateral stability.
Sweep-back:
Sweepback refers to the angle at which an aircraft’s wing is set back from a right angle to the body. Sweepback helps the aircraft gain lateral stability, therefore righting it if it is disturbed in roll.
In the diagram, you can see that when an aircraft rolls and slips, the relative airflow doesn’t meet the aircraft directly head on. This means, the effective span, and therefore the aspect ratio of each wing is different.
Aspect ratio = span / mean chord line
This means, the lower wing has a higher aspect ratio, and therefore produces more lift, this in turn raises the lower wing back to straight and level.
Wing position (Pendulum Effect):
High wing aircraft have a higher CG, thus when they are disturbed in roll, a moment is created between the forces of lift and weight, creating a righting moment.
This effect is not as strong in low wing aircraft.
This is also known as pendulum effect.