Stability Flashcards
Explain static stability and dynamic stability. And stability in general
- Static stability: initial reaction of a body after being disturbed
- Dynamic stability: subsequent motion once static stability reaction has taken place
- An aircraft must have static stability before it can be dynamically stable.
- Stability is the inherent ability of the aircraft to return to its original attitude after being disturbed, without any action being taken by the pilot
Differentiate between stability and controllability.
Stability - the inherent ability of the aircraft to return to its original attitude after being disturbed without any action being taken by the pilot.
Controllability - refers to the ease with which a pilot can manoeuvre the aircraft and change its attitude by using the control surfaces.
High stability = reduced controllability.
Define longitudinal stability and explain:
(a) the action of the tailplane in maintaining longitudinal stability;
Stability about the lateral axis. To be longitudinally stable, an aircraft must have a tendency to return to the same pitch attitude after a disturbance.
If theres attitude changes, a change of AoA on the tailplane produces a main stabilising force which returns the aircraft to its original place.
(E.g. nose up disturbance AoA and tailplane increased, tailplane lift increased to provide nose-down restoring moment)
What is positive, neutral and negative dynamic stability?
Positive - returns to original attitude of own accord
Neutral - oscillates evenly about original attitude
Negative - diverges from original
Explain wing pitching moments
Any pitching movement of the wing.
If CoP is positioned behind CoG, if aircraft is displaced nose-up, wing lift increases, which tends to increase the nose-down pitch about the CoG. Wing pitching moment from lift tends toward stability.
If CoP is positioned in front, wing pitching moment is unstable.
Tailplane let must be greater than wing lift for stability.
Explain the effect of CoG position.
Forward CoG position:
further forward, the longer the moment arm becomes for the tailplane. A given tailplane force therefore creates a more effective restoring moment and aircraft becomes more stable. Manoeuvrability is decreased however and feels more nose heavy
Aft CoG position:
Further aft, shorter tailplane moment arm and the less stable the aircraft becomes in pitch. Manoeuvrability is increased and feels more tail heavy.
CoG limits
Aft limit:
Insufficient longitudinal stability and difficulty to control. Or maybe impossible.
Forward limit:
Aircraft will lack manoeuvrability, elevator stick forces become excessive.
Define directional stability and explain the factors affecting it.
Stability in the yawing plane about the normal axis. The inherent ability of an aircraft to have its nose remain pointed into the oncoming airflow.
Factors:
The greater the fin area and keel surface behind the CoG the greater the moment arm, the greater the directional stability of the aircraft.
Thus having a forward CoG favours directional stability more than aft CoG as it gives a longer moment arm.
(Keel = the fuselage surface above the CoG)
Define lateral stability
The ability of an aircraft to recover from a disturbance in the lateral plane (roll about the longitudinal axis)
What’s dihedral and Anhedral?
Dihedral - upward inclination of the wingtips at the root.
Anhedral - downward inclination
Factors affecting lateral stability, (dihedral, shielding, wing position, keel surface/fin, sweep back.
Dihedral wings - As aircraft develops sideslip after an uninvited roll, the lower wing, because of dihedral, meets the RAF at a higher AoA than the upper.
lower wing produces more lift than upper wing and a rolling moment is produced which tends to return the aircraft into the original wings-level position.
Shielding - when aircraft sideslips, the up going wing Becomes shielded to some extent by the fuselage, which contributes to dihedral effect.
Wing position - high wing has greater lateral stability then low-wing.
When unwanted roll occurs, CoP of wing becomes laterally displaced in respect to CoG. CoP tend to move toward lower wing, increasing displacement and this couple which is set up laterally between VCL and weigh tend to restore aircraft to wings level.
Low wing aircraft are the same but since CoP and CoG are closer vertically the restoring moment is smaller.
Keel surface/fin - the sideslip drag force will tend to roll the aircraft away from the direction of the slip (provide stabilising influence). Tall fin designs make a positive contribution to lateral stability.
Sweep back - Lateral stability is increased if swept back.
As sideslip develops after an uninvited roll, the lower swept-back wing presents more of its span to oncoming airflow and the thickness/chord ratio is increased.
Aspect ratio effectiveness is then increased in the lower wing which results in more lift and a better restoring moment.
Explain the requirement to match lateral and directional stability.
After a disturbance in roll, lateral and directional stability characteristics of an aircraft are in conflict in that:
lateral stability wants to return to wings level and the directional stability wants to make the aircraft roll further.
Designer has to ensure both stability are correctly match and that neither predominates.
Explain spiral instability, dutch roll and snaking.
SPIRAL INSTABILITY
If aircraft have strong directional but weak lateral stability, once disturbed in roll it’ll tend to continue to roll in the same direction.
The increased AoA leads to more sideslip, which leads to more yaw and more roll and nose drops. This continues until a spiral dive if pilot does not stop it.
DUTCH ROLL
When lateral stability is too strong by comparison with its directional. Dutch roll forms.
Aircraft is disturbed in yaw, if restoring moment from fin and keel isn’t strong enough,aircraft begins oscillation.
Yawing motion heating a rolling motion through dihedral effect. Oscillatory motion is set up for both movements and are the same frequency. Because theres much stronger lateral stability, the recovery from each excursion is faster than the recovery in yaw.
SNAKING
When the yawing motion in Dutch roll is more pronounced than the oscillation in roll.
With respect to stability and control on the ground, explain: the importance of CoG position
The further the CoG is from any of the boundaries within the area bounded by the three wheels the less likely the aircraft will tip over in that direction.
Low CoG and widely spaced wheels reduce the tendency for the aircraft to tip over the ground.
With respect to stability and control on the ground, explain: the differences between nose wheel and tail wheel configurations
Nose wheel - inherently stable. Tendency to maintain a straight track when taxi.
If displaced from its straight course, the centrifugal reaction force, acting through the CoG and pivoting about the main wheels tend to straighten aircraft.
Tail wheel - inherently unstable, if tail wheel displacement occurs, centrifugal reaction force tens to increase the rate of turn. Meaning that once aircraft is in a turn, positive force must be applied.
With respect to stability and control on the ground, explain: handling of controls in strong crosswind
Crosswind behind - control column forward and put into wind.
In front - control column back and into wind
