Stability Flashcards
What are the 2 kinds of stability
Dynamic and Static
Static stability
Refers to the initial reaction of a body after being disturbed / displaced from a position of equilibrium
Positive static stability
Object is statically stable, it occurs if it tends to return to its original position
Neutral static stability
Occurs if it remains in its displaced position
Negative static stability
Occurs if it continues away from its original position
Dynamic stability
Refers to the subsequent motion of the disturbed body once the static stability reaction has taken place
An object that is dynamically stable
Will eventually returns of its own accord to its original position of equilibrium after a disturbance
An object that is dynamically unstable
Will continue to diverge from its original attitude
An object with neutral dynamic stability
If it continues to oscillate about the original attitude
Stability vs contolibility
Stability is the inherent tendency of the a/c to return to its original attitude after being disturbed without any action taken by the pilot
Controllability refers to the ease with which a pilot can manoeuvre the a/c and change its attitude by using control surfaces
An a/c must have _____ before it can be dynamically stable
Static stability
Longitudinal stability
Is in the pitching plane about the lateral axis, to be longitudinally stable the a/c must have an inherent tendency to return to the same pitch attitude after a disturbance
What is the main way to achieve longitudinal stability
Tailplane / horizontal stabiliser
Longitudinal stability after a nose up disturbance
As the nose up disturbance is taking place initially the a/c will tend to continue on its original path due to its momentum and the AoA of both the tailplane and wings is increased. The resulting increase in tailplane lift (or decrease in negative lift) produces a nose down pitching moment that returns the a/c to its original trimmed position
Wing pitching moments after a nose up disturbance with the CP positioned behind the CG
Wing lift increases which which tends to increase the nose down pitch about the CG (i.e the wing pitching moment from lift tends toward stability), however, for a cambered wing section the CP also moves forward with increasing AoA and this tends to reduce the stabilising effect of the increase in lift. In some circumstances the forward movement of the CP can be large enough to cause an unstable pitching moment.
Wing pitching moments after a nose up disturbance with the CP positioned forward the CG
Note this is uncommon, the wing pitching moment following a disturbance is unstable i.e if the nose is displaced upward the increase in wing pitching moment (through the increase in wing lift + movement of the CP) tends to increase displacement in pitch
Effect of forward CG on longitudinal stability
The further forward the CG the longer the moment arm becomes for the tailplane, so a given tailplane force therefore creates a more effective restoring moment and the a/c becomes more stable in pitch, at the same time however the further forward the CG the less effective the elevator becomes in changing the pitch attitude, this is because any given elevator deflection must now overcome a greater tailplane restoring moment
What happens if a/c is flown with CG further forward than normal
a/c feels nose heavy and higher stick forces are required to manoeuvre it in pitch
Effect of aft CG position on longitudinal stability
It has a shorter tailplane moment arm and is less stable in pitch, the elevator becomes more effective and lower stick force is required to achieve the same change in nose attitude pitch
What happens if a/c is flown with CG further back than normal
It feels tail heavy and becomes more sensitive to elevator movement
Directional stability
Is in the yawing plane about the normal axis, it is the inherent ability of the a/c to weathercock so that the nose remains pointed into the oncoming airflow, it relies on the a/c having a greater amount of keel (or side) surface behind the CG than ahead of it. A major part of this keel is the fin / vertical stabilizer
Fin
A symmetrical aerofoil which when the a/c is aligned with the oncoming airflow has 0° AoA and so produces no net side force
What happens when the a/c is disturbed in yaw
Its momentum will cause a skid to develop, the fin now develops an AoA and restoring moment which returns the fuselage back to allignment with the airflow
Factors effecting directional stability
The greater the keel surface behind the CG the greater the moment arm and the greater the directional stability of the a/c, thus a forward CG favors directional stability more than an aft CG as it gives a longer moment arm to the vertical stabiliser
