Stability

Question 1

What are the 2 kinds of stability

Answer 1

Dynamic and Static

Question 2

Static stability

Answer 2

Refers to the initial reaction of a body after being disturbed / displaced from a position of equilibrium

Question 3

Positive static stability

Answer 3

Object is statically stable, it occurs if it tends to return to its original position

Question 4

Neutral static stability

Answer 4

Occurs if it remains in its displaced position

Question 5

Negative static stability

Answer 5

Occurs if it continues away from its original position

Question 6

Dynamic stability

Answer 6

Refers to the subsequent motion of the disturbed body once the static stability reaction has taken place

Question 7

An object that is dynamically stable

Answer 7

Will eventually returns of its own accord to its original position of equilibrium after a disturbance

Question 8

An object that is dynamically unstable

Answer 8

Will continue to diverge from its original attitude

Question 9

An object with neutral dynamic stability

Answer 9

If it continues to oscillate about the original attitude

Question 10

Stability vs contolibility

Answer 10

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

Question 11

An a/c must have _____ before it can be dynamically stable

Answer 11

Static stability

Question 12

Longitudinal stability

Answer 12

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

Question 13

What is the main way to achieve longitudinal stability

Answer 13

Tailplane / horizontal stabiliser

Question 14

Longitudinal stability after a nose up disturbance

Answer 14

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

Question 15

Wing pitching moments after a nose up disturbance with the CP positioned behind the CG

Answer 15

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.

Question 16

Wing pitching moments after a nose up disturbance with the CP positioned forward the CG

Answer 16

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

Question 17

Effect of forward CG on longitudinal stability

Answer 17

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

Question 18

What happens if a/c is flown with CG further forward than normal

Answer 18

a/c feels nose heavy and higher stick forces are required to manoeuvre it in pitch

Question 19

Effect of aft CG position on longitudinal stability

Answer 19

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

Question 20

What happens if a/c is flown with CG further back than normal

Answer 20

It feels tail heavy and becomes more sensitive to elevator movement

Question 21

Directional stability

Answer 21

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

Question 22

Fin

Answer 22

A symmetrical aerofoil which when the a/c is aligned with the oncoming airflow has 0° AoA and so produces no net side force

Question 23

What happens when the a/c is disturbed in yaw

Answer 23

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

Question 24

Factors effecting directional stability

Answer 24

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