Stability

Question 1

What are the three axis?

Answer 1

Longitudinal, Lateral and Normal

Question 2

Which axis do ailerons act upon and what does it do?

Answer 2

Longitudinal and pitch

Question 3

Which axis do the ailerons act upon and what does it do?

Answer 3

Lateral and roll

Question 4

Which axis does the rudder act upon and what does it do?

Answer 4

Normal and yaw

Question 5

What is stability?

Answer 5

The reaction of any body when its equilibrium is disturbed

Question 6

What are the two types of stability?

Answer 6

Static and Dynamic

Question 7

What is static stability?

Answer 7

The initial response/tendency of an aircraft when disturbed

Question 8

What are the three types of static or dynamic stability?

Answer 8

Positive, Neutral and Negative

Question 9

What is positive stability?

Answer 9

The disturbed object returns to the original position

Question 10

What is neutral stability?

Answer 10

The disturbed object remains displaced

Question 11

What is negative stability?

Answer 11

The disturbed object continues to deviate further away

Question 12

What is dynamic stability?

Answer 12

The response over time after the disturbance

Question 13

What is longitudinal stability?

Answer 13

Stability around the lateral axis or pitch stability

Question 14

What impact on longitudinal stability does a CoG forward of the forward limit have?

Answer 14

Too stable

Question 15

What impact on longitudinal stability does a CoG aft of the aft limit have?

Answer 15

Unstable

Question 16

What impact does CoG have on longitudinal stability?

Answer 16

The further forward the CoG the larger the tailplane arm and so the greater the righting moment

Question 17

What impact does a larger tailplane area have on longitudinal stability?

Answer 17

It increases longitudinal stability

Question 18

What is angle of incidence?

Answer 18

The angle between the chord line and the longitudinal axis

Effectively the angle the wing is fixed to the airframe

Question 19

What is longitudinal dihedral?

Answer 19

The tailplane has a smaller angle of incidence so that it experiences a greater % increase in lift when a disturbance occurs, producing a righting moment

Question 20

What impact does a CoP forward of the CoG have on the longitudinal stability?

Answer 20

A disturbance that leads to an increase in AoA would lead to the pitch up moment continuing

I.e. Negative static & dynamic

Question 21

What impact does a CoP aft of the CoG have on the longitudinal stability?

Answer 21

A disturbance that leads to an increase in AoA would naturally be restored by the nose-down moment

I.e. Positive static & dynamic

Question 22

What impact does thrust have on longitudinal stability?

Answer 22

If thrust is increased, the nose will tend to pitch up

I.e. negative static & dynamic

Question 23

What is lateral stability?

Answer 23

Stability around the longitudinal axis or roll stability

Question 24

What is wing dihedral?

Answer 24

Wings are angled slightly upwards from the empennage

Question 25

What impact does wing dihedral have on lateral stability?

Answer 25

It improve it

Question 26

Why does wing dihedral improve lateral stability?

Answer 26

In a turn, the lower wing will be at a higher AoA and produce more lift, working to level the wings

Question 27

What is wing anhedral?

Answer 27

Wings are angled downwards from the empennage

Question 28

What impact does wing anhedral have on lateral stability?

Answer 28

It has a destabilising effect but increases manoeuvrability

Question 29

What is the pendulum effect?

Answer 29

When a high-wing aircraft is rolled, its low centre of gravity will also work to roll the wings level

Question 30

What impact do floats or keel surfaces have on lateral stability when they are above the axis?

Answer 30

They provide a stabilising moment

Question 31

What impact do floats or keel surfaces have on lateral stability when they are below the axis?

Answer 31

They provide a destabilising moment

Question 32

What is the impact of swept back wings on lateral stability?

Answer 32

In a turn, the lower wing will present more of its span to the RAF and so generate a righting moment

Question 33

What is directional stability?

Answer 33

Stability around the normal axis or yaw stability

Question 34

What impact do large keel surfaces have on directional stability?

Answer 34

Stabilising and the greater the arm, the greater the stability.

Question 35

What is spiral instability?

Answer 35

This occurs when directional stability is stronger than lateral stability

Question 36

Why does spiral instability occur?

Answer 36

In a roll, the RAF changes direction. At this point, lateral stability tries to roll wings level, while directional stability tries to yaw the aircraft into the new RAF. Directional stability is greater so the aircraft yaws.

Question 37

What happens in a turn with spiral instability?

Answer 37

As the aircraft yaws, the outboard wing travels faster and so produces more lift. This produces a further roll and if left unchecked will lead to a spiral dive

Question 38

What are the symptoms of a spiral dive?

Answer 38

High AoB Low nose attitude, high RoD High and increasing airspeed

Question 39

What is the recovery technique for a spiral dive?

Answer 39

Reduce power to idle Gently roll wings level with aileron Gently raise nose to glide attitude