Stability Flashcards
What are the three axis?
Longitudinal, Lateral and Normal
Which axis do ailerons act upon and what does it do?
Longitudinal and pitch
Which axis do the ailerons act upon and what does it do?
Lateral and roll
Which axis does the rudder act upon and what does it do?
Normal and yaw
What is stability?
The reaction of any body when its equilibrium is disturbed
What are the two types of stability?
Static and Dynamic
What is static stability?
The initial response/tendency of an aircraft when disturbed
What are the three types of static or dynamic stability?
Positive, Neutral and Negative
What is positive stability?
The disturbed object returns to the original position
What is neutral stability?
The disturbed object remains displaced
What is negative stability?
The disturbed object continues to deviate further away
What is dynamic stability?
The response over time after the disturbance
What is longitudinal stability?
Stability around the lateral axis or pitch stability
What impact on longitudinal stability does a CoG forward of the forward limit have?
Too stable
What impact on longitudinal stability does a CoG aft of the aft limit have?
Unstable
What impact does CoG have on longitudinal stability?
The further forward the CoG the larger the tailplane arm and so the greater the righting moment
What impact does a larger tailplane area have on longitudinal stability?
It increases longitudinal stability
What is angle of incidence?
The angle between the chord line and the longitudinal axis
Effectively the angle the wing is fixed to the airframe
What is longitudinal dihedral?
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
What impact does a CoP forward of the CoG have on the longitudinal stability?
A disturbance that leads to an increase in AoA would lead to the pitch up moment continuing
I.e. Negative static & dynamic
What impact does a CoP aft of the CoG have on the longitudinal stability?
A disturbance that leads to an increase in AoA would naturally be restored by the nose-down moment
I.e. Positive static & dynamic
What impact does thrust have on longitudinal stability?
If thrust is increased, the nose will tend to pitch up
I.e. negative static & dynamic
What is lateral stability?
Stability around the longitudinal axis or roll stability
What is wing dihedral?
Wings are angled slightly upwards from the empennage
What impact does wing dihedral have on lateral stability?
It improve it
Why does wing dihedral improve lateral stability?
In a turn, the lower wing will be at a higher AoA and produce more lift, working to level the wings
What is wing anhedral?
Wings are angled downwards from the empennage
What impact does wing anhedral have on lateral stability?
It has a destabilising effect but increases manoeuvrability
What is the pendulum effect?
When a high-wing aircraft is rolled, its low centre of gravity will also work to roll the wings level
What impact do floats or keel surfaces have on lateral stability when they are above the axis?
They provide a stabilising moment
What impact do floats or keel surfaces have on lateral stability when they are below the axis?
They provide a destabilising moment
What is the impact of swept back wings on lateral stability?
In a turn, the lower wing will present more of its span to the RAF and so generate a righting moment
What is directional stability?
Stability around the normal axis or yaw stability
What impact do large keel surfaces have on directional stability?
Stabilising and the greater the arm, the greater the stability.
What is spiral instability?
This occurs when directional stability is stronger than lateral stability
Why does spiral instability occur?
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.
What happens in a turn with spiral instability?
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
What are the symptoms of a spiral dive?
High AoB
Low nose attitude, high RoD
High and increasing airspeed
What is the recovery technique for a spiral dive?
Reduce power to idle
Gently roll wings level with aileron
Gently raise nose to glide attitude
