Stability Flashcards
Which pair of forces are greater in the magnitude than the other?
Lift/Weight
If CP is behind the CG, what pitching motion occurs?
Nose-down
If CP is in front of the CG, what pitching motion occurs?
Nose-up
If the drag line is above the thrust line, what pitching motion occurs?
Nose-up
If the drag line is below the thrust line, what pitching motion occurs?
Nose-down
In a standard designed aircraft, what moment does the thrust-drag couple have?
Nose-up
In a standard designed aircraft, what moment does the lift-weight couple have?
Nose-down
Function of the tailplane?
To counteract the residual pitching moments from the two main couples
Why is the area and size of the tailplane small compared with the mainplanes?
Its moment arm is long so can provide significant pitching effect
Generally, where does the CP and CG lie relatively?
The CP is behind the CG
Generally, what aerodynamic force does the tailplane provide?
Downwards
What does stability refer to?
Natural ability of aircraft to return to its original condition
The natural ability of aircraft to return to its original condition after being disturbed is given what term?
Stability
What is the trade-off for stability?
Controllability
A highly controllable aircraft will possess poor…?
Stability
An aircraft in trim is different from the state of equilibrium how?
Its pitch, roll and yaw moments are zero, not the aerodynamic forces or turning moments
Movement around the longitudinal axis is known as…
Rolling
Rolling’s axis
Longitudinal axis
Stability around the longitudinal axis is known as?
Lateral stability
Lateral stability is related to what motion?
Roll
Movement around the lateral axis is called?
Pitch
Pitch’s axis?
Lateral axis
Stability around the lateral axis is known as?
Longitudinal stability
Longitudinal stability is related to what motion?
Pitch
Movement about the normal axis is known as?
Yaw
Yaw’s axis
Normal axis
What is the third axis alongside the lateral and longitudinal axis?
Normal axis
Stability around the normal axis is known as?
Directional stability
Directional stability is related to what motion?
Yaw
Rotation around a point or axis is called?
Angular motion
The speed at which rotation occurs around a point or axis is called?
Angular velocity
The number of degrees that rotation occurs by is called?
Angular displacement
What is the key term used when discussing a rotation around a point or axis?
Angular
A body that returns to its original position after being disturbed is said to possess….
Static stability
A body that does not return to its original position after being disturbed is said to possess….
Dynamic stability, dynamic instability or neutral dynamic stability
Neutral dynamic stability is when what remains constant?
Amplitude and period of oscillation
If the departure of a body from its original position increases over time, it is said to have
Dynamic instability
A nose-up disturbance results in a nose-down moment how?
Greater AoA on tailplane
Tailplane rises and nose falls
What aircraft part provides the greatest contribution to longitudinal stability?
Tailplane
Relation of CG position and turning effect of tailplane
Further forward the CG, the greater the moment arm of the tailplane
High AoA are detrimental to longitudinal stability in what way?
High AoA may shield the tailplane or cause turbulent airflow and reduce tailplane effectiveness
What aircraft part provides the greatest contribution to directional stability?
Vertical fin
During a sideslip or yawing motion, a vertical fin experiences what to provide it with directional stability?
AoA
What power-related force can affect a fin’s effectiveness as a vertical stabiliser?
Propeller slipstream
A dihedral means the lower wing in a turn will experience what?
A greater AoA than the upper wing so there is a lift imbalance and a rolling moment is produced
What is a negative dihedral called?
Anhedral
High keel surfaces correct disturbances in roll how?
Their high position with a lower CG will create a restoring moment
Dihedral helps increas what in roll?
AoA on lower banked wing
In roll, what is essential for lateral stability to work?
Sideslip
Directional stability in a roll means the aircraft will…
Yaw its nose into the relative airflow
An aircraft is in a roll and yaws its nose into the relative airflow. What has ocurred?
Sideslip has exerted a force on the keel surfaces and given it directional stability
Directional stability essentially points an aircraft’s nose towards what?
Into the relative airflow
In a roll, the greater the directional stability of an aircraft, the greater the..
Tendency to turn away from the original heading in the direction of the lower wing
Relationship between directional stability strength and yaw
The greater the directional stability, the greater the yaw
Why does the nose drop in a roll with good directional stability?
Outside wing is moving faster and generates greater lift
What aerodynamic conflict occurs in a roll followed by yaw?
Directional stability wants to steepen the turn and drop the nose further
Lateral stability wants to level the wings
What counteracting force occurs during a roll as directional stability steepens the turn?
Lateral stability uses the dihedral to level the wings
Directional stability > lateral stability =
Spiral instability
Directional stability < lateral stability =
Dutch roll
Dutch roll is what experience?
Aircraft shows no tendency to turn in the direction of the sideslip
In a yaw followed by roll, directional stability < lateral stability =
Significant yaw, sideslip and resulting banked turn away from sideslip
In a yaw followed by roll, directional stability > lateral stability =
Sideslip towards lower wing occurs
On the ground, a low-thrust line has effect in high-power situations?
Lower tendency to pitch onto its nose
What two aerodynamic design features means crosswinds have a greater destabilising effect?
High keel surfaces
Dihedral
