Flight Stability and Dynamics CASA 8.4

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Questions and Answers

What does the stability of an aircraft mean?

The aircraft's ability to return to a particular flight condition after disturbance. (correct)

The aircraft's ability to fly faster than the speed of sound.

The aircraft's ability to turn without banking.

The aircraft's ability to maintain a constant altitude.

Which of the following describes positive static stability?

The aircraft's inability to return to its original attitude.

The tendency of an aircraft to return to its original attitude after displacement. (correct)

The aircraft's stability only during high-speed maneuvers.

The aircraft's tendency to move away from its original position.

How does stability differ from balance or 'trim' in an aircraft?

An aircraft can be out of trim and still stable. (correct)

There is no difference; they are synonymous.

Stability is concerned with level flight only.

Balance refers to the symmetry of the aircraft wings.

What is considered inherent stability in an aircraft?

The aircraft's ability to stabilize in a nosedive. Signup and view all the answers

What describes static stability?

The initial tendency of an object after equilibrium is disturbed. Signup and view all the answers

Which statement about aerodynamic stability is true?

A stable aircraft does not adjust its wings with respect to airflow. Signup and view all the answers

Which condition may affect an aircraft's stability?

Both A and B. Signup and view all the answers

Which of the following is not a type of static stability?

Dynamic static stability. Signup and view all the answers

What is the characteristic of negative static stability?

The aircraft moves farther away from its original attitude following a disturbance. Signup and view all the answers

What does neutral dynamic stability indicate about an aircraft's behavior?

It attempts to return to equilibrium but oscillations neither increase nor decrease. Signup and view all the answers

Which axis is associated with lateral stability in an aircraft?

Lateral axis Signup and view all the answers

What does positive dynamic stability allow an aircraft to do?

Return directly to its original attitude or through decreasing oscillations. Signup and view all the answers

What is the primary surface controlling longitudinal stability in an aircraft?

Horizontal stabiliser Signup and view all the answers

What is meant by an aircraft's centre of gravity (CG)?

The point where the entire weight of the aircraft is concentrated. Signup and view all the answers

Which motion is referred to as yawing in aircraft dynamics?

Rotational motion about the vertical axis. Signup and view all the answers

How does directional stability benefit an aircraft during flight?

It ensures the aircraft remains on course without pilot control. Signup and view all the answers

What typically counters a nose-down pitching moment in an aircraft?

Elevator trim Signup and view all the answers

What is one effect of sideslip on aircraft motion?

It produces both rolling and yawing motions. Signup and view all the answers

Which type of static stability allows an aircraft to return to its disturbed position without external control?

Positive static stability Signup and view all the answers

What indicates an aircraft has negative dynamic stability?

Oscillations increase in magnitude over time. Signup and view all the answers

Which aerodynamic feature enhances directional stability?

Sweptback wings Signup and view all the answers

What is the main purpose of the horizontal tail surface in aircraft stability?

To counteract disturbances in pitch. Signup and view all the answers

What does lateral stability refer to in an aircraft?

Stability around the longitudinal axis Signup and view all the answers

What is dihedral in the context of an aircraft wing?

The positive acute angle between the wing and the lateral axis Signup and view all the answers

Which wing configuration typically has a larger dihedral angle?

Low-wing aeroplanes Signup and view all the answers

What contributes to the lateral stability of an aeroplane?

The wing's dihedral Signup and view all the answers

Which angle is referred to as longitudinal dihedral?

Angle between the horizontal stabiliser and the wings Signup and view all the answers

What characterizes oscillatory instability in aircraft?

Combined rolling and yawing movement Signup and view all the answers

What is the difference between Dutch roll and snaking?

Dutch roll is rolling motion; snaking is yawing motion Signup and view all the answers

What effect does dihedral have on aircraft stability?

Produces stable rolling moments with sideslip Signup and view all the answers

What is the purpose of using washout in wing design?

To counteract torque effect Signup and view all the answers

What causes an aircraft to float during landing?

Cushion effect from air trapped between wing and ground Signup and view all the answers

What is spiral instability a result of?

Negative static stability in roll and yaw Signup and view all the answers

What does the pendulum effect in high-wing aircraft provide?

A righting action when rolling occurs Signup and view all the answers

What happens when an aircraft's vertical fin produces a greater aerodynamic force than the dihedral effect?

The aircraft may spiral into instability Signup and view all the answers

Study Notes

Aircraft Stability

Aircraft stability is essential for safe and controlled flight across different flight conditions
Stability refers to an aircraft's ability to return to its original flight condition after being disturbed.
Stability is not synonymous with balance or trim.
An aircraft can be stable in one flight condition (straight-and-level) but unstable in another (inverted).
Inherent stability describes an aircraft's natural stabilizing tendencies.

Static Stability

Static stability is the initial tendency of an aircraft after a disruption to equilibrium.
Three types of static stability exist, described by the aircraft's response to disturbance:
- Positive: The aircraft returns to its original attitude.
- Negative: The aircraft moves further away from its original attitude.
- Neutral: The aircraft remains in its displaced attitude
Static stability can also be called passive stability as it doesn't require active control.

Dynamic Stability

Dynamic stability describes how quickly an aircraft responds to its static stability.
It is determined by the aircraft's tendency to oscillate after initial displacement.
An aircraft can have positive static stability but positive, negative or neutral dynamic stability.
Positive dynamic stability: The aircraft returns to its original attitude directly or through decreasing oscillations
Negative dynamic stability: Oscillations increase in magnitude over time.
Neutral dynamic stability: The aircraft oscillates neither increasing nor decreasing in magnitude.

Aeroplane Axes (Stability)

Aircraft movements in flight are described in relation to three axes:
- Longitudinal: Runs from nose to tail.
- Lateral: Runs from wingtip to wingtip.
- Vertical: Runs perpendicular to the other two axes.
The centre of gravity (CG) is a point where the aircraft's total weight is concentrated. The three axes intersect at the CG.
Lateral stability refers to the aircraft's ability to return to its original attitude after rolling motion.
- The wing's dihedral angle, vertical stabiliser, and fuselage contribute to lateral stability.
Longitudinal stability refers to the aircraft's ability to maintain a constant angle of attack in pitch.
- The horizontal stabiliser is the primary surface controlling this stability.
- Elevator trim is adjusted to balance the aircraft for a desired flight path.
Directional stability refers to the aircraft's ability to maintain a straight course.
- The vertical stabiliser is the primary control surface, aided by the wing sweep and dorsal fin.
The pendulum effect is caused by the centre of gravity being below the centre of lift. This effect helps restore straight-and-level flight in high-wing aircraft.
The torque effect occurs due to the propeller's rotation, causing a yawing moment.
- Washout on one wing and wash in on the other help counter this effect.
Ground effect refers to the "floating" sensation an aircraft experiences near the ground due to the downwash and air trapped between the wing and the ground.
- This effect reduces stalling speed and is influenced by factors such as wing area, height, and wing loading.

Oscillatory Instability

Oscillatory instability involves a combined rolling and yawing motion.
It can be categorized into:
- Dutch Roll: When rolling motion predominates.
- Snaking: When yawing motion predominates.
The amount of dihedral and the vertical fin size affect oscillatory instability.
Dutch Roll occurs when dihedral effect is stronger than the vertical fin's effect.
- This causes an oscillation in the direction of the slip, leading to a Dutch roll.
Spiral instability occurs when the dihedral effect is not strong enough to counteract the yawing moment caused by the vertical fin.
- This can lead to a continuous rolling and yawing, resulting in a spiral dive.
A spin is distinct from spiral instability and involves a stalled condition followed by yawing.

Active Stability

Modern high-performance aircraft are designed to be aerodynamically unstable but rely on active stability control systems.
- These systems maintain stability through digital flight control, directly influencing the aircraft's motion around the three axes.

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Description

This quiz covers the essential principles of aircraft stability, focusing on the definitions and types of static stability. Understand how aircraft respond to disturbances in various flight conditions and learn the differences between positive, negative, and neutral stability. Test your knowledge on these critical aspects of aviation safety and control.