Stability

Question 1

Static stability

Answer 1

Initial response of an aircraft to a disturbance forms its equilibrium position.

Question 2

Positive static stability

Answer 2

If an aircraft returns towards its original position.

Question 3

Negative static stability

Answer 3

If an aircraft continues to move away from its original position after being disturbed.

Question 4

Neutral static stability

Answer 4

If an aircraft remains in its new position after being disturbed and doesn’t;t return towards its its original position or move further away.

Question 5

Dynamic stability

Answer 5

The motion of the aircraft after the initial response.

Question 6

Positive dynamic stability

Answer 6

Oscillates around its equilibrium position with decreasing amplitude and eventually returns to its original state.

Question 7

Negative dynamic stability

Answer 7

Oscillations increase in amplitude over time, leading the aircraft further away from its original state.

Question 8

Neutral dynamic stability

Answer 8

The aircraft oscillates round its equilibrium position with constant amplitude.

Question 9

Stability vs. controllability

Answer 9

Aircraft’s are designed to have positive static and dynamic stability.

Question 10

Longitudinal stability

Answer 10

The stability around its lateral axis, which affects the aircraft’s pitching motion.polppol

Question 11

Airspeed and tail plane stability maintenance

Answer 11

• Pitch down - airspeed increases

Question 12

Position of the CoG and the stability of the tail plane

Answer 12

• Dependent on the moment arm between the CoG and the tailplane.
• Longer arm = greater restoring force
• Shorter arm = smaller restoring force

Question 13

Longitudinal dihedral and stability of tail plane

Answer 13

• Tailplane is usually set at a smaller angle of incidence compared to the main wing
• For any decrease or increase of AoA, the change is more significant for the tailplane than for the main wing
• Greater restoring force

Question 14

Directional stability

Answer 14

An aircraft’s ability to. Aintain or return towards its its regional heading or direction of flight after being disturbed by external forces.

• Associated with the yaw motion
• Provided by the vertical stabiliser - dependent on airspeed and the position of the CoG

Question 15

Lateral stability

Answer 15

The aircraft’s ability to resist rolling motions around the longitudinal axis and maintain a level wind attitude without any control inoutm

Question 16

Dihedral

Answer 16

Wing design where the wingtip is higher than the wing root

• Helps prevent adverse effects of slipping
• Higher AoA on the down going wing, generates lift.

Question 17

Shielding

Answer 17

When one wing is shielded from the airflow by the fuselage during a sideslip, reducing its lift relative to the other wing.

Question 18

Wing position

Answer 18

When an aircraft rolls, the CoP and the CoG get misaligned in the vertical plane. The weight and lift will naturally create a restoring moment due to the misalignment.

Question 19

Keel/Fin surface area

Answer 19

• Helps align aircraft with RAF
• RAF over the tail fin produces an aerodynamic force which acts through the centre of the fin which is above the CoG
• This sideslip drag force will tend to roll the aircraft away from the direction of the slip.

Question 20

Sweep back

Answer 20

• Wings deigned with the wingtip further back than the wing root
• When distrurbed in roll, the down going wing has an increased effective span and the up going wing has a decreased effective span.
• The down going wing will then produce more lift compared to the up going one, producing a restoring moment

Question 21

Spiral instability

Answer 21

• If an aircraft has too much directional stability and not enough lateral stability, it will continue to yaw and roll towards the down going wing after being disturbed.
• If a pilot doesn’t take corrective action, it leads to a spiral dive.

Question 22

Dutch roll

Answer 22

• If an aircraft has too much lateral stability and a weak directional stability.
• After being disturbed in yaw, the aircraft skids. The outer wing travels faster producing more lift, making the aircraft roll. Due to the strong lateral stability, the aircraft tries to roll wings level.
• Creates an uncomfortable wallowing

Question 23

Cog on the ground position

Answer 23

• Aft CoG increases risk of the aircraft tipping backwards
• Particularly critical in a tricycle aircraft.

Question 24

Nosewheel vs. tailwheel

Answer 24

Nosewheel:

Better directional stability on ground

When turning of the ground, the nosewheel is inherently stable

Tailwheel:

More susceptible to over rotation and require more effort to maintain straight lines, especially during take-off and landing.

When turning on the ground, the tricycle is inherently unstable

Question 25

Handling of control in a headwind

Answer 25

Tricycle: - Slightly back Tail dragger: - Fully back

Question 26

Handling of controls in a strong tail wind

Answer 26

- Control column in forward position - Danger of prop strike

Question 27

Handling of controls in cross wind

Answer 27

Wind in front of wings: - Into wind and backwards Wind from behind: - Dive with wind and push forwards

Question 28

Cross-wind take-off technique

Answer 28

Controls into wind and gradually move to neutral as take-off progresses.

Question 29

Landing cross wind technique

Answer 29

- Approach with nose into wind - Use rudder to realign aircraft upon landing before touchdown - Lower wing facing the wind

Question 30

Why do aircraft yaw or veer off during take off roll?

Answer 30

- Slipstream - Asymmetric blade effect - Torque effect - Gyroscope effect

Question 31

Effect of slipstream on take off roll

Answer 31

- Slipstream = air accelerated rearward by the propeller - Results in a force pushing on the tail which forces the aircraft to yaw to the left

Question 32

Asymmetric blade effect on take off roll

Answer 32

- Plane of rotation is tilted - Down going blade has a greater angle of attack and a faster RAF than the up going blade - Down going blade produces more thrust - Difference of thrust yaws the aircraft to the left

Question 33

Torque effect on take off roll

Answer 33

- For every action there is an equal and opposite reaction - The reaction from the propeller rotating clockwise is the aircraft wanting to rotate anti-clockwise - Greater force on the left wheel - Yaws to the left

Question 34

Gyroscopic effect on the take off roll

Answer 34

- Precession - when a force is applied it is felt 90 degrees ahead of that point in the direction of rotation - Gyroscopic effect yaws the aircraft when there is a change in attitude - Only on tail draggers

Question 35

Ground effect

Answer 35

- Vortices generate a downwash on the trailing edge of the wing which tilts the lift backwards increasing the drag and decreasing the lift. - Can feel as is the aircraft in floating on an air cushion during the flare.