81 04 Stability And Controls

Question 1

Describe Static Stability

Answer 1

Describes what an AC will do immediately after a disrupting force (such as a gust) is applied

Question 2

Describe the three types of Static stability

Answer 2

• Positive Static Stability: The AC will initially try to return to the state of equilibrium
• Neutral Static Stability: The AC will initially make no attempt to return to equilibrium or diverge further from equilibrium
• Negative Static Stability: The AC will initially continue to diverge from the state of equilibrium

Question 3

What are the two ways an aircraft can maintain equilibrium?

Answer 3

1. Continuous pilot input
2. Trim
   - The AC can be in equilibrium but out of trim

Question 4

Describe Dynamic Stability

Answer 4

How the AC behaves after a period of time after a disruptive force (such as a gust)

Question 5

Describe the three types of Dynamic Stability

Answer 5

• Positive Dynamic Stability: The AC will continue to try and return to equilibrium over time
• Neutral Dynamic Stability: The AC will make no attempt to try and return to equilibrium or diverge further over time
• Negative Dynamic Stability: The AC will continue to diverge from the state of equilibrium over time

Question 6

Describe the two types of motion

Answer 6

• Periodic: The AC will overshoot when returning to equilibrium
• Aperiodic: The AC will return straight to equilibrium without overshooting
• Aka Deadbeat motion

Question 7

Longitudinal Stability is in relation to?

Answer 7

The pitch about the Latitudinal Axis

Question 8

Latitudinal Stability is in relation to?

Answer 8

The roll about the Longitudinal Axis

Question 9

Describe the Aerodynamic Centre (AC)

Answer 9

• Fixed (does not move) for a specific configuration
• The pitching moment: M = L x d where L is lift and d is the distance between the CP and AC
• As AOA increases, L increases but as CP moves forward, d gets smaller. This keeps the pitching moment the same.

Question 10

What are the following for Pitching, Rolling, Yawing:

• Symbol
• Coefficient
• Positive direction
• Axis

Answer 10

• Pitching: M / CM / Nose Up / Latitudinal
• Rolling: L’ / CL’ / Right / Longitudinal
• Yawing: N / CN / Right / Normal

Question 11

What factors increase Longitudinal Stability?

Answer 11

• A larger Tail plane area

- Larger distance between the CG and the Tail Plane (resulting in a bigger moment arm)

Question 12

What factors increase Directional Stability?

Answer 12

• Increase the Fin area

- Increase the distance between the CG and the Fin (thus increasing the moment arm)

Question 13

What factors increase Lateral Stability?

Answer 13

• The main contributor to lateral stability is the wings
• Dihedral wings: Down wing has bigger AOA
• Swept wings: Down wing has more wing span in the RAF
• High wings: CP moves towards the down wing and a low CG creates a turning moment upwards
• Larger Keel (fin) Area: Increased area above CG improves Lateral Stability. Dorsal fins above increase, Ventral fins below reduce.

Question 14

What are Stick Free and Stick Fixed and what is their comparable stability?

Answer 14

• Stick Free: Pilot has released the controls on an unpowered or power assisted AC. The controls are therefore free to do as they wish. Because the control surfaces are inclined to remain where they are in relation to the RAF the AC is now less stable.
• Stick Fixed: Pilot has hold of the controls or it is a powered control AC. Therefore the control surfaces will not move. The AC will therefore be stable.
• Stick Fixed is always more stable than Stick Free.

Question 15

How is the dynamic stability of an AC measured?

Answer 15

By the amount of time it takes for the oscillations to get to half the starting amplitude

Question 16

Dynamic Stability needs two things:

Answer 16

1. Positive Static Stability

2. Dampening to reduce oscillations to zero