1. 6.08 Aircraft Stability References: FTGU pages 31, 32
Theory of Flight
6.08 Aircraft Stability
References:
FTGU pages 31, 32

2. Review
Define load.
2. What is the difference between live load and dead load?
What is the load factor in a 60O turn?
Describe the forces acting in a turn.
Answers:
1. Gross weight of the aircraft divided by the area of lifting surfaces (wings)
2. live: Load due to acceleration or change in direction associated with manoeuvres in flight
dead: The load put on the aircraft when it is sitting on the ground.
3. 2G
4. Centripetal which pulls the aircraft into the turn, centrifugal which is a fictitious force pulling out of the turn, and weight

3. 6.08 Aircraft Stability MTPs: Definitions Longitudinal Stability
Lateral Stability
Directional Stability

4. Definitions
Stability: How an aircraft reacts to disturbances while in flight
Types of stability:
Dynamic (positive, neutral, negative)
Static
Inherent

5. Definitions Positive: airplane will return to its position
Dynamic Stability
The overall tendency of an aircraft, when disturbed, to return to its original position
Positive: airplane will return to its position
Negative: airplane will tend to move further away from its position
Neutral: airplane will neither return to its position nor continue to change

6. Definitions Static Stability
The initial tendency of an aircraft, when disturbed, to return to its original position

7. Definitions Inherent Stability
Built in characteristics to enable the airplane to be either stable or unstable
Stability may be built into each aircraft axis creating
Longitudinal stability
Lateral stability
Directional stability

8. Longitudinal Stability
Stability around the lateral axis
Also known as pitch stability

9. Longitudinal Stability
Longitudinal stability is affected by:
1. Size and position of horizontal stabilizer
2. Position of the C of G

10. Longitudinal Stability
Centre of Gravity (C of G)
C of G too far forward
Required loading on the horizontal tail surfaces to maintain angle of attack increases
Overall weight of aircraft increases
Stall speed increases

11. Longitudinal Stability
C of G too far aft
Decreased longitudinal stability because centre of gravity is behind the centre of pressure
Violent stall characteristics
Poor stall recovery (very dangerous!)
Stall speed decreases

12. Longitudinal Stability
A - C of G too far forward
STALL SPEED INCREASES
B - C of G too far aft
STALL SPEED DECREASES

13. Lateral Stability Stability around the longitudinal axis
Also known as roll stability

14. Lateral Stability Lateral stability is created through Dihedral
Keel effect
Sweepback wings

15. Lateral Stability Dihedral Downgoing wing = greater angle of attack
The angle that each wing makes with the horizontal of the aircraft
The lowered wing will produce more lift and will roll back into place
Downgoing wing
= greater angle of attack
= increased lift

16. Lateral Stability Keel Effect
When disturbed, weight of the aircraft acts like a pendulum to swing aircraft back into position
Natural feature of high wing aircraft
Weight of the aircraft lies under the wings

17. Lateral Stability Sweepback Leading edge of the wing slopes backward
When one wing is dropped, the lowered wing produces more lift than the raised wing and the original position is restored

18. Directional Stability
Stability around the vertical or normal axis

19. Directional Stability
Vertical Tail Surface (fin and rudder)
Airplanes have a tendency to fly directly into the relative airflow due to the vertical tail surface
When disturbed the relative airflow will hit the side of the vertical tail surface and push it back into position

20. Directional Stability
Sweep Back

21. Confirmation

22. Confirmation Match the axes with the factors affecting them
Longitudinal Stability
Lateral Stability
Directional Stability
Keel effect
C of G
Sweepback
Vertical Stabilizer
Horizontal Stabilizer
Dihedral

23. Confirmation
What is the difference between dynamic stability and static stability?
How does dihedral affect lateral stability?
How does the vertical stabilizer affect directional stability?

24. Snowbirds – CT-114