aircraft static and dynamic stability

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

2. Review
1. Define load.
2. What is the difference between live load
and dead load?
3. What is the load factor in a 60O turn?
4. Describe the forces acting in a turn.

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
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
1. Dihedral
2. Keel effect
3. Sweepback wings

15. Lateral Stability
Dihedral
• 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
1. Keel effect
2. C of G
3. Sweepback
4. Vertical Stabilizer
5. Horizontal Stabilizer
6. 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