Theory of Flight

6.08 Aircraft Stability References:

FTGU pages 31, 32

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.

6.08 Aircraft Stability

MTPs:

• Definitions

• Longitudinal Stability

• Lateral Stability

• Directional Stability

Definitions

Stability: How an aircraft reacts to

disturbances while in flight

Types of stability:

• Dynamic (positive, neutral, negative)

• Static

• Inherent

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

Definitions

• Static Stability

– The initial tendency of an aircraft, when

disturbed, to return to its original position

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

Longitudinal Stability

• Stability around the lateral axis

• Also known as pitch stability

Longitudinal Stability

• Longitudinal stability is affected by:

1. Size and position of horizontal stabilizer

2. Position of the C of G

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

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

Longitudinal Stability

A - C of G too far

forward

STALL SPEED

INCREASES

B - C of G too far aft

STALL SPEED

DECREASES

Lateral Stability

• Stability around the longitudinal axis

• Also known as roll stability

Lateral Stability

Lateral stability is created through

1. Dihedral

2. Keel effect

3. Sweepback wings

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

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

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

Directional Stability

Stability around the vertical or normal axis

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

Directional Stability

• Sweep Back

Confirmation

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

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?

Snowbirds – CT-114 CH146 Griffon and USCG HH-65 Dolphin