Theory of Flight 6 08 Aircraft Stability References

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 60 O 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 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. 2. 3. 4. 5. Keel effect C of G Sweepback Vertical Stabilizer 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