# 4 2 . 7 Stability.

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4 2 . 7 Stability

Stability Stability is the tendency of an airplane in flight to remain in straight, level, upright flight and to return to this attitude if displaced, without corrective action by the pilot. Regional Gliding School

Static Stability The initial tendency of an airplane when disturbed to return to its original position. Regional Gliding School

Dynamic Stability The overall tendency of an airplane to return to its original position, following a series of damped out oscillations. Regional Gliding School

Stability may be: Positive Neutral
meaning the airplane will develop forces or moments that tend to restore it to its original position. Neutral meaning the restoring forces are absent and the airplane will neither return from its disturbed position, nor move further away. Regional Gliding School

Stability may be: Negative
meaning it will develop forces or moments that tend to move it further away. Negative stability is Instability. Regional Gliding School

Longitudinal Stability
Longitudinal Stability is pitch stability or stability around the lateral axis of the airplane. Regional Gliding School

Two factors influence longitudinal stability
The Horizontal Stabilizer When a disturbance increases the angle of attack on the wings, the nose goes up and the tail down. As the stabilizer moves down it meets the air at a greater angle of attack, obtains more lift and tends to restore the aircraft to straight and level flight. Regional Gliding School

Two factors influence longitudinal stability
Center of Gravity C of G is very important in achieving longitudinal stability. If the airplane is loaded with the C of G too far aft, the airplane will have a nose up rather than a nose down attitude. The inherent stability will be lacking and even though putting the elevator down may correct the situation, control of the airplane in the longitudinal plane will be difficult and maybe impossible. Regional Gliding School

NOTE: You must use the buttons in the Confirmation Stage
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Theory of Flight Stability
Let's try a few review questions on Theory of Flight: Question #1 - When the restoring forces are absent and the airplane will neither return from its disturbed position, nor move further away is known as? A Static stability B Neutral stability C Positive stability D Dynamic stability Regional Gliding School

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Theory of Flight Stability
Let's try a few review questions on Theory of Flight: Question #2 - Longitudinal stability is pitch stability around or about what axis? A Longitudinal B Lateral C Normal D Vertical Regional Gliding School

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Lateral Stability Lateral stability is stability around the longitudinal axis or roll stability. Regional Gliding School

Lateral stability is achieved through:
Dihedral The Dihedral Angle is the angle that each wing makes with the horizontal. When a wing is displaced, a flow of air will strike the down going wing at a higher angle of attack with a resultant increase in lift. Regional Gliding School

Lateral stability is achieved through:
Keel Effect Most high wing airplanes are laterally stable simply because the wings are attached in a high position on the fuselage and because the weight is therefore low. When the airplane is disturbed and one wing dips, the weight acts as a pendulum returning the airplane to its original attitude. Regional Gliding School

Lateral stability is achieved through:
Sweepback When the aircraft is disturbed and a wing dips, the lower frontward wing is exposed to more airflow. This will result in an increase in lift in the front ward wing. Regional Gliding School

Directional Stability
Directional Stability is stability around the vertical or normal axis. The most important feature that affects directional stability is the vertical tail surface, that is, the fin and rudder. Regional Gliding School

The Fin An airplane has the tendency always to fly head on into the relative wind. If the airplane yaws away from its course, the airflow strikes the vertical tail surface from the side and forces it back to its original line of flight. Regional Gliding School

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NOTE: You must use the buttons in the Confirmation Stage
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Theory of Flight Stability
Let's try a few review questions on Theory of Flight: Question #3 - Name a type of lateral stability? A Keel effect B Centre of gravity C Fin D Horizontal stabilizer Regional Gliding School

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Theory of Flight Stability
Let's try a few review questions on Theory of Flight: Question #4 - What stability is around or about longitudinal axis? A Directional B Lateral C Normal D Longitudinal Regional Gliding School

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Theory of Flight Stability Congratulations!!
You have now completed the Stability lesson of the Theory of Flight Module. Of course, this lesson is always available to you for future reference if required. You are now ready to move along to the next Theory of Flight lesson you have not completed or to any other module you wish. You can advance to the Self Test Module if you feel ready to challenge the final exam. Good Luck! Regional Gliding School

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