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Regional Gliding School

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.


Static Stability• The initial tendency of an airplane when

disturbed to return to its original position.


Dynamic Stability• The overall tendency of an airplane to

return to its original position, following a series of damped out oscillations.


Stability may be:• Positive

– 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.


Stability may be:• Negative

– meaning it will develop forces or moments that tend to move it further away. Negative stability is Instability.


Longitudinal Stability• Longitudinal Stability is pitch stability or

stability around the lateral axis of the airplane.


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.


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.


Confirmation Stage

NOTE: You must use the buttons in the Confirmation Stage


A

B

C

D

Static stability

Neutral stability

Dynamic 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?

Positive stability

Theory of FlightTheory of FlightStabilityStability

I’m afraid that answer is incorrect

Let's try again...

Flying Scholarship Program


That answer is correct.

Let's move on...


Stability may be:• Positive

– meaning the airplane will develop forces ormoments that tend to restore it to its originalposition.

• Neutral – meaning the restoring forces are absent and the

airplane will neither return from its disturbedposition, nor move further away.


A

B

C

D

Longitudinal

Lateral

Vertical


Question #2 - Longitudinal stability is pitch stability around or about what axis?

Normal



Let's try again...




Let's move on...


Longitudinal Stability• Longitudinal Stability is pitch stability or

stability around the lateral axis of theairplane.


Lateral Stability• Lateral stability is stability around the

longitudinal axis or roll stability.


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.



• 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.



• 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.


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.


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.


AXIS MOTION CONTROL STABILITYLongitudinal Rolling Ailerons Lateral

Stabilityaround theLongitudinalAxis.

Lateral Pitching Elevators LongitudinalStabilityaround theLateral Axis.

Normal Yawing Rudder DirectionalStabilityaround theNormalAxis.


Confirmation Stage

NOTE: You must use the buttons in the Confirmation Stage


A

B

C

D

Keel effect

Centre of gravity

Horizontal stabilizer


Question #3 - Name a type of lateral stability?

Fin



Let's try again...




Let's move on...



• Keel Effect• Most high wing airplanes are laterally stable simply

because the wings are attached in a high position onthe 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 airplaneto its original attitude.


A

B

C

D

Directional

Lateral

Longitudinal


Question #4 - What stability is around or about longitudinal axis?

Normal



Let's try again...




Let's move on...


Lateral Stability• Lateral stability is stability around the

longitudinal axis or roll 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!


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Documents

negative stability

stability o

inherent stability

scholarship programim

original position

disturbed position

restoring forces

longitudinal plane