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Aerodynamic Stability

An introduction to

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Objective

Purpose

To ensure the applicant learns the purpose of and and can exhibit a clear understanding of basic aerodynamics and the various factors which affect airplane stability and controllability

A basic understanding of aerodynamics and airplane stability and controllability is essential for every pilot. When flying, pilots need to understand the four forces acting on an airplane and how the center of gravity affects controllability and maneuverability. This lesson in basic aerodynamics will introduce pilots to stability and many other important concepts that are fundamental to other aspects of flying.

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Stability

Airplanes flight Characteristics

Static Stability - Airplanes tendency to return to its original flight attitude when disturbed

Positive - Returns to original attitude

Neutral - Keeps new attitude

Negative - moves further away from attitude

Dynamic Stability - Airplanes tendency to oscillate over time

Positive - Oscillations become lesser

Negative - Oscillations become greater

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Maneuverability Vs Controllability

Two properties primarily affected by stability

Maneuverability - airplanes ability to to be maneuvered easily and withstand the stresses imposed by maneuvers

Controllability - Airplanes ability to respond to a pilots control

An airplanes stability affects two areas significantly

Maneuverability is the quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. It is governed by the aircraft's weight, inertia, size and location of flight controls, structural strength and powerplant. It is an aircraft design characteristic. Airplanes which are more stable tend to be less maneuverable, they may require more effort to maneuver and respond more slowly. Think about it, it is resisting being acted upon by an outside force.

Controllability is capability of an aircraft to respond to the pilots control, especially with regard to flight path and attitude. It is the quality of the aircraft;s response to the pilots control application when maneuvering the aircraft regardless of its stability characteristics. Airplanes with positive stability tend to be highly controllable, where negative stability may be uncontrollable in many situations. Divergent oscillations may lead to rapid loss of control.

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Longitudinal Stability and Stall Recovery

Longitudinal Stability - Stability in Pitch

Center of Gravity (CG) - a pint at which the aircraft would balance if it were suspended at that point.

Center of Lift (CL) - a point on an aircraft where all the lifting forces can be considered to act.

Forward CG	Aft CG
Stronger tail down force Higher load imposed on CL	Lighter tail down force Lighter load imposed CL

A well designed plane’s CG is always forward of the CL

Longitudinal Stability refers to an airplanes stability in pitch. Most airplanes are stable in pitch and have gentle up/down pitch oscillations over time. The most important factor in longitudinal stability is the location of the center of gravity relative to the center of lift.

CG is the point at which the aircraft would be balanced if it were suspended at that point

CL is a point on an aircraft where all the lifting forces can be considered to act

In conventionally designed airplanes the center of gravity is always in front of the center of lift. Because the horizontal stabilator is behind the center of lift, it produces downward force (lift, but downward) to keep the nose of the airplane level in flight. This downward force opposes the upward force of the main wing, requiring it to produce slightly more lift to compensate. Also, because an airfoil that produces more lift also produces more drag, the amount of drag caused by the horizontal stabilizer depends on the force it must produce.

A well designed airplanes CG is always forward of the CL. This design causes a natural tendency for the airplane to nose down, reducing the angle of attack. That means when the airplane stalls, if the elevator pressure is relaxed, the CG will naturally fall.

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Lateral Stability

Lateral Stability - Stability in Roll

Helped by dihedral wing design

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Directional Stability

Directional Stability - Stability in Yaw

Created by large distance from CG to Rudder