LESSON 16 Chapter 15 Longitudinal Stability and Control ANA Chapter 4

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Flight Theory

Chapter 15

Longitudinal Stability and Control

Static Stability

is the initial tendency of an aircraft to move, once it has been displace from its equilibrium position

Positive static stability

would be the initial tendency to return to the original position.

Negative static stability

Is the initial tendency to move away from the original position.

Neutral static stability

Is the initial tendency to stop at a random point neither moving farther away from or towards the original position

Dynamic Stability

Is the movement over a period of time.

This is usually explained in terms of oscillatory movement.

Positive dynamic

is when the oscillations are getting smaller over time

Negative dynamic

is when the oscillations are getting larger over time

Neutral dynamic

is when the oscillations are staying about the same size over time.

Combo Platters

1. positive static pos dynamic

2. pos static neutral dynamic

3. pos static neg dynamic

4. neutral static pos dynamic

5. Neutral static neutral dynamic

6. neutral static neg dynamic

7. neg static pos dynamic

8. neg static neutral dynamic

9. neg static neg dynamic

Longitudinal axis

runs down the center of the fuselage.

This axis deals with roll stability or lateral stability about the longitudinal axis.

Stability on this axis determines how well the plane will right itself when encountering gusts

Lateral axis

runs down the wings.

This axis deals with pitch stability or longitudinal stability about the lateral axis.

This stability is important because it determines the pitch characteristics of the plane.

Lateral axis

How easily the airplane stalls may be effected by this as well.

A symmetrical airfoil

will not experience any pitching because the forces acting on the top and bottom surfaces are located in the same position.

A cambered airfoil

Will experience a difference in forces causing a nose down pitching moment.

Negatively camber airfoils experience a nose up pitching moment.

The shape of the fuselage

May be a destabilizing influence on pitch control.

It may have neutral lift characteristics until a pitching up is caused by an gust then the pressure distribution may cause a pitching up moment.

Power Effects

are felt in two major areas, direct and indirect.

Direct effects include the vertical location of the thrust line with respect to the longitudinal axis.

If below, nose up

If above nose down.

Nacelles And Props

Engine Nacelles and props may cause an inherent pitch up force

the air has to turn a corner when entering striking the nacelle or prop shaft causing the action reaction.

Largest at high power, low airspeed, high AOA, inlet forward of cg


Indirect effects are the tail down force

This increases with an increase in airspeed or propeller slipstream

the pitch to increase.

The tail is the single largest contributor to longitudinal stability.


The center of lift related to cg determines a large part of stability.

If the cg is lined up with center of lift neutral stability will result.

If the cg is behind the center of lift, negative stability will result.

If the cg is in front of center of lift, positive stability will result.

Dynamic oscillations

occur in three flavors,

First mode of dynamic longitudinal or Phugoid

These are long period oscillations lasting 20 to 100 seconds and are easily controlled.

Dynamic oscillations

The second mode or short period oscillationsgenerally lasting 1 to 2 seconds or less that are impossible to control

Porposing of the plane is a good example (In Cruise flight)The pilot should let the controls go and let the inherent stability design take care of it

Dynamic oscillations

The third mode deals with the elevator flapping about the hinge line

These last .3 to 1.5 seconds.

Vertical axis

Runs vertically through the fuselage.

This axis deals with yaw stability or directional stability about the vertical axis.

Yaw angle or beta is the primary reference in lateral stability as well as directional stability considerations.

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