LESSON 17 Chapter 16 Directional and Lateral Stability and Control ANA Chapter 4

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Flight Theory

Chapter 16

Directional and Lateral Stability and Control

Rudder/Vertical Stabilizer

Directional stability about the vertical axis is obtained in our airplanes by use of the rudder/vertical stabilizer.

Sideslip angle or beta is the angle of the relative wind and the longitudinal axis.

The wings

The wings may be a small stabilizing influence if they are sweptback.

When the relative wind is shifted, the upwind wing produces more drag than the other because of increased frontal area being presented to the relative wind

This causes the yaw towards the relative wind.

The fuselage

The fuselage may be a destabilizing influence on yaw.

If the surface area ahead of the cg is larger than that behind, it will tend towards negative stability.

Engine nacelles

Engine nacelles can be a destabilizing force if ahead of the cg because of the reasons discussed in the previous chapter.

The turning of the air to go in the hole.

Can you say “Destabilization”

These two, the fuselage and the engine nacelles have the greatest potential for destabilization.

The Vertical Stabilizer

The vertical stabilizer is the primary stabilizing influence.

Strakes may be added under the fuselage to improve overall yaw characteristics.

Also in cases where yaw is a problem especially at higher speeds, a yaw damper may be required to be operational on the aircraft.

The Vertical Stabilizer

When at high angles of attack, there is a reduction in the vertical tails contribution and stability is decreased.

There is a decay in directional stability in planes with low aspect ratios.

The sweepback needs such high angles of attack that a lack of directional stability results


Has a minimal effect on directional stability

Lateral stability and control

Lateral stability is the stability displayed around the longitudinal axis of the airplane or specifically the stability in the roll.

Dihedral or Anhedral

Dihedral is a stabilizing design, whereas Anhedral is a destabilizing design.

The stabilizing effect of dihedral occurs when a sideslip is set up as the result of turbulence or gust displacing the plane.


The side slip results in the downward wing having a greater angle of attack than the upward wing. The extra lift then rights the airplane.

Wing position

A high wing sets up a pendulum type of situation this can result in the equivalent of a 1 to 3 degree dihedral.

So not as much dihedral is needed.

In some planes, negative dihedral is needed.

The low wing however the reverse is true.

Wing Sweepback

When a side slip is set up in a sweepback wing, the upwind side wing will have a greater angle of attack because of the more favorable relative wind.

Vertical Tail or Keel Effect

Side forces on the tail cause a weather vaining effect which tends to bring the plane back into alignment with the relative wind.

If there is a large enough portion of the rudder above the longitudinal axis, then there is a rolling moment induced by the side ways force of the relative wind on the tail; this is a stabilizing force.

Lateral control

Ailerons, elevons and ailevator (delta wing aircraft) are all ways the pilot can use to control the plane.

The elevons or ailevator work together when elevator action is required and work opposite when roll action is required.

A negative aspect of lateral control is the overbanking tendency which shows up at bank angles of greater than 30 degrees.

Lateral control

The outer wing is moving faster than the inner wing and thus developing more lift.

This causes the plane to want to continue the roll and it takes opposite aileron to counteract it.

This may lead to the curious condition of cross controls but having a coordinated airplane.

Adverse Yaw

When rolling into a turn, the upward wing’s lift vector is tilted aft because of the change in the relative wind components being up and parallel to the flight path.

The downward wing’s lift vector is tilted forward because of the change in relative wind components being down and parallel to the flight path.

Adverse Yaw

These two forces oppose the turn entry and cause adverse yaw.

Aileron drag is another common cause of adverse yaw.

Frise ailerons and differential aileron travel are common ways of offsetting the effects of aileron drag.

Using spoilers to turn solves this problem.

Bad things

1. spiral divergence happens when directional stability is greater than lateral stability.

What happens is there is little sideslip when the aircraft is displaced and next to no dihedral effect.

The plane enters an ever tightening spiral.

Bad things

2. directional divergence is where the plane will eventually end up yawing sideways to the relative wind.

Bad things

3. Dutch roll is directional and lateral oscillation caused by strong dihedral effect and weak directional stability.

The yaw will cause greater lift on one wing more than the other this causes a rolling moment which in turn causes a sideslip to be set up whereby the process starts all over again.

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