The tendency for the roll rate to become constant (i.e. roll moment to become zero) is called roll damping. This helps to establish lateral stability but it is not enough. At most roll damping would be expected to produce neutral lateral stability.

Roll Moments Induced by Yaw

If a pilot steps on a rudder pedal causing the aircraft to yaw one wing will advance and the other will retreat. The faster moving wing produce more lift than the other which will cause a roll in the same direction as the yaw. This will be exaggerated by wing dihedral which we will discuss below.

 Lateral Stability

Most light aircraft display approximately neutral lateral static stability. In other words when rolled into a given bank and released the aircraft remains more or less at that angle of bank.

If the aircraft rolled back to zero bank by itself we would say it had positive static lateral stability. If it rolled further, into a steeper bank we would say it was displaying negative static lateral stability.

Most light aircraft if left with no pilot input will eventually roll into a steeper turn, usually entering a spiral dive. This means that most aircraft exhibit negative dynamic lateral stability. Despite this they can be flown quite well, except they require considerable attention when flying in IMC conditions.

As indicated above lateral stability and directional stability are closely linked. If an aircraft has a lot of directional stability (most do) it tends to become unstable laterally. This is because the bank angle starts the aircraft turning, which speeds up the wing on the outside of the turn (high wing.) The faster wing produces more lift, which rolls the aircraft into a steeper bank.

Dihedral

Dihedral is the most common design feature used to increase the lateral stability. The movie below shows how dihedral works. 

In order to understand dihedral you must realize that the side force created by a bank angle will pull the aircraft sideways through the air. In other words it will cause the aircraft to slip.

As mentioned above the aircraft will slip once it enters a banked attitude. However, the directional stability will quickly turn the aircraft into this new relative wind (as discussed on the previous page.) As a result the slip will quickly disappear and the dihedral effect will be eliminated.

High Wing Effect

Aircraft with high wings (wings which are above the c of g) will have more lateral stability than aircraft which have low wings (below the c of g.) This is often referred to as the pendulum effect. Essentially the mass of the aircraft tends to swing back under the wing when disturbed laterally. Conversely the low wing aircraft is unstable laterally, just as a pendulum would be if you balanced it upside down. The movie below demonstrates this effect.

Swept wings

Swept wings are one of the most effective ways of increasing lateral stability. However, they are usually only used on high speed jet aircraft. Therefore, this effect is reserved only for those lucky enough to fly a jet.

Just as with the dihedral effect, the swept wing affects lateral stability because the aircraft tends to slip initially one banked. The movie below shows how the slip angle changes the relative sweep of the two wings. This in turn changes the amount of lift each wing produces.

Dutch Roll

Many swept wing aircraft suffer a dynamic instability problem known as Dutch Roll.

Dutch roll happens when the aircraft has relatively strong static lateral stability (usually due to the swept wings) and somewhat weak directional stability (relatively.) In a Dutch roll the aircraft begins to yaw due to a gust or other input. The yaw is slow damping out so the aircraft begins to roll before the yaw is stopped (due to the increased speed of the advancing wing and the increased lift due to the swept wing effect.)

By the time the yaw stops and begins to swing back toward zero slip the aircraft has developed a considerable roll rate and due to momentum plus the slip angle the aircraft continues to roll even once the nose has begun returning to the original slip angle. 

Eventually the yaw overshoots the zero slip angle causing the wings to begin rolling back in the opposite direction.

The whole procedure repeats, sometimes with large motions, sometimes witch just a small churning motion. Like all dynamic stability problems, Dutch roll is much worse at high altitudes where the air is less dense.

Dutch roll is almost certain to happen in a jet aircraft is the Yaw dampener is turned off at high altitude. Therefore, the first thing to check if an aircraft begins to exhibit Dutch roll is that the Yaw Dampener is on. The pilot should then try to minimize the yawing oscillations by blocking the rudder pedals (i.e. hold the rudder pedals in the neutral position.) Next apply aileron (spoiler) control opposite to the roll. The best technique to use is short jabs of ailerons applied opposite to the roll. Try to give one quick jab on each cycle (i.e. turn the wheel toward the rising wing, then return it to neutral.) Finally accelerate to a higher speed, where directional stability will be better, or descend into more dense air, for the same reason.

The movie below shows graphically what a steady Dutch roll looks like. However, it is critical to realize that Dutch rolls are often dynamically divergent. In other words in this movie the Dutch roll is exhibiting neutral dynamic stability, but it may well be negative dynamic stability in a given aircraft  (at certain speed and air densities.)

Reducing Lateral Stability

The Dutch Roll tendency described above is exacerbated when an aircraft has too much lateral stability. Generally pilots prefer to fly aircraft which exhibit neutral static lateral stability, or very slightly positive. 

As a result aircraft with both swept and high wings often are too stable. This can be "fixed" by incorporating anhedral (negative dihedral.) The BAE-146 to the left is an example of an aircraft with anhedral.