Aircraft Stabliity

Aircraft Stability

Aircraft stability refers to the aircraft’s ability to correct for conditions that disturb it’s equilibrium. The stability of an aircraft is, generally, inversely proportional to the controllability/maneuverability of an aircraft. Some aircraft are designed to be more stable, which makes them less controllable/maneuverable. Some aircraft are more maneuverable/controllable which makes them less stable. These features are baked into the aircraft's control surfaces and design characteristics. Generally speaking, transport category aircraft are more stable but less controllable. The flight controls are typically more stiff, slower to react, and the aircraft is is generally sluggish in movement. Conversely, fighter jets are designed to be extremely maneuverable. Their flight controls are lighter, extremely quick to react, and the aircraft is generally very quick to maneuver. Fighter jets, however, are very unstable and use advanced fly by wire computers to automatically apply flight control inputs to make the aircraft more stable. Without these fly by wire systems, the aircraft would be extremely difficult to fly.

There are two types of stability, static and dynamic.

Static Stability

Static stability refers to the aircraft’s initial tendency after its equilibrium is disturbed. There are three types of static stability. To explain these types of stability, we will use this example: when flying straight and level, in a properly trimmed single engine high wing conventional tail aircraft, you could let go of the flight controls, and the airplane will remain where it’s at. If you were to add flaps the nose of the aircraft will pitch up because of the increase in lift on the wings and the increased momentary downwash on the horizontal stabilizer.

Positive Static Stability

Positive static stability is the aircraft’s initial tendency to return to its original state of equilibrium before being disturbed. 

If we continue with the example provided above, the aircraft will pitch the nose back down in an attempt to return to that straight and level flight configuration it was in before being disturbed.

Neutral Static Stability

Neutral static stability is the aircraft’s initial tendency to stay in it’s new configuration after its equilibrium is disturbed.

If we continue with the example provided above, the aircraft will remain in this nose up attitude after this additional thrust is introduced. It will remain there and not want to return to it’s original state of equilibrium.

Negative Static Stability

Negative static stability is the aircraft’s initial tendency to want to continue away from it’s initial point of equilibrium after being disturbed.

If we continue with the example provided above, the aircraft will want to continue away from its original state of equilibrium. So after excess thrust has been introduced, the nose pitches up and the nose will want to continue to pitch up, away from its point of equilibrium.

Dynamic Stability

Dynamic stability is the aircraft’s tendency over time after its state of equilibrium is disturbed. There are three types of dynamic stability, positive dynamic, neutral dynamic, and negative dynamic. It is important to note that an aircraft with a neutral static or negative static stability cannot have a dynamic stability. This is because if the initial tendency is not to return to the original state of equilibrium then the aircraft’s tendency over time would be to continue the characteristics of the static stability. I will provide more examples of this below with each section.

We will use the same example provided with static stability. When flying straight and level, in a properly trimmed aircraft you could let go of the flight controls, and the airplane will remain where it’s at. If you were to add flaps the nose of the aircraft will pitch up because of the increase in lift on the wings and the increased momentary downwash on the horizontal stabilizer

Positive Dynamic Stability

This is the aircraft’s tendency over time to want to return to its original state of equilibrium. So continuing with the example provided above, if the aircraft’s initial tendency is to return back to its original state of equilibrium then the nose would pitch itself back down (that is the static stability). As the nose comes down to the horizon, it would pass through the horizon and the nose would pitch down. Then the nose would come back up and back down and so on. The nose would oscillate up and down, slightly less with each osculation, until it has reached back to straight and level flight. (Side note: the reason the aircraft would pitch back down through the horizon instead of just leveling off at the horizon is because of momentum and inertial. It would also apply to a ball on a string. If you take a ball attached to a string, move it from the center and then let go, the ball would not return to center and stop. It would pass through the center and oscillate to the other side. Then back and forth until reaching equilibrium again. This is also be a demonstration of positive static and dynamic stability) This is positive dynamic stability. The aircraft’s tendency over time to return back to that original state of equilibrium (straight and level flight)

If the aircraft had a neutral static stability, it would be unable to “oscillate” up and down until reaching straight and level again, because it doesn’t have the initial tendency to return back to straight and level. The nose just remained in that nose up pitch attitude.

This video provides a perfect example of positive static and positive dynamic stability. Once the balls are removed from their equilibrium (hanging in the center) their initial tendency is to return to their point of equilibrium (the center). After that initial tendency, their tendency over time is to return to that equilibrium. So they oscillate back and forth, less and less each time, until reaching that equilibrium again. While this is not an aircraft, it does provide a very good visual of what positive static and positive dynamic stability looks like in the real world.

This video provides a great overview of the different types of stability. You will notice that the only one of those three that has a tendency over time is the positive static (or stable in this video) object.

The neutral static object just continues to do what it is already doing(its initial tendency is just keep rolling in the same way at the same speed. It is not negative static stability, for it to be negative, it would need to continue to accelerate even after the force is no longer being applied).

The negative (or unstable) object continues away from its point after being disturbed. (To make sure there is no confusion, the one that is labeled unstable is the negative static stability. If it were to be pushed and then just remain sitting at an angle that would be neutral static. But because it continues to fall even after the force is no longer being applied it is negative static stability)

Neutral Dynamic Stability

This is the aircraft’s tendency over time to stay where it is at. Continuing the example provided above, the aircraft would have a positive static stability, so the nose would pitch back down. Once it starts to pitch back down the nose will pass through the horizon and pitch nose down. In an aircraft which has neutral dynamic stability, the nose would then remain in that nose down position and not want to pitch back up. It would remain at this new pitch attitude which would be its new equilibrium point.

Negative Dynamic Stability

This is the aircraft’s tendency over time to continue away from its point of equilibrium. If we continue the example provided above, as the nose of the aircraft would pitch back down through the horizon (positive static stability) and then it would continue to pitch further and further nose down. This is neutral static stability

If the aircraft had a negative static stability, it would not pitch down initially, the nose would just continue to pitch up which is the initial tenancy and would never return back to equilibrium again.

Dihedral

Dihedral in an airplane refers to the upward angle of an aircraft's wings relative to the horizontal plane. It is a key design feature that helps improve the aircraft's stability, especially in roll. When an airplane has dihedral, if the plane tilts to one side, the wing on the lower side will experience a higher angle of attack, generating more lift and naturally bringing the aircraft back to level flight. This self-correcting tendency helps to counteract unwanted rolling motions, making the aircraft more stable and easier to control, particularly during turbulent conditions or when the pilot makes small input errors. Dihedral is often used in the wing design of many aircraft, though the degree of dihedral can vary depending on the specific stability requirements of the plane.

Anhedral

Anhedral is the opposite of dihedral; it refers to the downward angle of an aircraft's wings relative to the horizontal plane. While dihedral increases an aircraft's roll stability, anhedral is used to reduce it. Aircraft with anhedral wings tend to have a more neutral or even unstable roll response, which can make them more agile and responsive to pilot inputs. This design feature is often found in high-performance or military aircraft where maneuverability and quick roll rates are prioritized over inherent stability. By reducing roll stability, anhedral allows the aircraft to roll more freely, which is useful in combat situations or for aircraft designed for tight turns and rapid changes in direction. However, anhedral can also make the aircraft more susceptible to rolling in turbulent air or during crosswind landings, so it must be carefully balanced with other aspects of the plane's stability and control.

Keel Effect

The keel effect, also known as the pendulum effect, refers to the stabilizing influence of an aircraft's side area above its center of gravity, particularly in the vertical plane. When an aircraft experiences a sideslip—such as from a gust of wind or a yawing moment—the side area above the center of gravity, including the vertical stabilizer, fuselage, and sometimes parts of the wing or engine nacelles, catches the relative wind and produces a force that tends to rotate the aircraft back into coordinated flight. This action increases directional stability, helping the aircraft resist unwanted yawing motions. The greater the side area above the center of gravity, the stronger the keel effect, which is especially noticeable in high-wing aircraft, where the fuselage and vertical surfaces act like a weathervane to keep the nose pointed into the relative wind.