Control and Stability in Aircraft

An aircraft requires three things to take flight and maintain it: lift, propulsion, and control. Lift is provided by the aircraft’s wings, and propulsion by its engines, but establishing control is the most challenging to maintain.

Control of an aircraft is broken down into three different axes, all based on the aircraft’s center of gravity. These axes are:

  1. The longitudinal axis, which runs from nose to tail. It is also called the axis of roll, as it involves one wing rolling up while the opposite rolls down.
  2. The lateral axis, which runs from wingtip to wingtip. Also called the axis of pitch, the lateral axis determines if the aircraft’s nose is pointed up or down.
  3. The normal axis, which runs from the top of the cabin to the bottom of the landing gear. Also called the axis of yaw, it relates to the nose pointing left or right.               

Modern conventional aircraft have control surfaces for each of these axes. The ailerons, panels in the trailing edge of the wings, adjust the aircraft’s longitudinal axis. Panels in the tail (or in some aircraft the entire tail) act as a horizontal elevator, controlling the aircraft’s pitch. Lastly, a vertical tail plane features a aircraft rudder (like a boat’s) that controls the aircraft’s axis of yaw.

Other control systems and methods have been invented, of course. Some combine control elements like the ailerons and elevators into a single control surface called a taileron, while others warp the entire wing’s shape to provide longitudinal control, but these methods are not as prevalent.

Stability refers to the aircraft’s ability to return to its original equilibrium state after a small displacement, without the pilot interfering. In other words, if the plane is disturbed in any of its axis, such as by turbulence, it will return to its original orientation. This is referred to as static stability. Dynamic stability is when an aircraft continuously tries to return to its original state, and may overcorrect and oscillate, or diverge completely and behave uncontrollably.

Longitudinal stability is the stability around the pitching axis. It is influenced by both the aircraft’s center of gravity, and the center of pressure. The further forward towards the nose the center of gravity is, the more stable the aircraft is with respect to pitching. However, the further forward the center of gravity is, the more difficult the aircraft is to control. Meanwhile, the center of pressure is the point that aerodynamic lift forces are assumed to act if they were combined onto a single point. If the center of pressure does not match the center of gravity, pitching movements will be induced around the center of gravity. The problem is that the center of pressure is not static and moves in flight depending on the angle of incidence on the wings. The pilot’s control over longitudinal stability comes with the design of the tail plane and elevators. By using the elevators, the pilot can correct undesired pitching motions.

Lateral stability is the stability of the aircraft when rolling one wing down and the other up. As it does this, the wings are no longer generating equal amounts of lift. This creates a vertical lift component in the direction of gravity, and a horizontal load component, causing the aircraft to sideslip. A sideslip load can contribute to returning the aircraft to the original configuration and can be achieved with wings that are either upward-inclined or swept-back.                 


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