Relaxed stability
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In aeronautical engineering, relaxed stability refers to airplanes with no inherent natural stability, at least at low speeds. Lowering stability allows the plane to be designed purely for aerodynamic efficiency, as opposed to handling or "flyability", and can have noticeable performance improvements in some designs.
A parachute is stable (easy to understand) as are uncontrolled model aeroplanes (not easy to understand, even all the maths in Flight dynamics does not fully suffice). Reentry vehicles are stable beside the space shuttle. Rockets and manned aeroplanes need a pilot (autopilot) otherwise the rocket flies a parbola and the plane a phugoid or a spiral. The very first aeroplanes also flew in parobolas due to an aft center of mass (flying flat), leading to many crashes. Now with the advent of reliable autopilots, strong electric motors and fast sensors leading to a reaction time under 1 ms, this trim is used again.
Most older aeroplane designs are naturally stable across their entire performance envelope, allowing the aircraft to be flown without the pilot needing to constantly correct the flight path. However this leads to a loss of maneuverability: the more stable the plane is the more it will naturally continue flying in its current direction, and the more effort has to be applied to maneuver it. This stability increases as speed increases, because of the higher airflow over the control surfaces. This presents a design problem for planes that need to be manoeuverable at high speeds (like fighter aircraft) because at lower speeds the decreased stability makes control difficult. Since takeoffs and landings occur at low speeds, the need for low-speed control limits high-speed maneuverability.
The introduction of automatic flight controls and fly-by-wire systems changed this equation considerably. Now low speed control is no longer an issue: any "drift" from the flight path not commanded by the pilot can be detected and automatically corrected. This gives the plane an artificial stability to make it flyable at any speed within the aeroplane's flight envelope, and the lowered inherent stability makes the plane maneuverable at higher speeds.
Although such systems have been in use for many years in military aircraft, notably for yaw damping, the first true relaxed stability fighter was the F-16 Fighting Falcon, which used an advanced (for the time) flight control system that allowed the plane to fly very "flat" to the air as well, increasing performance and range. It was able to completely outperform the F-17 Cobra in head-to-head competition, and went on to become one of the most produced post-war US designs. Today almost all fighter designs make extensive use of relaxed stability and automatic flight controls, and watching the rapid movement of their control surfaces as they land will demonstrate how unstable these designs are.
Relaxed stability has since moved into the civilian market as well, although for other reasons. Today most modern airliners use a fly-by-wire system in order to save weight, but a side effect of this is that the planes can use relaxed stability to fly flatter and thus save fuel. First introduced on the Airbus A320, today almost all large airliners use some sort of relaxed stability in their designs for this reason.
