In aviation, relaxed stability is the tendency of an aircraft to change its attitude and angle of bank of its own accord. An aircraft with relaxed stability will oscillate in simple harmonic motion around a particular attitude at an increasing amplitude.
This can be contrasted with the tendency of an aircraft with positive stability, which, when trimmed to fly at a certain attitude, will continue to do so in the absence of control input, and will oscillate in simple harmonic motion on a decreasing scale around the trimmed attitude, eventually returning to its trimmed attitude. A positively stable aircraft will also resist any bank movement. A Cessna 152 is an example of a stable aircraft. Similarly, an aircraft with neutral stability will not return to its trimmed setting without control input, but will oscillate in simple harmonic motion around the trimmed setting continuously and be susceptible to bank influences.
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The differing concepts of stability marked the first heavier-than-air flight attempts made until 1908. Most aeronautical investigators regarded flight as if it were not so different from surface locomotion, except the surface would be elevated. They thought of changing direction in terms of a ship's rudder, so the flying machine would remain essentially level in the air, as did an automobile or a ship at the surface. The idea of deliberately leaning, or rolling, to one side seemed either undesirable or did not enter their thinking.[1]
Some of these early investigators, including Langley, Chanute, later Santos-Dumont, and Voisin brothers, sought the ideal of inherent stability in a very strong sense, believing a flying machine should be built to automatically roll to horizontal (lateral) position after any disturbance. They achieved it with the help of Hargrave cellular wings (having box-kite structure including the vertical panels) and strongly dihedral wings. In most cases they did not include any means for a pilot to control the aircraft roll[2] — they could only control elevator and rudder. The effect not predicted was that without roll, it was very hard to turn the aircraft.[2][3] They were also very susceptible to the side winds.
The Wright brothers designed their 1903 first powered Flyer with anhedral (drooping) wings, which are inherently unstable. They showed that pilot can maintain roll (lateral) control himself anyway. And it was a good way for a flying machine to turn—to "bank" or "lean" into the turn just like a bird or just like a person riding a bicycle.[4] Equally important, this method would enable recovery when the wind tilted the machine to one side. Although used in 1903, it would not become widely known in Europe until August 1908. At that time Wilbur Wright demonstrated to European aviators the importance of the coordinated use of elevator, rudder and roll control for making effective turns.
The latest generation of fighter aircraft often employ design elements which reduce stability to increase maneuverability. The BAE Harrier GR7/GR9 employs a significant and obvious anhedral angle to its wings, this reduces the inherent lateral stability of the wings mounted high on the fuselage.
Modern military aircraft and in particular low observable designs often exhibit instability as a result of their design. The Lockheed F-117 Nighthawk for instance employs a highly non-traditional fuselage and wing shape in order to reduce its radar cross section and enable it to penetrate air defences with relative impunity. However the flat facets of the design reduce its stability to the point where a digital fly-by-wire system was required to allow safe operation.[5] The difficulty in flying such an aircraft without fly-by-wire can be illustrated by the proof-of-concept vehicle developed prior to the Nighthawk, Have Blue.
Though it is not just fighter jets that have relaxed stability designs. The McDonnell Douglas MD-11 has a relaxed stability design which was implemented to save fuel. To ensure stability for safe flight, a LSAS(longitudinal stability augmentation system) was introduced to compensate for the MD-11's rather short horizontal stabilizer to ensure that the aircraft would remain stable. However, there have been incidents in the past in which the MD-11's relaxed stability caused an "inflight upset."
The vertical positioning of the wing changes the roll stability of an aircraft.
This is commonly explained through the analogy of a pendulum-style effect, but this explanation is incorrect (see Pendulum rocket fallacy). Instead, this effect is due to the aircraft's response to sideslip. An aircraft which is rolled to one side will tend to start to sideslip towards the low side of the airplane. A high wing tends to cause the aircraft to roll away from the sideslip, which tends to level the aircraft. A low wing tends instead to roll into the sideslip, increasing the roll angle and therefore increasing the sideslip further.