Aerodynamically unstable

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 behaviour 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 if perturbed will oscillate in simple harmonic motion on a decreasing scale around and eventually return to the 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 a stable simple harmonic motion around the trimmed setting continuously, neither increasing nor decreasing oscillation amplitude, and be susceptible to bank influences.

Early attempts at heavier-than-air flight were marked by a differing concept of stability than is used today. Most aeronautical investigators regarded flight as if it were not so different from surface locomotion, except the surface was 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 either seemed undesirable or did not enter their thinking.

Some of these early investigators, including Langley, Chanute, and later Santos-Dumont and the Voisin brothers, sought the ideal of inherent stability in a very strong sense, believing a flying machine should be built to automatically roll to a horizontal (lateral) position after any disturbance. They achieved this with the help of Hargrave cellular wings (wings with a 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 — they could only control elevator and rudder. The unpredicted effect of this was that it was very hard to turn the aircraft without rolling. They were also strongly affected by side gusts and side winds upon landing.

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