Relaxed stability

In aviation, relaxed stability is the tendency of an aircraft to change its attitude and angle of bank on 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.

Methods of decreasing stability in design

Expand-section|date=June 2008Modern military aircraft and in particular low observable designs often exhibit relaxed stability 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. 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. This was a smaller design based upon similar principles of reducing radar cross section, and was successfully implemented, however both prototypes were lost in accidents while testing due to their instability.

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, while the wings are mounted high on the fuselage: the anhedral reduces inherent stability in the roll axis.

Vertical wing position

The vertical positioning of the wing changes the roll stability of an aircraft.
*An aircraft with a "high" wing position (i.e., set on top of the fuselage) has a higher roll stability. A Cessna 152 is an example.
*An aircraft with a "low" wing (i.e., underneath the fuselage) has less roll stability. A Piper Pawnee is an example of a "low" wing.

In order to visualize how this works, consider the difference between balancing and hanging. Because the center of mass is under the wing in a high wing, the wing essentially "carries" the aircraft from above. Because the center of gravity is above the wing in a low wing, the wing "holds up" the aircraft from below. This means that if the aircraft begins to roll, it will tend to roll more, since the weight of the fuselage will no longer be centered over the wings.