In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...
, relaxed stability refers to an
aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
with low or negative
stability
Stability may refer to:
Mathematics
*Stability theory, the study of the stability of solutions to differential equations and dynamical systems
** Asymptotic stability
** Linear stability
** Lyapunov stability
** Orbital stability
** Structural sta ...
.
An aircraft with negative stability will have a tendency to change its
pitch and bank angles spontaneously. An aircraft with negative stability cannot be
trimmed
''Trimmed'' is a 1922 American silent Western film directed by Harry A. Pollard and featuring Hoot Gibson. It is not known whether the film currently survives, and it may be a lost film.
Cast
* Hoot Gibson as Dale Garland
* Patsy Ruth Miller ...
to maintain a certain
attitude
Attitude may refer to:
Philosophy and psychology
* Attitude (psychology), an individual's predisposed state of mind regarding a value
* Metaphysics of presence
* Propositional attitude, a relational mental state connecting a person to a pro ...
, and will, when disturbed in pitch or roll, continue to pitch or roll in the direction of the disturbance at an ever-increasing rate.
This can be contrasted with the behaviour of an aircraft with positive stability, which can be trimmed to fly at a certain attitude, which it will continue to maintain in the absence of control input, and, if perturbed, will
oscillate
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
in
simple harmonic motion
In mechanics and physics, simple harmonic motion (sometimes abbreviated ) is a special type of periodic motion of a body resulting from a dynamic equilibrium between an inertial force, proportional to the acceleration of the body away from the ...
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
The Cessna 152 is an American two-seat, fixed- tricycle-gear, general aviation airplane, used primarily for flight training and personal use. It was based on the earlier Cessna 150 incorporating a number of minor design changes and a slightly ...
is an example of a stable aircraft. Similarly, an aircraft with neutral stability will not return to its original attitude without control input, but will continue to roll or pitch at a steady (neither increasing nor decreasing) rate.
Early aircraft
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
A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally aircraft, air or watercraft, water). On an aircraft the rudder is used primarily to ...
, 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
Alberto Santos-Dumont ( Palmira, 20 July 1873 — Guarujá, 23 July 1932) was a Brazilian aeronaut, sportsman, inventor, and one of the few people to have contributed significantly to the early development of both lighter-than-air and heavier ...
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
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
* Lateral cricoarytenoid muscle
* Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral co ...
) position after any disturbance. They achieved this with the help of
Hargrave cellular wings (wings with a
box kite
A box kite is a high performance kite, noted for developing relatively high lift; it is a type within the family of cellular kites. The typical design has four parallel struts. The box is made rigid with diagonal crossed struts. There are two s ...
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 control only the 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.
The
Wright brothers designed their
1903 first powered Flyer with
anhedral (drooping) wings, which are inherently unstable. They showed that a pilot can maintain control of lateral roll and it was a good way for a flying machine to turn—to "
bank
A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets.
Because ...
" or "lean" into the turn just like a bird or just like a person riding a bicycle. 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, when
Wilbur Wright demonstrated to European aviators the importance of the
coordinated use of elevator, rudder and roll control for making effective turns.
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. For example, the
Cessna 152
The Cessna 152 is an American two-seat, fixed- tricycle-gear, general aviation airplane, used primarily for flight training and personal use. It was based on the earlier Cessna 150 incorporating a number of minor design changes and a slightly ...
.
* An aircraft with a "low" wing (i.e., underneath the fuselage) has less roll stability. The
Piper Pawnee
The PA-25 Pawnee is an agricultural aircraft produced by Piper Aircraft between 1959 and 1981. It remains a widely used aircraft in agricultural spraying and is also used as a tow plane, or tug, for launching gliders or for towing banners. In ...
uses a "low" wing.
Unstable aircraft
Modern military aircraft, particularly low observable ("
stealth") designs, often exhibit instability as a result of their shape. The
Lockheed F-117
The Lockheed F-117 Nighthawk is a retired American single-seat, twin-engine stealth attack aircraft developed by Lockheed's secretive Skunk Works division and operated by the United States Air Force (USAF). It was the first operational aircr ...
Nighthawk, for instance, employs a highly non-traditional fuselage and wing shape in order to reduce its
radar cross section
Radar cross-section (RCS), also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected.
An object reflects a limited amount of radar energy back to the source. ...
and enable it to penetrate air defenses with relative impunity. However, the flat facets of the design reduce its stability to the point where a computerized
fly-by-wire
Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control ...
system was required to allow safe operation.
Relaxed stability designs are not limited to military jets. The
McDonnell Douglas MD-11
The McDonnell Douglas MD-11 is an American tri-jet wide-body airliner manufactured by American McDonnell Douglas (MDC) and later by Boeing.
Following DC-10 development studies, the MD-11 program was launched on December 30, 1986.
Assembly of ...
has a neutral stability design which was implemented to save fuel. To ensure stability for safe flight, an LSAS (Longitudinal
Stability Augmentation System) was introduced to compensate for the MD-11's rather short horizontal stabilizer and ensure that the aircraft would remain stable.
However, there have been incidents in which the MD-11's relaxed stability caused an "inflight upset."
Intentional instability
The latest generation of fighter aircraft often employ design elements which reduce stability to
increase maneuverability. Greater stability leads to lesser control surface authority; therefore, a less stable design will have a faster response to control inputs. This is highly sought after in fighter aircraft design.
A less stable aircraft requires smaller control deflections to initiate maneuvering; consequently drag and control surface imposed stresses will be reduced and aircraft responsiveness will be enhanced. Since these characteristics will typically make control by the pilot difficult or impossible, an artificial stability will typically be imposed using computers, servos, and sensors as parts of a
fly-by-wire
Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control ...
control system.
Citations
General and cited references
*
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{{DEFAULTSORT:Relaxed Stability
Aerodynamics