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 to maintain a certain attitude, 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 in simple harmonic motion on a decreasing scale around, and eventually return to, the trimmed attitude.[citation needed] 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 original attitude without control input, but will continue to roll or pitch at a steady (neither increasing nor decreasing) rate.[citation needed]
Early aircraft
Early attempts at heavier-than-air flight were marked by a differing concept of stability from that 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.[3]
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[4][pageneeded]—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.[4][pageneeded][5] They were also strongly affected by side gusts and side winds upon landing.[citation needed]
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" or "lean" into the turn just like a bird or just like a person riding a bicycle.[6] 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.[citation needed]
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.
An aircraft with a "low" wing (i.e., underneath the fuselage) has less roll stability. The Piper Pawnee uses a "low" wing.
Modern military aircraft, particularly low observable ("stealth") designs, often exhibit instability as a result of their shape. The LockheedF-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 defenses with relative impunity. However, the flat facets of the design reduce its stability to the point where a computerized fly-by-wire system is required for it to fly.[7]
Relaxed stability designs are not limited to military jets. The McDonnell Douglas MD-11 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.[8] However, there have been incidents in which the MD-11's relaxed stability caused an "inflight upset".[9]
Many modern fighter aircraft often employ design elements that 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, artificial stability will typically be imposed using computers, servos, and sensors as parts of a fly-by-wire control system.[citation needed]
Tobin, James (2004). To Conquer The Air: The Wright Brothers and the Great Race for Flight. New York: Simon & Schuster. ISBN978-0-7432-5536-3.
Related Research Articles
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 computers determine how to move the actuators at each control surface to provide the ordered response. Implementations either use mechanical flight control backup systems or else are fully electronic.
A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using aerodynamic lift. Fixed-wing aircraft are distinct from rotary-wing aircraft, and ornithopters. The wings of a fixed-wing aircraft are not necessarily rigid; kites, hang gliders, variable-sweep wing aircraft, and airplanes that use wing morphing are all classified as fixed-wing aircraft.
A tailplane, also known as a horizontal stabiliser, is a small lifting surface located on the tail (empennage) behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed-wing aircraft such as helicopters and gyroplanes. Not all fixed-wing aircraft have tailplanes. Canards, tailless and flying wing aircraft have no separate tailplane, while in V-tail aircraft the vertical stabiliser, rudder, and the tail-plane and elevator are combined to form two diagonal surfaces in a V layout.
An aileron is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.
In flight dynamics a spin is a special category of stall resulting in autorotation about the aircraft's longitudinal axis and a shallow, rotating, downward path approximately centred on a vertical axis. Spins can be entered intentionally or unintentionally, from any flight attitude if the aircraft has sufficient yaw while at the stall point. In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different.
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.
In aeronautics, dihedral is the angle between the left and right wings of an aircraft. "Dihedral" is also used to describe the effect of sideslip on the rolling of the aircraft.
The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. The X-29 was developed by Grumman, and the two built were flown by NASA and the United States Air Force. The aerodynamic instability of the X-29's airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, and to reduce weight. The aircraft first flew in 1984, and two X-29s were flight tested through 1991.
A conventional fixed-wing aircraft flight control system (AFCS) consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered flight controls as they change speed.
Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's pitch, and therefore the angle of attack and the lift of the wing. The elevators are usually hinged to the tailplane or horizontal stabilizer. They may be the only pitch control surface present, and are sometimes located at the front of the aircraft or integrated into a rear "all-moving tailplane", also called a slab elevator or stabilator.
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The empennage, also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow. The term derives from the French language verb empenner which means "to feather an arrow". Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise the flight dynamics of yaw and pitch, as well as housing control surfaces.
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An aircraft stabilizer is an aerodynamic surface, typically including one or more movable control surfaces, that provides longitudinal (pitch) and/or directional (yaw) stability and control. A stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are hinged, or it can itself be a fully movable surface such as a stabilator. Depending on the context, "stabilizer" may sometimes describe only the front part of the overall surface.
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In flight dynamics, longitudinal stability is the stability of an aircraft in the longitudinal, or pitching, plane. This characteristic is important in determining whether an aircraft pilot will be able to control the aircraft in the pitching plane without requiring excessive attention or excessive strength.
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