Relaxed stability
Encyclopedia
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.
, 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.
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 — they could only control elevator and rudder. The effect not predicted was that without roll, it was very hard to turn the aircraft. 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. 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.
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.
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."
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.
Simple harmonic motion
Simple harmonic motion can serve as a mathematical model of a variety of motions, such as the oscillation of a spring. Additionally, other phenomena can be approximated by simple harmonic motion, including the motion of a simple pendulum and molecular vibration....
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
Cessna 152
The Cessna 152 is an American two-seat, fixed tricycle gear, general aviation airplane, used primarily for flight training and personal use.-Development:...
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.
Early aircraft
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 rudderRudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
, 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.
Some of these early investigators, including Langley
Langley
Langley is often used as a metonym in U.S. government and military parlance to refer to the Central Intelligence Agency, whose headquarters is located in Langley, Virginia.Langley may also refer to:-Persons:...
, Chanute
Chanute
The name Chanute may refer to:*Chanute, Kansas, United States*Octave Chanute, railroad engineer and aviation pioneer*Chanute Air Force Base, Illinois, United States*Octave Chanute Award, awarded by the Western Society of Engineers since 1901...
, 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
Lateral
Lateral may refer to:*Lateral , an anatomical direction*Lateral canal, a canal built beside another stream*Lateral consonant, an ℓ-like consonant in which air flows along the sides of the tongue...
) 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 — they could only control elevator and rudder. The effect not predicted was that without roll, it was very hard to turn the aircraft. They were also very susceptible to the side winds.
The Wright brothers
Wright brothers
The Wright brothers, Orville and Wilbur , were two Americans credited with inventing and building the world's first successful airplane and making the first controlled, powered and sustained heavier-than-air human flight, on December 17, 1903...
designed their 1903 first powered Flyer with anhedral
Anhedral
* Anhedral angle, the downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft* Anhedral , a rock texture without crystal faces or cross-section shape in thin section...
(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
Banked turn
A banked turn is a turn or change of direction in which the vehicle banks or inclines, usually towards the inside of the turn. The bank angle is the angle at which the vehicle is inclined about its longitudinal axis with respect to its path....
" 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. 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.
Methods of decreasing stability in design
BAE Systems
BAE Systems plc is a British multinational defence, security and aerospace company headquartered in London, United Kingdom, that has global interests, particularly in North America through its subsidiary BAE Systems Inc. BAE is among the world's largest military contractors; in 2009 it was the...
Harrier GR7/GR9 employs a significant and obvious anhedral
Anhedral
* Anhedral angle, the downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft* Anhedral , a rock texture without crystal faces or cross-section shape in thin section...
angle to its wings, this reduces the inherent lateral
Lateral
Lateral may refer to:*Lateral , an anatomical direction*Lateral canal, a canal built beside another stream*Lateral consonant, an ℓ-like consonant in which air flows along the sides of the tongue...
stability of the wings mounted high on the fuselage.
Unstable aircraft
Modern military aircraft and in particular low observable designs often exhibit instability as a result of their design. The LockheedLockheed Martin
Lockheed Martin is an American global aerospace, defense, security, and advanced technology company with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area....
F-117 Nighthawk for instance employs a highly non-traditional fuselage and wing shape in order to reduce its radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
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
Fly-by-wire
Fly-by-wire 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...
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.
Though it is not just fighter jets that have relaxed stability designs. The McDonnell Douglas MD-11
McDonnell Douglas MD-11
The McDonnell Douglas MD-11 is a three-engine medium- to long-range widebody jet airliner, manufactured by McDonnell Douglas and, later, by Boeing Commercial Airplanes. Based on the DC-10, it features a stretched fuselage, increased wingspan with winglets, refined airfoils on the wing and smaller...
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."
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 152Cessna 152The Cessna 152 is an American two-seat, fixed tricycle gear, general aviation airplane, used primarily for flight training and personal use.-Development:...
is an example. - An aircraft with a "low" wing (i.e., underneath the fuselage) has less roll stability. A Piper PawneePiper PA-25 PawneeThe PA-25 Pawnee was an agricultural aircraft produced by Piper Aircraft between 1959 and 1982. 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. On April 15, 1998, The New Piper Aircraft, Inc....
is an example of a "low" wing.
This is commonly explained through the analogy of a pendulum-style effect, but this explanation is incorrect (see Pendulum rocket fallacy
Pendulum Rocket Fallacy
Pendulum rocket fallacy is a common fundamental misunderstanding of the mechanics of rocket flight and how rockets remain on a stable trajectory. The first liquid-fuel rocket, constructed by Robert Goddard in 1926, differed significantly from modern rockets in that the rocket engine was at the top...
). 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.