Phugoid
Encyclopedia
A phugoid or fugoid is an aircraft motion where the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "uphill" and "downhill." This is one of the basic flight dynamics
modes of an aircraft
(others include short period, dutch roll
, and spiral divergence), and a classic example of a positive feedback
system.
but varying pitch, caused by a repeated exchange of airspeed
and altitude
. It can be excited by an elevator
singlet (a short, sharp deflection followed by a return to the centered position) resulting in a pitch increase with no change in trim from the cruise
condition. As speed decays, the nose will drop below the horizon. Speed will increase, and the nose will climb above the horizon. Periods can vary from under 30 seconds for light aircraft
to minutes for larger aircraft
. Microlight aircraft typically show a phugoid period of 15–25 seconds, and it has been suggested that birds and model airplanes show convergence between the phugoid and short period modes. A classical model for the phugoid period can be simplified to about (0.85 × speed in knots) seconds, but this only really works for larger aircraft.
Phugoids are often demonstrated to student pilot
s as an example of the speed stability of the aircraft and the importance of proper trimming. When it occurs, it is considered a nuisance, and in lighter airplanes (typically showing a shorter period) it can be a cause of pilot-induced oscillation
.
The phugoid, for moderate amplitude, occurs at an effectively constant angle of attack, although in practice the angle of attack actually varies by a few tenths of a degree. This means that the stalling angle of attack is never exceeded, and it is possible (in the <1g section of the cycle) to fly at speeds below the known stalling speed. Free flight models with badly unstable phugoid typically stall or loop, depending on thrust.
An unstable or divergent phugoid is caused, mainly, by a large difference between the incidence angles of the wing and tail. A stable, decreasing phugoid can be attained by building a smaller stabilizer on a longer tail, or, at the expense of pitch and yaw "static" stability, by shifting the center of gravity to the rear.
The term "phugoid" was coined by Frederick W. Lanchester, the British aerodynamicist who first characterized the phenomenon. He derived the word from the Greek words and to mean "flight-like" but recognized the diminished appropriateness of the derivation given that meant flight in the sense of "escape" rather than vehicle flight. The use of a canard
in aircraft design exploits this phenomenon and ensures that the main wing is prevented from stalling.
In 1985, Japan Airlines Flight 123
lost all hydraulic controls and its vertical stabiliser, and went into a phugoid before crashing into a mountain. With 520 deaths it remains the deadliest single-aircraft disaster in history.
In 1989, United Airlines Flight 232
suffered an engine
failure which caused total hydraulic system failure. The crew flew the aircraft with throttle
only. Suppressing the phugoid tendency was particularly difficult. The pilots were able to reach Sioux Gateway Airport
but crashed during the landing attempt. The pilots and a majority of the passengers survived.
Another aircraft that lost all hydraulics was a DHL
operated Airbus A300B4 that was hit by a surface-to-air missile
fired by the Iraqi resistance in the 2003 Baghdad DHL attempted shootdown incident. This was the first time that a crew was able to land an air transport aircraft safely only adjusting engine thrust.
The 2003 crash of the Helios solar powered aircraft
was precipitated by reacting to an inappropriately diagnosed phugoid oscillation the results of which ultimately resulted in the aircraft structure exceeding design loads.
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
modes of an aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
(others include short period, dutch roll
Dutch roll
Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic modes...
, and spiral divergence), and a classic example of a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
system.
Detailed description
The phugoid has a nearly constant angle of attackAngle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
but varying pitch, caused by a repeated exchange of airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
and altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
. It can be excited by an elevator
Elevator (aircraft)
Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...
singlet (a short, sharp deflection followed by a return to the centered position) resulting in a pitch increase with no change in trim from the cruise
Cruise (flight)
Cruise is the level portion of aircraft travel where flight is most fuel efficient. It occurs between ascent and descent phases and is usually the majority of a journey. Technically, cruising consists of heading changes only at a constant airspeed and altitude...
condition. As speed decays, the nose will drop below the horizon. Speed will increase, and the nose will climb above the horizon. Periods can vary from under 30 seconds for light aircraft
Light-sport Aircraft
A Light-sport aircraft, also known as light sport aircraft or LSA, is a small aircraft that is simple to fly and which meets certain regulations set by a National aviation authority restricting weight and performance...
to minutes for larger aircraft
Airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...
. Microlight aircraft typically show a phugoid period of 15–25 seconds, and it has been suggested that birds and model airplanes show convergence between the phugoid and short period modes. A classical model for the phugoid period can be simplified to about (0.85 × speed in knots) seconds, but this only really works for larger aircraft.
Phugoids are often demonstrated to student pilot
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
s as an example of the speed stability of the aircraft and the importance of proper trimming. When it occurs, it is considered a nuisance, and in lighter airplanes (typically showing a shorter period) it can be a cause of pilot-induced oscillation
Pilot-induced oscillation
Pilot-induced oscillations, as defined by MIL-HDBK-1797A, are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft and occurs when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions, each...
.
The phugoid, for moderate amplitude, occurs at an effectively constant angle of attack, although in practice the angle of attack actually varies by a few tenths of a degree. This means that the stalling angle of attack is never exceeded, and it is possible (in the <1g section of the cycle) to fly at speeds below the known stalling speed. Free flight models with badly unstable phugoid typically stall or loop, depending on thrust.
An unstable or divergent phugoid is caused, mainly, by a large difference between the incidence angles of the wing and tail. A stable, decreasing phugoid can be attained by building a smaller stabilizer on a longer tail, or, at the expense of pitch and yaw "static" stability, by shifting the center of gravity to the rear.
The term "phugoid" was coined by Frederick W. Lanchester, the British aerodynamicist who first characterized the phenomenon. He derived the word from the Greek words and to mean "flight-like" but recognized the diminished appropriateness of the derivation given that meant flight in the sense of "escape" rather than vehicle flight. The use of a canard
Canard (aeronautics)
In aeronautics, canard is an airframe configuration of fixed-wing aircraft in which the forward surface is smaller than the rearward, the former being known as the "canard", while the latter is the main wing...
in aircraft design exploits this phenomenon and ensures that the main wing is prevented from stalling.
Aviation incidents
In the 1975 Tan Son Nhut C-5 accident, USAF C-5 68-021 with flight controls damaged by loss of the rear cargo door, encountered phugoid oscillations while the crew was attempting a return to base, and crash-landed in a rice paddy. Of the 328 people on board, 153 died, making it the deadliest accident involving a US military aircraft.In 1985, Japan Airlines Flight 123
Japan Airlines Flight 123
Japan Airlines Flight 123 was a Japan Airlines domestic flight from Tokyo International Airport to Osaka International Airport on August 12, 1985. The Boeing 747-146SR that made this route, registered , suffered mechanical failures 12 minutes into the flight and 32 minutes later crashed into two...
lost all hydraulic controls and its vertical stabiliser, and went into a phugoid before crashing into a mountain. With 520 deaths it remains the deadliest single-aircraft disaster in history.
In 1989, United Airlines Flight 232
United Airlines Flight 232
United Airlines Flight 232 was a scheduled flight from Stapleton International Airport in Denver, Colorado, to O'Hare International Airport in Chicago, with continuing service to Philadelphia International Airport...
suffered an engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
failure which caused total hydraulic system failure. The crew flew the aircraft with throttle
Aircraft engine controls
Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the...
only. Suppressing the phugoid tendency was particularly difficult. The pilots were able to reach Sioux Gateway Airport
Sioux Gateway Airport
-History:The construction of Sioux City Army Air Base began in March 1942, about three months after the Japanese attack on Pearl Harbor. Opened on 5 July 1942, it became a major training center during World War II for crew members of B-24 Liberators and B-17 Flying Fortresses...
but crashed during the landing attempt. The pilots and a majority of the passengers survived.
Another aircraft that lost all hydraulics was a DHL
DHL
DHL Express is a division of the German logistics company Deutsche Post providing international express mail services. DHL is a world market leader in sea and air mail....
operated Airbus A300B4 that was hit by a surface-to-air missile
Surface-to-air missile
A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...
fired by the Iraqi resistance in the 2003 Baghdad DHL attempted shootdown incident. This was the first time that a crew was able to land an air transport aircraft safely only adjusting engine thrust.
The 2003 crash of the Helios solar powered aircraft
Helios Prototype
The NASA Pathfinder, NASA Pathfinder Plus, NASA Centurion and NASA Helios Prototype were an evolutionary series of solar- and fuel-cell-system-powered unmanned aerial vehicles. AeroVironment, Inc. developed the vehicles under NASA's Environmental Research Aircraft and Sensor Technology program...
was precipitated by reacting to an inappropriately diagnosed phugoid oscillation the results of which ultimately resulted in the aircraft structure exceeding design loads.