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Spin (flight)
Encyclopedia
In aviation, a spin is an aggravated stall
resulting in autorotation
about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw rate while an aircraft is stalled. In either case, however, a specific and often counterintuitive set of actions may be needed for an effective recovery to be made. If the aircraft exceeds published limitations regarding spins, or is loaded improperly, or if the pilot uses incorrect technique to recover, the spin can lead to a crash.
In a spin, one or both wings are in a stalled condition, if both are stalled one wing will be in a deeper stall condition than the other. This causes the aircraft to autorotate (yaw) towards the deeper-stalled wing due to its higher drag. Spins are also characterized by high angle of attack
, low airspeed
, and high rate of descent.
Spins differ from spiral dives which are characterized by low angle of attack and high airspeed. A spiral dive is not a type of spin because neither wing is stalled. In a spiral dive the airplane will respond conventionally to the pilot's inputs to the flight controls.
Certificated, light, single-engine airplanes must meet specific criteria regarding stall and spin behavior. Many types of airplane will only spin if the pilot simultaneously yaws and stalls the airplane (intentionally or unintentionally). Under these circumstances, one wing tends to stall more deeply than the other. The wing that stalls first will drop, increasing its angle of attack
and deepening the stall. Both wings must be stalled for a spin to occur. The other wing will rise, decreasing its angle of attack
, and the aircraft will yaw towards the more deeply-stalled wing. The difference in lift
between the two wings causes the aircraft to roll, and the difference in drag
causes the aircraft to yaw.
One common scenario that can lead to an unintentional spin is an uncoordinated turn
towards the runway during the landing sequence. A pilot who is overshooting the turn to final approach may be tempted to apply rudder to increase the rate of turn. The result is twofold: the nose of the airplane drops below the horizon and the bank angle increases. Reacting to these unintended changes, the pilot may then begin to pull the elevator control aft (thus increasing the angle of attack) while applying opposite aileron to decrease bank angle. Taken to its extreme, this can result in an uncoordinated turn with sufficient angle of attack
to cause the aircraft to stall. This is called a cross-control stall, and is very dangerous if it happens at low altitude where the pilot has little time to recover. In order to avoid this scenario, pilots are taught the importance of always making coordinated turns.
Spins can also be entered intentionally for training, flight testing, or aerobatics.
During the 1970s NASA
used its spin tunnel at the Langley Research Center to investigate the spinning characteristics of single-engine general aviation airplane designs. A 1/11-scale model was used with nine different tail designs.
Some tail designs that caused inappropriate spin characteristics had two stable spin modes – one steep or moderately steep; and another that was either moderately flat or flat. Recovery from the flatter of the two modes was usually less reliable or impossible. The further aft that the center of gravity was located the flatter the spin and the less reliable the recovery. For all tests the center of gravity of the model was at either 14.5% of Mean Aerodynamic Chord
(MAC) or 25.5% of MAC.
Single-engine airplane types certified in the normal category must be demonstrated to recover from a spin of at least one turn, while single-engine aircraft certified in the utility category must demonstrate a six turn spin that cannot be unrecoverable at any time during the spin due to pilot action or aerodynamic characteristic. NASA recommends various tail configurations and other strategies to eliminate the flatter of the two spin modes and make recovery from the steeper mode more reliable.
The spin was initially explored by individual pilots performing ad-hoc experiments (often accidentally) and by aerodynamicists. Lincoln Beachey
was able to exit spins at will according to Harry Bruno in Wings over America (1944). In August 1912, Lieutenant Wilfred Parke
RN
became the first aviator to recover from an accidental spin when his Avro biplane
entered a spin at 700 feet AGL in the traffic pattern at Larkhill
. Parke attempted to recover from the spin by increasing engine speed, pulling back on the stick, and turning into the spin, with no effect. The aircraft descended 450 feet, and horrified observers braced themselves for a fatal crash.
Parke was disabled by centrifugal forces but was still considering a means of escape. In an effort to neutralize the forces pinning him against the right side of the cockpit, he applied full right rudder, and the aircraft leveled out fifty feet above the ground. With the aircraft now under control, Parke climbed, made another approach, and landed safely.
In spite of the discovery of "Parke's technique," pilots were not taught spin-recovery procedures until the beginning of World War I.
The first documented case of an intentional spin and recovery is that of Harry Hawker
. In the summer of 1914, Hawker recovered from an intentional spin over Brooklands, England, by centralizing the controls.
In 1917, the English physicist Frederick Lindemann
conducted a series of experiments that led to the first understanding of the aerodynamics of the spin.
Spin-entry procedures vary with the type and model of aircraft being flown but there are general procedures applicable to most aircraft. These include reducing power to idle and simultaneously raising the nose in order to induce an upright stall. Then, as the aircraft approaches stall, apply full rudder in the desired spin direction while holding full back-elevator pressure for an upright spin. Sometimes a roll input is applied in the direction opposite of the rudder (i.e., a cross-control).
If the aircraft manufacturer provides a specific procedure for spin recovery, that procedure must be used. Otherwise, to recover from an upright spin, the following generic procedure may be used: Power is first reduced to idle and the ailerons are neutralized. Then, full opposite rudder (that is, against the yaw) is added and held to counteract the spin rotation, and the elevator control is moved briskly forward to reduce the angle of attack
below the critical angle. Depending on the airplane and the type of spin, the elevator action could be a minimal input before rotation ceases, or in other cases the elevator control may have to be moved to its full forward position to effect recovery from the upright spin. Once the rotation has stopped, the rudder must be neutralized and the airplane returned to level flight. This procedure is sometimes called PARE
, for Power idle, Ailerons neutral, Rudder opposite the spin and held, and Elevator through neutral. The mnemonic "PARE" simply reinforces the tried-and-true NASA standard spin recovery actions—the very same actions first prescribed by NACA in 1936, verified by NASA during an intensive, decade-long spin test program overlapping the 1970s and '80's, and repeatedly recommended by the FAA and implemented by the majority of test pilots during certification spin-testing of light airplanes.
Inverted spinning and erect or upright spinning are dynamically very similar and require essentially the same recovery process but use opposite elevator control. It must be noted that in an upright spin both roll and yaw are in the same direction but that an inverted spin is composed of opposing roll and yaw. It is crucial that the yaw be countered to effect recovery. The visual field in a typical spin (as opposed to a flat spin) is heavily dominated by the perception of roll over yaw, which can lead to an incorrect and dangerous conclusion that a given inverted spin is actually an erect spin in the reverse yaw direction (leading to a recovery attempt in which pro-spin rudder is mistakenly applied and then further exacerbated by holding the incorrect elevator input).
In some aircraft that spin readily upright and inverted—such as Pitts- and Christen Eagle-type high-performance aerobatic aircraft—an alternative spin-recovery technique may effect recovery as well, namely: Power off, Hands off the stick/yoke, Rudder full opposite to the spin (or more simply "push the rudder pedal that is hardest to push") and held (aka the Mueller/Beggs technique). An advantage of the Mueller/Beggs technique is that no knowledge of whether the spin is erect or inverted is required during what can be a very stressful and disorienting time. Even though this method does work in a specific subset of spin-approved airplanes, the NASA Standard/PARE procedure can also be effective provided that care must be taken to ensure the spin does not simply cross from positive to negative (or vice versa) and that a too-rapid application of elevator control is avoided as it may cause aerodynamic blanketing of the rudder rendering the control ineffective and simply accelerate the spin. The converse, however, may not be true at all—many cases exist where Beggs/Mueller fails to recover the airplane from the spin, but NASA Standard/PARE will terminate the spin. Before spinning any aircraft the flight manual should be consulted to establish if the particular type has any specific spin recovery techniques that differ from standard practice.
Although entry techniques are similar, modern military fighter aircraft often tend to require yet another variation on spin recovery techniques. While power is still typically reduced to idle thrust and pitch control neutralized, opposite rudder is almost never used. Adverse yaw created by the rolling surfaces (ailerons, differential horizontal tails, etc.) of such aircraft is often more effective in arresting the spin rotation than the rudder(s), which usually become blanked by the wing and fuselage due to the geometric arrangement of fighters. Hence, the preferred recover technique has a pilot applying full roll control in the direction of the rotation (i.e., a right-hand spin requires a right stick input), generally remembered as "stick into the spin." Likewise, this control application is reversed for inverted spins.
If the center of gravity of the airplane is behind the aft limit approved for spinning, any spin may prove to be unrecoverable except by using some special spin-recovery device such as a spin-recovery parachute
specially installed in the tail of the airplane which was offered for aircraft starting in the mid-1930s; or by jettisoning specially installed ballast at the tail of the airplane.
In the past, some airplanes displayed an unrecoverable spin in which the nose was higher, relative to the horizon, than in conventional spins. This is sometimes called a flat spin, although whether a flat spin is indeed unrecoverable depends on aircraft type and loading. The plane spins on its belly around the normal axis. The empennage
will feel very light and loose. Depending on the aircraft, changing the rudder and aileron inputs or engine power settings may have little effect. There is a small number of accounts where pilots recovered from flat spins by loosening their restraint harnesses and leaning forward in an attempt to alter the position of the center of gravity.
Some World War II
airplanes were notoriously prone to flat spins when loaded erroneously, such as the Bell P-39 Airacobra. The P-39 was a unique design with the engine behind the pilot's seat and a large cannon in the front. Without ammunition or a counterbalance load in the nose compartment, the P-39's center of gravity was too far aft to recover from a spin. Soviet pilots did numerous tests of the P-39 and were able to demonstrate its dangerous spinning characteristics. Bell then issued a recommendation to bail out if the airplane entered a spin. North American P-51 Mustangs
with auxiliary fuel tanks not originally designed for the P-51 suffered from the same problem. Similarly, the Vought
F4U Corsair
was reputed to have appalling stall and spin recovery characteristics, even in the "clean" (no stores) configuration.
Modern fighter aircraft are not immune to the phenomena of unrecoverable spin characteristics. Although highly resistant to entering into a spin, once caught in one the Grumman F-14 Tomcat
can exhibit a fast, flat spin from which it is nearly impossible to recover. Another example of a nonrecoverable flat spin occurred in 1963, with Chuck Yeager
at the controls of the NF-104A rocket-jet hybrid: during his fourth attempt at setting an altitude record, Yeager lost control and entered a flat spin, then ejected and survived.
An airplane spin tends to flatten as it progresses because then its mass is distributed furthest from its center of rotation, as rotating objects tend to rotate about their axis of maximum rotational inertia. Aircraft have their maximum rotational inertia when spinning on their normal axis, i.e. flatly.
In purpose-built aerobatic aircraft, spins may be intentionally flattened through the application of power and aileron within a normal spin. Rotation rates experienced are dramatic and can exceed 400 degrees per second in an attitude that may even have the nose above the horizon. Such maneuvers must be performed with the center of gravity in the normal range and with appropriate training, and consideration should be given to the extreme gyroscopic forces generated by the propellor and exerted on the crankshaft.
, including certificated glider
s, must meet specified criteria regarding stall and spin behavior. Complying designs typically have a wing with greater angle of attack
at the wing root than at the wing tip, so that the wing root stalls first, reducing the severity of the wing drop at the stall and possibly also allowing the aileron
s to remain somewhat effective until the stall migrates outward toward the wing tip. One method of tailoring such stall behavior is known as washout
. Some designers of recreational aircraft seek to develop an aircraft that is characteristically incapable of spinning, even in an uncoordinated stall
.
Some airplanes have been designed with fixed leading edge slot
s. Where the slots are located ahead of the ailerons they provide strong resistance to stalling and may even leave the airplane incapable of spinning.
The flight control systems
of some gliders and recreational aircraft are designed so that when the pilot moves the elevator control close to its fully aft position, as in slow speed flight and flight at high angle of attack
, the trailing edges of both ailerons are automatically raised slightly so that the angle of attack
is reduced at the outboard regions of both wings. This necessitates an increase in angle of attack at the inboard (center) regions of the wing, and promotes stalling of the inboard regions well before the wing tips.
A US certification standard for civil airplanes up to 12,500 lb maximum takeoff weight is Part 23 of the Federal Aviation Regulations
, applicable to airplanes in the normal, utility and acrobatic categories. Part 23, §23.221 requires that single-engine airplanes must demonstrate recovery from either a one-turn spin if intentional spins will be prohibited or six-turn spins if intentional spins will be approved. Even large, passenger-carrying single-engine airplanes like the Cessna Caravan
must be subjected to one-turn spins by a test pilot and repeatedly demonstrated to recover within no more than one additional turn. With a small number of airplane types the FAA
has made a finding of equivalent level of safety (ELOS) so that demonstration of a one-turn spin is not necessary. For example, this has been done with the Cessna Corvalis and the Cirrus SR20/22
. Successful demonstration of the one-turn spin does not allow an airplane type to be approved for intentional spinning. If an airplane is to be approved for intentional spinning it must be repeatedly subjected to a spin of six turns and then demonstrated to recover within one and a half additional turns. Spin testing is a potentially hazardous exercise and the test aircraft must be equipped with some spin-recovery device such as a tail parachute or jettisonable ballast, or some method of rapidly moving the center of gravity forward.
Agricultural airplanes are typically certificated in the normal category at a moderate weight. For single-engine airplanes this requires successful demonstration of the one-turn spin. However, with the agriculture hopper full these airplanes are not intended to be spun and recovery is unlikely. For this reason, at weights above the maximum for the normal category, these airplanes are not subjected to spin testing and, as a consequence, can only be type certificated in the restricted category. As an example of an agricultural airplane see the Cessna AG series.
is available from the manufacturer.
Many training aircraft may appear to be resistant to entering a spin even though some are intentionally designed and certified for spins. A well-known example of an aircraft designed to spin readily is the Piper Tomahawk
, which is certified for spins, though the Piper Tomahawk's spin characteristics remain controversial. Aircraft that are not certified for spins may be difficult or impossible to recover once the spin exceeds the one-turn certification standard.
Although it has been removed from most flight test syllabuses, there are some countries that still require flight training
on spin recovery. In the U.S. spin training is required only for civilian flight instructor candidates and military pilots. A spin occurs only after a stall
, so the FAA emphasizes training pilots in stall recognition, prevention, and recovery as a means to reduce accidents due to unintentional stalls and/or spins.
A spin is often intimidating to the uninitiated, however many pilots trained in spin entry and recovery find that safely spinning is an interesting experience. In a spin, the occupants of the airplane will only feel reduced gravity
during the entry phase and then will experience normal gravity, except that the extreme nose-down attitude will press the occupants forward against their restraint harnesses. The rapid rotation, combined with the nose-down attitude, can also be disorienting.
The recovery procedure from a spin requires using rudder to stop the rotation, then elevator to reduce angle of attack to stop the stall, then pulling out of the dive without exceeding the maximum permitted airspeed (VNE) or maximum G loading. The maximum G loading for a light airplane in the normal category is usually 3.8 G. For a light airplane in the acrobatic category it is usually at least 6 G.
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
resulting in autorotation
Autorotation
In aviation, autorotation refers to processes in both fixed-wing and rotary-wing aircraft. The term means significantly different things in each context....
about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw rate while an aircraft is stalled. In either case, however, a specific and often counterintuitive set of actions may be needed for an effective recovery to be made. If the aircraft exceeds published limitations regarding spins, or is loaded improperly, or if the pilot uses incorrect technique to recover, the spin can lead to a crash.
In a spin, one or both wings are in a stalled condition, if both are stalled one wing will be in a deeper stall condition than the other. This causes the aircraft to autorotate (yaw) towards the deeper-stalled wing due to its higher drag. Spins are also characterized by high angle of attack
Angle 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...
, low 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 high rate of descent.
Spins differ from spiral dives which are characterized by low angle of attack and high airspeed. A spiral dive is not a type of spin because neither wing is stalled. In a spiral dive the airplane will respond conventionally to the pilot's inputs to the flight controls.
How a spin occurs
Angle 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...
and deepening the stall. Both wings must be stalled for a spin to occur. The other wing will rise, decreasing its angle of attack
Angle 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...
, and the aircraft will yaw towards the more deeply-stalled wing. The difference in lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
between the two wings causes the aircraft to roll, and the difference in drag
Parasitic drag
Parasitic drag is drag caused by moving a solid object through a fluid medium . Parasitic drag is made up of many components, the most prominent being form drag...
causes the aircraft to yaw.
One common scenario that can lead to an unintentional spin is an uncoordinated turn
Coordinated flight
In aviation, coordinated flight of an aircraft is flight without sideslip.When an aircraft is flying with zero sideslip a turn and bank indicator installed on the aircraft’s instrument panel usually shows the ball in the center of the spirit level...
towards the runway during the landing sequence. A pilot who is overshooting the turn to final approach may be tempted to apply rudder to increase the rate of turn. The result is twofold: the nose of the airplane drops below the horizon and the bank angle increases. Reacting to these unintended changes, the pilot may then begin to pull the elevator control aft (thus increasing the angle of attack) while applying opposite aileron to decrease bank angle. Taken to its extreme, this can result in an uncoordinated turn with sufficient angle of attack
Angle 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...
to cause the aircraft to stall. This is called a cross-control stall, and is very dangerous if it happens at low altitude where the pilot has little time to recover. In order to avoid this scenario, pilots are taught the importance of always making coordinated turns.
Spins can also be entered intentionally for training, flight testing, or aerobatics.
Phases
In aircraft that are capable of recovering from a spin, the spin has four phases. For all or some types of spin some airplanes are not recoverable. At low height recovery may also be impossible. In both cases, only the first three phases occur.- Entry – The pilot stalls the plane while in uncoordinated flight.
- Incipient – With one wing more stalled than the other, the rotation starts.
- Developed – The aircraft's rotation rate, airspeed, and vertical speed are stabilized. At least one wing of the aircraft is stalled.
- Recovery – After appropriate control inputs, the angle of attackAngle of attackAngle 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...
of both wings decreases below the critical angle of attack, rotation slows. The nose attitude of the aircraft steepens, airspeed increases, autorotation stops, the aircraft is no longer stalled. The controls respond conventionally and the airplane can be returned to normal flight.
Modes
The US National Aeronautics and Space Administration (NASA) has defined four different modes of spinning. These four modes are defined by the angle of attack of the airflow on the wing.| Spin mode | Angle-of-attack range, degrees |
|---|---|
| Flat | 65 to 90 |
| Moderately flat | 45 to 65 |
| Moderately steep | 30 to 45 |
| Steep | 20 to 30 |
During the 1970s NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
used its spin tunnel at the Langley Research Center to investigate the spinning characteristics of single-engine general aviation airplane designs. A 1/11-scale model was used with nine different tail designs.
Some tail designs that caused inappropriate spin characteristics had two stable spin modes – one steep or moderately steep; and another that was either moderately flat or flat. Recovery from the flatter of the two modes was usually less reliable or impossible. The further aft that the center of gravity was located the flatter the spin and the less reliable the recovery. For all tests the center of gravity of the model was at either 14.5% of Mean Aerodynamic Chord
Chord (aircraft)
In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...
(MAC) or 25.5% of MAC.
Single-engine airplane types certified in the normal category must be demonstrated to recover from a spin of at least one turn, while single-engine aircraft certified in the utility category must demonstrate a six turn spin that cannot be unrecoverable at any time during the spin due to pilot action or aerodynamic characteristic. NASA recommends various tail configurations and other strategies to eliminate the flatter of the two spin modes and make recovery from the steeper mode more reliable.
History
In aviation's early days, spins were poorly understood and often fatal. Proper recovery procedures were unknown, and a pilot's instinct to pull back on the stick served only to make a spin worse. Because of this, the spin earned a reputation as an unpredictable danger that might snatch an aviator's life at any time, and against which there was no defense.The spin was initially explored by individual pilots performing ad-hoc experiments (often accidentally) and by aerodynamicists. Lincoln Beachey
Lincoln Beachey
Lincoln J. Beachey was a pioneer American aviator and barnstormer. He became famous and wealthy from flying exhibitions, staging aerial stunts, helping invent aerobatics, and setting aviation records....
was able to exit spins at will according to Harry Bruno in Wings over America (1944). In August 1912, Lieutenant Wilfred Parke
Wilfred Parke
Lieutenant Wilfred Parke RN was a British airman and became the first aviator to recover from an accidental spin.-Family:Parkes's father was Alfred Watlington Parke, the Rector of Uplyme and his mother was Hilda Fort...
RN
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
became the first aviator to recover from an accidental spin when his Avro biplane
Avro Type G
-References:*...
entered a spin at 700 feet AGL in the traffic pattern at Larkhill
Larkhill
Larkhill is a garrison town in the civil parish of Durrington, Wiltshire, England. It is a short distance west of Durrington village proper and north of the prehistoric monument of Stonehenge. It is about north of Salisbury....
. Parke attempted to recover from the spin by increasing engine speed, pulling back on the stick, and turning into the spin, with no effect. The aircraft descended 450 feet, and horrified observers braced themselves for a fatal crash.
Parke was disabled by centrifugal forces but was still considering a means of escape. In an effort to neutralize the forces pinning him against the right side of the cockpit, he applied full right rudder, and the aircraft leveled out fifty feet above the ground. With the aircraft now under control, Parke climbed, made another approach, and landed safely.
In spite of the discovery of "Parke's technique," pilots were not taught spin-recovery procedures until the beginning of World War I.
The first documented case of an intentional spin and recovery is that of Harry Hawker
Harry Hawker
Harry George Hawker MBE, AFC, was an Australian aviation pioneer and co-founder of Hawker Aircraft, the firm that would later be responsible for a long series of successful military aircraft.-Early life:...
. In the summer of 1914, Hawker recovered from an intentional spin over Brooklands, England, by centralizing the controls.
In 1917, the English physicist Frederick Lindemann
Frederick Lindemann, 1st Viscount Cherwell
Frederick Alexander Lindemann, 1st Viscount Cherwell FRS PC CH was an English physicist who was an influential scientific adviser to the British government, particularly Winston Churchill...
conducted a series of experiments that led to the first understanding of the aerodynamics of the spin.
Entry and recovery
Some aircraft cannot be recovered from a spin using only their own flight control surfaces and must not be allowed to enter a spin under any circumstances. If an aircraft has not been certified for spin recovery, it should be assumed that spins are not recoverable and are unsafe in that aircraft. Important safety equipment, such as stall/spin recovery parachutes, which generally are not installed on production aircraft, are used during testing and certification of aircraft for spins and spin recovery.Spin-entry procedures vary with the type and model of aircraft being flown but there are general procedures applicable to most aircraft. These include reducing power to idle and simultaneously raising the nose in order to induce an upright stall. Then, as the aircraft approaches stall, apply full rudder in the desired spin direction while holding full back-elevator pressure for an upright spin. Sometimes a roll input is applied in the direction opposite of the rudder (i.e., a cross-control).
If the aircraft manufacturer provides a specific procedure for spin recovery, that procedure must be used. Otherwise, to recover from an upright spin, the following generic procedure may be used: Power is first reduced to idle and the ailerons are neutralized. Then, full opposite rudder (that is, against the yaw) is added and held to counteract the spin rotation, and the elevator control is moved briskly forward to reduce the angle of attack
Angle 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...
below the critical angle. Depending on the airplane and the type of spin, the elevator action could be a minimal input before rotation ceases, or in other cases the elevator control may have to be moved to its full forward position to effect recovery from the upright spin. Once the rotation has stopped, the rudder must be neutralized and the airplane returned to level flight. This procedure is sometimes called PARE
PARE (aviation)
In aviation, PARE is a mnemonic for a generic spin recovery technique applicable to many types of fixed-wing aircraft.-Overview:PARE stands for:*Power: off*Ailerons: neutral *Rudder: full opposite to the spin and held...
, for Power idle, Ailerons neutral, Rudder opposite the spin and held, and Elevator through neutral. The mnemonic "PARE" simply reinforces the tried-and-true NASA standard spin recovery actions—the very same actions first prescribed by NACA in 1936, verified by NASA during an intensive, decade-long spin test program overlapping the 1970s and '80's, and repeatedly recommended by the FAA and implemented by the majority of test pilots during certification spin-testing of light airplanes.
Inverted spinning and erect or upright spinning are dynamically very similar and require essentially the same recovery process but use opposite elevator control. It must be noted that in an upright spin both roll and yaw are in the same direction but that an inverted spin is composed of opposing roll and yaw. It is crucial that the yaw be countered to effect recovery. The visual field in a typical spin (as opposed to a flat spin) is heavily dominated by the perception of roll over yaw, which can lead to an incorrect and dangerous conclusion that a given inverted spin is actually an erect spin in the reverse yaw direction (leading to a recovery attempt in which pro-spin rudder is mistakenly applied and then further exacerbated by holding the incorrect elevator input).
In some aircraft that spin readily upright and inverted—such as Pitts- and Christen Eagle-type high-performance aerobatic aircraft—an alternative spin-recovery technique may effect recovery as well, namely: Power off, Hands off the stick/yoke, Rudder full opposite to the spin (or more simply "push the rudder pedal that is hardest to push") and held (aka the Mueller/Beggs technique). An advantage of the Mueller/Beggs technique is that no knowledge of whether the spin is erect or inverted is required during what can be a very stressful and disorienting time. Even though this method does work in a specific subset of spin-approved airplanes, the NASA Standard/PARE procedure can also be effective provided that care must be taken to ensure the spin does not simply cross from positive to negative (or vice versa) and that a too-rapid application of elevator control is avoided as it may cause aerodynamic blanketing of the rudder rendering the control ineffective and simply accelerate the spin. The converse, however, may not be true at all—many cases exist where Beggs/Mueller fails to recover the airplane from the spin, but NASA Standard/PARE will terminate the spin. Before spinning any aircraft the flight manual should be consulted to establish if the particular type has any specific spin recovery techniques that differ from standard practice.
Although entry techniques are similar, modern military fighter aircraft often tend to require yet another variation on spin recovery techniques. While power is still typically reduced to idle thrust and pitch control neutralized, opposite rudder is almost never used. Adverse yaw created by the rolling surfaces (ailerons, differential horizontal tails, etc.) of such aircraft is often more effective in arresting the spin rotation than the rudder(s), which usually become blanked by the wing and fuselage due to the geometric arrangement of fighters. Hence, the preferred recover technique has a pilot applying full roll control in the direction of the rotation (i.e., a right-hand spin requires a right stick input), generally remembered as "stick into the spin." Likewise, this control application is reversed for inverted spins.
Center of gravity
The characteristics of an airplane with respect to spinning are significantly influenced by the position of the center of gravity. In general terms, the further forward the center of gravity the less readily the airplane will spin, and the more readily it will recover from a spin. Conversely, the further aft the center of gravity the more readily the airplane will spin, and the less readily it will recover from a spin. In any airplane the forward and aft limits on center of gravity are carefully defined. In some airplanes that are approved for intentional spinning the aft limit at which spins may be attempted is not as far aft as the aft limit for general flying. Intentional spinning should not be attempted casually, and the most important pre-flight precaution is to determine that the airplane's center of gravity will be within the range approved for intentional spinning.Unrecoverable spins
Parachute
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag, or in the case of ram-air parachutes, aerodynamic lift. Parachutes are usually made out of light, strong cloth, originally silk, now most commonly nylon...
specially installed in the tail of the airplane which was offered for aircraft starting in the mid-1930s; or by jettisoning specially installed ballast at the tail of the airplane.
In the past, some airplanes displayed an unrecoverable spin in which the nose was higher, relative to the horizon, than in conventional spins. This is sometimes called a flat spin, although whether a flat spin is indeed unrecoverable depends on aircraft type and loading. The plane spins on its belly around the normal axis. The empennage
Empennage
The empennage , also known as the tail or tail assembly, of most aircraft gives stability to the aircraft, in a similar way to the feathers on an arrow...
will feel very light and loose. Depending on the aircraft, changing the rudder and aileron inputs or engine power settings may have little effect. There is a small number of accounts where pilots recovered from flat spins by loosening their restraint harnesses and leaning forward in an attempt to alter the position of the center of gravity.
Some World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
airplanes were notoriously prone to flat spins when loaded erroneously, such as the Bell P-39 Airacobra. The P-39 was a unique design with the engine behind the pilot's seat and a large cannon in the front. Without ammunition or a counterbalance load in the nose compartment, the P-39's center of gravity was too far aft to recover from a spin. Soviet pilots did numerous tests of the P-39 and were able to demonstrate its dangerous spinning characteristics. Bell then issued a recommendation to bail out if the airplane entered a spin. North American P-51 Mustangs
P-51 Mustang
The North American Aviation P-51 Mustang was an American long-range, single-seat fighter and fighter-bomber used during World War II, the Korean War and in several other conflicts...
with auxiliary fuel tanks not originally designed for the P-51 suffered from the same problem. Similarly, the Vought
Vought
Vought is the name of several related aerospace firms. These have included, in the past, Lewis and Vought Corporation, Chance Vought, Vought Sikorsky, LTV Aerospace , Vought Aircraft Companies, and the current Vought Aircraft Industries. The first incarnation of Vought was established by Chance M...
F4U Corsair
F4U Corsair
The Vought F4U Corsair was a carrier-capable fighter aircraft that saw service primarily in World War II and the Korean War. Demand for the aircraft soon overwhelmed Vought's manufacturing capability, resulting in production by Goodyear and Brewster: Goodyear-built Corsairs were designated FG and...
was reputed to have appalling stall and spin recovery characteristics, even in the "clean" (no stores) configuration.
Modern fighter aircraft are not immune to the phenomena of unrecoverable spin characteristics. Although highly resistant to entering into a spin, once caught in one the Grumman F-14 Tomcat
F-14 Tomcat
The Grumman F-14 Tomcat is a supersonic, twin-engine, two-seat, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experimental program following the collapse of the F-111B project...
can exhibit a fast, flat spin from which it is nearly impossible to recover. Another example of a nonrecoverable flat spin occurred in 1963, with Chuck Yeager
Chuck Yeager
Charles Elwood "Chuck" Yeager is a retired major general in the United States Air Force and noted test pilot. He was the first pilot to travel faster than sound...
at the controls of the NF-104A rocket-jet hybrid: during his fourth attempt at setting an altitude record, Yeager lost control and entered a flat spin, then ejected and survived.
An airplane spin tends to flatten as it progresses because then its mass is distributed furthest from its center of rotation, as rotating objects tend to rotate about their axis of maximum rotational inertia. Aircraft have their maximum rotational inertia when spinning on their normal axis, i.e. flatly.
In purpose-built aerobatic aircraft, spins may be intentionally flattened through the application of power and aileron within a normal spin. Rotation rates experienced are dramatic and can exceed 400 degrees per second in an attitude that may even have the nose above the horizon. Such maneuvers must be performed with the center of gravity in the normal range and with appropriate training, and consideration should be given to the extreme gyroscopic forces generated by the propellor and exerted on the crankshaft.
Aircraft design
For safety, all certificated, single-engine fixed-wing aircraftFixed-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...
, including certificated glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
s, must meet specified criteria regarding stall and spin behavior. Complying designs typically have a wing with greater angle of attack
Angle 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...
at the wing root than at the wing tip, so that the wing root stalls first, reducing the severity of the wing drop at the stall and possibly also allowing the aileron
Aileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
s to remain somewhat effective until the stall migrates outward toward the wing tip. One method of tailoring such stall behavior is known as washout
Washout (aviation)
Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...
. Some designers of recreational aircraft seek to develop an aircraft that is characteristically incapable of spinning, even in an uncoordinated stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
.
Some airplanes have been designed with fixed leading edge slot
Leading edge slot
A leading edge slot is an aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading edge slot is a span-wise gap in each wing, allowing air to flow from below the wing to its upper surface...
s. Where the slots are located ahead of the ailerons they provide strong resistance to stalling and may even leave the airplane incapable of spinning.
The flight control systems
Aircraft flight control systems
A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight...
of some gliders and recreational aircraft are designed so that when the pilot moves the elevator control close to its fully aft position, as in slow speed flight and flight at high angle of attack
Angle 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...
, the trailing edges of both ailerons are automatically raised slightly so that the angle of attack
Angle 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...
is reduced at the outboard regions of both wings. This necessitates an increase in angle of attack at the inboard (center) regions of the wing, and promotes stalling of the inboard regions well before the wing tips.
A US certification standard for civil airplanes up to 12,500 lb maximum takeoff weight is Part 23 of the Federal Aviation Regulations
Federal Aviation Regulations
The Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal Regulations...
, applicable to airplanes in the normal, utility and acrobatic categories. Part 23, §23.221 requires that single-engine airplanes must demonstrate recovery from either a one-turn spin if intentional spins will be prohibited or six-turn spins if intentional spins will be approved. Even large, passenger-carrying single-engine airplanes like the Cessna Caravan
Cessna 208
The Cessna 208 Caravan is a single turboprop engine, fixed-gear short-haul regional airliner and utility aircraft built in the United States by Cessna. The airplane typically seats nine passengers, with a single person crew, although with a FAR Part 23 waiver, it can seat up to fourteen passengers...
must be subjected to one-turn spins by a test pilot and repeatedly demonstrated to recover within no more than one additional turn. With a small number of airplane types the FAA
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
has made a finding of equivalent level of safety (ELOS) so that demonstration of a one-turn spin is not necessary. For example, this has been done with the Cessna Corvalis and the Cirrus SR20/22
Cirrus SR22
The Cirrus SR22 is a single-engine, four-seat, composite aircraft, built by Cirrus Aircraft starting in 2001. It is a more powerful version of the Cirrus SR20, with a larger wing, higher fuel capacity, and a 310 horsepower engine...
. Successful demonstration of the one-turn spin does not allow an airplane type to be approved for intentional spinning. If an airplane is to be approved for intentional spinning it must be repeatedly subjected to a spin of six turns and then demonstrated to recover within one and a half additional turns. Spin testing is a potentially hazardous exercise and the test aircraft must be equipped with some spin-recovery device such as a tail parachute or jettisonable ballast, or some method of rapidly moving the center of gravity forward.
Agricultural airplanes are typically certificated in the normal category at a moderate weight. For single-engine airplanes this requires successful demonstration of the one-turn spin. However, with the agriculture hopper full these airplanes are not intended to be spun and recovery is unlikely. For this reason, at weights above the maximum for the normal category, these airplanes are not subjected to spin testing and, as a consequence, can only be type certificated in the restricted category. As an example of an agricultural airplane see the Cessna AG series.
Spin kit
To make some sailplanes spin easily for training purposes or demonstrations a spin kitSpin kit
A few sailplanes are very difficult, if not impossible, to spin under normal conditions.To make these sailplanes spin easily for training purposes or demonstrations a spin kit is available from the manufacturer....
is available from the manufacturer.
Many training aircraft may appear to be resistant to entering a spin even though some are intentionally designed and certified for spins. A well-known example of an aircraft designed to spin readily is the Piper Tomahawk
Piper PA-38 Tomahawk
|-References:*Taylor, John W.R. Jane's All The World's Aircraft 1982-83. London:Jane's Yearbooks, 1982. ISBN 0-7106-0748-2.-External links:**-Related content:Comparable aircraft:* Aerotec Tangará* Alpha 2000 * Beechcraft Skipper...
, which is certified for spins, though the Piper Tomahawk's spin characteristics remain controversial. Aircraft that are not certified for spins may be difficult or impossible to recover once the spin exceeds the one-turn certification standard.
Although it has been removed from most flight test syllabuses, there are some countries that still require flight training
Flight training
Flight training is a course of study used when learning to pilot an aircraft. The overall purpose of primary and intermediate flight training is the acquisition and honing of basic airmanship skills....
on spin recovery. In the U.S. spin training is required only for civilian flight instructor candidates and military pilots. A spin occurs only after a stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
, so the FAA emphasizes training pilots in stall recognition, prevention, and recovery as a means to reduce accidents due to unintentional stalls and/or spins.
A spin is often intimidating to the uninitiated, however many pilots trained in spin entry and recovery find that safely spinning is an interesting experience. In a spin, the occupants of the airplane will only feel reduced gravity
Weightlessness
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
during the entry phase and then will experience normal gravity, except that the extreme nose-down attitude will press the occupants forward against their restraint harnesses. The rapid rotation, combined with the nose-down attitude, can also be disorienting.
The recovery procedure from a spin requires using rudder to stop the rotation, then elevator to reduce angle of attack to stop the stall, then pulling out of the dive without exceeding the maximum permitted airspeed (VNE) or maximum G loading. The maximum G loading for a light airplane in the normal category is usually 3.8 G. For a light airplane in the acrobatic category it is usually at least 6 G.
External links
- Narrated spin videos in Cessna & Pitts performed by Spencer Suderman
- Stalls and spins
- 60-turn spin in a Cessna 152
- Video of a spin in a Cessna 152
- Videos of Accelerated, Flat, Inverted, Crossover, and Knife Edge spins
- Narrated, in-flight videos of all spin modes taken from an Extra 300 aerobatic aircraft