Spin (flight)

In aviation, a spin is an aggravated stall resulting in rotation about the center of gravity wherein the aircraft follows a downward corkscrew path. Spins can be entered unintentionally or intentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw at the moment an aircraft stalls. An incipient spin is typically driven by inputs made and held by the pilot, whereas a fully developed spin is a self-sustaining maneuver. In either case, however, a specific and often counterintuitive set of actions may be needed to effect recovery. 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 fatal crash.

In a spin, one wing is sufficiently stalled and generates significant drag but little or no lift, and the other is either not stalled or not stalled as fully as the other, and generates significant lift. This causes the aircraft to autorotate due to the non-symmetric lift and drag. Spins are 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 stall because the wing is not stalled and the airplane will respond to the pilot's inputs to the flight controls.

How a spin occurs

Certificated, light, single-engine aircraft must meet specific criteria regarding stall and spin behavior. Even so, it is generally true that such an airplane will only depart into a 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. 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 always taught the importance of making coordinated turns.

A famous example of an unexpected spin was the death of Major Thomas McGuire. He attempted to fight a Nakajima Ki-43 Hayabusa piloted by Akira Sugimoto at low altitudes over Negros island in WWII. He flew a Lockheed P-38 Lightning. Instead of the "zoom and dive" tactics usually employed by P-38 pilots, he attempted to dogfight the nimble Japanese fighter without releasing his auxiliary fuel tanks. The plane was heavier than normal, and sudden drop of airspeed combined with a tight aileron and rudder turn made the plane stall and spin, from which he did not have altitude to recover. McGuire's plane crashed in the jungle and exploded.

Spins can also be entered intentionally for training, flight testing, or aerobatics.

Phases

A spin has four phases in aircraft that are capable of recovering from a spin. In aircraft that cannot recover from a spin, there are only three phases—the developed phase continues until the aircraft hits the ground.

Entry

The pilot provides the necessary elements for the spin, either accidentally or intentionally.

Incipient

The aircraft stalls and rotation starts.

Developed

The aircraft's rotation rate, airspeed, and vertical speed are stabilized.

Recovery

The angle of attack of the wings decreases below the critical angle of attack and autorotation slows. The nose steepens, after which autorotation stops.

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. [NASA Technical Paper 1009. p.11]

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. [NASA Technical Paper 1009. p.8]

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. [NASA Technical Note TN D-6575. p.15] 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. [NASA Technical Paper 1009. p.9]

Single-engine airplane types must be demonstrated to recover from a spin of at least one turn. [US Federal Aviation Regulations, Part 23, §23.221] 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. [NASA Technical Paper 1009. p.14]

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. 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 [ [http://www.flightsimbooks.com/jfs2/chapter1.php History of Aerobatics - Jet Fighter School 2] by Richard G. Sheffield] 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, Frederick Lindemann, 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 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 "P"ower idle, "A"ilerons neutral, "R"udder opposite the spin and held, and "E"levator 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 1970's 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 direction.

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 disorientating time. Even though this method does work in a specific subset of spin-approved airplanes, the NASA Standard/PARE procedure will also be effective, but care must be taken to ensure the spin does not simply cross from positive to negative or vice versa. 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

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; or by jettisoning lead pellets specially installed as 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 along the transverse axis. The empennage will feel very light and loose. Depending on the aircraft, rudder and aileron inputs and changing engine power settings may have little effect. There is a small number of accounts of heroic pilots recovering from flat spins by loosening their restraint harnesses and leaning forward in an attempt to favourably alter the position of the center of gravity. Unfortunately, the great majority of pilots who have experienced an unrecoverable spin have not lived to talk about it if they could not bail out of/eject from the aircraft. Generally speaking, such characteristics are confined to high-performance aircraft, primarily fighters.

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. This was instrumental to the plot of the movie "Top Gun" where a flat spin results in the death of Nick "Goose" Bradshaw (portrayed by Anthony Edwards). Another example of a nonrecoverable flat spin occurred in 1963, with Chuck Yeager at the controls of the NF-104A rocket-jet hybrid: after setting an altitude record, Yeager lost control and entered a flat spin, then ejected and survived. (The plane did not.)

The mathematics of the flat spin are that if the center of lift force is ahead of the center of gravity on longitudinal axis, the real number components of the eigenvalues of the stability matrix exceed zero and the poles of the stability matrix migrate to the positive half of the complex number plane. This will indicate positive feedback on attempts at control: the plane will resist any attempts at recovery and stabilizing the plane. Some modern fighter aircraft, like the F-16 Fighting Falcon and the Saab Gripen have, for greater maneuverability, been intentionally designed to be unstable and are controlled by a computer to stabilize the plane.

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 aircraft, including certificated gliders, must meet published criteria regarding stall and spin behavior. These 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, while the ailerons may 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 slots. Where the slots are located ahead of the ailerons they provide strong resistance to spinning 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.

The 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 Columbia 300/350 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.

pin Kit

To make some sailplanes spin easily for training purposes or demonstrations a spin kit 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 this is the Piper Tomahawk, which is certified for spins, though the Piper Tomahawk's spin characteristics remain [http://www.pipertomahawk.com/stallspin.htm 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 [http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=62510df6166eef6322c923fc4ed332a7&rgn=div5&view=text&node=14:2.0.1.1.2&idno=14#14:2.0.1.1.2.8.1.4 required] only for flight instructor candidates. A spin occurs only after a stall, so the FAA emphasizes training pilots in [http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/a2fdf912342e575786256ca20061e343/$FILE/AC61-67C.pdf 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 3.8G. For a light airplane in the acrobatic category it is usually at least 6G.

References

* [http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf Airplane Flying Handbook] , ch. 4: Slow Flight, Stalls, and Spins

* [http://ntrs.nasa.gov/search.jsp?R=523481&id=1&qs=Ntt%3DSummary%252Bof%252BSpin%252BTechnology%252Bas%252BRelated%252Bto%252BLight%26Ntk%3Dall%26Ntx%3Dmode%2520matchall%26N%3D0%26Ns%3DHarvestDate%257c1 NASA Technical Note TN D-6575] "Summary of Spin Technology as Related to Light General-Aviation Airplanes". Retrieved 2008-06-13

* [http://ntrs.nasa.gov/search.jsp?R=93056&id=1&qs=Ntt%3D1009%257CSpin-tunnel%252BInvestigation%252Bof%252Bthe%252BSpinning%26Ntk%3Dall%7Call%26Ntx%3Dmode%2520matchall%7Cmode%2520matchall%26N%3D0%26Ns%3DHarvestDate%257c1 NASA Technical Paper 1009] "Spin-tunnel Investigation of the Spinning Characteristics of Typical Single-engine General Aviation Airplane Designs". Retrieved 2008-06-13

Notes

External links

* [http://www.av8n.com/how/htm/spins.html#sec-nasa-spin-studies Stalls and spins]
* [http://www.aceaerobaticschool.com/spins.html 60-turn spin in a Cessna 152]
* [http://www.bestofyoutube.com/video.asp?videoid=308 Video of a spin in a Cessna 152]
* [http://www.tutimaacademy.com/video_spin.html Videos of Accelerated, Flat, Inverted, Crossover, and Knife Edge spins]
* [http://www.aerobats.com/training.html Narrated, in-flight videos of all spin modes taken from an Extra 300 aerobatic aircraft]