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What is a Spin?

A spin is a dangerous combination of a stall and yaw. Spins occur when a stalled aircraft experiences too great a yaw rate, which can be the result of an incorrect rudder input or a pre-existing yawing moment as would occur if an airplane is stalled while performing an uncoordinated turn. During the ensuing spin, an aircraft rapidly loses altitude as it rotates about its spin axis, driven by an asymmetric stall condition between the two wings. The pilot often loses the ability to control the aircraft because of disorientation or loss of control authority, making spins dangerous and harrowing. During a spin, the aircraft experiences low airspeed and a high angle of attack. It is worth noting that this is different from a spiral dive in which an aircraft experiences high airspeed and a low angle of attack, and during which more control authority is preserved. Some spins are recoverable by experienced pilots, but this requires high situational awareness, proper training, and often more than 1000 feet of altitude. Other spins are unrecoverable, even by the most experienced pilot.

Stalls/spins are a significant source of serious accidents in General Aviation accounting for 41% of fatal accidents that occurred because of “pilot-related factors,” according to the Aircraft Owners and Pilots Association (AOPA) Air Safety Institute’s 2010 Nall Report. Pilot-related factors are responsible for 70% of all accidents, with the remainder being mechanical, unknown, or undetermined in cause. Stall/spin accidents are particularly dangerous because they usually occur at low altitude and low airspeeds, such as in the traffic pattern during maneuvering when the pilot’s attention is diverted from maintaining sufficient airspeed by other tasks. In fact, 80% of stall/spin accidents occur at 1000 feet AGL (above ground level) or below. Surprisingly, the highest portion of stall/spin accidents happened to private and commercial pilots and not to students, likely because of students’ close supervision and the fact that more experienced pilots may have grown complacent in their skills.

History of Spin Resistance

The earliest attempts to create a spin-resistant aircraft date back to the early days of flight, well before World War II. The ERCO Ercoupe was developed in an attempt to be safer than comparable aircraft by being less susceptible to spins. Through the simple measures of limiting control-surface deflection and center-of-gravity range, the aircraft was certified as “characteristically incapable of spinning” by the Civil Aeronautics Administration (predecessor to today’s FAA). However, to achieve this, the Ercoupe did not have rudder pedals, which prevented the pilot from actively controlling aircraft yaw. An aircraft’s tendency to spin is extremely sensitive to the location of its center of gravity (CG), which is the result of how much weight it is carrying and where the weight is located. The farther back the CG, the less effective the horizontal tail is at providing longitudinal stability and the more likely the plane is to spin.


The ERCO Ercoupe
The ERCO Ercoupe is one of the earliest aircraft with spin-resistant characteristics, which were achieved by limiting the deflection of control surfaces (including the elimination of rudder pedals from the cockpit), as well as the location of its center of gravity. Credit: Smithsonian National Air and Space Museum

In the 1970s and 1980s, researchers at NASA’s Langley Research Center studied spin resistance in depth, with a focus on aerodynamic characteristics and techniques to make aircraft more resistant to spins without highly unconventional approaches like the Ercoupe’s elimination of rudder pedals. They performed extensive modifications to four existing General Aviation aircraft and flew thousands of test flights to determine how changes to the airframe would affect spin characteristics. What they discovered was that small changes could dramatically affect performance during spins. They were able to create an aircraft that “gives plenty of warning, lots of buffet, very little roll-off laterally—a long period of telling the pilot ‘Hey, you’re doing something wrong,’ ” according to NASA experimenters. This work eventually evolved into techniques to make aircraft that are resistant to entering spins.


A low-wing spin research aircraft
James Patton Jr. (center) stands with James Bowman Jr. (left) and Sanger Burk (right) in front of a low-wing spin research aircraft. A radio-controlled model and a spin-tunnel model of the same configuration are in the foreground. Credit: NASA


A Cessna 172 equipped with wing cuffs
One of NASA’s spin test aircraft, a Cessna 172 equipped with wing cuffs (in red on the leading edge of the outboard parts of the wing), in flight. Credit: NASA

One of the key findings of the NASA studies was that a critical component of spin resistance is controlling the way the wing stalls. They concluded that having the stall initiate near the root of the wing (where it attaches to the fuselage) while the outboard panels of the wing continue to fly is ideal because it prevents the stall from ever fully developing or “breaking” because the outboard panels are still generating lift. Without a stall, a spin cannot initiate. This progressive stall is achieved with a wing cuff, or a discontinuity on the leading edge of the wing that separates the wing into two distinct parts. The outboard segments of cuffed wings have a different airfoil with a drooped leading edge, compared to the main wing, which causes that portion of the wing to stall later than the inboard part of the wing as angle of attack increases. Because the ailerons are located on the outboard panel which is still flying, roll control is preserved even after the inboard panel of the wing has stalled.

The FAA recognized the significance of the NASA work, as well as the danger of spins, and introduced standards for spin-resistance for Part 23-certified aircraft in 1991. The standards carefully define what the behavior of an aircraft under specific tests should be in order for it to be considered spin resistant, with five maneuvers completed across the entire center of gravity range of the aircraft, and across the full spectrum of configurations, including landing-gear position, power setting, and flap setting. Depending on the complexity of the aircraft, it must pass hundreds of test cases to be considered spin resistant by the FAA.

Since the establishment of the Part 23 spin-resistance standards in 1991, a few aircraft companies attempted to produce aircraft that fully meet those standards; however, no conventional production aircraft without canards ever truly succeeded, either for technical reasons or because the aircraft was not successfully brought to market. It is worth noting, however, that both the Cirrus SR 20/22 models and Cessna Corvalis aircraft employed a cuffed wing design to advance stall- and spin-resistance characteristics in General Aviation aircraft, although they did not meet the full Part 23 spin-resistance standards. The Jetcruzer, a canard airplane, is another aircraft that advanced spin resistance and was even certified as spin resistant, although it never entered production. While the idea of controlling the stall is quite simple, it has proven extraordinarily challenging to get the exact airflow patterns required for a plane to pass the Part 23 standard completely.

Further reading:


References and Image credits:
http://www.aopa.org/asf/publications/10nall.pdf
http://www.airspacemag.com/flight-today/cit-bourque.html?c=y&page=5
http://www.aopa.org/asf/ntsb/stall_spin.html
http://www.nasa.gov/centers/langley/news/researchernews/rn_halloffame.html
http://www.nasm.si.edu/images/collections/media/full/A19790677000CP04.jpg
Joseph R. Chambers, Concept to Reality, NASA SP 2003-4529 (2003)