Spin-Resistant Airframe (SRA)

Spin-Resistant Airframe (SRA) ICON Aircraft

What is a Spin? A spin is a dangerous combination of a stall and harrowing. During a spin, the aircraft experi- yaw. Spins occur when a stalled aircraft expe- ences low airspeed and a high angle of attack. riences too great a yaw rate, which can be the It is worth noting that this is different from a result of an incorrect rudder input or a pre-exist- spiral dive in which an aircraft experiences high ing yawing moment as would occur if an airplane airspeed and a low angle of attack, and during is stalled while performing an uncoordinated turn. which more control authority is preserved. Some During the ensuing spin, an aircraft rapidly loses spins are recoverable by experienced pilots, but altitude as it rotates about its spin axis, driven by this requires high situational awareness, proper an asymmetric stall condition between the two training, and often more than 1,000 feet of alti- wings. The pilot often loses the ability to control tude. Other spins are unrecoverable, even by the the aircraft because of disorientation or loss of most experienced pilot. control authority, making spins dangerous and

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Stalls/spins are a significant mechanical, unknown, or undeter- (above ground level) or below. source of serious accidents in mined in cause. Stall/spin acci- Surprisingly, the highest portion of General Aviation accounting for dents are particularly dangerous stall/spin accidents happened to 41% of fatal accidents that oc- because they usually occur at low private and commercial pilots and curred because of “pilot-related altitude and low airspeeds, such not to students, likely because of factors,” according to the Aircraft as in the traffic pattern during students’ close supervision and Owners and Pilots Association maneuvering when the pilot’s at- the fact that more experienced pi- (AOPA) Air Safety Institute’s 2010 tention is diverted from maintain- lots may have grown complacent Nall Report. Pilot-related factors ing sufficient airspeed by other in their skills. are responsible for 70% of all ac- tasks. In fact, 80% of stall/spin cidents, with the remainder being accidents occur at 1,000 feet AGL

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History of Spin-Resistance characteristics. They discovered FAA recognized the significance The earliest attempts to create a that small changes could dramat- of the introduced standards for spin-resistant aircraft date back to ically affect performance during spin-resistance for Part 23-certi- the early days of flight, well before spins. They were able to create fied aircraft in 1991. The standards World War II. The ERCO Ercoupe an aircraft that “gives plenty of carefully define what the behavior was developed in an attempt to warning, lots of buffet, very little of an aircraft under specific tests be safer than comparable air- roll-off laterally—a long period of should be in order for it to be craft by being less susceptible to telling the pilot ‘Hey, you’re doing considered spin-resistant, with spins. Through the simple mea- something wrong,’” according to five maneuvers completed across sures of limiting control-surface NASA experimenters. This work the entire center-of-gravity range deflection and center-of-gravity eventually evolved into techniques of the aircraft, and across the range, the aircraft was certified to make aircraft that are resistant full spectrum of configurations, as “characteristically incapable of to entering spins. including landing-gear position, spinning” by the Civil Aeronautics power setting, and flap setting. Administration (predecessor to One of the key findings of the Depending on the complexity of today’s FAA). However, to achieve NASA studies was that a critical the aircraft, it must past hundreds this, the Ercoupe did not have component of spin resistance is of test cases to be considered rudder pedals, which prevented controlling the way the wing stalls. spin-resistant by the FAA. the pilot from actively controlling They concluded that having the aircraft yaw. An aircraft’s tenden- stall initiate near the root of the cy to spin is extremely sensitive wing (where it attaches to the fu- Depending on the to the location of its center of “ selage) while the outboard panels gravity (CG), which is the result complexity of the of the wing continue to fly is ideal of how much weight it is carrying aircraft, it must pass because it prevents the stall from and where the weight is located. ever fully developing or “breaking” hundreds of test cases The farther back the CG, the less because the outboard panels are to be considered spin- effective the horizontal tail is at still generating lift. Without a stall, providing longitudinal stability and resistant by the FAA. a spin cannot initiate. This pro- ” the more likely the plane is to spin. gressive stall is achieved with a wing cuff, or a discontinuity on the Since the establishment of the In the 1970s and 1980s, research- leading edge of the wing that sep- Part 23 spin-resistance stan- ers at NASA’s Langley Research arates the wing into two distinct dards in 1991, a few aircraft Center studied spin resistance parts. The outboard segments companies attempted to produce in depth, with a focus on aero- of cuffed wings have a different aircraft that fully meet those dynamic characteristics and airfoil with a drooped leading standards; however, no conven- techniques to make aircraft more edge, compared to the main wing, tional production aircraft without resistant to spins without high- which causes that portion of the canards ever truly succeeded, ly unconventional approaches wing to stall later than the inboard either for technical reasons or like the Ercoupe’s elimination of part of the wing as angle of attack because the aircraft was not rudder pedals. They performed increases. Because the ailerons successfully brought to market. extensive modifications to four are located on the outboard panel It is worth noting, however, that existing General Aviation aircraft which is still flying, roll control is both the Cirrus SR 20/22 mod- and flew thousands of test flights preserved even after the inboard els and Cessna Corvalis aircraft to determine how changes to the panel of the wing has stalled. The employed a cuffed wing design to airframe would affect spin

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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 stan- dard completely.

Behind the Scenes of ICON’s Spin-Resistance Program ICON’s engineers and management team were aware of the NASA work on spin resistance, as well as the sobering statistics around stall/spin accidents. Delivering an aircraft that provides both excellent control throughout the stall and resistance to entering a spin dramatically raises the bar for light aircraft safety by decreasing the likelihood of inadvertent stall/spin loss of control by the pilot. This is especially important at low altitude where the majority of sport flying occurs. To say that this was risky is a tremendous under- statement. Not only had spin resistance (to the Part 23 standards) never been accomplished by legacy Top: One of NASA’s spin-test aircraft, a Cessna Above: James Patton Jr. (center) stands with 172 equipped with wing cuffs (in red on the leading James Bowman Jr. (left) and Sanger Burk (right) aircraft manufacturers on a conven- edge of the outboard parts of the wing), in flight. in front of a low-wing spin-research aircraft. A tional production aircraft, but ICON Credit: NASA radio-controlled model and a spin-tunnel model of the same configuration are in the foreground. was a fledgling company that had Bottom Left: The ERCO Ercoupe is one of the Credit: NASA. earliest aircraft with spin-resistant characteristics, not yet delivered any production which were achieved by limiting the deflection aircraft. However, ICON manage- of control surfaces (including the elimination of rudder pedals from the cockpit), as well as the ment felt that the benefits of a location of its center of gravity.Credit: Smithsonian spin-resistant aircraft were too National Air and Space Museum great not to include, especially when considering that the A5 is intended

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to be used at low altitudes and the pilot must wear a parachute, has completed spin testing for 25 low speeds, where a spin entry and the aircraft itself is also fitted different types, which made him is especially unforgiving. They with a parachute to stop an unre- ideally suited for spin-resistance also trusted the competence of coverable spin, should one occur. testing of the A5. ICON’s extremely talented engi- Because ICON’s usual testing neering team to systematically occurs above Tehachapi, whose The FAA Part 23 spin-resistance approach the problem and deliver altitude is 4,000 feet, a special standards require tests across a production-ready solution. test site was selected with lower the range of configurations and elevation to provide more space center-of-gravity (CG) locations The design process began with for the pilot to recover from a spin. that the aircraft will fly with, and an all-new cuffed wing. All told, Most tests were completed with a the tests become progressively ICON’s wing uses several differ- starting altitude of at least 8,000 more difficult as the CG moves ent proprietary airfoils across its feet above ground level. aft. For each configuration, the air- span. The resulting wing provides craft must successfully complete a stall that is more progressive ICON engineers designed, built, five different maneuvers ranging than that of an aircraft not de- and installed a boom to mount the from a relatively mild wings level signed for spin resistance. spin parachute on the back of the or coordinated turning stall to an A5 prototype and also retained aggressive abused input (uncoor- Collaborating with aerodynami- globally recognized spin-test dinated with full deflection of el- cist John Roncz, ICON engineers pilot Len Fox to put the aircraft evator, full rudder, and full aileron designed a new wing and then through its spin-resistance test- input opposite to rudder), which built a physical subscale model, ing regimen. Fox has nearly 40 must be held for seven seconds which they tested in a wind years of experience and has flown without a spin initiating. With all tunnel. The NASA studies had almost 200 aircraft types. He was configurations and permutations, demonstrated an association be- a United States Naval Aviator for the A5 was subjected to over 360 tween certain airflow patterns on 20 years, flying 17 military types test cases. the wing and spin resistance, so including F-15, F-16, and FA-18. He when engineers observed similar flow patterns on the ICON model in the wind tunnel, they were very encouraged.

The wing design was rapidly fabricated in full scale in ICON’s shop and installed on the prototype aircraft. Initial validation flight tests were promising, and ICON began to prepare the aircraft for its full range of spin-resistance tests. Spin testing is one of the more dangerous types of testing and requires a pilot with consider- able specialized experience and skills. Because of the possibility of entering an unrecoverable spin,

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During the testing process, the A5 Gionta and COO Steen Strand called while being resistant to entering a was continuously optimized. As the entire company together to spin dramatically raises the bar for the tests became more difficult, it announce the news, a spontaneous light aircraft safety by decreasing became necessary to make a variety celebration erupted in a moment of the likelihood of inadvertent stall/ of aerodynamic changes, which were elation, a reflection of the extraordi- spin loss of control by the pilot. This iteratively flight tested. After several nary challenges and risks the team is especially important at low altitude weeks of iterations and testing, the had taken on to achieve such an where the majority of sport flying will A5 finally passed its last and most ambitious goal. “I’m incredibly proud occur. This is just another example difficult test, the 7-second “abused of our engineering and fabrication of ICON going above and beyond controls” or “pro spin” test (control team,” said ICON Aircraft CEO Kirk the call of duty to deliver not only the inputs of rudder and aileron that Hawkins. “While creating a full-en- world’s coolest sport plane, but also would promote a spin) at aft CG. velope spin-resistant airplane was one of the world’s safest.” It was a momentous day at ICON, extraordinarily difficult and took lon- representing the successful comple- ger than we expected, it was abso- tion of the riskiest and most tech- lutely the right thing to do for safety nically ambitious part of the entire and is a game-changing innovation. development program. When ICON Delivering an aircraft that provides Aircraft VP of Engineering Matthew excellent control throughout the stall

ICON Videos Resources

A5 Spin-Resistance Development AIR & SPACE Smithsonian NASA https://www.youtube.com/watch?v=bsQcfzNWJWc http://www.airspacemag.com/flight-today/ http://www.nasa.gov/centers/langley/news/ cit-bourque.html?c=y&page=5 researchernews/rn_halloffame.html

ICON Spin-Resistance Reactions AOPA http://www.aopa.org/asf/ntsb/stall_spin.html http://www.aopa.org/asf/ntsb/stall_spin.html

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ICON A5 Spin-Resistant Airframe

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1. Wing Cuff 3. Wing Flaps The wing cuff is a discontinuity on Wing flaps provide additional lift at the leading edge of the wing that low speeds and are particularly valu- separates it into two distinct parts able during water takeoffs A spin-re- that have different airfoils. The sistant airplane must demonstrate outboard panel of the wing has a that it is resistant to entering spins drooped leading edge, which allows with the flaps both up and down. it to continue generating lift after the inboard panel has stalled. This 4. Planing Wingtips gives the A5 a progressive stall, which is a signicant contributor to The wingtips have flat surfaces on spin resistance. the bottom to ensure that the wings skim along the surface of the water during extreme or unintentional 2. Ailerons water maneuvering. They also pro- Ailerons are located on the outboard vide hydrostatic stability when the panel of the wing, which continues aircraft is not in forward motion. to fly even while the rest of the wing is stalled. This ensures that the pilot maintains roll control during a stall. Control authority was not limited in any axis to achieve spin resistance.

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