Spin-Resistant Airframe (SRA) - ICON Aircraft

Spin-Resistant Airframe (SRA)ICON AircraftWhat is a Spin?A spin is a dangerous combination of a stall andyaw. Spins occur when a stalled aircraft experiences too great a yaw rate, which can be theresult of an incorrect rudder input or a pre-existing yawing moment as would occur if an airplaneis stalled while performing an uncoordinated turn.During the ensuing spin, an aircraft rapidly losesaltitude as it rotates about its spin axis, driven byan asymmetric stall condition between the twowings. The pilot often loses the ability to controlthe aircraft because of disorientation or loss ofcontrol authority, making spins dangerous andharrowing. During a spin, the aircraft experiences low airspeed and a high angle of attack.It is worth noting that this is different from aspiral dive in which an aircraft experiences highairspeed and a low angle of attack, and duringwhich more control authority is preserved. Somespins are recoverable by experienced pilots, butthis requires high situational awareness, propertraining, and often more than 1,000 feet of altitude. Other spins are unrecoverable, even by themost experienced pilot.i co n a ircra ft .co m

S pin - Resista nt Airfram e (SR A)03History of Spin-ResistanceThe earliest attempts to create aspin-resistant aircraft date back tothe early days of flight, well beforeWorld War II. The ERCO Ercoupewas developed in an attempt tobe safer than comparable aircraft by being less susceptible tospins. Through the simple measures of limiting control-surfacedeflection and center-of-gravityrange, the aircraft was certifiedas “characteristically incapable ofspinning” by the Civil AeronauticsAdministration (predecessor totoday’s FAA). However, to achievethis, the Ercoupe did not haverudder pedals, which preventedthe pilot from actively controllingaircraft yaw. An aircraft’s tendency to spin is extremely sensitiveto the location of its center ofgravity (CG), which is the resultof how much weight it is carryingand where the weight is located.The farther back the CG, the lesseffective the horizontal tail is atproviding longitudinal stability andthe more likely the plane is to spin.Stalls/spins are a significantsource of serious accidents inGeneral Aviation accounting for41% of fatal accidents that occurred because of “pilot-relatedfactors,” according to the AircraftOwners and Pilots Association(AOPA) Air Safety Institute’s 2010Nall Report. Pilot-related factorsare responsible for 70% of all accidents, with the remainder beingmechanical, unknown, or undetermined in cause. Stall/spin accidents are particularly dangerousbecause they usually occur at lowaltitude and low airspeeds, suchas in the traffic pattern duringmaneuvering when the pilot’s attention is diverted from maintaining sufficient airspeed by othertasks. In fact, 80% of stall/spinaccidents occur at 1,000 feet AGL(above ground level) or below.Surprisingly, the highest portion ofstall/spin accidents happened toprivate and commercial pilots andnot to students, likely because ofstudents’ close supervision andthe fact that more experienced pilots may have grown complacentin their skills.In the 1970s and 1980s, researchers at NASA’s Langley ResearchCenter studied spin resistancein depth, with a focus on aerodynamic characteristics andtechniques to make aircraft moreresistant to spins without highly unconventional approacheslike the Ercoupe’s elimination ofrudder pedals. They performedextensive modifications to fourexisting General Aviation aircraftand flew thousands of test flightsto determine how changes to theairframe would affect spincharacteristics. They discoveredthat small changes could dramatically affect performance duringspins. They were able to createan aircraft that “gives plenty ofwarning, lots of buffet, very littleroll-off laterally—a long period oftelling the pilot ‘Hey, you’re doingsomething wrong,’” according toNASA experimenters. This workeventually evolved into techniquesto make aircraft that are resistantto entering spins.One of the key findings of theNASA studies was that a criticalcomponent of spin resistance iscontrolling the way the wing stalls.They concluded that having thestall initiate near the root of thewing (where it attaches to the fuselage) while the outboard panelsof the wing continue to fly is idealbecause it prevents the stall fromever fully developing or “breaking”because the outboard panels arestill generating lift. Without a stall,a spin cannot initiate. This progressive stall is achieved with awing cuff, or a discontinuity on theleading edge of the wing that separates the wing into two distinctparts. The outboard segmentsof cuffed wings have a differentairfoil with a drooped leadingedge, compared to the main wing,which causes that portion of thewing to stall later than the inboardpart of the wing as angle of attackincreases. Because the aileronsare located on the outboard panelwhich is still flying, roll control ispreserved even after the inboardpanel of the wing has stalled. TheFAA recognized the significanceof the introduced standards forspin-resistance for Part 23-certified aircraft in 1991. The standardscarefully define what the behaviorof an aircraft under specific testsshould be in order for it to beconsidered spin-resistant, withfive maneuvers completed acrossthe entire center-of-gravity rangeof the aircraft, and across thefull spectrum of configurations,including landing-gear position,power setting, and flap setting.Depending on the complexity ofthe aircraft, it must past hundredsof test cases to be consideredspin-resistant by the FAA.“Depending on thecomplexity of theaircraft, it must passhundreds of test casesto be considered spinresistant by the FAA.”Since the establishment of thePart 23 spin-resistance standards in 1991, a few aircraftcompanies attempted to produceaircraft that fully meet thosestandards; however, no conventional production aircraft withoutcanards ever truly succeeded,either for technical reasons orbecause the aircraft was notsuccessfully brought to market.It is worth noting, however, thatboth the Cirrus SR 20/22 models and Cessna Corvalis aircraftemployed a cuffed wing design toiconaircraft .com

S pin - Resista nt Airfram e (SR A)05advance stall and spin-resistancecharacteristics in General Aviationaircraft, although they did not meetthe full Part 23 spin-resistancestandards. The Jetcruzer, a canardairplane, is another aircraft thatadvanced spin resistance and waseven certified as spin-resistant, although it never entered production.While the idea of controlling thestall is quite simple, it has provenextraordinarily challenging to getthe exact airflow patterns requiredfor a plane to pass the Part 23 standard completely.Behind the Scenes of ICON’sSpin-Resistance ProgramTop: One of NASA’s spin-test aircraft, a Cessna172 equipped with wing cuffs (in red on the leadingedge of the outboard parts of the wing), in flight.Credit: NASABottom Left: The ERCO Ercoupe is one of theearliest aircraft with spin-resistant characteristics,which were achieved by limiting the deflectionof control surfaces (including the elimination ofrudder pedals from the cockpit), as well as thelocation of its center of gravity.Credit: SmithsonianNational Air and Space MuseumAbove: James Patton Jr. (center) stands withJames Bowman Jr. (left) and Sanger Burk (right)in front of a low-wing spin-research aircraft. Aradio-controlled model and a spin-tunnel modelof the same configuration are in the foreground.Credit: NASA.ICON’s engineers and managementteam were aware of the NASA workon spin resistance, as well as thesobering statistics around stall/spinaccidents. Delivering an aircraftthat provides both excellent controlthroughout the stall and resistanceto entering a spin dramatically raises the bar for light aircraft safety bydecreasing the likelihood of inadvertent stall/spin loss of control by thepilot. This is especially important atlow altitude where the majority ofsport flying occurs. To say that thiswas risky is a tremendous understatement. Not only had spin resistance (to the Part 23 standards)never been accomplished by legacyaircraft manufacturers on a conventional production aircraft, but ICONwas a fledgling company that hadnot yet delivered any productionaircraft. However, ICON management felt that the benefits of aspin-resistant aircraft were toogreat not to include, especially whenconsidering that the A5 is intendedto be used at low altitudes andlow speeds, where a spin entryis especially unforgiving. Theyalso trusted the competence ofICON’s extremely talented engineering team to systematicallyapproach the problem and delivera production-ready solution.The design process began withan all-new cuffed wing. All told,ICON’s wing uses several different proprietary airfoils across itsspan. The resulting wing providesa stall that is more progressivethan that of an aircraft not designed for spin resistance.Collaborating with aerodynamicist John Roncz, ICON engineersdesigned a new wing and thenbuilt a physical subscale model,which they tested in a windtunnel. The NASA studies haddemonstrated an association between certain airflow patterns onthe wing and spin resistance, sowhen engineers observed similarflow patterns on the ICON modelin the wind tunnel, they werevery encouraged.the pilot must wear a parachute,and the aircraft itself is also fittedwith a parachute to stop an unrecoverable spin, should one occur.Because ICON’s usual testingoccurs above Tehachapi, whosealtitude is 4,000 feet, a specialtest site was selected with lowerelevation to provide more spacefor the pilot to recover from a spin.Most tests were completed with astarting altitude of at least 8,000feet above ground level.ICON engineers designed, built,and installed a boom to mount thespin parachute on the back of theA5 prototype and also retainedglobally recognized spin-testpilot Len Fox to put the aircraftthrough its spin-resistance testing regimen. Fox has nearly 40years of experience and has flownalmost 200 aircraft types. He wasa United States Naval Aviator for20 years, flying 17 military typesincluding F-15, F-16, and FA-18. Hehas completed spin testing for 25different types, which made himideally suited for spin-resistancetesting of the A5.The FAA Part 23 spin-resistancestandards require tests acrossthe range of configurations andcenter-of-gravity (CG) locationsthat the aircraft will fly with, andthe tests become progressivelymore difficult as the CG movesaft. For each configuration, the aircraft must successfully completefive different maneuvers rangingfrom a relatively mild wings levelor coordinated turning stall to anaggressive abused input (uncoordinated with full deflection of elevator, full rudder, and full aileroninput opposite to rudder), whichmust be held for seven secondswithout a spin initiating. With allconfigurations and permutations,the A5 was subjected to over 360test cases.The wing design was rapidlyfabricated in full scale in ICON’sshop and installed on the prototype aircraft. Initial validation flighttests were promising, and ICONbegan to prepare the aircraft forits full range of spin-resistancetests. Spin testing is one of themore dangerous types of testingand requires a pilot with considerable specialized experience andskills. Because of the possibilityof entering an unrecoverable spin,iconaircraft .com