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 will occur.
To say that this was risky is a tremendous understatement. Not only had spin resistance (to the Part 23 standard) never been accomplished by legacy aircraft manufacturers on a conventional production aircraft, but ICON was a fledgling company that had not yet delivered any production aircraft. However, ICON management felt that the benefits of a spin-resistant aircraft were too great not to include, especially when considering that the A5 is intended to be used at low altitudes and low speeds, where a spin entry is especially unforgiving. They also trusted the competence of ICON’s extremely talented engineering team to systematically approach the problem and deliver a production-ready solution.
The design process began with an all-new cuffed wing, a feature that emerged from the NASA studies and proved successful on the Cirrus and Lancair/Columbia/Cessna aircraft for improving stall and spin characteristics. All told, the wing uses several different proprietary airfoils across its span. These unique airfoils were not suited to the no-flap wing design ICON had previously planned to use on the A5, so ICON engineers chose to reintroduce wing flaps to preserve takeoff performance on the water. The resulting wing provides a stall that is more progressive than that of an aircraft not designed for spin resistance.
Collaborating with aerodynamicist John Roncz, ICON engineers designed a new wing and then built a physical subscale model, which they tested in a wind tunnel. The NASA studies had demonstrated an association between certain airflow patterns on the wing and spin resistance, so when engineers observed similar flow patterns on the ICON model in the wind tunnel, they were very encouraged.
|A subscale model of ICON’s spin resistant wing undergoes testing in the wind tunnel.|
The wing design was rapidly fabricated in full scale in ICON’s shop and installed on the prototype aircraft. Initial validation flight tests proved 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 considerable specialized experience and skills. Because of the possibility of entering an unrecoverable spin, the pilot must wear a parachute, and the aircraft itself is also fitted with a parachute to stop an unrecoverable spin, should one occur. Additionally, the pilot must be able to jettison the airplane parachute to resume flying the aircraft or else there is a risk of severe damage or total loss. Because ICON’s usual testing occurs above Tehachapi, whose altitude is 4000 feet, a special test site was selected with lower elevation to provide more space for the pilot to recover from a spin. Most tests were completed with a starting altitude of at least 8000 feet above ground level.
|An ICON shop technician prepares sheets of carbon fiber for lamination in the wing mold.|
|The partially completed spin-resistant wing with main spar and ribs.|
|This photo of the completed and installed spin-resistant wing clearly shows the cuff on the outboard panel of the wing.|
ICON engineers designed, built, and installed a boom to mount the spin parachute on the back of the A5 prototype and also retained globally recognized spin-test pilot Len Fox to put the aircraft through its spin-resistance testing regimen. Fox has nearly 40 years of experience and has flown almost 200 aircraft types. He was a United States Naval Aviator for 20 years, flying 17 military types including F-15, F-16, and FA-18. He has completed spin testing for 25 different types, which made him ideally suited for spin-resistance testing of the A5.
|The installed boom-mounted spin parachute.|
|The A5 as equipped for spin-resistance testing with yarn tufts, spin parachute, and GoPro cameras used for data collection.|
|ICON ground crew looks on as the A5 performs its spin-resistance tests. Their purpose is to recover the spin parachute and/or pilot in the event of a deployment. Fortunately, the parachute never needed to be deployed.|
The FAA Part 23 spin-resistance standards require tests across the range of configurations and center-of-gravity (CG) locations that the aircraft will fly with, and the tests become progressively more difficult as the CG moves aft. For each configuration, the aircraft must successfully complete five different maneuvers ranging from a relatively mild wings level or coordinated turning stall to an aggressive abused input (uncoordinated with full deflection of elevator, full rudder, and full aileron input opposite to rudder), which must be held for seven seconds without a spin initiating. With all configurations and permutations, the A5 was subjected to over 360 test cases.
During the testing process, the A5 was continuously optimized. As the tests became more difficult, it became necessary to make a variety of aerodynamic changes, which were iteratively flight tested. In mid-December, after several weeks of iterations and testing, the A5 finally passed its last and most difficult test, the 7-second “abused controls” or “pro spin” test (control inputs of rudder and aileron that would promote a spin) at aft CG. It was a momentous day at ICON, representing the successful completion of the riskiest and most technically ambitious part of the entire development program. When ICON Aircraft VP of Engineering Matthew Gionta and COO Steen Strand called the entire company together to announce the news, a spontaneous celebration erupted in a moment of sheer elation, a reflection of the extraordinary challenges and risks the team had taken on to achieve such an ambitious goal.
“I’m incredibly proud of our engineering and fabrication team,” said ICON Aircraft CEO Kirk Hawkins. “While creating a full-envelope spin-resistant airplane was extraordinarily difficult and took longer than we expected, it was absolutely the right thing to do for safety and is a game-changing innovation. Delivering an aircraft that provides excellent control throughout the stall while being resistant 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 will occur. This is just another example of ICON going above and beyond the call of duty to deliver not only the world’s coolest sport plane, but also one of the world’s safest.”