The ASK 21 was designed by Rudolf Kaiser to replace the popular ASK 13, providing a modern two-seat aircraft bridging the gap between initial training and single-seat performance flying. The ASK 21 is the first full-GRP two-seater produced by Schleicher, flying for the first time in February 1979 (6.2.1979). Production also began in 1979, remaining in production to this day (May 2013), over 900 units completed. In December 2004 the ASK 21 Mi, a self-launching version, made its first flight.
The two seats are in a tandem arrangement with dual controls, adjustable rudder pedals and seatbacks. The fuselage consists of a tubular sandwich and thus offers passive safety with low weight. The two-piece, mid-set, cantilever wing is a single-spar glass-fiber construction without flaps, but with upper side Schempp-Hirth-type spoilers. The wingtips curve downward, providing tip skids that allow take-offs without a wing helper(small wheel added with ASK-21mi). The T-tail possesses a fixed horizontal stabilizer and an elevator with spring trim and automatic connections. The undercarriage consists of two or three fixed wheels. The main wheel lies behind the centre of gravity and has a hydraulically operated disc brake. Checking the tire pressure is hindered by the need for removing the wheel fairing (which many gliding clubs remove permanently to aid regular maintenance and to reduce the risk of trapped grass being ignited by the wheel brake as it dissipates kinetic energy as heat). The tail unit has a rubber skid or a tailwheel.
The thick wing profile gives good low-speed characteristics (nominal stall speed approximately 65 km/h or 35 knots). The flight characteristics are docile; its stall is gentle, with ample vibration warning, and is easily recoverable. The nose does not noticeably drop, but the variometer indicates a high rate of descent and the vibration continues until back pressure is released and stall recovery is initiated.
Spin kit installation on the tail of C-FASK at the Bromont Airport in Quebec
The glider has little tendency to enter a spin. In order to make spinning possible for training purposes or demonstrations a spin kit is available from the manufacturer. It consists of ballast discs applied to the tail to change the center of gravity according to the weight of the crew. With this device the ASK 21 when slowed to stall speed with rudder input drops one wing and begins to rotate. Recovery is standard, utilizing full opposite rudder, stick easing forward until the rotation stops and then centralizing the rudder and pulling out of the dive.
When entering a side slip, which is not uncommon as an energy management technique, the rudder can aerodynamically over centre, requiring an input from the pilot for the rudder pedals to return to neutral.