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 two-piece, mid-set, cantilever wing is a single-spar glass-fiber construction without flaps, but with upper side Schempp-Hirth-type air brakes.

The wingtips curve downward, providing tip skids that allow take-offs without a wing helper (small wheel added with ASK-21 mi).

The thick wing profile gives good low-speed characteristics (nominal stall speed approximately 65 km/h or 35 knots).

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.