Schleicher ASK 21

ASK 21
Schleicher ASK 21
Role Two Seater-class or Club-class sailplane
National origin Germany
Manufacturer Alexander Schleicher GmbH & Co
Designer Rudolf Kaiser
First flight December 1979
Introduction 1979
Number built Over 900

|} The ASK 21 is a glass-reinforced plastic (GRP) two-seater mid-wing glider aircraft with a T-tail. The ASK 21 is designed primarily for beginner instruction, but is also suitable for cross-country flying and aerobatic instruction.

Design and development

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. In 2003 the operating time was extended to 18,000 hours[1]


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 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 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. The tail unit has a rubber skid or a tailwheel.

Flight characteristics

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.


Production sailplane
ASK 21 Mi
Self-launching sailplane, as ASK 21 with a retractable IAE R50-AA rotary engine with a fixed pitch propeller mounted behind the wing.
Vanguard TX.1
The ASK-21 acquired by the UK Ministry of Defence for use by the Air Cadet organisations.

Specifications (ASK 21)

Data from Jane's All the World's Aircraft 1988-89[2]

General characteristics

  • Crew: 2
  • Length: 8.35 m (27 ft 5 in)
  • Wingspan: 17 m (55 ft 9 in)
  • Height: 1.55 m (5 ft 1 in)
  • Wing area: 17.95 m2 (193.2 sq ft)
  • Aspect ratio: 16:1
  • Airfoil: FX S02-196 / FX 60-126
  • Empty weight: 360 kg (794 lb)
  • Max takeoff weight: 600 kg (1,323 lb)


  • Stall speed: 62 km/h (39 mph; 33 kn)
  • Never exceed speed: 280 km/h (174 mph; 151 kn) in smooth air
180 km/h (97 kn) in rough air
175 km/h (94 kn) on aero-tow
130 km/h (70 kn) on winch launch
  • g limits: +6.5 -4 at 180 km/h (97 kn)
  • Maximum glide ratio: 34 at 90 km/h (49 kn)
  • Rate of sink: 0.64 m/s (126 ft/min) at 67 km/h (36 kn)
  • Wing loading: 33.4 kg/m2 (6.8 lb/sq ft)






 United Kingdom

See also


  1. Technical note No. 29, Extension of service life beyond 12000 operation hours.
  2. John W.R. Taylor, ed. (1988). Jane's All the World's Aircraft 1988-89. London: Jane's Information Group. p. 630. ISBN 0-7106-0867-5.
  3. "Alexander Schleicher: AAFC". Retrieved 2014-02-28.

Further reading

  • Milgram, Fred Thomas; Judah; translator; contributor (1999). Fundamentals of sailplane design (3rd ed.). College Park, MD: College Park Press. ISBN 978-0966955309. 
  • Simons, Martin (2005). Sailplanes 1965-2000 (2nd revised ed.). Königswinter: EQIP Werbung und Verlag G.m.b.H. ISBN 978-3-9808838-1-8. 
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