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	February 1979
Introduction	1979
Number built	Over 1000

The ASK 21 is a glass-reinforced plastic (GRP) two-seat 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 2024), 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 A revised and improved version, the ASK 21B was introduced in 2018.

Construction

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 neutralized ailerons, full opposite rudder until the rotation stops and then centralizing the rudder, relaxing back pressure to break the stall, and then easing the stick back to pull 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.

Variants

ASK 21

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.

ASK 21B

Improved version of the original ASK 21. New features: automatic control connections, internal spin ballast, revised canopy, more effective ailerons.

Operators

Military

Current

Australia

• Royal Australian Air Force
  • Australian Air Force Cadets - 11

Portugal

• Portuguese Air Force
  • Portuguese Air Force Academy

Former

United Kingdom

• Royal Air Force for Air Training Corps (Vanguard TX.1)

Civilian

Canada

• Association de Vol à Voile Champlain - 2
• Cu Nim Gliding Club
• Great lakes Gliding Club
• Rideau Valley Soaring School
• Southern Ontario Soaring Association - 3
• York Soaring
• Youth Flight Canada

United Kingdom

• Yorkshire Gliding Club - 2
• Devon and Somerset Gliding Club
• London Gliding Club - 7
• Buckminster Gliding Club
• Midland Gliding Club
• Cranwell Gliding Club
• Kent Gliding Club
• Scottish Gliding Union

United States

• Mid Atlantic Soaring Association - 2
• Texas Soaring Association
• Caesar Creek Soaring Club - 2

Specifications (ASK 21)

K21 Three-view drawing

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

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)

Performance

• Stall speed: 65 km/h (40 mph, 35 kn)
• Never exceed speed: 280 km/h (170 mph, 150 kn) in smooth air

200 km/h (110 kn) in rough air

180 km/h (97 kn) on aero-tow

150 km/h (81 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)

See also

• IS 28 B2
• Schleicher ASK 23
• ZS Jezow PW-6U
• Grob G103a Twin II
• PZL Bielsko SZD-50

Further reading

• Thomas, Fred; Milgram, Judah (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.

External links

• Alexander Schleicher GmbH & Co English version
• Johnson R, A Flight Test Evaluation of the ASK 21, Soaring, July 1985
• European Aviation Safety Agency Type Certificate Data Sheet