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Glasflugel 401 Kestrel

Slingsby T-59 Kestrel

Vickers-Slingsby T-59 Kestrel

 

glas401

 

Designed by Hanle and Prasser, the 17m Kestrel high performance Open Class single-seater was designed to meet the demand for a successor to the Libelle variants with a longer wing span and roomier cockpit; it was known originally as the 17m Libelle and has a new fuselage and wing profile and a T-tail. The fuselage is a fiberglass monocoque (not sandwich) for greater resilience and pilot protection. The cockpit has room enough for a 198 cm / 6 ft pilot and features a control stick that slides fore and aft (rather than rotating) to reduce likelihood of pilot induced oscillations and which has a press-to- trim-push-button trimmer. It has a nose and a center of gravity release.
 
The cantilever two piece shoulder wings are of glassfibre and balsa and/or foam sandwich construction, with unidirectional glassfibre spar caps and glassfibre and balsa shear webs. To cure a small airflow separation problem at the wing root fuselage junction at low speeds large wing root fillets were added to production aircraft; these were actually developed by Vickers-Slingsby, who built the Kestrel 17 under licence. High lift camber-changing flaps are featured which operate in conjunction with the ailerons between 8° and +12°, and can be lowered to 35° for a landing; both ailerons and flaps are partially mass-balanced. It features airbrakes, drogue chute, water ballast and retractable gear. Up to 99lb of water ballast can be carried. There are flush fitting air brakes on the wing upper surfaces, and also a tail braking parachute which can be streamed for short-field landings.
 
The Kestrel 17 tail unit is similar in construction to the wings, the fixed T-tail being secured by three attachments. Both the elevator and rudder are mass balanced. The monowheel is retractable, with an internally expanding brake, and there is an interchangable tailwheel or tailskid.
 
The prototype Kestrel first flew at Karlsruhe-Forchheim on 9 August 1968 and production deliveries began the following year, reaching a total of 129 by January 1978.
 
 Glasf-401-02
Kestrel 17m

 

The Kestrel 17 has several records to its credit. Goran Ax won the second Smirnoff Transcontinental Sailplane Race in a Kestrel in 1973. Sue Martin of Australia gained both the womens world 100 km speed record (113.2 kph / 70.36 mph ) and 300 km triangle speed record (114.4 kph/ 71.11 mph). The 100km closed circuit speed record of 102.74mph was set by K. B.Briegleb of the USA on 18 July 1974 (since broken by an AS-W 17), and the ladies' 300km closed circuit speed record of 71.1mph set up by Susan Martin of Australia on 11 February 1972.
 
Glas-Kes-17
Glasflugel 401 Kestral 17
 
The Italian firm of Glasfaser Italiana SrL has also built 25 Kestrel 17s, as well as 130 complete fuselage assemblies for the Kestrel.
 
After Slingsby Aircraft Company Ltd had gone into liquidation in July 1969 the firm was reorganised as part of the Vickers Group, at first as Slingsby Sailplanes, later trading as Vickers-Slingsby and then as Slingsby Engineering Ltd. It was decided in September 1969 to produce a modern glassfibre design, and a licence to build the Kestrel was negotiated with Glasflugel. Construction of the first Slingsby-built T 59 Kestrel 17 began in March 1970, and it first flew on 15 August that year at Rufforth, Yorkshire.
 
After putting the Glasflugel Kestrel 17 into production, Vickers-Slingsby (later Slingsby Engineering Ltd) began to pursue its own line of development of this Open Class single-seater, which resulted in the 19m span T59B, T59C and T59D. The first 19m span Kestrel, the T 59B, being flown by Mr G Burton in the 1970 World Championships in Texas. The 19m T 59C, which had a carbon-fibre main spar, first flew on 7 May 1971. This was followed by the T 59D of the same span, which first flew in July 1971, the extra span being in the form of 0.5m at each wing root and 0.5m at each wing tip. The T 59D also featured a larger fabric-covered rudder and an antibalance tab in the elevator.
 
 Slings-T59-02
Slingsby T59D ZK-GHU

 

The D model was still further developed into the T59H of 22m (72ft 2.25in) span, the extra span consisting of two 1.5m stub wings inserted into the exisiting wing at the roots. The fuselage is similar to the Kestrel 17 up to just aft of the canopy, beyond which an additional section 29 1/2 in long is inserted which considerably reduced the 'waisting' of the earlier version. The fin and rudder area are increased by about 25%, although the tailplane is the same size as the Kestrel 17's; the rudder is lightened to prevent flutter by fabric-covered cut-out sections.
 
The T.59H Kestrel 22 is a special version of the single-seat high performance Kestrel 19 and the prototype was built by using a Kestrel 19 with two 1.5 metre stub wings, complete with flaps, inserted at the wing roots and an extra fin extension mounted above the tailplane. The idea was to make these modifications available to any owner of the Kestrel 19 who wanted to increase the performance of his sailplane.
 
Two prototype T59Hs were built, the first of these flying in 1974, but the new variant was found to suffer from a wing flutter at 140kts (161mph); Vickers-Slingsby had to recover the prototype T59H from the original customer who had bought it, while the flutter problem was investigated by the College of Aeronautics, where it was still being studied early in 1978.
 
 Slings-T59-05
T.59H Kestrel 22
 
Like the T59C, the H has a carbon-fibre main spar and, apart from the longer fuselage and long-span four-piece wing, joined at the flap/aileron junction, incorporate carbon-fibre spars and large Schempp-Hirth airbrakes on both upper and lower surfaces, it is structurally similar to the Kestrel 17 with the same cantilever T-tail, up to 220lb of water ballast can be carried. There are Schempp-Hirth air brakes in the upper and lower wing surfaces, and there is a monowheel. The two T59H prototypes are known as the Kestrel 22 Series 1 and Series 2.
 
The fuselage has been lengthened by 750 mm (2 ft 4½ in) by adding a section to the 19m Kestrel just aft of the wing trailing edge. The tail unit is like that of the Kestrel 19 with the weight of the rudder reduced to prevent flutter by cutting panels out of the glassfibre skin and covering with fabric. Landing gear comprises a large retractable unsprung Gerdes monowheel with a disc brake, plus a fixed tailwheel. The disc brake is operated by a hydraulic unit coupled to the airbrake system.
The T59H was abandoned in favour of a new aircraft, the Kestrel 22.
 
A total of 101 Kestrels had been built by Vickers-Slingsby by the end of 1974, plus two 22m span T 59H Kestrel 22s. Most of these have been 17m span versions.
 
 
Kestrel 17m
Wing span: 17m / 55.7ft
Wing area: 11.6 sq.m / 124.9 sq.ft
Length: 6.72 m / 22ft 0.5 in
Height: 1.52 m / 5 ft 0 in
Empty Weight: 260 kg / 573 lb
Payload: 140 kg / 309 lb
Gross Weight: 400 kg / 882 lb
Max Wing Load: 34.5 kg/sq.m / 7.06 lb/sq.ft
Water Ballast: 45 kg / 99 lb
Aspect ratio: 25
Wing section: Wortmann FX-67-K-1707150
Max speed: 155 mph / 135 kt / 250 km/h
Stalling speed: 33.5 kt / 63 km/h
L/DMax: 43.1 at 97 kph / 52 kt / 60 mph
MinSink: 0.55 m/s / 1.80 fps / 1.06 kt at 46mph / 40 kt / 74 km/h
Seats: 1

No. Built: 129


Kestrel 19M
Wing span: 19m / 62.3ft
Wing area: 12.87sq.m / 138.5sq.ft
Empty Weight: 330kg / 728lb
Payload: 140kg / 309lb
Gross Weight: 472kg / 1041lb
Wing Load: 36.65kg/sq.m / 7.51lb/sq.ft
Aspect ratio: 28
Water Ballast: 0kg / 0lb
L/DMax: 44 97 kph / 52 kt / 60 mph
MinSink: 0.52 m/s / 1.71 fps/ 1.01 kt
Seats: 1

 

Slingsby T.59D Kestrel 19
Wing span: 19.0 m (62 ft 4 in)
Length: 6.6 m (21 ft 7 3/4 in)
Height: 1.47 m (4ft 9¼ in)
Wing area: 12.87 sq.m (138.5 sq.ft)
Wing section: Wortmann FX-67-K-1707150
Aspect ratio: 28
Empty weight: 330 kg (728 lb)
Max weight: 472 kg (1,041 lb)
Water ballast: 63.6 kg (140 lb)
Max wing loading: 36.65 kg/sq.m (7.51 lb/sq ft)
Max speed: 135 kt (250 km/h)
Stalling speed: 33 kt (61 km/h)
Min sinking speed: 0.52 m/sec (1.71 ft/sec) at 40 kt (74 km/h)
Max rough air speed: 105 kt (195 km/h)
Best glide ratio: 44 at 52.5 kt (97 km/h)

 

Slingsby T-59H
Span: 22.0 m / 72 ft 2.25 in
Length: 7.55 m / 24 ft 9.25 in
Height: 1.94 m / 6 ft 4.25 in
Wing area: 15.44 sq.m / 166.2 sq.ft
Aspect ratio: 31.35
Wing section: Wortmann FX-67-K-170/150
Empty weight: 390 kg / 860 lb
Max weight: 659 kg / 1,453 lb
Water ballast: 100 kg / 220 lb
Max wing loading: 42.68 kg/sq.m / 8.74 lb/sq.ft
Max speed: 155 mph / 110 kt / 204 km/h (in smooth air)
Max aero-tow speed: 93 mph / 105 kt / 194.6 km/h
Min sinking speed: 0.48m/sec / 1.57 ft/sec at 53 mph / 46 kt / 85 km/h

Best glide ratio: 51.5:1 at 64.5 mph / 56 kt / 104 km/h

 

 Glasf-401-01
Kestrel 17m
 
 Slings-T59-03
Slingsby T.59D Kestrel 19
 
 Slings-T59-04
Slingsby T.59H Kestrel 22

 

 

 

 


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