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Sigma
Role	Open class sailplane
National origin	United Kingdom
Manufacturer	Operation Sigma Ltd
Designer	John Sellars
First flight	12 September 1971
Status	still flying
Number built	2

The Sigma is an experimental glider developed in Britain from 1966 by a team led by Nicholas Goodhart. After disappointing performance during flight testing the Sigma was passed on to a Canadian group which carried out modifications, making the Sigma more competitive.^[1]

Design and development

Designed to compete in the 1970 World Gliding Championships, the team aimed to develop a wing that would climb well through a high lift coefficient and a large wing area, but equally had the "maximum possible reduction of area for cruise at low lift coefficients". At the same time for the minimum possible drag they aimed for "extensive" laminar flow. To achieve this they employed flaps that would alter both wing area and wing camber. Based on analysis of the nature of thermals encountered in cross-country flying, they reasoned that by having a slow turning circle, their sailplane could stay close to the central (and strongest) part of the thermal and gain maximum benefit.^[1]

Its unusual feature is its ability to vary its wing area using Fowler flaps. It had been tried before by the Hannover Akaflieg in 1938 with their AFH-4, the South African Beatty-Johl BJ-2 Assegai and the SZD Zefir gliders.^[2]

Operation Sigma Ltd. was formed, attracting sponsorship from Hawker Siddeley and assistance from eminent aerodynamicists, and construction of a prototype was started at the Slingsby Kirbymoorside factory. A fire at the Slingsby factory in 1968 destroyed the prototype, which was nearing completion. Slingsby Sailplanes Ltd. soon went into receivership after the disastrous fire so the remaining prototype Sigma Type C was relocated to the British European Airways workshops at London Heathrow Airport (LHR). After completion it eventually flew for the first time on 12 September 1971 at the College of Aeronautics, Cranfield.^[1]

The wings and tail boom are of aluminum alloy construction and the front of the fuselage is glass-fibre composite, with a welded steel tube centre-section connecting the fuselage components and wings. Other equipment fitted include a retractable main wheel, speed limiting air brakes and a tail parachute housed in a compartment in the rudder.^[1]

The hydraulically actuated flaps increased the wing area to 177 square feet (an increase of 35%), lowering the wing loading and stalling speed (37 knots), allowing the desired tight circling when thermalling.^[3] The hydraulic pressure needed to move the flaps was provided by the pilot pumping on the rudder pedals, this proving to be tiring, not to mention that moving the flaps in flight was found to be almost impossible due to bending in the wings. Performance testing revealed a disappointing^[4] best L/D of 41:1 and the project was wound up in 1977.^[1]

The group offered the Sigma up to further development by other parties, selecting a proposal by David Marsden a professor of mechanical engineering at the University of Alberta (on sabbatical at Cranfield Institute of Technology and a glider pilot holding records with his own glider designs such as the Marsden Gemini).^[5]

The aircraft was moved to Canada in 1979 by Marsden, modified with a new flap system, conventional ailerons in lieu of outboard flap sections and the tail parachute was removed from the rudder. Despite the glide ratio only increasing to 47:1, its good climb rate made it competitive with contemporary Open Class gliders of the time, breaking the US 300 km triangle record in 1997 at 151 km/h (82 kn).^[1]

Variants

Data from:British Gliders and Sailplanes 1922-1970^[6]

Sigma Type A: This version was under construction at the Slingsby factory at Kirbymoorside in 1969 when they were completely destroyed in the 1969 Slingsby factory fire.
Sigma Type B: A version differing in detail from the Type A, not completed.
Sigma Type C: After the fire the project was taken on by Operation Sigma Ltd., ostensibly to supply a world beating sailplane for the World Gliding Championships. Of similar outline, shape and size to the original prototypes, the Type C was completely revised in detail, but retained the construction methods of the earlier aircraft.
(Marsden) Sigma: The Sigma after modification in Canada by David Marsden.

Specifications (before Marsden conversion)

Data from Sailplanes 1965-2000^[1]

General characteristics

Crew: 1
Length: 28 ft 11 in (8.81 m)
Wingspan: 68 ft 11 in (21 m)
Wing area: 131 sq ft (12.2 m²) (flaps retracted)

16.5 m² (177.60 sq ft) (flaps extended)

Aspect ratio: 36.2(flaps retracted)

26.8 (flaps extended)

Airfoil: Wortmann FX-67-Σ136-017^[7]
Empty weight: 1,338 lb (607 kg)
Max takeoff weight: 1,550 lb (703 kg)

Performance

Maximum glide ratio: 41:1 as completed (47:1 with Marsden modifications)
Rate of sink: 126.6 ft/min (0.643 m/s) (1.25 kts)(flaps retracted)

151 ft/min (1.5 kts) (0.772 m/s) (flaps extended)

Wing loading: 11.8 lb/sq ft (57.6 kg/m²) (flaps retracted)

42.6 kg/m² (8.725 Lb/sqft) (flaps extended)

Notes

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Simons, Martin (2005). Sailplanes 1965-2000 (2nd revised ed.). Königswinter: EQIP Werbung & Verlag GmbH. ISBN 3 9808838 1 7.
^ The SZD Zefir series gliders of the 1960s also had Fowler flapped wings. Zefirs had been piloted to 2. and 3. (Cologne 1960), 1. and 2. (Junin 1962) places at World Championships, with L/D of Zefir-3 (1965) exceeding 43. Also LET L-13 Blaník used Fowler flaps, however the latter was not intended to be a high performance glider. (Development of "Zefir" gliders; Lecture by Jerzy Śmielkiewicz (co-designer). Polish Aviation Technology, Historical Notes, issue 26, Sep 2006)
^ "Sigma up for disposal" Flight International 1977
^ The SZD Zefir series gliders of the previous decade overcame the control problems with Fowler flaps (abandoning hydraulic and designing minimalistic low friction mechanical controls), but the Zefir designers ultimately concluded that the Fowler flaps are incompatible with the laminar flow required (over 70% of the chord) on the wing of a high performance glider (Jerzy Śmielkiewicz, op.cit.)
^ "Sigma finds a home" Flight International, 5 November 1977 p.1353
^ Ellison, Norman (1971). British Gliders and Sailplanes 1922-1970 (1st ed.). London: Adam & Charles Black. ISBN 0 7136 1189 8.
^ Flight International, 4 March 1971 p318

References

Simons, Martin (2005). Sailplanes 1965-2000 (2nd revised ed.). Königswinter: EQIP Werbung & Verlag GmbH. ISBN 3 9808838 1 7.
Goodhart, N (27 March 1969), "Sigma—design of a super-glider", Flight International