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Operation Sigma Sigma 1
Operation Sigma was set up in 1966 with the object of building a pre-eminent sailplane for the British entry at the 1970 World Championships at Marfa, Texas. Rear Admiral Nick Goodhart was appointed project manager and manufacture of this very high performance sailplane was financially assisted by companies both within and outside the aviation industry.
The idea was that Sigma should have two sets of wings: one for thermalling at low speeds with low minimum sink, and the other for good performance at high speeds for cross-country flying. The two wing sections were specially developed for the project by Dr F.X. Wortmann.
Sigma's wing consists of a light alloy box structure with full span flaps. These hydraulically operated flaps move in a similar manner to Fowler flaps but the gap between wing and flap is sealed with a flexible closure plate on the lower surface of the wing and spoilers on the upper surface to form a continuous extension of the wing. Each flap carries a full-span trailing edge aileron on the outer panel, and a camber-changing flap on the inner panel. These surfaces remain exposed when the flap is retracted. With the flap extended the circling speed and low sink rate should give a better rate of climb in weak thermals. Forward of the aileron on the upper surface is a light alloy spoiler to assist with lateral control at low speeds. With the flaps retracted the high wing loading and low drag should give a high lift/drag ratio at high speeds. The braking system is operated by lowering the camber-changing flaps and raising the spoilers on the upper surfaces to control the speed.
The fuselage is of the pod and boom type with welded steel tube centre structure faired by the wood framed glassfibre cockpit pod and with the tailboom of light alloy monocoque bolted to it. The long undercarriage is used to ensure that the wing tips have satisfactory ground clearance, necessary due to the large span and the degree of wing flexibility. The retractable sprung tail wheel is mounted in the rudder base and is operated by cables from the main undercarriage. The rudder incorporates a brake parachute. The tail unit is of light alloy construction with an all-moving T-tailplane incorporating a full-span anti-balance trim tab. The flying controls are all manually operated except for the flaps and the undercarriage, which are hydraulically operated. The hydraulic pressure is achieved by the pilot pushing both rudder pedals back and forth to operate the hydraulic pump.
The aircraft flew for the first time on 12 September 1971 with Nick Goodhart at the controls. Many aspects of handling and performance had been developed to a satisfactory level, but it did not prove possible to overcome difficulties associated with the flaps and flexible closure plates.
In 1977 the project was taken over by Prof. David J. Marsden of the University of Alberta, who designed and built the Gemini two-seater sailplane. He proposes to replace Sigma's present flaps by simple slotted flaps in the course of his research on variable geometry sailplanes.
Sigma 1
Wing span: 21.0 m (68 ft 10 in)
Length: 8.81 m (28 ft 10 in)
Height: 1.83 m (6 ft 0 in)
Wing area: 12.2 sq.m (131.3 sq ft)
Wing section: Wortmann FX-67 Series
Aspect ratio: 36.2
Empty weight: 607 kg (1,338 lb)
Max weight: 703 kg (1,550 lb)
Water ballast: None
Max wing loading: 57.6 kg/sq.m (11.79 lb/sq ft)
Max speed: 140 kt (259 km/h)
Stalling speed: 3.5 kt (69.5 km/h)
Max rough air speed: 110 kt (204 km/h)
Best glide ratio: 48 at 63 kt (117 km/h)

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