Operation Sigma Sigma

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	1

The Sigma is an experimental glider developed in Britain from 1966 by a team led by Nicholas Goodhart.

Design and devleopment

Designed to compete in the 1970 World 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 achive 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.

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, in the South African Beatty-Johl BJ-2 and in the LET L-13 Blaník^{[citation needed]}.

A company was set up which attracted sponsorship from Hawker Siddeley and assistance from eminent aerodynamicists. Despite a fire at Slingsby Aviation, who were constructing the prototype, in 1968 and a relocation, it eventually flew for the first time on 12 September 1971 at Cranfield.

The wings and tail boom are of aluminum alloy construction and the front of the fuselage is glass-fibre composite. It features a retractable main wheel and air brakes. A tail parachute was originally fitted.

When operated the flaps increased the wing area to 177 square feet (an increase of 35%). The resulting lower wing loading and stalling speed (37 knots) allowed the desired tight circling.^[1] The hydraulic pressure needed to move the flaps was provided by the pilot pumping on the rudder pedals and this proved to be tiring. Due to bending in the wings, moving the flaps in flight was found to be almost impossible. The glide ratio also proved to be disappointing at 41:1. Consequently the project was wound up in 1977.

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 - at the time on sabatical at Cranfield Institute of Technology and a glider pilot holding records with his own glider design.^[2]

The aircraft was moved to Canada in 1979 by Marsden. Among the modifications, the flap system was replaced and the tail parachute was removed. The glide ratio only increased to 47:1. However its good climb rate made it comparable with other Open Class gliders of the time. It broke the US 300 km triangle record in 1997 at 151 km/h.

Specifications (before conversion)

Data from Sailplanes 1965-2000

General characteristics

• Crew: One
• Length: 28 ft 10 in (8.8 m)
• Wingspan: 68 ft 11 in (21.00 m)
• Wing area: 131.3 - 177.6 ft² (12.2 - 16.5 m²)
• Aspect ratio: 36.2 - 26.8
• Empty weight: 1,338 lb (607 kg)
• Gross weight: 1,550 lb (703 kg)

Performance

• Maximum glide ratio: 41

Notes

1. ^ "Sigma up for disposal" Flight International 1977
2. ^ "Sigma finds a home" Flight International, 5 November 1977 p.1353

References

• Martin Simons, Sailplanes 1965-2000, Equip, 2004, ISBN 3 9808838 1 7

• Goodhart, N (27 March 1969), "Sigma—design of a super-glider", Flight International, http://www.flightglobal.com/pdfarchive/view/1969/1969%20-%200541.html 

External links

• Photograph - Sigma