- Cierva Air Horse
W.11 Air Horse Rear view, the three rotors can be seen Role Single-engine three-rotor helicopter Manufacturer Cierva Autogiro Company, Ltd. Designer Cyril G. Pullin First flight December 1948 Retired 1950 Primary user Ministry of Supply Number built 2 Developed from Weir W.5/W.6
The Cierva W.11 Air Horse was a British helicopter which was the largest helicopter in the world when it first flew in 1948. The aircraft was unusual for using three rotors mounted on outriggers, and driven by a single engine mounted inside the fuselage.
The W.11 "Air Horse" heavy lift helicopter was developed by the G & J Weir, Ltd., Aircraft Department, reconstituted in 1943 as the Cierva Autogiro Company. The "W" in the designation is a continuation of the autogyro and helicopter series developed by G & J Weir, Ltd., during the period 1932–1940.
The W.11 was a development of the Weir W.6 dual transverse rotor helicopter. It is the only helicopter of its type ever built and included three lifting rotors all turning in the same direction. The adoption of three rotors was due to concerns over the capability of a single large rotor to generate the required lift.
Torque balance was provided by inclining each rotor axis to generate rotor thrust components in the corrective sense. The three rotor configuration was foreseen by Belgian helicopter experimenter Nicolas Florine in his patent of 1926 which presented means for balancing the reaction on the fuselage of two or more torque driven lifting rotors turning in the same direction.
Work on the W.11 commenced in 1945. The original W.11 configuration used two rotors transversely mounted either side of the front of the fuselage and a single rotor mounted on the centerline at the tail. This configuration was tested in 1947 with a scale-model in a wind tunnel at the Royal Aircraft Establishment at Farnborough, and much useful data on its performance was acquired. This determined that a single rotor at the front and the pair at the back of the fuselage was the preferred configuration for optimum stability and control.
A Rolls-Royce Merlin piston engine in the fuselage drove three 47 feet (14 m) diameter three-blade wooden rotors mounted on outriggers which projected from the fuselage. The blades, made of resin-impregnated wood which provided enormous strength, were manufactured by the Glasgow woodworking engineering firm H. Morris & Co., Ltd. The W.11 rotor control system was hydraulically powered. It was the second helicopter ever to fly using such a system, the first being the Cierva W.9. The landing gear had a stroke of five feet to cater for high descent rates in the event of engine failure during low altitude operations.
Roles envisaged for the W.11 included passenger transport, air ambulance, and aerial crane. In September 1945 the design was modified to meet a requirement from Pest Control, Ltd., for use as a crop sprayer ("Spraying Mantis") in Africa for the groundnut scheme. Two aircraft were ordered under Air Ministry Specification E.19/46 in July 1946.
Cunliffe-Owen Aircraft, Ltd. at Eastleigh Airport, Southampton, UK, were contracted to build the two W.11s under the direction of the Cierva Autogiro Company, Ltd. With a payload of 6,720 lb (3,048 kg) it would have been a very capable sprayer and following the first flights in December 1948 a grant was received from the Colonial Office to assist in development. However, the exit of Cunliffe-Owen from the aircraft business in 1947 delayed development of the W.11.
A projected enlarged development using two Merlins or two Rolls-Royce Dart turboprops was designated as the W.11T. This was abandoned after an accident with the first W.11. The death of three long-time colleagues in the accident prompted financier James G. Weir to decline to provide additional funds since the Cierva Autogiro Company required ever increasing investment. As result all its development contracts were transferred to Saunders Roe. Development of the W.11 continued for a short time thereafter but was terminated by the British Government and the remaining airframe, which flew for less than 20 hours total time, was scrapped. Saunders-Roe, however, continued development of the Cierva W.14 Skeeter which used a main/tail rotor configuration.
W.11 G-ALCV made its first flight on 7 December 1948 and was displayed at the Farnborough Air Show in 1949.
G-ALCV crashed on 13 June 1950, the accident claiming the lives of Alan Marsh (Chief Test Pilot) John "Jeep" Cable, (Ministry of Supply Test Pilot), and Joseph K. Unsworth (Flight Engineer). The cause of the crash was due to fatigue failure of a swashplate carrier driving link in the front rotor hub.
The second W.11, G-ALCW, after additional development work, was scrapped about one year later.
- Prototype three-rotor helicopter powered by a Rolls-Royce Merlin 24 inline piston engines, two built.
- Proposal for an enlarged variant powered by two Rolls-Royce Merlin 502 engines, to meet Air Ministry Specification 10/48 for a crop spraying helicopter, requirement was cancelled and the W.11T was not built.
- Proposed freighter variant using Rolls-Royce Dart turboprops, not built
Data from The Encyclopedia of World Aircraft 
- Crew: 3
- Capacity: 800 cu feet of cabin
- Length: 88 ft 7 in (27.00 m)
- Rotor diameter: 95 ft 0 in  (28.96 m)
- Height: 17 ft 9 in (5.41 m)
- Disc area: 5,205 ft² (483.5 m²)
- Empty weight: 12,140 lb (5,507 kg)
- Max takeoff weight: 17,500 lb (7,940 kg)
- Powerplant: 1 × Rolls-Royce Merlin 24 liquid cooled V-12, 1,620 hp (1,208 kW)
- Propellers: 3 rotor
- Propeller diameter: 47 ft 0 in (14.33 m)
- Maximum speed: 140 mph (225 km/h)
- Cruise speed: 95 mph (153 km/h)
- Range: 330 miles (531 km)
- Service ceiling: 28,000 ft (8,540 m)
- ^ Flight 14 April 1949 p427
- ^ http://www.flightglobal.com/pdfarchive/view/1948/1948%20-%202049.html
- ^ http://www.flightglobal.com/pdfarchive/view/1950/1950%20-%201236.html
- ^ Donald 1997, p.262.
- ^ http://www.flightglobal.com/pdfarchive/view/1950/1950%20-%200193.html Flight 1950
- ^ Width rotors turning.
- The Illustrated Encyclopedia of Aircraft (Part Work 1982-1985). Orbis Publishing.
- Donald, David (editor) (1997). The Encyclopedia of World Aircraft. Leicester, UK: Blitz Editions. ISBN 1-85605-375-X.
- Cierva W.11 Air Horse Copies of entries in "The Complete Encyclopedia of World Aircraft", 1997. by D.Donald and "Helicopters and Autogyros of the World", 1958 by P.Lambermont. Hosted at www.aviastar.org. Accessed January 2008
- The Cierva Air Horse. Jacob Shapiro. Journal of the Helicopter Association of Great Britain, Vol.2 No.4, 1949.
- Flight 1950 GIVING THE AIR HORSE ITS HEAD
- Discussing the Helicopter - the Air Horse described Flight 1948
- Air Horse Flight 1949
- Air Horse Part 2 Flight 1949
- British Portfolio
- US Patent 1,783,011: Helicopter. Nicolas Florine. 
- US Patent 2,529,629: Rotary Wing Aircraft with Plural Rotors. Cyril George Pullin, Kenneth Watson. 
- US Patent 2,623,711: Multirotor Helicopter. Cyril George Pullin, Kenneth Watson. 
External images Flying Cierva W11 Air Horse in colour Phlairline.com's Photo Database  Aircraft produced by Cierva/Cierva Autogiro Company/Weir Lists relating to aviation General Military Accidents/incidents Records
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