The Hillson Bi-mono was a British experimental aircraft of the 1940s. It was designed to test the idea of "slip-wings", where the aircraft could take off as a biplane, jettison the upper, disposable wing, and continue flying as a

monoplane A monoplane is a fixed-wing aircraft configuration with a single mainplane, in contrast to a biplane or other types of multiplanes, which have multiple planes. A monoplane has inherently the highest efficiency and lowest drag of any wing con ...

. A single example was built, which successfully demonstrated jettisoning of the slip wing in flight.

Design and development

In the 1930s, as take-off weights of aircraft continued to increase, designers grew increasingly concerned about the effects that these weights would have on take-off runs, and several designers investigated the concept of a "slip-wing", which could be jettisoned after take-off. Amongst the proponents of the "slip-wing" was

Noel Pemberton Billing Noel Pemberton Billing (31 January 1881 – 11 November 1948), sometimes known as Noel Pemberton-Billing, was a British aviator, inventor, publisher and Member of Parliament for Hertford. He founded the firm that became Supermarine and promoted ...

, the founder of

Supermarine Supermarine was a British aircraft manufacturer that is most famous for producing the Spitfire fighter plane during World War II as well as a range of seaplanes and flying boats, and a series of jet-powered fighter aircraft after World War II ...

, who wrote several articles in the aviation press promoting the idea, either with a manned, reusable auxiliary wing, or a disposable or "scrap-wing".Jarrett ''Aeroplane Monthly'' April 1990, p. 204.Jarrett ''Air Enthusiast'' August to October 1993, pp. 3–7. Blackburn Aircraft also investigated "slip-wings".Ellis ''Air Enthusiast'' September/October 2003, p. 49. Following the outbreak of the

Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...

, F Hills and Son, a light aircraft manufacturer based at

Trafford Park Trafford Park is an area of the Metropolitan Borough of Trafford, Greater Manchester, England, opposite Salford Quays on the southern side of the Manchester Ship Canal, southwest of Manchester city centre and north of Stretford. Until the l ...

Manchester Manchester () is a city in Greater Manchester, England. It had a population of 552,000 in 2021. It is bordered by the Cheshire Plain to the south, the Pennines to the north and east, and the neighbouring city of Salford to the west. The t ...

offered a design for a light fighter aircraft to the British

Air Ministry The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State ...

. The fighter would be cheap to build and could be operated from small fields or open roads. To give the required take-off performance, the design was to be fitted with a disposable "slip-wing". While the proposal was not accepted by the Air Ministry, Hills and Sons decided to continue with the project as a private venture, and so built a scale test-bed, to prove the slip-wing process. The test bed, known as the Bi-mono, was a small

tractor A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most commo ...

with a fabric-covered steel-tube construction fuselage and a wooden wing. It had a fixed tailwheel undercarriage while an enclosed cockpit was provided for the pilot. The auxiliary wing was attached to the top of the cockpit canopy and to the lower wing by

interplane struts In aeronautics, bracing comprises additional structural members which stiffen the functional airframe to give it rigidity and strength under load. Bracing may be applied both internally and externally, and may take the form of strut, which act in ...

. Two different upper wings were flown. The one used initially had a wingspan of but the later, shorter wing's span of was the same as that of the lower one. The aircraft was powered by a single

de Havilland Gipsy Six The de Havilland Gipsy Six is a British six-cylinder, air-cooled, inverted inline piston engine developed by the de Havilland Engine Company for aircraft use in the 1930s. It was based on the cylinders of the four-cylinder Gipsy Major and ...

engine.Jarrett ''Aeroplane Monthly'' April 1990, p. 205.Ellis ''Air Enthusiast'' September/October 2003, p. 50. The Bi-mono was not the only slip-wing project built by Hills and Sons, as they were also contracted by Pemberton Billing to build his PB.37 design for a slip-wing dive-bomber, with a pusher monoplane lower component powered by a 290 hp (216 kW) engine, while the manned slip-wing upper component was a tractor monoplane powered by a 40 hp (30 kW) engine. Construction work started on the PB.37 early in 1940, but work was abandoned in July 1940 when construction was almost finished but the aircraft was unflown.Jarrett ''Air Enthusiast'' August to October 1993, pp. 3–4.

Operational history

The Bi-mono, which carried no serial number or

civil registration Civil registration is the system by which a government records the vital events ( births, marriages, and deaths) of its citizens and residents. The resulting repository or database has different names in different countries and even in differ ...

, made its maiden flight from Barton Aerodrome during 1941.Ellis ''Air Enthusiast'' September/October 2003, p. 51. Test flights were made both as a monoplane and as a biplane, with the shorter upper wing being chosen. In order to avoid the potential hazards to people on the ground of dropping the wing, wing jettisoning tests were carried out from Squires Gate Airport, Blackpool, with the upper wing being successfully dropped over the

Irish Sea The Irish Sea or , gv, Y Keayn Yernagh, sco, Erse Sie, gd, Muir Èireann , Ulster-Scots: ''Airish Sea'', cy, Môr Iwerddon . is an extensive body of water that separates the islands of Ireland and Great Britain. It is linked to the Ce ...

on 16 July 1941. The test proved successful, with no great change in trim and a few hundred feet in altitude being lost when the upper wing was jettisoned.Jarrett ''Aeroplane Monthly'' April 1990, p. 206. The Bi-mono was subject to further testing by the

Aeroplane and Armament Experimental Establishment The Aeroplane and Armament Experimental Establishment (A&AEE) was a research facility for British military aviation from 1918 to 1992. Established at Martlesham Heath, Suffolk, the unit moved in 1939 to Boscombe Down, Wiltshire, where its wo ...

Boscombe Down MoD Boscombe Down ' is the home of a military aircraft testing site, on the southeastern outskirts of the town of Amesbury, Wiltshire, England. The site is managed by QinetiQ, the private defence company created as part of the breakup of the Def ...

from October 1941. The A&AEE found that the maximum speed of the biplane configuration was slower than the stalling speed of the monoplane configuration. Its landing characteristics were likened to a

kangaroo Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern ...

.Mason ''The Secret Years: Flight testing at Boscombe Down'' p209 Hills and Sons went on to further develop the slip-wing concept, flying the Hillson FH.40, a

Hawker Hurricane The Hawker Hurricane is a British single-seat fighter aircraft of the 1930s–40s which was designed and predominantly built by Hawker Aircraft Ltd. for service with the Royal Air Force (RAF). It was overshadowed in the public consciousness b ...

fitted with a slip-wing.Ellis ''Air Enthusiast'' September/October 2003, pp. 50–51.

Specifications (short upper wing)

Notes

References

* Ellis, Ken. "Back to the Biplane: The 'Slip Wing' and the Hurricane". ''

Air Enthusiast ''Air Enthusiast'' was a British, bi-monthly, aviation magazine, published by the Key Publishing group. Initially begun in 1974 as ''Air Enthusiast Quarterly'', the magazine was conceived as a historical adjunct to ''Air International'' maga ...

'', No 107, September/October 2003. . pp. 47–51. * Jarrett, Philip. "Nothing Ventured...: Part 1". ''

Aeroplane Monthly ''Aeroplane'' (formerly ''Aeroplane Monthly'') is a British magazine devoted to aviation, with a focus on aviation history and preservation. __TOC__ ''The Aeroplane'' The weekly ''The Aeroplane'' launched in June 1911 under founding edito ...

'', April 1990, Vol 18 No 4. . pp. 204–206. * Jarrett, Philip. "PB's Dream Machines". ''Air Enthusiast'', Fifty-one, August to October 1993. . pp. 1–7. {{refend 1940s British experimental aircraft Aircraft first flown in 1941 Low-wing aircraft Biplanes Single-engined tractor aircraft