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 single example was built, which successfully demonstrated jettisoning of the slip wing in flight.
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, the founder of Supermarine, who wrote several articles in the aviation press promoting the idea, either with a manned, reusable auxiliary wing, or a disposable or “scrap-wing”. Blackburn Aircraft also investigated “slip-wings”.
Following the outbreak of the Second World War, F Hills and Son, a light aircraft manufacturer based at Trafford Park, Manchester offered a design for a light fighter aircraft to the British Air Ministry. 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 monoplane 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. Two different upper wings were flown. The one used initially had a wingspan of 32 ft (9.8 m) but the later, shorter wing’s span of 20 ft (6.1 m) was the same as that of the lower one. The aircraft was powered by a single de Havilland Gipsy Six engine.
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.
Sources: YouTube; Wikipedia