Back to bip the line

UNTIL THE dawning of World War Two, the Trafford Park, 4 Manchester-based woodworking firm F Hills and Son could hardly be described as having made a major contribution to the development of aviation in the UK.Trading in aeronautical matters as Hillson, the company’s first venture was the construction of an ultra-light Mignet HM. 14 ‘Flying Flea’ which first flew at Manchester’s Barton aerodrome in October 1935, but was quickly put to one side.

This foray prompted the firm to adopt aviation as an additional outlet for the combined skills of its workforce.A small licensed production run of the Czech Praga E. I 14 Air Baby monoplane under the name Hillson Praga was begun at Barton in 1936, with 28 being completed.Two original designs by Norman Sykes came about for the touring and training market, the Pennine (in 1937) and the Helvellyn (1939) respectively, but these did not go beyond the prototype stage.

By 1939 and the outbreak of war, F Hills and Son was an expanding sub-contractor.There was plenty of work available from its neighbour Avro and many Anson sub-assembly contracts were taken on.

Being largely woodworkers, the employees were also kept busy in the mass manufacture of propellers. Additionally, the company ran a flourishing Civilian Repair Depot on behalf of the Air Ministry.


Apart from a flirtation with light aviation, the design department had little to really test itself upon.W R Chown,the Managing Director, was keen to project Hills as more than a ‘jobbing’ sub-contractor.

Chown became interested in a notion investigated by Blackburn Aircraft from 1938 until late 1940.

The Yorkshire-based manufacturer was investigating what today would be called ‘lift augmentation devices’, with several possible uses in mind, mostly military. Included in this study wa: a look at the so-called ‘scrap wing’ theory — a disposable extra flying surface for use from takeoff to operating height.

Under the guidance of the famous ‘Monospar’ designer, J H Steiger, Blackburn carried out trials on a specially converted Miles M.5 Sparrowhawk, which flew briefly from February 1940.

Fighter design by 1940 was such that ultimate war-winners were available in the form of the Supermarine Spitfire and Hawker Hurricane.

In technology they were not too far removed from their biplane predecessors, but in terms of performance, they represented a major leap forward in speed, range and agility.This boost was achieved at the loss of some very desirable biplane attributes, notably short-field take-off and endurance.

Airfields could be made less vulnerable if fighters could be persuaded to use smaller amounts of their area on take-off.A means was needed to boost current performance, while not affecting the much-enhanced fighting abilities of the new generation. (This line of thought was, eventually, to find its only real practical solution in the HS Harrier — but in 1940 this was 21 years away!)

Be it 1940 or 2008, war-planes have a very high disposable weight.They carry a large amount of fuel for their high-performance engines and an ever-increasing warload. All of this needs to be up-lifted at the commencement of a sortie, but only a small proportion is brought back.

Consequently, there is a high wing loading at the beginning of the mission and a lower one at the end. As warload increases, either an inordinately long take-off run from scarce and open-to-attack runways must be endured, or a means of reducing the distance taken (and hopefully improving time-to-height) must be found. Long term, the solution lies with increasing the efficiency of the powerplant. Short term, airframe modifications or adaptations may help.


In the 1920s W S Gray put forward the idea of the ‘scrap wing’, which was later termed the‘slip-wing’ which more accurately defined its role.An auxiliary airfoil would enhance lift on take-off and through the climb, but would be jettisoned (Gray even talked of a mechanism to stow the extra wing) to enable the aircraft to carry out a more agile existence.With the biplane still very much king in those days, the idea was not developed.

Alongside the re-evaluation of the idea by Blackburn at Barton, Hillson’s W R Chown became interested in the theory. He advocated a simple-to-build,easy to maintain and store, fighter that could be produced in large numbers and operated from a wide range of flying fields without jeopardising its operational performance.

Augmented lift was the answer to this design problem and Hills’ Chief Designer, Norman Sykes, set to the task. Air Ministry minds were not convinced and official backing for a full-scale prototype was not forthcoming. Instead, a scaled-down flying test-bed was to be built and, if trials were successful beyond this, a production fighter would be modified.

The ubiquitous Hurricane was chosen as the production fighter.The scale test-bed, the Hillson Bi-Mono, was not unlike a ‘shrunken’ Hurricane in appearance. Just how much recognition the Bi-Mono got from officialdom throughout its brief test career is unclear.

Perhaps a measure of how unofficial the project was lies with the fact that it was never allocated any kind of serial or ‘B Condition’ (or ‘trade-plate) identity.The diminutive aircraft carried the standard ‘prototype’ camouflage of the period, dark green and dark earth with yellow undersides and a small P-for-prototype marking was carried.

The fuselage was a welded tube while the wings were all-wood, both with plywood covering. Construction of the upper wing had to be to the same air-worthiness standards as any other part of the airframe.This made it an expensive item to consider throwing away.

It had been hoped that the aircraft Wr would not have inter-plane struts, so as to avoid complicating the separation. Ideally, the only mounting point for the upper wing would be at the top of the pilot’s cabin.The structure needed to make this possible imposed several penalties and in the end this outweighed the complexities of inter-plane struts.

There were five attachment points: one on top of the canopy and two on each wing, at 40-per cent span, at the ends of ‘V’ struts, which would detach with the wing.A purely mechanical method of separation was employed, as electrical or hydraulic methods would be complicated.

Powerplant for the Bi-Mono was a 200hp (l49kW) de Havilland Gipsy Six. Originally it drove a two-bladed fixed pitch wooden propeller designed and built by the firm.This was later replaced by a two-bladed constant-speed metal prop from de Havilland.

A model was tested in the wind tunnel at the Royal Aircraft Establishment (RAE) at Farnborough, Hampshire.These tests were successful; proving that separation was simple and at no time put the tail assembly at risk. A wide range of speeds were evaluated and it was found that this was not a major factor in deciding when to let go of the wing.

Flight testing began at Barton in early 1941, in both configurations.Wing-dropping trials were to follow but for this, coastal Squires Gate, Blackpool, Lancashire, was chosen as the venue.The wing was too large — and potentially lethal — to be dropped over land, so the Irish Sea was the preferred target With P H Richmond at the controls, and with a Lockheed Hudson brim-full of observers chasing it, the Bi-Mono undertook the first live separation on July 16, 1941. Reaction from the little craft to such an apparently traumatic change in its circumstances was a drop of a couple of hundred feet while the process took place.

The Bi-Mono was also tested by the Aeroplane & Armament Experimental Establishment (A&AEE), Boscombe Down,Wiltshire.Thoughts by this time had faded away from purpose-built short-field fighters and the Bi-Mono project was shelved. It was decided to develop the theme by adapting it to the Hurricane, to establish if there was a place for such a modification within RAF operations.


At the RAE, former Royal Canadian Air Force Hurricane I (serial 321,formerly LI884) was allocated to be used by Hills on April 13, 1942. It was taken to Barton where it was converted to ‘slip-wing’ configuration, under the company designation FH.40.

The conversion involved the construction of a second wing, to essentially the same platform and airfoil sections as the Hurricane’s, but without the ailerons or other internal fittings. Including the release gear and struts, the ‘biplane’ weighed in at 6931b (314kg) above normal monoplane weight.

‘N’ type inter-plane struts supported the wing, being based just above the undercarriage bracing points. Extra support struts went from the rear top inter-plane strut to a location just below the cockpit, giving the whole structure lateral stability. Jettison gear on the FH.40 was electrically actuated, with a mechanical back-up and a warning circuit. On the ‘real’ main plane, the ailerons were increased in area by 10 per cent to impart better control when configured as a biplane.

Successful testing, again conducted over water, was carried out from Sealand, near Chester. All trials were conducted under the charge of the A&AEE.The FH.40 was noted as being under repair after an unspecified accident in February 1944. It was ready for collection for the following May, but nothing else is known of its career, or fate.

So the slip-wing proved itself in operation, but not in practicality.The need for short-field performance disappeared with the availability of greater power for fighter powerplants, and with the achievement in air superiority after the Battle of Britain. Most runways would survive and more airfields were coming on-line almost daily.

Apart from the changing operational scene, the extra wing was too costly to be regarded as a disposable item. Collection systems, most likely a form of parachute recovery, would prove difficult to develop and would be a further complication for operations ‘in the field’.

The technical achievement for F Hills and Son was certainly considerable, although it did not seem to bring the company any more development work. Hills’ main ‘bread and butter’ for most of the war lay with the production of large numbers of Percival Proctors, when the original I manufacturer’s Luton production line was switched to higher priority DH Mosquitos.

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