HURRICANE BIPLANE

Aug. 29

There was a time before computer modelling when all sorts of concepts were given an aerodynamic chance at life. It was a time of great experimentation and excitement. The aviation world was full of some pretty strange one-off aircraft that were either outright failures or before or after their time. In the case of the biplane Hurricane, the builders first designed and constructed a proof-of-concept airplane—a tiny aircraft that could begin its flight as a biplane and end it as a monoplane.

If you do any amount of research or web-surfing on the internet about Second World War aviation, or any type of aviation for that matter, you will chance upon some fascinating facts and some fairly obscure images from time to time. If you are like me, you have a folder in your electronic files into which you dump these facts and images, with the hope of someday following them through to a story befitting their obscurity and unique qualities. I call my folder Random Beauty, and it is full of some pretty strange aviation material, from which many stories are nurtured and eventually harvested.

Many years ago, I came upon an image of a Hawker Hurricane, photographed from below as it banks gently away to the left. What made this image so compelling for me was the fact this was no ordinary Hurricane—it had a second and identical wing! It was a Hurricane Biplane! I was aware of all the fabulous Hawker Biplanes which led up to the development of the monoplane Hurricane itself—Nimrod, Audax, Hind, Fury, Hart, etc.— but I was never aware of this two-winged Hurricane. Over the past three years, I have dumped additional images into my Random Beauty folder and looked to someday putting together a story about it and its development. That day has come.

By the beginning of the Second World War, in late 1939, the writing was on the wall for the biplane combat aircraft—both fighters and bombers. Though many biplanes were still in use on both sides, manufacturing was steadily drawing down as designers and governments turned their creative talents and procurement budgets to the monoplane. The reasons why biplanes existed at all were simple. First, materials strengths in the first 30 years of flight did not allow for highly manoeuvrable monoplanes. Secondly, these early materials strengths led to a box-structure for wings to lift the heavy, underpowered engines of the day. The advent of better steel and aluminum structures spelled the end of the need for these lightweight box structures.

Soon, the monoplane was not just ascendant, but the only way forward. Biplanes, for the most part, were used for liaison and training activities in the last years of the war. There were exceptions, but the days of the military biplane were over. In the case of the fighter aircraft, three things were of absolute necessity—manoeuvrability, strength/payload and speed. The biplane could often match and even surpass the monoplane on the first two of these requirements, but not the last and most important—speed. The addition of a second wing, and its attendant struts and wires, made for increased lift but extra drag. An aircraft built with two main wings was capable of lifting up to 20 percent more weight than a monoplane of similar wingspan and weight. Traditionally then, biplanes were given shorter wingspans than the equivalent monoplane, which provided the benefit of greater manoeuvrability in the roll. A biplane typically produces more drag than a monoplane, especially as speed increases and each wing of a biplane has a negative effect on the aerodynamics of the other wing. This increase in drag gave limits to the top speed of late model biplane fighters, despite their other positive qualities.

It was those positive qualities of increased lift that led some designers to continue to consider the biplane configuration for special duties. In the case of the defensive fighter, called upon to scramble from a field and get to or above an attacking enemy as soon as possible, the lift provided by a second wing was definitely missed, but once the battle was engaged, a higher speed than the enemy was an advantage a fighter pilot would never sacrifice. Some designers, such as those at the small Manchester-based aircraft components and light aircraft manufacturer called F. Hills and Son Ltd, played with an idea of a jettisonable second wing (a slip wing) that would give more wing area during takeoff and less during flight.

F. Hills and Son gained valuable aircraft construction experience by license building a small wooden general aviation aircraft called the Praga during the 1930s. The Praga was built largely by ČKD-Praga in Czechoslovakia, but of the more than 300 built before and after the war, 28 were constructed by F. Hills and Son (Hillson). The aviation knowledge gained by Hillson on this small aircraft gave them the experience they needed to attempt a concept for a “slip wing” fighter. First they would build a proof-of-concept slip wing aircraft and then apply this knowledge to no less an aircraft than the Hawker Hurricane.

The Hillson Praga E-114 “Baby” built in Manchester. F. Hills and Son Ltd gained valuable experience building light aircraft when they were licensed to construct the civilian Baby aircraft by Czech aircraft builder ČKD-Praga. The Praga was a high wing, cantilever monoplane seating two in a side-by-side cabin. This Praga (G-AEON) was sold to an Australian and re-registered as VH-UVP and is photographed here likely at Barton Airfield, Manchester. Photo via R.A. Scholefield Collection

The Hillson Praga was a very uncomplicated aircraft. F. Hills and Son produced 28 copies of the little Czech monoplane, but when the war started, they retooled to produce sub-assemblies for the Avro Anson and other projects. Photo via R.A. Scholefield Collection

The year 1940 saw thousands upon thousands of defensive fighter sorties being scrambled from airfields throughout the south of England during the Battle of Britain, some of which were too late to engage the enemy at altitude let alone gain a height advantage. It was at this time that Mr. W.R. Chown, the Managing Director of Hillson, considered designing a small, lightweight fighter that would employ an extra wing during takeoff, which could be dumped once the aircraft had climbed to an advantage. The Hillson slip wing fighter concept was proposed as a method of producing a cheap fighter that could take off from minimally prepared fields and roads and climb quickly to altitude. The company had no official support from government, so they financed, designed and built a prototype of a small test aircraft with a detachable wing—in just seven weeks!

The aircraft conceived was a small low-wing airplane with two different sizes of detachable upper wing. Officially it was only known as “Experimental Aeroplane N°133” in the wartime series of prototypes. The nickname given it by Hillson was “Bi-Mono”, for its hoped-for ability to change from a biplane to a monoplane mid-flight. Officials were skeptical and thought that throwing away the wing while aloft would cause problems such as fatal damage to the aircraft or even fatal injury to innocent people on the ground.

The aircraft, wearing RAF livery and official Prototype roundels, was test flown in both the biplane and monoplane configurations at Manchester, but without a jettisoning test of the upper wing. Hillson designers experimented with two different spans for the upper slip wing and settled on the smaller of the two as the one that gave the overall best performance. After evaluation by RAF test pilots, it was moved to the west coast city of Blackpool for a test of the detachable wing. The one and only test flight where the wing was detached at altitude was made on 16 July 1941 and proved that separation could be made without any problems. The upper wing flipped off, lifted by its own aerodynamics and fell to the sea and was lost.

During tests, the pilot reported that the maximum speed in the biplane configuration was actually slower than the stall speed of the monoplane configuration. Naturally, the second the wing was detached, the remaining monoplane aircraft went into an immediate stall and dropped “a few hundred feet”. After the Blackpool tests, the aircraft went to RAF Boscombe Down for further testing by the Aeroplane and Armament Experimental Establishment.

The Hillson Bi-Mono slip wing concept aircraft

Over the past month, I have collected several images of this unique and diminutive aircraft from numerous sources on the internet. While some are redundant, I have placed ALL of them together in this article so that there is a repository of all the images found on the web. Perhaps this will intrigue others enough to search for and bring to light other photographs of the Bi-Mono and hopefully images of the test where the wing was jettisoned.

The Hillson FH.40 Slip Wing Hawker Hurricane

Despite the less than spectacular results of flight and jettison tests, there was just enough promise left in the project for the Air Ministry to grant Hillson the use of a somewhat clapped out former Royal Canadian Air Force Hawker Hurricane I to test the concept out further on a full-sized aircraft before committing to further design. The result was the Hillson FH.40, a Hawker Hurricane with a massive and identical second wing propped on slender N-struts high above the fuselage. I was not able to find any report that suggested that the biplane Hurricane ever attempted to jettison its wing, but it seems the project had changed to more of a study of how an extra wing might benefit a Hurricane for ferry flights and getting off the ground with heavier loads than as a possible slip-wing defensive fighter.

The strange aircraft was tested at RAF Sealand during May 1943, and at the Aeroplane and Armament Experimental Establishment at RAF Boscombe Down from September 1943. The upper wing was not released in flight before the program was terminated due to poor performance. As with the Hillson Bi-Mono above, I have collected and published here all of the images I was able to scour from the internet.