Bristol Boxkite - Design and Development

Design and Development

The Boxkite was a two-bay biplane with an elevator carried on booms in front of the wings and an empennage consisting of a pair of fixed horizontal stabilisers, the upper bearing an elevator, and a pair of rudders carried on booms behind the wing. There were no fixed vertical surfaces. Lateral control was effected by ailerons on both upper and lower wings. These were single-acting, the control cables arranged to pull them down only, relying on the airflow to return them to the neutral position. The wings and fixed rear horizontal surfaces were covered by a single layer of fabric: the other surfaces were covered on both sides. Power was usually provided by a 50 hp (37 kW) Gnome rotary engine, although other engines were also used. This was mounted on a pair of substantial wooden beams mounted above the lower wing: these continued forward to carry the seats, which were arranged in tandem, with the pilot sitting over the leading edge of the wing. The undercarriage consisted of a pair of long skids, each bearing a pair of wheels sprung by bungee cords, and a single sprung tailskid mounted below the leading edge of the lower tailplane.

The first two Boxkites, assigned works numbers 7 and 8, differed in detail from the later production aircraft: the front outrigger booms were braced by a pair of vertical struts and were attached to the ends of the interplane struts. This arrangement was inherited from the Zodiac, being necessary in that aircraft because the front spar of the wing did not also form the leading edge. Additionally the rear elevator had a straight trailing edge. No. 8 also had double-surfaced wings, the wings of No. 7 being single-surfaced with the ribs enclosed in pockets, like production aircraft. No. 7 was initially fitted with a 50 hp (37 kW) Grégoire, but for its first flight this was replaced by a Gnome, although the Grégoire was later refitted for trial purposes: No. 8 had a 50 hp (37 kW) E.N.V.

The first examples built had upper and lower wings of equal span, although most of the aircraft eventually produced had an extended upper wing and were known as the Military Version. The examples of this type sold to the Russian government and the first aircraft sold to the British Army were fitted with a third rudder hinged to the centre leading edge interplane strut of the tailplane, but this was not made standard.

Two modified Boxkites were produced for competition purposes. The first, No.44, was a single-seater built to compete in the 1911 Circuit of Europe air race and had reduced wingspan and a nacelle for the pilot, similar to the Bristol Type T. The second, No.69, was a redesign by Gabriel Voisin, who was employed as a consultant by Bristol. This had no front elevator, monoplane tail with a single rudder, and a reduced gap between the wings. It was tested at Larkhill in February 1912, but was evidently unsuccessful since it was soon rebuilt as a standard Boxkite.

A total of 76 were built, 61 of which were the extended military version, and it continued in production until 1914.

Although satisfactory by the standards of the day, the Farman design was obsolescent even in 1910, and no serious development of the Boxkite was attempted.