page_title: De Havilland DH.88 Comet
section_title: Design phase
hierarchical_section_title: De Havilland DH.88 Comet / Development / Design phase
caption_attribution_description: English: Cockpit of the DH.88 Comet Racer G-ACSS 'Grosvenor House' as it is in 2010 at the Shuttleworth Collection.
context_page_description: The de Havilland DH.88 Comet is a British two-seat, twin-engined aircraft built by the de Havilland Aircraft Company. It was developed specifically to participate in the 1934 England-Australia MacRobertson Air Race from the United Kingdom to Australia.
  Development of the Comet was seen as both a prestige project and an entry into modern techniques. It was designed around the specific requirements of the race. Despite being made of wood, it was the first British aircraft to incorporate all the elements of the modern fast monoplanes in one airframe.
  Three Comets were produced for the race, all for private owners at the discounted price of £5,000 per aircraft. The aircraft underwent a rapid development cycle, performing its maiden flight only six weeks prior to the race. Comet G-ACSS Grosvenor House emerged as the winner. Two further examples were later built. The Comet went on to establish many aviation records, both during the race and in its aftermath, as well as participating in further races. Several examples were bought and evaluated by national governments, typically as mail planes.
context_section_description: Although designed around the requirements for the MacRobertson race, owing to its unusual requirements the Comet did not fit the standard technical specification for a racing aircraft, nevertheless it was classed as a "Special, sub-division (f), Racing or Record". De Havilland paid special attention to the non-stop range necessary for the long official stages. They initially intended to produce a twin-engined two-seat development of the DH.71 experimental monoplane. However it would have insufficient performance so the designer, A. E. Hagg, turned to a more innovative design. He chose a modern cantilever monoplane with enclosed cockpit, retractable undercarriage and flaps. In order to achieve take-off at a reasonable speed and with high all-up weight, combined with a satisfactory high-speed cruise, it would be necessary to fit variable-pitch propellers.
  The resulting design had a low, tapered high aspect ratio wing and was powered by two Gipsy Six R engines, a specially-tuned version of the new Gipsy Six. The aircraft was composed almost entirely of wood, the limited use of metal being confined to high-stress components, such as the engine bearers and undercarriage, and to complex curved fairings such as the engine cowlings and wing root fairings. The sheet metal parts comprised a lightweight magnesium-aluminium alloy. Manually-actuated split flaps were fitted beneath the wing's inboard rear sections and lower fuselage, while the Frise ailerons were mass-balanced by lead strips within the aileron's leading edges. Both the rudder and elevators fitted to the conventional tail had horn mass balances. In order to validate the wing design, a half-scale model wing was built and tested to destruction. The exterior skin was treated via a time-consuming and repetitive process of painting and rubbing down to produce a highly smooth surface to reduce air friction and increase overall speed.
  Aerodynamic efficiency was a major design priority and it was therefore decided to use a thin wing of RAF34 section. This was not thick enough to contain spars of sufficient depth to carry the flight loads and so the wing skin would have to carry most of the loads in a "stressed-skin" construction. However, the complex curves required for aerodynamic efficiency could not be manufactured using plywood. Hagg, who also had experience as a naval architect, adapted a construction technique previously used for building lifeboats. The majority of the wing was covered using two layers of 2-inch (50 mm) wide spruce planking laid diagonally across the wing, with the outer layer laid crosswise over the inner. These strips were of variable thickness, according to the loads they carried, reducing over the span of the wing from 0.5 in (12.5 mm) at the root to 0.14 in (3.5 mm) towards the tips. It was built as a single assembly around three box-spars located at 21, 40 and 65 percent chord: there was an intermediate spruce stringer between each pair of spars to prevent buckling.  The ribs were made of birch ply and spruce. The outboard 6 ft (1.5 m) were skinned with various thicknesses of ply because of the difficulty of machining spruce planking to less than 0.07 in thickness. The leading edge, forward of the front spar, was also ply covered. The centre section was reinforced with two additional layers of 0.07 in spruce. This method of construction had been made possible only by the recent development of high-strength synthetic bonding resins and its success took many in the industry by surprise.
  The fuselage was built principally from plywood over spruce longerons, while the upper and lower forward section were built up from spruce planking in order to achieve the necessary compound curves. As with the wing, the strength of the structure was dependent upon the skin. Fuel was carried in three fuselage tanks. The two main tanks filled in the nose and centre section in front of the cockpit. A third auxiliary tank, of only 20 gallon capacity, was placed immediately behind it and could be used to adjust the aircraft's trim.  The pilot and navigator were seated in 
caption: Cockpit of G-ACSS Grosvenor House (2010).  This differs considerably from the original state.
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