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 the use of modern techniques. It was designed to meet the specific requirements of the race. It was the first British aircraft to incorporate in one airframe all the elements of the modern high speed aircraft - stressed-skin construction, cantilever monoplane flying surfaces, retractable undercarriage, landing flaps, variable-pitch propellers and an enclosed cockpit.
Three Comets were produced for the race, all for private owners, at the discounted price of £5,000 per aircraft. The aircraft had a rapid development process, performing its maiden flight only six weeks before the race. Comet G-ACSS Grosvenor House eventually won the race. Another two Comets were built after the race. Comets established many aviation records, both during the race and afterwards, and also took part in further races. Three were bought and evaluated by national governments, typically as mail planes. Two Comets, G-ACSS and G-ACSP, survived while a number of full-scale replicas have also been constructed.
Development Background: The Great Air Race
During 1933, the MacRobertson Air Race, a multi-stage flight from the United Kingdom to Australia, was being planned for October 1934, to celebrate the 100th anniversary of the Australian State of Victoria. Sponsored by Macpherson Robertson, an Australian confectionery manufacturer, the race would be flown in stages from England to Melbourne.Lewis 1970, p. 257."The England–Australia Race: £10,000 for Fastest Flight from London to Melborne". Flight, Vol. XXV, No. 1284, 3 August 1933, p. 770.
The new generation of monoplane airliners which were then being developed in America had no rival in Britain at the time, so Geoffrey de Havilland, a British aviation pioneer and founder of aircraft manufacturing firm de Havilland, was determined that, for the sake of national prestige, Britain should put up a serious competitor.Ogilvy 1988, p. 16. While the company board recognised that there would be no prospect of recouping the full investment of producing such a machine, they believed that the project would enhance the company's prestige and also provide experience in the development of fast monoplanes.Ogilvy 1988, p. 19.
They therefore announced in January 1934 that, if three orders could be obtained by 28 February, a specialist racer which could achieve a guaranteed speed of to be named the Comet would be built and sold for £5,000 each.Flight, 18 Jan 1934, p. 59. "...that a limited number of these machines will be built, and that the price will be £5,000. A deposit of 20 per cent. will be demanded with the order, and the company will guarantee a top speed of at least 200 m.p.h. If that speed is not attained, the customer will be at liberty to cancel his order, and all money paid by him will be refunded. In order to ensure ample time for development and tests, it is pointed out that instructions to begin construction should be placed before the end of February." This price was estimated as being half of the cost of manufacture. Three orders were received by the deadline; one from Jim Mollison, to be flown by him and his wife Amy (better known as Amy Johnson), one from Arthur Edwards, a hotel owner and manager, and the last from motorcar racer Bernard Rubin.Ogilvy 1988.
Design phase 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".Ogilvy 1988, p. 155. 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, this would have had inadequate performance so the designer, A. E. Hagg, turned to a more innovative design. This was a cantilever monoplane with enclosed cockpit, retractable undercarriage and flaps. In order to achieve take-off at a reasonable speed with a high all-up weight and 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 built almost entirely of wood, the use of metal being limited 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 used 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.NACA 1935, p. 4. 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.Ogilvy 1988, p. 22. The exterior skin was treated using 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.Flight 20 September 1934, p. 971.
Aerodynamic efficiency was a 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, so the wing skin had to carry most of the loading in a "stressed-skin" construction.Flight 20 September 1934, pp. 968, 971. 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 wide spruce planking laid diagonally across the wing, with the outer layer laid crosswise over the inner. These strips varied in thickness according to the loads they carried, reducing over the span of the wing from at the root to towards the tip. 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 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 (1.8mm) spruce.Ogilvy 1988, pp. 17–18. 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.Winter, H.T. Flight Today and Tomorrow. 1st Edition, Blackie, ca. 1949. p. 232.
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 depended on the skin. Fuel was carried in three fuselage tanks; the two main tanks were in the nose and centre section, in front of the cockpit. A third auxiliary tank, of only 20 gallon capacity, was placed immediately behind the cockpit and could be used to adjust the aircraft's trim. The pilot and navigator were seated in tandem in a cockpit set aft of the wing. Although dual flight controls were fitted, only the forward position had a full set of flight instruments. The rear crew member could also see many of the pilot's instruments by craning sideways while seated. The cockpit was set low in order to reduce drag and forward visibility was very poor. The engines were uprated versions of de Havilland's newly developed Gipsy Six, race-tuned for optimum performance with a higher compression ratio and with a reduced frontal area; the aircraft could maintain an altitude of up to on one engine. The main undercarriage retracted backwards into the engine nacelles and was operated manually, requiring 14 turns of a large handwheel located on the right hand side of the cockpit.
The challenging production schedule meant that flight tests of the DH.88 began just six weeks prior to the start of the race. Hamilton-Standard hydromatic variable-pitch propellers were initially fitted.Ogilvy 1988, p. 20. During testing, the propeller blade roots were found to interfere unacceptably with the airflow into the engine. Instead, a French two-position pneumatically actuated Ratier type was substituted. Its blades were manually set to fine pitch before takeoff using a bicycle pump, and in flight they were repositioned automatically to coarse (high-speed) pitch via a pressure sensor. A drawback was that the propellers could not be reset to fine pitch except on the ground. Other changes included the installation of a large landing light fitted in the nose, and a higher cockpit profile to give the pilot marginally improved visibility.Ogilvy 1988, p. 24.
Operational history
MacRobertson Race All three Comets lined up for the start of the race at Mildenhall, a newly established airfield in Suffolk shortly to be handed over to the RAF. G-ACSP was painted black and named Black Magic, G-ACSR was green and unnamed and G-ACSS was red and named Grosvenor House. The three aircraft took their places among 17 other entrants, which ranged from new high-speed Douglas DC-2 and Boeing 247 airliners to old Fairey Fox biplanes.Lewis 1970, p. 270.
G-ACSP Black Magic G-ACSP was the only Comet to be flown by its owner. Jim Mollison and his wife Amy were the first to start, taking off at 6:30a.m. G-ACSP, named Black Magic began a non-stop flight to the first compulsory staging point at Baghdad, the only crew who flew this leg non-stop.Taylor 1979, p. 54. Their next stop was at Karachi, landing at 4:53a.m, and setting a new England-India record. They made two unsuccessful attempts to depart, returning after the first when their landing gear failed to retract, and after the second on discovering that they had the wrong map. They finally left Karachi at 9:05p.m. for Allahabad. After drifting off course, they made an unscheduled stop at Jabalpur to refuel and determine their position. Since there was no aviation fuel available, they had to use motor car petrol provided by a local bus company; a piston seized and an oil line ruptured. They flew on to Allahabad on one engine but, by now needing completely new engines, were forced to retire.
G-ACSS Grosvenor House
Arthur Edwards named his red Comet G-ACSS after the Grosvenor House Hotel, of which he was the managing director. He engaged C. W. A. Scott and Tom Campbell Black to fly it in the race.Lewis 1970, pp. 269–270.
Having landed at Kirkuk to refuel, they arrived at Baghdad after the Mollisons had left but took off again after a half-hour turnaround. Scott and Campbell Black missed out Karachi and flew non-stop to Allahabad where they were told they were the first to arrive, having overtaken the Mollisons. Despite a severe storm over the Bay of Bengal, in which both pilots had to wrestle with the controls together, they reached Singapore safely, eight hours ahead of the DC-2.