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de Havilland DH-88 Comet
Twin-engine Two-seat Low-wing Long-distance Racing Monoplane, U.K.
Archive Photos 1
de Havilland DH-88 Comet (N88XD, G-ACSS, replica) on display (c.1994) at the Hawthorne Air Faire, Hawthorne, California (Photos by John Shupek)
de Havilland DH-88 Comet (G-ACSS, original) on display 10 September 2002 at the Shuttleworth Collection, Old Warden Aerodrome, Nr Biggleswade, Bedfordshire, England (Photos by John Shupek)
Overview 2
The de Havilland DH-88 Comet is a British two-seat, Twin-engine 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 DH-88 Comet was initiated at the behest of British aviation pioneer Geoffrey de Havilland, along with the support of de Havilland’s board, being keen to garner prestige from producing the victorious aircraft as well as to gain from the research involved in producing it. The DH-88 Comet was designed by A. E. Hagg around the specific requirements of the race; Hagg produced an innovative design in the form of a stressed-skin cantilever monoplane, complete with an enclosed cockpit, retractable undercarriage, landing flaps, and variable-pitch propellers.
Three DH-88 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. DH-88 Comet G-ACSS Grosvenor House emerged as the winner. Two further examples were later built. The DH-88 Comet went on to establish a multitude of 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. Two DH-88 Comets, G-ACSS and G-ACSP, survived into preservation, while a number of full-scale replicas have also been constructed.
Development 2
During 1933, the MacRobertson Air Race, a long distance multi-stage journey 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 confectionary manufacturer, the race would be flown in stages from England to Melbourne.
Despite a number of previous air racing successes by British companies, a new generation of monoplane airliners that were then being developed in America had no viable rival in Britain at the time. Geoffrey de Havilland, a British aviation pioneer and founder of aircraft manufacturing firm de Havilland, was determined that the MacRobertson Air Race ought to be won by a British aircraft. While it was recognized that there would be no prospect of recouping the full investment in developing a new design, the De Havilland board decided to subsidize the project; it was hoped that the endeavor would serve to both enhance the company’s prestige and also provide benefits that would result from the research involved.
Accordingly, it was announced in January 1934 that if three orders could be obtained by 28 February, a specialist racer to be named the Comet would be built and sold for £5,000 each, that would be capable of achieving a guaranteed speed of 200 miles per hour (320 km/h). This price was estimated as being half of the cost of manufacture. Three orders were indeed 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 racing motorist Bernard Rubin.
Design Phase 2
Background: The Great Air Race
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-engine 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&ldquol;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 center 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 center 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 tandem in a cockpit set aft of the wing. While 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 DH-88 could maintain altitude up to 4,000 feet (1,200 m) 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. 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 revised, higher profile to the cockpit to give the pilot marginally improved visibility.
Operational History 2
MacRobertson Race
All three DH-88 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 green and unnamed, G-ACSS 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.
G-ACSP Black Magic
Jim Mollison and his wife Amy (born Amy Johnson) were both famous aviators in their own right and were the first entrants to take off[a] in their own G-ACSP Black Magic.
At 6:30 a.m. on 20 October 1934, they began a non-stop leg to the first compulsory staging point at Baghdad, the only crew who managed to fly this first leg non-stop. Arriving next at Karachi at 4:53 a.m they set a new England-India record. They made two attempts to leave Karachi, the first time they returned when their landing gear failed to retract, they left a second time but had to return after finding they had the wrong map. They finally departed Karachi at 9:05 p.m. for Allahabad. After drifting off course, they made an unscheduled stop at Jabalpur to refuel and discover their position. With no aviation fuel available, they had to use motor car fuel 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 managing director. He engaged C. W. A. Scott and Tom Campbell Black to fly it in the race.
Having landed at Kirkuk to refuel, they arrived at Baghdad after the Mollisons had left but took off again after a fast half-hour turnaround. This time it was Scott and Campbell Black who missed out Karachi and flew non-stop to Allahabad. There they were told they were the first to arrive - they had overtaken the troubled 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.
They took off for Darwin, losing power in the port engine over the Timor Sea but struggled on to Darwin. While mechanics were working on the engine its designer, Frank Halford, saw a news placard back in England and telephoned through to Darwin. Talking it over he concluded that, despite the warning indicator, they should be able to fly on at reduced power. Despite this their lead was now unassailable and after the final mandatory stop and more engine work at Charleville they flew on to cross the finish line at Flemington Racecourse at 3.33 p.m. (local time) on 23 October. Their official time was 70 hours 54 minutes 18 seconds.
G-ACSR
The third Comet, G-ACSR had been painted in British racing green by Bernard Rubin who was a successful motor race driver. He had intended to fly it himself along with Ken Waller but had to pull out at the last minute due to ill health and instead engaged Owen Cathcart Jones to take his place.
On reaching Baghdad, they overshot it in the dark, landing by a village when they ran low on fuel. Leaving at first light, they just made it to Baghdad on empty tanks. On taking off again they found that they had a serious oil leak and had to return for repairs. These repairs were carried out by T.J. Holmes RAF (while in Baghdad on RAF secondment. More trouble was encountered on the Darwin leg so they landed at Batavia, where engineers employed by KLM, who had entered the DC-2, made repairs for them. They were the fourth aircraft to reach Melbourne, in a time of 108 h 13 min 30 sec. Cathcart Jones and Waller promptly collected film of the Australian stages of the race and set off the next day to carry it back to Britain. Their return time of 13 days 6 hr 43 min set a new record.
After the Race
Grosvenor House was dismantled and shipped back to England. It was later bought by the Air Ministry, given the military serial K5085, painted silver overall with RAF markings and flown to RAF Martlesham Heath for evaluation by the Aeroplane and Armament Experimental Establishment. It made several flights before being written off in a heavy landing and sold for scrap. It was subsequently sold on, rebuilt and fitted with Gypsy Six series II engines and a castoring tailwheel. In this form it made several race and record attempts under various names. It claimed fourth place in the 1937 Istres-Damascus-Paris race and later the same year lowered the out-and-home record to the Cape to 15 days 17 hours. In March 1938, A. E. Clouston and Victor Ricketts made a return trip to New Zealand covering 26,450 mi (42,570 km) in 10 days 21 hours 22 minutes.
In G-ACSR, the day after they finished the race Cathcart Jones and Waller took off on the return journey. Suffering engine trouble, at Allahabad they found the Mollisons still there and were generously given two good pistons from Black Magic to allow them to continue. Arriving back in England they set a new round-trip record of 13 days, 6 hours and 43 minutes. That December, named Reine Astrid in honor of the Belgian queen, G-ACSR flew the Christmas mail from Brussels to Leopoldville in the Belgian Congo.[page needed] It was then sold to the French government and modified as mail plane F-ANPY, its delivery flight setting a Croydon-Le Bourget record on 5 July 1935. It subsequently made Paris-Casablanca and Paris’Algiers high-speed proving flights. Formerly believed destroyed alongside F-ANPZ (see below), F-ANPY was last seen in an unflyable condition at Étampes in France in 1940.
Black Magic was sold to Portugal for a projected flight from Lisbon to Rio de Janeiro. Re-registered CS-AAJ and renamed Salazar it was damaged on its attempted takeoff at Lisbon Cintra Airport for the Atlantic crossing. On a later return flight from Hatfield it made a record flight from London to Lisbon, setting a time of 5 hr, 17 min in July 1937.
Other Comets
Following the French government’s acquisition of F-ANPY (see above), they ordered a fourth Comet, F-ANPZ, with a mail compartment in the nose. It was later taken on charge by the French Air Force before being destroyed in a hangar fire at Istres in France in June 1940.
The fifth and last Comet, registered G-ADEF and named Boomerang, was built for Cyril Nicholson. It was piloted by Tom Campbell Black and J. C. McArthur in an attempt on the London-Cape Town record. It reached Cairo in a record 11 hr, 18 min, but the next leg of the journey was cut short due to oil trouble while in flight over the Sudan. On 21 September 1935, Campbell Black and McArthur took off in Boomerang from Hatfield in an another attempt at the Cape record. The aircraft crashed while flying over Sudan on 22 September 1935 due to propeller problems, the crew escaping by parachute.
Operators 2
de Havilland DH-88 Specifications 2
General Characteristics
Performance
References