De Havilland DH93 Don
Why was this advanced design rejected?
The de Havilland Don prototype as originally built, complete with turret. At this stage, it has not yet acquired the finlets under the tailplane. It is carrying its "B" class registration number of "E-3". An extremely "clean" aircraft, the air-cooled Gypsy XII did not need a radiator. The wheels protruded a little when retracted, which would have helped to reduce damage in the event of a wheels-up crash-landing. Note the small "bump" under the fuselage, a feature of both the turret equipped aircraft, it was probably associated with a prone bomb-aimer's position. It should also be noted that the dark colour of the fuselage and wing under-surfaces is probably due to the use of orthochrome film, they were most likely to have been yellow.
Designed to Air Ministry specification T6/36 the beautiful looking DH93 Don was an all-in-one training workhorse. A student pilot and instructor sat side-by-side up front, while behind in the cabin was accommodation for a trainee WT (radio) operator and behind that a hand-cranked turret for a trainee air gunner with a Lewis gun. In the wing was a Browning machine gun so that the student pilot could get in some air gunnery training, and there were racks for 16 practice bombs (little 2 pounders) it is presumed that there was a hatch in the floor for a bomb-sight to allow a trainee bomb-aimer to practice his art. Power was provided by de Havilland's air-cooled Gypsy XII ¹ engine of 425 hp (the air was taken in by two inlets at the root of the wings and directed over the engine from the rear). With a retracting undercarriage and variable pitch prop, the Don was very state-of-the-art when the prototype took to the air on the 18th June 1937. The Don was named after the title used by British university professors, clearly reflecting its role as a trainer. Its wooden construction was based on the practices pioneered in the DH 88 Comet racer and DH 91 Albatross airliner, construction methods bought to perfection in the DH98 Mosquito. The Air Ministry ordered 250 examples of the Don even before the prototype flew, specifying that it also needed a version capable of being used as a 4-6 seat communications aircraft.
The Don prototype L2387 in later configuration with finlets under the tailplane which were added after initial testing and used on all subsequent production aircraft. Note the small practice bomb under the wing. The "bump" under the fuselage is more pronounced in this view.
Initial testing of the prototype showed up unsatisfactory stalling characteristics and heavy controls, both highly undesirable in a training aircraft. De Havilland fixed the issues with the addition of strips along the leading edge of the wing towards the wingtips and small finlets under the tailplane. The Don already had anti-spin strakes in front of the tailplane (like those used on the de Havilland Chipmunk a decade later). Various other modifications were carried out to improve control.
Another view of the prototype, this time in flight. Note the prominent anti-spin strakes in front of the tailplane and the dark lines on the leading edge of the wings where strips had been added to improve the stalling behavior. Only L2387 and L2388 were built with turrets.
The Don Turret in close up. The purpose of the "box" to the rear of the turret is unknown, it may simply be a crude way of avoiding the trainee gunner shooting his own tail off! Another suggestion is that it contained a small emergency anti-spin parachute, a device often fitted to aircraft under evaluation at the Aeroplane and Armament Experimental Establishment (A&AEE). The small fin on top of the turret counteracted the drag of the gun barrel to make the turret easier to move around. Note that a direction-finding (DF) radio loop is mounted on the top of the cabin.
The philosophy behind the design was that just one type of training aircraft could meet the bulk of the needs of training the whole range of aircrew; pilots, observers (navigator/bomb aimers), radio operators and air gunners. With the Airspeed Oxford complimenting it in smaller numbers to finish off the training of pilots for multi-engined aircraft. However, it quickly became apparent that large numbers of multi-engined trainers would be needed to train the pilots of the many multi-engined bombers ordered into production for the RAF expansion, and that these multi-engined trainers would be better "flying classrooms" than the cramped DH93. It was also evident that a trainer with higher performance would be required to prepare pilots for the new Hurricane and Spitfire fighters recently ordered into production. So the Air Ministry cancelled the DH93 and instead ordered more examples of the Avro Anson (which had started as a small airliner adapted for coastal patrol aircraft) for use as a trainer and increased orders for the Airspeed Oxford (23/36). Also the Miles Master² was ordered as a trainer for fighter pilots (later augmented by the Harvard from the USA) and the smaller and lower-performance single-engined Percival Proctor (20/38) was obtained specifically for training WT operators. This meant the only role left open to the Don was that of communications aircraft, which had been an afterthought to the original specification. Don L2391 was evaluated in this role at A&AEE Martlesham Heath and was found to have some shortcomings. The passenger cabin was cramped and it was difficult to get into, with a large drop from the door to the floor inside. Access onto the wing to get to the cabin door was also difficult and it was felt that entry into the Don would be a problem for anyone with even a slight disability. The front seats were uncomfortable and there was no provision for heating the cabin.
The production run for Dons was cut to 50 aircraft and 48 of these were in the 6-4 seat passenger configuration with the rear turret deleted. Only 30 were built to full flying condition. One (L2394) was on the strength of 24 Squadron and 10 were allocated to various RAF stations as communication "hack" aircraft. One (L2412) was allocated to the Royal Aircraft Establishment, one to A&AEE at Martlesham Heath (L2391) and another (L2407) went to No 1 Electrical and Wireless School at Cranwell. Another ten aircraft were allocated to various flying training establishments around the UK, but some of these may have only been used for static instruction. The rest were delivered without engines to be used as instructional airframes at RAF technical schools. A full list of Don production number, serial numbers and brief notes on their use can be found in a table published in the Summer (June) 2013 edition of Air Britain Aeromilitaria magazine at the end of a short article by Phil Butler. This list is at odds in several respects with the information in Putnam's "De Havilland Aircraft since 1909" by AJ Jackson.
Some sources say the Don's Gypsy XII engine proved to be a bit troublesome, overheating being the main problem reported. Only 95 examples of the Gypsy XII were ever built and once the production of the only two aircraft to use it were cancelled (the Don and the DH 91 Albatross airliner) de Havilland would have had little incentive to develop it. Only one Don was ever lost in a crash, that was Don L2391 on 22nd of September 1938 at A&AEE. Its engine stopped responding to the throttle and a cross-wind landing had to be attempted; the aircraft stalled a few feet above the ground and the undercarriage collapsed, no one was seriously hurt.
The DH98 Don as it entered into brief service, as a communication aircraft with turret deleted.
Various commentators explain away the perceived "failure" of the DH 93 by saying the aircraft was rejected after Air Ministry testing at Martlesham Heath, implying it was found to be too heavy for the power of its engine. One source lays the blame on the extra weight of equipment to be carried by the Don specified by the Air Ministry³, while another strong possibility is that the Gypsy XII never developed the full power expected of it. I find it highly suspicious that contemporary descriptions of the Gypsy XII engine list it as rated as 525 horsepower, yet modern ones (including the authoritative Lumsden's "British Piston Engines and their Aircraft") list it as only 425 horsepower. If the Gypsy XII was expected to produce 525 hp but only produced 425 hp that would certainly explain a lack of performance. The Don did not reach the top speed of 200 mph (322 kph) originally specified in 6/36, falling short by 11 mph (17 kph). But surely the major cause for the Don's cancellation was a realisation by the Air Ministry that one type of aircraft was never going to meet all its training requirements. In the late '30s, the priorities for aircrew training changed considerably. The need for longer and better training in the skills of navigation becoming particularly evident (read C.G. Jeffords book "Observers and Navigators" or this short article from the old "Tee Emm" magazine for details of the big changes in the roles and training of navigators in this period), and the extra room in twin-engined aircraft such as the Anson and Oxford had many advantages for such training.
Another view of a Don in Communications form. The RAF, Royal Navy and Air Transport Auxiliary had a desperate need for aircraft in this class for ferrying staff and equipment around. Various civil types were impressed for service to fill this gap. .
De Havilland was not as dependent on RAF orders as other British aircraft companies, preferring to build up their position in the civil aviation marketplace, a policy that had been a resounding success, by the mid-1930's they were the most profitable British aviation company. With them setting up a production line for 250 Don aircraft and then seeing the order reduced to only 50 you can see one drawback of doing government work! However, de Havilland was rewarded with extra orders for Tiger Moth trainers and was even asked to build some of the Oxford trainers to be used in place of the Dons. Furthermore, at the start of hostilities, the DH Dragon Rapide small airliner was retained in production (renamed the Dominie) to be used as flying classrooms to train navigators and radio operators. Oxfords were produced at the de Havilland factory at Hatfield alongside Dominies and Queen Bee target drones until the Mosquito was ordered, when the Dominie line went to Loughborough the Oxford line was closed down.⁴
This photo appeared in the British aeronautical press in March 1940 showing a Don in use as an instructional airframe at No 2 school of technical training (RAF Cosford). Only some 33 months after the first flight of the prototype Don. The trainees would have found the experience of working on the Don of great benefit if they later worked on the DH Mosquito, which used the same construction techniques.
All the published sources I can find indicate that, by the start of WW II, there were no Dons left flying with the RAF. They had all been relegated to instructional airframes (it was the availability of the Gypsy XII engines stripped from them which allowed the small fleet of DH91 Albatross airliners, to be kept flying until 1943). However Janic Geelen, the author of "Magnificent Enterprise - Moths Majors and Minors" suggest that de Havilland may have retained one Don for use for communications and testing duties into the early war years. A table of aircraft histories in Putnam's "De Havilland Aircraft since 1909" lists Don L2412 as still being flown in October 1940 as "E-0232" which suggests it was indeed being used for some testing purpose (de Havilland gave their prototypes and testing machines B-class "E" numbers as in the photo of "E3" at the top of this page). Apparently, a Don was used to test a de Havilland designed propeller with reverse-pitch that could act as an airbrake, perhaps that was L2412. Bill Gunston, the prolific aviation author, said he encountered a Don at de Havilland's airfield at Hatfield when he visited there as an Air Cadet in 1942 and was allowed to get inside to have a look around, he noted that even the pilot's position seemed cramped. Gunston also said he talked to a pilot who had just flown one into the RAF airfield at Halton; he reports the pilot as saying the Don was "Not a patch on the Proctor".⁵ All of this implies that at least one Don was kept flying well into the Second World War.
A photo published in the September 2013 edition of Air Britain's "Aeromilitaria" magazine showed a Don fuselage (minus engine) being used as a children's climbing frame at the April 22nd 1947 "open day" at the Walsall (Aldridge) airfield, perhaps the last surviving Don airframe, it was probably an old instructional airframe from either the nearby RAF Hednesford or RAF Cosford.
Max Speed: A respectable 189 mph (304 kph), quite high for an aircraft of this size considering its engine only produced 425 hp. Speed was virtually identical to the Anson and 20mph (32 kph) faster than the Oxford
Range: About 900 miles (1,448 km), again quite good for a training aircraft of this vintage and surprisingly superior to both the twin-engined Anson and Oxford.
Ceiling: 23,300 ft (7,102 metres). More than enough for its intended role and considerably higher than either the Anson or Oxford.
DH 93 Don, Note how similar the tailplane is to the famous DH88 Comet racer.
What If ?
Above: I've painted these Don's as if they had entered service in their original role - In this case in the markings of No 1 Air Observers School at Desford around 1939 (they actually used Avro Ansons). If the Don had entered large-scale production maybe de Havilland would have been satisfied they were "doing their bit" - and never gone on to develop the outstanding DH 98 Mosquito?
¹ The Gypsy XII was later renamed the Gypsy King I.
² It is worth noting that the Miles Master started life as the Miles M9 Kestrel project which was itself tendered to the same specification as the DH93 (Air Ministry Spec T6/36).
³ Industry and Air Power, The Expansion of British Aircraft Production, 1935-1941 by Sebastian Ritchie, Chapter 3,(page 106).
⁴ Thanks to Janic Geelen for putting me right on the way de Havilland moved production around to cope with the demands of the war - see his book "Magnificent Enterprise - Moths Majors and Minors".
⁵ Quoted in Gunston's book "Back to the Drawing Board" - see sources below.
"Out-moded Teacher - De Havilland's Don Crew Trainer" - An article by Daniel Ford in Air Enthusiast magazine edition 105 May/June 2003
"The British Aircraft Specification File" by KJ Meekoms and EB Morgan, an Air-Britain publication. ISBN 0 85130 220 3.
"The Discarded Don" a two page article in the Winter 2000 (issue 104) of Air Britain Aeromilitaria magazine. Unattributed , probably by the magazine editors, James Halley and Ray Sturivant.
"The de Havilland Don" a 3-page article by Phil Butler in the Summer (June) 2013 edition (no 154) of Air Britain "Aeromilitaria" magazine.
"DH - A History of de Havilland" by C. Martin Sharp has a mention of the Don being used to test reverse pitch propellers. Airlife ISBN 0 906393 20 5.
"De Havilland Aircraft since 1909" by AJ Jackson, Putnam ISBN 0 370 30022 X (2nd Edition).
"Back To The Drawing Board - Aircraft That Flew But Never took Off" by Bill Gunston, published by Airlife in 1996. ISBN 1 85310 758 1.