Airco DH.6

The Airco DH.6 was a British military trainer  biplane  used by the Royal Flying Corps  during the

First World War. Known by various nicknames, including the "Skyhook", the trainer became a widely used light civil aircraft in the postwar era.[1]

Design and development
The DH.6 was specifically designed as a military trainer, at a time when it was usual for obsolete service types to be used in this role.[2] Geoffrey de Havilland  seems to have had two design criteria in mind.[3] The first was that it should be cheap and easy to build, and above all, simple to repair after the mishaps common in ab initio training. The top and bottom wings were "brutally" square cut, and were interchangeable. (Hence the roundels in unconventional positions on many wartime photographs of the type.) They were heavily cambered, and braced with cables rather than streamlined wires. On the original version of the type there was no stagger. Even the rudder, on the prototype of the usual curved de Havilland outline, was on production machines cut square. The fuselage structure was a straight box with no attempt at refinement of outline – instructor and pupil sat in tandem on basketwork seats in a single cockpit that was Spartan even by the standards of the time.[3] The standard engine was the ubiquitous and readily available 90 hp (67 kW) RAF 1a. Because of its use in the B.E.2 [4] the engine had the advantage of being very familiar indeed to RFC mechanics. It was stuck onto the front of the DH.6 in the most straightforward way possible, without any type of cowling, and the usual crudely upswept exhaust pipes of this type of engine were fitted. Eventually even stocks of the RAF 1a ran short, and various other engines were fitted to DH.6s, including the 90 hp (67 kW) Curtiss OX-5  and the 80 hp (60 kW) Renault.

This was an era when instructors in the RFC referred to their pupils as “Huns” (the term used for enemy airmen) and casualties at training schools were high.[5] The second design criterion was that the new trainer should be "safe" to fly, both for a new pupil and his instructor. One way to obtain this safety was a "decouple" on the dual controls so that the instructor could take control at any time without having to wrestle with a panicking pupil.[3] Another route to the desired safety was through the new trainer’s flying characteristics. De Havilland’s work at the Royal Aircraft Factory, where much basic research had been carried out into the nature of stability and control in aircraft, left him well qualified to design a "safe" aircraft.[3] In the event, the DH.6 had very gentle flying characteristics; it was probably the most "forgiving" aircraft of its time, allowing itself to be flown “crab wise” in improperly banked turns, and being almost impossible to stall or spin , as it was able to maintain sustained flight at speeds as low as 30 miles per hour (48 km/h).[6]

In fact, the DH.6 has been frequently described as "too safe" to make a good trainer;[7] this referred to its gentle reaction to inexpert piloting rather than to excessive stability however, as it was designed with a degree of inherent instability about all three axes.[6]

With the "Skyhook's" low power, strong but rather heavy construction and lack of streamlining, its maximum speed was naturally very low, even by the standards of the time.

Wartime production
At least 2,282 DH.6s were built in the UK during the war, out of orders totalling about 3,000. Besides Airco, batches were built by Grahame-White , Kingsbury Aviation,Harland and Wolff , Morgan, Savages, Ransomes, Sims & Jefferies, and Gloucestershire. A single DH.6 was constructed in July 1917 by Canadian Aeroplanes Ltd.  as a prototype for projected production should availability of the Curtiss JN-4  prove inadequate; it was the first British-designed aircraft built in Canada. In the event, there was no shortage of "Jennies", and it remained a one-off.[8]

Operational history
In 1917, training of RFC pilots began to receive a long overdue overhaul. TheSchool of Special Flying at Gosport in Hampshire was established by Maj. Robert R. Smith-Barry  with the aim of making flight instructors into specially trained experts – rather than entrusting the role to novices who had barely completed their own training,[9] and operational pilots being “rested” to recover from combat fatigue. The Avro 504K was adopted as the standard trainer by the end of 1917, with the DH.6 becoming “surplus” as far as the training role was concerned.[8]

At the end of 1917, about 300 DH.6s were transferred to the RNAS  for anti-submarine patrols. While far from ideal for this work, the type proved surprisingly “seaworthy”, being known to float for as long as ten hours after ditching.[10] On operations, the underpowered trainer had to be flown solo, to allow a token bomb load to be carried. The “built in” instability designed to keep a student pilot alert proved tiring for pilots on long patrols over water, and experimental changes were made in mid-1918 to improve stability. These included the introduction of 10 in (25 cm) of back-stagger to wings of reduced chord and camber, with narrower elevators and rudder. DH.6s modified to this standard were unofficially dubbed "DH 6As".[10]

Over 1,000 DH.6s were still in service in second line roles with the RAF at the end of the war.

Nicknames
Many RFC/RAF aircraft of this period received nicknames (some of which, like the "zoo" names of Sopwith  types, reached semi-official status) but the DH.6 must hold the record for the number and variety of humorous but highly disrespectful epithets. The “skyhook”, a favourite appellation of Australian airmen, probably referred to its lack of speed, although the shape of the exhaust pipes has also been mentioned. Other nicknames for the type included "crab," "clockwork mouse," "flying coffin" and "dung hunter" (these last two on account of the shape of the plywood cockpit, thought to resemble either a coffin or an outside toilet).[11] The type’s over-forgiving nature was probably behind yet another nickname, the "clutching hand," although this may also have been associated with its notorious lack of speed.[7]

Postwar and civil use
There was no place for the DH.6 in the postwar RAF, and survivors of the type became surplus. In 1919, many were sold to civilian operators – especially for "joy riding". Some were exported as far as South Africa and Australia, where they could be found flying into the late 1930s.

Some 60 aircraft were licence-built in Spain from 1921 onward with Hispano-Suiza 8 engines, refined fuselages that included separate cockpits, and rounded "de Havilland style" rudder/fin assemblies. At least some of these found their way into the inventory of two Spanish Air Force training establishments.[12]

A D.H.6 became the first powered aircraft to be owned by a British Scout Troop, when it was presented to 3rd Hampden (Middlesex) Scouts in 1921.[13]

Variants

 * DH.6
 * Two-seat training aircraft.


 * DH.6A
 * Modified to improve stability for patrol duties.

Operators

 * Australia  – Australian Flying Corps
 * No. 5 (Training) Squadron AFC in the United Kingdom.
 * No. 7 (Training) Squadron AFC  in the United Kingdom.
 * Central Flying School AFC at Point Cook, Victoria.
 * Greece
 * Hellenic Naval Air Service
 * Spain
 * Spanish Air Force
 * United Kingdom
 * Royal Flying Corps  / Royal Air Force
 * No. 67 Squadron RAF
 * No. 99 Squadron RAF
 * No. 105 Squadron RAF
 * No. 110 Squadron RAF
 * No. 144 Squadron RAF
 * No. 187 Squadron RAF
 * No. 190 Squadron RAF
 * No. 236 Squadron RAF
 * No. 244 Squadron RAF
 * No. 250 Squadron RAF
 * No. 251 Squadron RAF
 * No. 252 Squadron RAF
 * No. 253 Squadron RAF
 * No. 254 Squadron RAF
 * No. 255 Squadron RAF
 * No. 256 Squadron RAF
 * No. 258 Squadron RAF
 * No. 260 Squadron RAF
 * No. 272 Squadron RAF
 * No. 274 Squadron RAF
 * Royal Naval Air Service

Specifications (DH.6)
Data from De Havilland Aircraft since 1909[14]

General characteristics
 * Crew: As trainer: two, pilot and instructor
 * As patrol aircraft : one
 * As postwar joy riding aircraft: up to three, pilot plus one or two passengers

Performance
 * Length: 27 ft 3½ in (8.32 m)
 * Wingspan: 35 ft 11 in (10.95 m)
 * Height: 10 ft 9½ in (3.29 m)
 * Wing area: 436 ft² (40.5 m²)
 * Empty weight: 1,460 lb (664 kg)
 * Loaded weight: 2,030 lb (923 kg)
 * Powerplant: 1 × RAF 1a air-cooled V8 engine, 90 hp (70 kW)
 * Maximum speed: 70 mph (61 kn, 113 km/h)
 * Stall speed: 40 mph (35 kn, 64 km/h)
 * Rate of climb: 225 ft/min (1.1 m/s)
 * Wing loading: 4.66 lb/ft² (22.8 kg/m²)
 * Power/mass: 0.044 hp/lb (0.076 kW/kg)

Armament
 * Endurance: 2¾ hours

Usually none – although a few small bombs sometimes carried on anti-submarine patrol.