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 . 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 . Geoffrey de Havilland seems to have had two design criteria in mind . 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 . The standard engine was the ubiquitous and readily available 90 hp ( 67 kW ) RAF 1 a . Because of its use in the B . E . 2 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 . 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 . 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 . 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 . In fact , the DH . 6 has been frequently described as " too safe " to make a good trainer ; this referred to its gentle reaction to inexpert piloting rather than to excessive stability however , as it was designed with a degree of inherent " in " stability about all three axes . 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 . Operational history . In 1917 , training of RFC pilots began to receive a long overdue overhaul . The School 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 , and operational pilots being “ rested ” to recover from combat fatigue . The Avro 504 K 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 . 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 . 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 " . 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 ) . 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 . 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 . 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 .