DH.6: Two-seat training aircraft.
DH.6A: Modified to improve stability for patrol duties.
The DH.6, nicknamed the "Skyhook", was specifically designed as a military trainer for the RFC. Geoffrey de Havilland 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 training. The second was that it should be safe for inexperienced pilots and their instructors.
The top and bottom wings were square cut and were interchangeable. (Hence the roundels in unconventional positions on many wartime photographs of the DH.6.) They were heavily cambered, and braced with cables rather than streamlined wires. On the original version of the DH.6 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. The 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 readily available 90 hp (67 kW) RAF 1a. Because of its use in the B.E.2 the engine had the advantage of being very familiar 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.
The safety criterion was partly achieved by adding 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. The DH.6 was designed to have 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).
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.
Flying instructors were often newly trained pilots with little experience or combat fighters who were brought home for light duties while recovering from combat fatigue. In 1917 the School of Special Flying at Gosport in Hampshire was established with the aim of training expert flight instructors. As a training aircraft, the DH.6 was being phased out and replaced by the Avro 504K, which became the standard trainer by the end of 1917. Consequently, 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 DH.6 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 heavy bombs to be carried. The “built in” instability designed to keep a student pilot alert proved tiring for pilots on long patrols over water, and design 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.
Adapted from various sources including Wikipeadia.