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The earliest British training aircraft to be used in very large numbers was the famous and well-loved Avro 504. The prototype made its first flight at Brooklands on 18 September l9l3, and its classical and simple lines were very advanced for that time: Shortly before the outbreak of the First World War it was adopted as a general purpose machine for use by both the RFC and the RNAS. Small numbers were used operationally by both services, a notable bombing raid being carried out in November 1914 by three naval machines on the Zeppelin sheds at Frieclrichshafen‘ on the shores of Lake Constance.
Despite its advanced design‘ the 504 was not essentially a military machine‘ and consequently had its limitations for front-line use The main problem was that the observer was situated in the front seat, where he was surrounded by wings‘ struts and wires‘ making his task difficult. and this was compounded when operational needs led to the installation of a gun. By 1915, therefore, the 504 was largely relegated to the training role, in which it was to prove the mainstay of the RFC and later RAF for nearly two decades.
Prior to the arrival of the first few trainers from the production line‘ the manufacturers provided a number of self-contained dual control conversion units for existing machines. Early versions of the 504 were mainly powered by the 80 hp Gnome engine‘ this being fitted to the 504A, the first true dual control training version‘ Built in quantity by both the parent company and a number of contractors. this appeared in 1915 and had shorter ailerons than the early 504s, broad-chord struts, and a tailskid attached to the bottom of the rudder‘ This version gave way in 1917 to the 504J, fitted with the rotary 100 hp Gnome Monosoupape, though difficulties in supply of this engine led to mixed production batches of 504As and 504Js. Various modifications were made in service, and it was not uncommon to see variations to the undercarriage, fuel system and cowling.
The subsequent fame of the 504, however. undoubtedly stemmed from the adoption in August 1917 of the 504J by Major R.R, SmitheBarry as the standard trainer at his new School of Special Flying at Gosport With the more powerful engine he was able to introduce instructors to the art of recovery from the various manouevres likely to be encountered in aerial fighting, and they were then able to pass on this knowledge to their own pupils,
The success of the 504J led to its being ordered in quantity for the numerous training units by then in existence at home and overseas. but this only served to exacerbate the engine supply difficulties‘ ln any event, the Monosoupape Gnome was regarded as obsolete by the end of l9l7,and trials had already taken place at Gosport with a 130 hp Clerget. Avros were therefore asked to redesign the engine mounting so that a variety of rotary engines could be fitted. The new design had an overhung mounting consisting of two bearer plates, and allowed the fitment of a more streamlined open~fronted cowling.
ln this form the machine became the 504K, and on its introduction numerous surplus 80 hp Le Rhone, 100 hp Le Rhone and l30 hp Clerget engines were recalled from RFC units for fitment in production machines. lt was originally intended that the 504K would supplant the 504J on the production lines, but in practice there were never enough engines available, and therefore many of the initial batches comprised a mixture of the two versions, and sometimes also the 504A. By February l9l8 the 504K had been adopted as the standard trainer aircraft, a role it was destined to fulfil for ten years with the Royal Air Force‘ which formed shortly afterwards, Planned production of 100 machines a week had almost been achieved by the time of the Armistice‘
Despite having the same engine as aircraft at the front, the 504K was sufficiently docile for it to be handled easily by novice pilots, lt highlighted the shortcomings of the less able pilots, but was capable of being flown at its best only by those of outstanding ability‘ Under the Smith-Barry system, pupils had control of the machine from the front seat, and could keep in touch with the instructor behind him, initially by waggling the control column, but later by the introduction of a ‘Gosport speaking tube, which in appearance and use was somewhat similar to those used by waiters to pass on orders to the kitchen. In its original form it was essentially a long flexible metal hose, to one end of which was fitted a triangular-shaped mouthpiece‘ this being secured to the instructors head by an elasticated strap‘ he passed the other end back to the pupil, who plugged it into a V-shaped earpiece on his helmet It was strictly a oneeway means of communication‘ and much was left to guesswork or hand signals. Use of the tube was rather hazardous in winter, as an instructor with a streaming cold could find his top lip stuck to the canvas bag.
A set routine of instruction was followed in the Smith-Barry system‘ comprising demonstration of controls, straight and level flying, turns. misuse of controls in a turn, then take-offs and landings. Manoeuvres would be analysed and commented on during flight, and after landing there would be a further discussion, during which the instructor would point out any bad habits which were developing. When the pupil was considered sufficiently advanced he would be sent up for his first solo flight.
The 504Jwas declared obsolete in September l92l, but the 504K remained in service after the war with the new Flying Training Schools, until it, too, gradually became outdated, though it was not until September 1926 that the last one left the Avro production line at Hamble. Meanwhile. Avros had been contracted to fit two machines (E9265 and E9266) experimentally with a 175 hp Siddeley Lynx, and in this Specification, redesignated 504N, they were flown to Croydon by their test pilot, Bert Hinkler, in May l9Z2, for demonstrations. The modification showed promise, and was developed with a new oleo-pneumatic undercarriage in place of the original skid type.
There were no resources available at that time for a completely new design of RAF trainer, but this modernisation of the 504 had produced an acceptable compromise. Production orders for the 504N were placed early in l927, and later that year these began to enter service with the Flying Training Schools as well as many other units such as University Air Squadrons and reserve squadrons. Most service squadrons also had one or two on strength, for instrument and refresher flying, plus communications work. In addition, many of the surviving 504Ks were reconditioned from 1927 by the Home Aircraft Depot at Henlow and at the same time brought up to 504N standard‘ Fitting a more powerful engine was not the only modification distinguishing the new version from the 504K, other changes including the fitment of large twin l8-gallon fuel tanks just outside the upper centre section struts, and additional stringers making a more rounded fuselage. The wooden fuselages and tapered ailerons of the early production machines later gave way to welded steel fuselage construction and rectangular Prise ailerons Various versions of the engine were fitted‘ final production aircraft having the 2l5 hp Lynx IVC.
An in-service modification was the fitment in 1931 of blind flying equipment on several aircraft of the Central Flying School at Wittering. This comprised a specially designed blind flying hood over the pupils cockpit, Reid and Sigrist turn indicators and wings with slightly less dihedral to reduce inherent stability. In the early 1930s, crowds at the annual RAF Display at Hendon were treated as a star turn to an exhibition of soecalled ‘crazy flying‘ by experienced pilots flying 504Ns, but by then the type was nearing the end of its useful life. It was gradually withdrawn from 1933, to be largely superseded by the Avro Tutor, and later the de Havilland Tiger Moth. ‘[1]
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- ↑ The History of Britian's Military Training Aircraft. Ray Sturtivant. Haynes Publishing Group. 1987. ISBN 0-85429-579-8. Pages 56 to 58