A.Jackson Avro Aircraft since 1908 (Putnam)

Avro 521

   Designed late in 1915, the Avro 521 two seat fighter-trainer was a hybrid embodying the features of several Avro 504 variants. In side elevation the straight top longerons proclaimed it a derivative of the 504 prototype, yet the short span ailerons and the rudder/tail skid assembly were pure 504A, the cockpit positioning and centre section struts were 504E, the Vee strut undercarriage was contributed by the 504G and the streamlined headrest was copied from the Avro 519. Standard Avro 504 mainplanes were shortened to a span of approximately 27 ft. 6 in., cut away at all four wing roots to improve upward and downward vision and rigged with only a single set of interplane struts on each side. The engine was a 110 h.p. Clerget rotary in characteristic Avro cowlings.
   The initial order was for one machine, test flown at Trafford Park, Manchester by F. P. Raynham with H. E. Broadsmith standing up in the rear cockpit and brandishing a dummy machine gun to enable the effect of the extra drag to be assessed. Raynham found the Avro 521 longitudinally unstable and unpleasant to fly; nevertheless it was delivered to Farnborough in February 1916 and 25 production machines were ordered for the R.F.C.
   Proposals were also made for interchangeable wings to suit different roles but it is not known if any aircraft were actually modified. Designation Avro 521A was allotted to a version with mainplanes of 46 ft. span, while with standard Avro 504 mainplanes of 36 ft. span it was to have been known as the Avro 521B. It is doubtful if all were completed and there is no record of any production Avro 521 having been delivered. This strengthens the belief that the Avro 521 in which Capt. Garnett crashed and was killed at Gosport in 1917 must have been the prototype.

SPECIFICATION AND DATA
   Manufacturers: A. V. Roe and Co. Ltd., Clifton Street, Miles Platting, Manchester
   Power Plant: One 110 h.p. Clerget
   Production: One unmarked prototype Works Order number believed 1811; and twenty-five production aircraft 7520 to 7544, believed not all built
   Service Use: At the Advanced Training School, Gosport, Hants.

P.Lewis The British Fighter since 1912 (Putnam)

The unqualified success of the Avro 504 prompted the parent company to construct during early 1916 a two-seat fighter variant under the type number 521. The wing structure was cleaned up by conversion to single-bay cellules and the same streamlining process applied to the undercarriage resulted in a simple V-strut structure. The rear cockpit was set some distance behind that of the pilot, above whom was a generous cut-out in the trailing edge of the centre-section.
   The 110 h.p. Clerget powered the Avro 521, which was test-flown by F. P. Raynham and found to have disagreeable flying characteristics. Nevertheless, despite the crash of the prototype in the hands of an R.F.C. pilot, twenty-five production 521s were built but did not apparently reach operational service in their intended role.

F.Mason The British Fighter since 1912 (Putnam)

Avro Type 521

   This single-bay derivative of the famous Avro 504 was intended to be a two-seat fighter. It was designed late in 1915 and embodied features from several 504 subvariants, including the straight upper longerons of the 504 prototype, the short-span ailerons and tail unit of the 504A, the cabane struts peculiar to the 504E and the V-strut undercarriage of the 504G.
   The wing span was reduced from the 504’s 36ft to about 27ft 6in, and generous cut-outs were provided in upper and lower wings, as were ailerons. Power was provided by a 110hp Clerget rotary in cowlings reminiscent of the 504.
   Mr J M Bruce has speculated that the Type 521 was evolved by way of a naval version of the 504, which may or may not have been completed; however, he points out that, not being intended for the RNAS, the 521 was fitted with a balanced rudder without fixed tailfin.
   The prototype was flown early in 1916 at Trafford Park, Manchester, by Fred Raynham, who complained that it was longitudinally unstable and ‘unpleasant to fly’; it was however sent to the Royal Aircraft Factory for evaluation in mid-February.
   In due course a production order followed for 25 aircraft, intended for the RFC, but it seems that few, if any, were delivered, and that the Type 521 which is known to have crashed at Gosport in 1917 was in all likelihood the prototype.


   Type: Single-engine, two-seat, single-bay scout biplane.
   Manufacturer: A V Roe & Co Ltd, Miles Platting, Manchester.
   Powerplant: One 110hp Clerget rotary engine driving two-blade propeller.
   Structure: All-wood construction with fabric covering.
   Dimensions; Span, approx 27ft 6in.
   Prototype and Production: One prototype, No 1811 (first flown by F P Raynham in January or February 1916), 25 production aircraft ordered, Nos 7520-7544, but it is not known how many, if any, were completed.

W.Green, G.Swanborough The Complete Book of Fighters

AVRO 521 UK

   The Avro 521 two-seat fighter, which was flown late in 1915, was something of a hybrid in that it embodied a number of Avro 504 components. Powered by a 110 hp Clerget 9Z nine-cylinder rotary, the prototype had provision for a free-mounted 0.303-in (7,7-mm) Lewis gun fired from the rear cockpit. The prototype underwent official trials early in 1916, and 25 aircraft were ordered for the RFC, but this contract was subsequently cancelled, and there is no evidence that any Avro 521 other than the prototype (which crashed at Upavon on 21 September 1916) was built.

Max speed, 90 mph (145 km/h) at sea level.
Time to 6,000 ft (1830 m), 14 min.
Endurance, 4.5 hrs.
Empty weight, 1,150 lb (522 kg).
Loaded weight, 1,995 lb (905 kg).
Span, 30 ft 0 in (9,14m).
Length, 28 ft 2 in (8,58 m).
Wing area, 266 sq ft (24,71 m2).

J.Bruce British Aeroplanes 1914-1918 (Putnam)

Avro 521

  THE Avro 521 was a derivative of the Avro 504, and was intended to be a two-seat fighter. It bore a family likeness to its great progenitor, and was probably derived via a R.N.A.S. version of the 504 which had (or was to have) a 110 h.p. Clerget engine, an elongated vee-type undercarriage, and widely separated cockpits. This machine existed at least as a design, and was to weigh 1,990 lb loaded; drawings show that it was designed to have the standard two-bay wings.
  It is believed that the illustration shows the Avro 521. This aircraft had the same fuselage and vee-type undercarriage as the R.N.A.S. Clerget-powered two-seater mentioned above, but it was an R.F.C. type and therefore had no fin. The type of rudder which was used seemed to be identical to that of the Avro 504A, for the tail-skid was attached directly to it.
  The wing structure was characterised by single-bay bracing and by the unusual arrangement of the centre-section struts. The wings were staggered, and ailerons were fitted to both upper and lower mainplanes.
  The Avro 521 appeared early in 1916 and was tested by F. P. Raynham; it proved to be unpleasant to fly. The prototype crashed while being flown by a R.F.C. pilot some time later. An order was placed for twenty-five production machines, but it is doubtful whether any went further than training units. At least one was at Gosport in 1917 but crashed and killed its pilot, Captain Garnett.


SPECIFICATION
  Manufacturers: A. V. Roe & Co., Ltd., Clifton. Street, Miles Platting, Manchester.
  Power: 110 h.p. Clerget.
  Service Use: Flown at Gosport.
  Production: Twenty-six Avro 521s were built.
  Serial Numbers: 1811, 7520-7544.

H.King Armament of British Aircraft (Putnam)

521. Designed in 1915 as a two-seat 'fighting scout#, the 521 (completed 1916) was intended to have a rear-mounted Lewis gun, though this does not appear to have materialised.