Avro 531 Spider and 538

Single seat sesquiplane fighter, a private venture submission to RAF Specification Type II using many Type 504 components for ease of production. To this end a shortened rear fuselage of conventional fabric-covered wooden construction with spruce longerons was married to a standard Avro 504K front fuselage. To simplify rigging all flying and landing wires were replaced by very rigid welded steel Warren girder interplane bracing. This comprised six faired steel tubes arranged in three inverted triangles on each side, anchored to the main spars of the upper mainplane and to the front spar of the lower. Ailerons were fitted only to the upper wing, the lower being shorter and with a chord of only 2 ft. 6 in. To improve substantially the pilot's field of vision, the cockpit was located under a circular aperture in the centre section. This was mounted close to the fuselage so that the pilot's head protruded above it. Armament consisted of a single synchronised Vickers gun on top of the fuselage and slightly to starboard of centre. One 110 h.p. Le Rhone or 130 h.p. Clerget powerplant.

As Type 531 with conventional equal span, equal chord, staggered two-bay wire-braced mainplanes.

Type 531 Spider Specification
Span	Length	Height	Wing Area	Empty Wt	Max AUW	Cruise Speed	Maximum Speed	Range	Service Ceiling
28 ft 6 in	20 ft 6 in	7 ft 10 in	189 sq ft	963 lb	1517 lb		120 mph/ 104 kn		19000 ft
8.69 m	6.25 m	2.39 m	17.56 m2	437 kg	688 kg		193 km/h		5791 m

Single seat communications and racing aircraft, converted from the incomplete Type 531A. 150 h.p. Bentley B.R.1 powerplant.

Type 538 Specification
Span	Length	Height	Wing Area	Empty Wt	Max AUW	Cruise Speed	Maximum Speed	Range	Service Ceiling
28 ft	20 ft 6 in	8 ft 6 in	210 sq ft	975 lb	1400 lb	108 mph/ 94 kn	125 mph/ 109 kn	320 mi
8.53 m	6.25 m	2.59 m	19.51 m2	442 kg	635 kg	174 km/h	201 km/h	515 km