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Blackburn CA15C
Comparative Aircraft

Experimental commercial transports used to compare the efficiency of biplanes and monoplanes.
Blackburn CA15C Comparative Aircraft biplane and monoplane A direct comparison between the two versions of the CA15C at Brough in October 1932.
 
In 1931, Blackburn Aircraft was commissioned by the Air Ministry to build two commercial aircraft to the same specification, one being a biplane and the other a monoplane.
 
The purpose was to investigate the comparative efficiency of the two designs (in terms of structural weight and payload fraction, and aspects of performance including speed, fuel consumption, take-off and landing performance, climb rate, performance with one engine failed, and so on).
 
The designs were to be identical in all respects, other than wing design. The specification sought accommodation for ten passengers and two crew, baggage and mail carriage. Both types were to be powered by two 400 hp Armstrong Siddeley Jaguar IVC engines driving two blade fixed-pitch propellers.
 
Blackburn CA15C Civil Biplane G-ABKW port side A side view of G-ABKW, the biplane variant of the Blackburn CA15C.
 
The two-bay biplane (G-ABKW) was of conventional configuration and the monoplane (G-ABKV) featured a high wing of increased span and chord when compared with the biplane. The monoplane had two external bracing struts on each side, running down to the fixed undercarriage mounting. The undercarriage position and track were the same for both aircraft, producing a rather complex strutted arrangement for the monoplane.
 
Blackburn CA15C Civil Biplane G-ABKWV port rear The ten-seat biplane variant of the Blackburn CA15C.
 
Both types were of all-metal construction with fabric-covered wings. The wing areas of both types were similar (1,068 sq ft for the biplane; 1,037 sq ft for the monoplane). The engines of the biplane example were mounted between the wings on complex strutted pylons, whereas those of the monoplane were mounted on the wing leading edge. Neither machine was fitted with landing flaps.
 
G-ABKW was first to fly on 10th June 1932 before being shown later in the month at the Hendon RAF Pageant and SBAC Show. This aircraft then went to Martlesham for trials in October where it experienced brake problems. It was also found to be rather heavy on the controls and to have inadequate engine output and performance. Full trials in comparison with the monoplane were conducted from February 1933 onward.
 
Blackburn CA15C Civil Monoplane G-ABKV stbd front The monoplane variant of the Blackburn CA15C G-ABKV was first flown in October 1932.
 
The Monoplane (G-ABKV) made its first flight on 4th October 1932 and as might be expected, the biplane was slower than the monoplane but had a lighter structural weight and more payload at a given all up weight.  It was already known that a wire-braced biplane structure provides high stiffness in torsion and bending with little weight penalty.
 
The biplane’s empty weight was 887 lb lower than that of the monoplane whilst the difference in maximum speed was only 10 mph (128 mph versus 118 mph for the biplane).
 
Blackburn CA15C Monoplane RAF K4241 The monoplane CA15C was taken on charge by the RAF as K4241.
 
The biplane was scrapped after these trials but the monoplane was used for automatic pilot trials in February 1934 and then taken on charge by the RAF (as K4241) for Wireless Telegraphy trials. It was used as a taxi aircraft supporting No 2 Aircraft Storage Unit at Cardington before returning to Martlesham Heath in 1937, where it was used as a ground target and subsequently scrapped.
 
Footnote: Monoplane designs became successful only with the combined use of stressed skin construction (providing torsional stiffness with low weight and drag penalties), retractable undercarriage, variable pitch propellers and effective landing flaps. These features enable reduced wing area, while allowing operation from existing airfields, and delivering better take-off and cruise performance. The monoplane payload fraction was still slightly inferior to a contemporary biplane, but a much higher cruise speed became available. The reduction in journey times proved to be very popular with passengers. The Douglas DC-2, for example, had a payload fraction of 33%, compared with the Handley Page HP42s 36%, but the DC-2 cruised at nearly twice the speed (190 mph compared with 100 mph).

Variants & Numbers Built

Two aircraft only G-ABKV (Monoplane) and G-ABKW (Biplane).

Specification

  Monoplane Biplane
Powerplant Two 400 hp Armstrong Siddeley Jaguar IVC
Span 86 ft 0 in 64 ft 0 in
Maximum Weight 13,074 lb   
Capacity Two crew, ten passengers
Maximum Speed 128 mph (fighter) 118 mph
Max cruise speed 110 mph 110 mph
Range  350 miles

Survivors

Nil: both aircraft scrapped.