In 1931, Blackburn Aircraft Company was commissioned by the Air Ministry to build two commercial aircraft to the same specification, one being a biplane and the other a monoplane. These were to become known as the Blackburn CA15.
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.
The two-bay biplane (G-ABKW) was of conventional configuration whilst 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.
Both types were of all-metal construction, with fabric-covered wings. The wing areas of both types of were similar dimensions at 1,068 sq ft for the biplane and 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.
The biplane (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 on to the Aeroplane & Armaments Experimental Establishment at Martlesham Heath 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 'poor performance'. Full trials for direct comparison with the monoplane were conducted from February 1933 onward.
The Monoplane (G-ABKV) made its first flight on 4th October 1932 and as might be expected, the biplane was slower than the monoplane, despite having a lighter structural weight. It had a greater payload at a given all up weight, as it was already known that a wire-braced biplane structure provided greater 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).
The biplane was scrapped after these trials but the monoplane was used for automatic pilot trials in February 1934. Thereafter, it was 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.
The purpose of the Blackburn CA15 was as a comparison between Biplanes and Monoplanes and some of the significant lessons learnt or confirmed were as follows.
Monoplane designs only became successful with the combined use of stressed skin construction techniques, providing torsional stiffness with low weight and drag penalties. They also demanded retractable undercarriages, variable pitch propellers and effective landing flaps. These features enable a reduced wing area, whilst allowing operation from existing airfields, delivering better take-off and cruise performance.
The monoplane payload fraction was still slightly inferior to that of a contemporary biplane, but a much higher cruise speed becomes available. This reduction in journey times proved to be very popular with passengers.
For example, the Douglas DC-2 had a payload fraction of 33%, compared with the Handley Page HP42 at 36%. However, the DC-2 could cruise at nearly twice the speed (190 mph compared with 100 mph).
Variants & Number Built
Two aircraft only G-ABKV (Monoplane) and G-ABKW (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|