Blackburn Cubaroo

The Blackburn T.4 Cubaroo was a British prototype biplane torpedo bomber of the 1920s. Built by Blackburn Aircraft and intended to carry a large 21 in torpedo, the Cubaroo was claimed to be the largest single-engined aircraft in the world at the time of its first flight.

Design and development
In 1921, the British Air Ministry issued Specification 8/21 (previously Directorate of Research type IX) to Blackburn for a Coastal Defence Torpedo Aeroplane, for a long-range torpedo bomber with a range of over 800 mi and the ability to carry a full size 21 in naval torpedo, which was at the time was thought capable of sinking the most heavily armoured warships. Previous torpedo bombers had used smaller, less capable torpedoes.

Blackburn Aircraft's chief designer, Major F. A Bumpus, submitted the Blackburn T.4 Cubaroo, which was a massive biplane powered by a 1000 hp 3681 cuin Napier Cub X-16 engine. Avro submitted and then withdrew their similarly powered Type 556, and replaced it with the Avro 557 Ava to a revised 16/22 specification which differed mainly in calling for two engines. The Ava was also a large biplane, but powered by two 600 hp 2137.5 cuin Rolls-Royce Condor V-12 engines.

To carry the heavy torpedo weighing over 2000 lb for the required range, the Cubaroo was massive. With a wingspan of 88 ft, it was claimed at the time to be the largest single-engine military aircraft built, and it had the most powerful aircraft engine then available, the Napier Cub, which weighed over a ton, excluding the radiator. The first example, N166, had a curved radiator ahead of the engine in the nose, while the second, N167, had a flat one.

The Cubaroo, with a mainly steel tube metal structure, had an 11 ft deep fuselage to accommodate the pilot who sat, with a navigator beside him, sitting over the engine. The rear of the fuselage had a triangle cross section to improve the gunner's fields of fire from hatches located near each lower wing root which permitted them to cover the underside of the tail. Bomb aiming was accomplished through a hatch in the floor of the cabin that also contained radio equipment and a navigational chart table. A passage ran aft to a ladder, which led up to another gunner's position, with a gun ring mounted on top of the fuselage just aft of the wing trailing edge.

The 88 ft span, 11 ft constant chord folding, two-bay wings had dihedral, and semi-circular wing tips, with inset unbalanced ailerons on all four wings running for about half the span. The empennage consisted of three balanced rudders, with the outer two having small fins, and an adjustable biplane horizontal stabilizer with a balanced elevator.

To carry the torpedo, the Cubaroo was fitted with a split undercarriage attached to the lower wings, each comprising two sets of two 4 ft diameter Palmer wheels, with the torpedo or bombs being carried on a crutch under the fuselage, between them.

Due to the change of British Air Ministry policy favouring twin-engine designs for the role, a new specification, 16/22 was drawn up, and the type was not ordered into service. Although drawings were made for three different versions with two engines, none of these were built.

Operational history
The first prototype (with serial N166) flew in secrecy in the summer of 1924, proving to have good handling characteristics, with the engine not causing problems, as the Napier Cub had already been test flown in an Avro Aldershot. It was then fitted with a metal, three-blade adjustable-pitch propeller and was delivered for testing at RAF Martlesham Heath but was written off after its undercarriage collapsed on 2 February 1925. A second prototype flew in 1925, but the Air Ministry had by then lost interest in single-engine heavy bombers, so the second prototype was used as an engine testbed, flying with the experimental 1100 hp Beardmore Simoon diesel engine.

Variants

 * T.4 Cubaroo Mk.I
 * Both prototypes as built, powered with one 1000 hp X-16 Napier Cub.


 * T.4A Cubaroo Mk.II
 * Unbuilt 1927 project, powered with two 650 hp direct-drive V-12 Rolls Royce Condor IV


 * T.4B Cubaroo Mk.III
 * Unbuilt 1927 project, powered with two 650 hp geared V-12 Rolls-Royce Condor III


 * T.4C Cubaroo Mk.IV
 * Unbuilt 1927 project, powered with two 650 hp direct-drive V-12 Rolls-Royce Condor IV

Operators

 * Royal Air Force - evaluation and engine testbed only.