The Blackburn T4, known more commonly as the Cubaroo, was a particularly large biplane
that was designed in the early 1920s. It was developed during a time of great activity
in the field of naval aircraft design. WW1 had demonstrated how air power could influence
the course of the war at sea, and the first use of air-dropped torpedoes against large
ships proved that the aircraft could be used as an anti-capital ship weapon. In 1922 the
British Air Ministry issued specification 16/22. This called for a long-range torpedo
bomber, it had to be capable of carrying a large 21-inch torpedo over a range of 800
miles. Two aircraft manufacturers would submit designs to meet the ministry’s specification
– Avro and Blackburn. Avro were working on a twin-engine design, but Blackburn designer
F.A Bumpus wanted to build his aircraft around a new prototype engine being developed by
D Napier and Son. The engine, which was dubbed the Napier Cub,
was a very large, very experimental design. It was a liquid-cooled engine that featured
16 cylinders in an X-pattern, which comprised of four banks of four cylinders. It weighed
over a ton, had a total displacement of 3,681 cubic inches, or 60.3 Litres, and it could
produce 1000hp at 1800 rpm. The engine had already flown successfully in 1922 in the
Avro 549 Aldershot, and Blackburn were confident that it could serve well in their design.
Carrying 21 inch torpedo in 1922 was no easy feat, they weighed on average one-and-a-half
tons, and it was hoped that this single, powerful engine would allow them to keep the design
more simple, and thus cheaper, than an equivalent design that utilised two smaller less powerful
engines. Bumpus submitted his design, and the ministry
awarded a contract for the production of two prototype aircraft at the end of 1922. Work
began on the first Cubaroo in 1923 at Blackburn’s Olympia Works. During its construction the
plane was shrouded in secrecy, and it was not until August 1924 that select members
of the Press, the Air Ministry, and some international Government representatives were invited to
inspect the aircraft for the first time. When it first rolled out of the hangar, the
Cubaroo would have been quite the sight to behold, being one of the largest single-engine
aircraft in the world at the time. It was built around a central fuselage that comprised
of a tubular steel structure, and this structure was then clad in a mix of stressed fabric
and duraluminium panels. The fuselage, which was no less than 11 feet deep at its forward
end, occupied almost the entire gap between the wings, but tapered sharply towards the
rear. It was built integral with two tubular-steel
girders which projected out on each side. These girders were used to mount the two separate
undercarriages that were required to support the heavy structure above; they also formed
the front and rear spars of the bottom centre section of the lower wing, from which two
more spars rose up at a diagonal angle to connect to the fuselage. This formed a very
rigid support structure for the enormous proportions of the front fuselage, which of course was
dictated by the size of the Cubaroo’s engine. The undercarrgiages were each fitted with
a pair of wheels that had a diameter of 4 feet, and they were spaced well apart with
an unobstructed space for the torpedo. The rear fuselage was supported on a bungee-sprung
tail skid, this was braced to the bottom longerons by two slanting struts and to the top by two
others lying parallel and external to the sides of the fuselage. A biplane tail unit
was chosen, for at the time they were still in high favour due to their structural advantages.
it was equipped with two normal fins with balanced rudders, a large central rudder,
horn-balanced elevators, and variable-incidence tailplanes.
The two main wings of the aircraft were massive, at least by the standards of single engine
aircraft of the day, and they both had a span of 88 feet. They maintained a constant chord
of 11 ft 6 inches and had a slight dihedral to maintain lateral stability. One source
also stated that the outer section of the wings also had a slight sweepback, but from
the photos I have seen and the aircraft drawings I think this is probably not the case. To
reduce the hangar space required to store an aircraft of this size, the outer panels
of the wings were hinged to the centre section and could be folded back for storage.
In terms of overall dimensions and performance the Cubaroo had a length of 54ft, a height
of 19ft 4 inches, and of course as mentioned earlier a wingspan of 88ft. It had an empty
weight of 9,632 lbs and a maximum take-off weight of 19,020 when carrying the torpedo.
When loaded it had a maximum speed of 115mph and an absolute ceiling of 11,800ft. It’s
operational range would differ depending on its configuration, but with maximum payload
it had a range of 1,800 miles and an endurance of 10 hours.
The Cubaroo was operated by a crew of four: a pilot, navigator, a bomb-aimer who also
served as a gunner, and another gunner. The pilot and navigator sat side-by-side in an
open cockpit ahead of the leading edge of the upper wing and over the rear part of the
engine. In front of the pilot’s windscreen was a curved radiator conforming to the rounded
shape of the nose, and it provided a convenient way of warming the cockpit in cold weather.
All crew members were in direct communication with each other. A hatch in the floor of the
cockpit gave access to an enclosed cabin below that housed the bomb-aimer. This cabin was
lit by portholes and equipped with wireless equipment, a chart table, and the floor of
the cabin incorporated a window for the bomb-aimer. It also provided prone position for two gunners
firing through rotating trap doors in the fuselage adjacent to the lower wing root.
The rear gunner position could by accessed by a passe that led aft from the cabin, it
was housed in an elevated gun ring on top of the rear fuselage that gave a wide field
of fire over the tail unit. The Napier Cub engine dominated the front
of the aircraft. Though it was built in an x arrangement, it was not completely symmetrical.
The upper banks were closer together than the lower banks, being inclined 26 ¼ degrees
to the vertical, whereas the lower banks lay 26 ¼ degrees to the horizontal. This feature
enabled the designer to enclose the upper portion of the engine completely within the
nose, with only the lower cylinder blocks and part of the exhause being exposed. Despite
its size and complexity, the Cub could be cold-started in under two minutes thankts
to a 5h gas starter that was installed in the engine bay.
After the initial inspection, the Cubaroo was painted in RAF colours and given the serial
N166. It flew for the first time in the summer of 1924. At this point the aircraft was still
kept fairly secret, and the flight was not publicly shown. It was flown by Flight Leftenant
Bulman, who would become well known as the chief test pilot for Hawker in later years.
He was chosen as the test pilot because he had previously flown the Avro Aldershot, which
had also been powered by the experimental Napier Cub engine.
Later on in August the aircraft would perform a demonstration flight at Brough. Bulman took
off in an 8 knot wind, and according to one onlooker the aircraft floated into the air
in half the available distance of the airstrip. The praise was not limited to those on the
ground, Bulman said reported that the aircraft felt very easy to fly, which considering it
was flying at a maximum load weight of 9 tons – including a torpedo – was high praise
indeed. Bulman put the Cubaroo through a series of slow and fast runs and climbing turns before
conducting a gentle landing that the plane achieved in 100 yards.
Following this, the Cubaroo then went to Martlesham heath for official performance trials. Only
one change was made during this time, and that was the replacement of its two-blade
wood propeller for a three-blade adjustable-pitch model.
A second Cubaroo was built in 1924, and given the serial N167. It first flew in 1925 and
was almost identical to the first, with the exception that the radiator was of a different
shape and that it was fitted with a metal three-blade propeller from the outset. It
made a brief appearance at the Hendon RAF display on June 27th 1925, where it dwarfed
all other aircraft. It greatly impressed onlookers during the main fly-pass with its huge size
and massive engine providing a symphony of noise that drowned out all others.
Despite impressing the public, the press, and several promiment members of the air staff,
the Cubaroo would not go into production. Later in 1925 the Air Ministry abandoned its
specification for a large bomber to carry the 21 inch torpedo, and at the same time
put more focus on twin-engine aircraft due to their higher reliability.
In response to this, Blackburn made detailed design studies for twin-engined versions of
the Cubaroo that would be powered by Rolls-Royce Condor engines. However, none of these were
built, and the two Cubaroo prototypes finished out their days as experimental platforms.
The first prototype was written off after its port undercarriage collapsed in 1925,
but the second one carried on for another year, being used as a testbed for the experimental
Beardmore Simoon 8 cylinder engine.