Blackburn Perth from Above |
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The Blackburn Iris was a large flying boat that served in small numbers with the RAF in the early 1930s, carrying out some impressive long range flights.
In the early 1920s Blackburn had produced the very large Blackburn Cubaroo torpedo bomber, with a wingspan of 88ft. This helped when the company decided to produce a design in response to Air Ministry Specification R.14/24, which called for a long range reconnaissance flying boat.
The Blackburn Iris was designed by Major J.D. Rennie. It was a three engined biplane, with the engines carried in the gap between the wings.
The hull of the prototype was of wooden construction with a plywood covering. It had a sharply pointed bottom with two steps which meant it ran cleanly on the water.
The wings were built in three sections. In the centre was a large three bay section that carried all three engines. The lower wing of this section used tubular steel main spars, the upper wing (and the outer sections) used wooden spars. All sections used wooden ribs, braced by tubular duralumin drag struts and steel tie rods. The outer sections added another two bays on each side and used panels identical in appearance to those of the Blackburn Dart. There were ailerons on all four wings. Large wooden wing tip floats were carried below the outer struts.
The tail was also of biplane construction with a 30ft span upper plane and 15ft span lower plane, and three aerodynamically balanced rudders.
It was powered by three 650hp Rolls-Royce Condor III water cooled engines, each with its own oil tank, starting gear and radiators. The aircraft could fly on two engines. A 302 gallon fuel tank was carried under the upper wing above each engine, although the fuel from these came to a central distribution system before going back to the engines.
The aircraft carried a crew of five. The two pilots sat in an open cockpit 12ft in front of the wings. A second open cockpit was positioned just behind them, with access to the rear gunner’s position and to a large cabin inside the hull. This contained the navigator’s table, wireless gear, four bunks and a small galley. Standard armament was two flexibly mounted .303 Lewis guns but it could also carry two more guns alongside the aft portholes in the cabin. Bombs were carried under the wing roots, and it could carry two 520lb bombs, four 230lb bombs or eight 20lb practice bombs.
The prototype, which became the Iris I, made its maiden flight on 18 June 1926. In July-August it was tested by the MAEE at Felixstowe.
Iris II
Work on a version of the Iris with an all-metal hull began before the maiden flight of the Iris I. The new hull was made of duralumin, with a series of transverse frames joined by longitudinal stringers (the upper superstructure was unmodified). The frames were attached to a central keelson, and the planing bottom was part of the hull (rather than being fairing added below the real hull). It was otherwise similar to the design of the Iris I, apart from a wider upswept rear to the tail, which had enough space for a tail gunner. This also required the removal of the central rudder. The new hull replaced the original wooden hull of the Iris I, and the new aircraft kept the same serial number, N185. The modified aircraft made its first flight on 2 August 1927.
The Iris II was then delivered to the RAF, and only ten days later left for a tour of Scandinavian capitals, with the Short Singapore I and Saunders Valkyrie prototypes. This was a combination of a trip to ‘show the flag’, a 3,000 mile long service test, and to carry Sir Samuel Hoare to the Air Traffic Exhibition at Copenhagen. The Iris II carried out a series of long flights during this trip, carrying Sir Samuel from Oslo to Copenhagen then back to Felixstowe before rejoining the flight in Poland. The trip lasted one month, from 12 August-11 September 1927.
This was followed by an even longer trip, to carry Sir Philip Sasson, Under Secretary of State for Air and Air Commodore Sir Arthur Longmore on a tour of RAF stations in the Mediterranean and the Middle East. The aircraft left Felixstowe on 27 September 1928 and visited Marseilles, Naples, Athens, Aboukir, Alexandretta, Baghdad, Basra, Karachi, Basra, Benghazi and Malta. Over a period of three weeks the aircraft flew 11,360 miles in 125 hours 5 minutes. One flight was cut short by engine problems and another by choked filters, but otherwise it performed well.
Iris III
The success of the Iris II was rewarded by an order for three examples of the improved Iris III, which would be used to equip one squadron.
The Iris III was similar to the Iris II. The remaining wooden elements of the superstructure were replaced with duralumin. Narrow chord servo tabs were added to the trailing edge of the two rudders. It was given three Condor IIIB engines in cleaner nacelles, with vertical radiators at the rear. Internally the pilot’s cockpit was finally connected to the main cabin. The original second cockpit was replaced by a navigator’s compartment with map table and roof hatch. The main cabin was behind and below that, and contained the wireless, sleeping quarters, engineer’s seat, cooker and dinghy storage. It was armed with three .303in Lewis guns each on Scarff rings, one in the tail, one amidships and one in the nose.
Only three examples of the Iris III were ordered. The prototype, N238, made its maiden flight on 21 November 1929. Tests showed that take-off, climb and two engined performance were improved compared to the Iris II. The second aircraft, S1263, made its first flight on 5 February 1930. The third aircraft entered service on 4 June.
The Iris III was used to equip No.209 (Flying Boat) Squadron, which reformed at Mount Batten, Plymouth, on 15 January 1930. When they entered service the three Iris IIIs were the largest aircraft in RAF service. During their time in service the Iris IIIs took part in some impressive flights. In June 1930 S1264 flew to Iceland to represent Britain at the celebration of 1,000 years of the Icelandic parliament. Her return trip was the first non-stop RAF flight between Iceland and Britain. In August S1264 made the first crossing of the Bay of Biscay by flying boat.
The prototype was lost in a crash in Plymouth Sound on 4 February 1931 in which nine men were killed. A fourth Iris III, S1593, was built to replace it, arriving on 6 July 1931. This aircraft had a modified nose compartment that could carry a 37mm Coventry Ordnance Works quick firing cannon.
Iris IV
The Iris IV was the designation given to the prototype N185 when it was given three 700hp Armstrong Siddeley Leopard two-row air cooled radial engines in 1931. These engines were expected to save 2,100lb of weight and give improved performance. The new engines were installed in an unusual configuration, with the outer pair as tractors and the middle engine as a pusher. It made its first flight in this configuration on 6 May 1931. This aircraft set the record for the heaviest Iris take-off, at an all-up weight of 35,000lb.
Iris V
By 1931 increases in fuel and equipment weights meant that the Iris III rode too lower in the water and its performance was also reduced. Blackburn suggested replacing the 675hp Condor III engines with more powerful 825hp Rolls-Royce Buzzard IIMS engines, which could use new larger airscrews and new low drag nacelles that could be mounted directly onto the wing struts, reducing drag. The Air Ministry agreed to carry out these changes.
S1263 was the first to be converted, and made its maiden flight as an Iris V on 5 March 1932. The new engines had greatly improved the flying characteristics, but she still sat lower in the water than was ideal. On 12 January 1933 she sank after colliding with a naval steam pinnace while landing in Plymouth Sound.
S1264 was next, but work was delayed after she sank at her moorings in Plymouth Sound and she didn’t reach Brough until March 1932. She made her first flight as an Iris V on 2 January 1933, flew to Felixstowe, where she sank at her moorings on 3 January and was written off.
S1593 made her first flight as an Iris V on 31 March 1933. She carried out one flight to Malta, before being sent back to Brough for reconditioning on 12 June 1934. She was then used as a flying test bed for the 720hp Napier Culverin Series I engine (a Junkers Jumo IVC built under licence), making her first flight with these engines on 9 June 1937. These tests lasted until 1938. She was then used as a test bed for anti-corrosive paints.
The Iris was replaced at No.209 Squadron by the larger Blackburn Perth, which was similar to the last Iris with its 37mm Coventry Ordnance Works cannon.
Iris Mk III
Engine: Three Rolls-Royce Condor IIIBs
Power: 675hp each
Crew: 5
Span: 97ft
Length: 67ft 4.75in
Height: 25ft 6in
Tare weight: 19,048lb
All-up weight: 29,489lb
Max speed: 118mph at sea level
Climb Rate: 503ft/ min
Service ceiling: 10,600ft
Range: 800 miles
Armament: Up to four machine guns
Bomb load: Two 520lb, four 230lb or eight 20lb bombs