The Blackburn Aeroplane and Motor Company RB1 Iris three-engine, long range, maritime reconnaissance flying boat was designed to meet Specification R.14/24.
The first prototype (N185) was powered by three 650 hp Rolls-Royce Condor III engines mounted between the wings, driving tractor propellers within a three-bay centre section, to which two-bay outer wings were attached on each side. With a resultant wingspan of more than 95 ft and a length of over 66 ft, the Iris was a most impressive design.
It initially flew with a white-painted wooden hull whose two-step design gave very clean operation on the water. A biplane tail unit was fitted with the upper tailplane carrying the elevators whilst the reduced span lower surface acted solely as a trimming surface. Three aerodynamically balanced rudders were fitted in between.
The choice of three powerful engines was intended to allow the aircraft to maintain height at its maximum weight following loss of one engine.
A 302-gallon fuel tank was mounted on the underside of the upper wing above each engine. These tanks gravity-fed a collector unit in the lower wing centre section from which supply to the individual engines was controlled.
Two pilots sat side-by-side in an open cockpit with a second cockpit was provided behind the pilots for use by the navigator and other crew members. There was access from this cockpit to a central cabin which was equipped with four bunks, a chart table, wireless telephone equipment and a galley. The normal crew complement was five which included two pilots, a navigator and two additional crew variously acting as engineers, gunners or as observers.
The Blackburn RB1 Iris I (N185) was flown from the River Humber on 19th June 1926 before leaving the next day for trials at Marine Aircraft Experimental Establishment (MAEE) Felixstowe, which were conducted during July and August that year.
Before the first prototype flew however, the company had begun construction of an all-metal hull for the Blackburn Iris. It was of generally similar external design to that of the prototype, other than the inclusion of small circular portholes to provide light to the cabin area. It featured an upswept tail section to accommodate a defensive gun position, aft of the tail unit.
On its return from official trials, the prototype was fitted with the new metal hull and re-designated as the Blackburn Iris II. At the same time, the central rudder was deleted and metal wing-tip floats were fitted along with more powerful Condor IIIA engines, fitted in a more refined installation.
In this form the Blackburn RB1 Iris II made its first flight on 2nd August 1927, before being delivered to the MAAE Felixstowe on the following day.
A few days later, the Blackburn RG1 Iris II set off with the Short Singapore and Saunders Valkyrie prototypes on a 3,000-mile tour of Scandinavian capitals. Between 12th August and 11th September, the routes comprised Felixstowe to Oslo and Copenhagen before returning to Felixstowe.
Thereafter, they flew from Felixstowe to Gdynia, Danzig, Helsinki, Stockholm, Copenhagen and then back to Felixstowe.
In 1928, longer flights were made around the Mediterranean and Middle East lasting between 27th September and 14th November, covering some 11,360 miles and extending as far as Karachi. During these expeditions engine trouble was experienced en route, resulting in a delay at Jask in Iran, where an engine change was carried out.
Toward the end of 1928, further trials were carried out with three-blade propellers, as well as with two blade propellers outboard, combined with a four-blade propeller on the central engine. Whilst testing with three-blade propellers, the Blackburn RB1 Iris II had to be beached to allow the starboard engine to be changed.
The merits of the Iris II resulted in an order being placed for three production examples of a further revised Blackburn RB1B Iris III built to Specification R.31/27.
The Blackburn Iris Iris III was generally similar to the Iris II although it used Rolls-Royce Condor IIIB engines in a further refined installation (with cleaner nacelles and rear radiators).
The wing structure was now all-metal (the Blackburn RB1 Iris I and Iris II having used a wooden structure, with duralumin drag struts and steel tie rods).
The cabin could now be accessed from the pilots cockpit and additional portholes were also added. Gun positions were provided in the nose, midships and at the tail with bomb racks provided under the wing roots, typically carrying a load of two 520 lb bombs.
The first Blackburn RB1 Iris III (N238) was first flown on 21st November 1928, with two production aircraft being built (S1263 and S1264). At the time, these were the largest aircraft in service with the RAF and they were used predominantly for a number of long-distance flights including destinations such as Reykjavik, Lisbon, Gibraltar, Malta, Aboukir, Crete and Athens.
Sadly, the first prototype was lost in a fatal accident on 4th February 1931 and as a result a further Iris III (S1593) was ordered as a replacement. Flying for the first time on 25th June 1931, this aircraft was painted battleship grey and had a squarer bow with an enlarged forward gun compartment allowing it to take a 37mm Coventry Ordnance Works cannon.
In parallel with the production of the Blackburn RB1 Iris III, the original Iris II was returned to Brough for conversion to a new model, the Iris IV.
The major change included the replacement of the liquid-cooled 675 hp Condor IIIA engines with the 800 hp Armstrong Siddeley Leopard III air cooled radial engines. Apart from the increase in power these provided, the other main key advantage was a significant reduction (some 1,430 lb) in empty weight.
Unusually on the Blackburn Iris IV, the outer two engines were installed driving tractor propellers, whereas the central engine was faced backwards to drive a pusher propeller.
It first flew in this configuration on 6th May 1931. Although the standard all up weight was 30,250 lb, tests at Felixstowe soon demonstrated that the Blackburn Iris IV could take-off after just a 60-second run, at a weight of 35,000 lb which was a much higher weight than any other Blackburn Iris.
The final development was the Blackburn Iris V. The significant increase in weight between the Blackburn Iris I and the final Iris III had seen an adverse effect on ride height on the water. This affected take-off, flight performance and engine reliability. Consequently, it was decided that the three remaining Blackburn Iris III aircraft should be re-engined with three 825 hp Rolls-Royce Buzzard IIMS engines.
Fortunately, this could be achieved with only small structural changes and with very little weight penalty. A more streamlined engine nacelle was adopted and a reduction drive allowed larger diameter propellers, which were a better match to the power available.
The first modified aircraft (S1263 designated the Blackburn RB1D Iris V) flew for the first time on 5th March 1932. It demonstrated excellent flying qualities, despite sitting fairly low in the water.
The next aircraft to be converted (S1264) was launched on 2nd January 1934, to Felixstowe on the same day. Unfortunately, a gale sprang up during the night and it sank at its moorings, never to fly again.
S1263 was written off on 12th January following collision with a vessel, when alighting at Plymouth.
The last aircraft (S1593) took to the air as an Blackburn Iris V on 31st March 1933, and was used to conduct trials with various airscrews. It was flown down to Malta but had difficulty getting airborne in calm conditions and was withdrawn from service in June 1934.
The final Iris (S1593) was selected as a test bed to investigate the characteristics of the 720 hp Napier Culverin I diesel engine (which was actually a licence-built Junkers Jumo IVC engine).
The first flight in this configuration took place on 9th June 1937 and limited trials were carried out (totalling only 15 hr 35 min flying time) before being relegated to use as a test article for corrosion-resistant paint finishes.
Variants & Number Built
|Blackburn RB1 Iris I||One aircraft N185 Condor III engines, wooden hull, converted to Iris II|
|Blackburn RB1A Iris II||One aircraft N185 metal hull Condor IIIA engines, converted to Iris IV|
|Blackburn RB1B Iris III||Four aircraft N238, S1263, S1264, S1593 Condor IIIB engines. S1263, S1264 and S1593 converted to Iris V|
|Blackburn RB1C Iris IV||One aircraft N185 with Armstrong Siddeley Leopard III engines|
|Blackburn RB1D Iris V||Three aircraft converted from S1263, S1264, S1593 with RR Buzzard IIMS engines. S1593 later converted to test Napier Culverin diesel engines.|
|Total production||5 airframes (N185, N238, S1263, S1264, S1593)|
Specification (Blackburn Iris III)
|Powerplant||Three 675 hp Rolls-Royce Condor IIIB engines|
|Span||97 ft 0 in|
|Maximum Weight||29,489 lb|
|Capacity & Armament||Five crew; three defensive Lewis guns; typical bomb load two 520 lb bombs|
|Maximum Speed||118 mph at sea level|
|Max cruise speed||97 mph|