What engine did an Iris V flight-test?

Short answer

One of the Blackburn Iris V flying boats was converted into a flying test bed for the Napier Culverin, a licence-built Junkers Jumo 204 diesel; it flew in that configuration in June 1937 and remained under test until April 1938. The Culverin, built by D. Napier & Son, was a liquid-cooled, six-cylinder vertically opposed engine.

Published: November 5, 2026 · Updated: November 5, 2026 · Edited by Above Ten Thousand · Editorial policy

Blackburn Iris Mk III S1263

The Culverin, a German diesel under British licence

The Napier Culverin was a version built under licence by D. Napier & Son of the Junkers Jumo 204, a six-cylinder vertically opposed, liquid-cooled aviation diesel engine. Its rated power was 821 hp at 2,050 rpm. The engine's tests were carried out in a Blackburn Iris V biplane flying boat.

An Iris V as a test bed

One of the Iris Vs was converted to serve as a test bed for the Napier Culverin and flew in that configuration in June 1937; flight testing continued until April 1938.

The Iris and its time with No. 209 Squadron

The Iris V was the final variant of the Blackburn Iris: three Iris Mk IIIs received 825 hp (615 kW) Rolls-Royce Buzzard IIMS piston engines. The three re-engined aircraft returned to service in 1932 as the Iris V and were replaced in squadron service in 1934. Although only five Irises were built, the Royal Air Force used them as long-range maritime reconnaissance aircraft. The first Iris III had flown on 21 November 1929, and the three Iris IIIs equipped No. 209 Squadron, re-formed at RAF Mount Batten, Plymouth, in January 1930. One Iris III, S1263 of No. 209 Squadron, based at Mount Batten, sank on 12 January 1933 after colliding with a dockyard launch after landing in Plymouth Sound: of nine airmen, one drowned and seven were injured.