Blackburn pursued a three-engine flying boat because long-range reconnaissance left little margin for a powerplant failure over the sea. The Iris was intended to remain fully capable after losing one engine and still return after losing two, a form of redundancy unavailable to the single-engine aircraft then serving from carriers and warship catapults. Its engines sat in self-contained nacelles on steel struts, while wooden and tubular-steel structural elements shared the loads. This mixture of powerplant redundancy and composite construction supported Royal Air Force reconnaissance beyond the British coast. The small production run does not diminish the design's value as an example of reliability shaping an entire maritime aircraft rather than one component.
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Iris uses three engines for maritime-reconnaissance reliability
The Blackburn Iris used three engines and composite construction to make long-range maritime reconnaissance more survivable when engine failure occurred far offshore.