Saro Lerwick - Design and Development

Design and Development

Air Ministry Specification R.1/36 was issued in March 1936 to several companies that had experience in building flying boats. The specification was for a flying boat to replace the Royal Air Force's Saro London, Supermarine Stranraer and Short Singapore biplane flying boats, for anti-submarine, convoy escort and reconnaissance.

An order was placed for a prototype of Blackburn's radical response to the specification - the Blackburn B-20 - but this would only be delivered in 1940. Meanwhile, a contract was issued in June 1937 to buy 21 of Saunders-Roe's proposed aircraft - the S.36 Lerwick - even though this only existed on paper. The Lerwick was a compact twin-engined, high-winged monoplane of all-metal construction. It had a conventional flying boat hull with a planing bottom and two stabilising floats carried under the wings on long struts. It was powered by two Bristol Hercules engines and initially had twin fins and rudders. For defense, the Lerwick was equipped with three powered Gun turrets. The nose turret had a Vickers K gun; the other two had Browning machine guns - two guns in the FN.8 turret in the dorsal position and four in the Nash & Thomson FN4.A turret at the tail. The offensive weapons - 2000 lb (900 kg) of bombs or depth charges - were carried in two streamlined nacelles behind the engines; a configuration shared by the Martin PBM Mariner.

The first three aircraft were used as prototypes, with the first being launched on 31 October 1938 after numerous delays during design and construction. The Lerwick was immediately found to be unstable aerodynamically and on the water and not suited to "hands off" flying, a major problem in an aircraft designed for long-range patrols. Numerous adjustments, including the addition of a greatly enlarged single fin and increasing the wing incidence, failed to remedy its undesirable characteristics, which included a vicious stall and unsatisfactory rates of roll and yaw. In service, several aircraft would be lost because of wing floats breaking off, suggesting this was a structural weakness. Persistent problems with the aircraft's hydraulics would see the bomb doors sometimes dropping open during flight

On one engine, the Lerwick could not maintain height, nor could it maintain a constant heading as the controls couldn't counter the torque of one engine on maximum power. Thus, an engine failure would inevitably see the aircraft flying in slowly descending circles. On one occasion, the loss of an engine forced a Lerwick to make an emergency landing in the Caledonian Canal. The aircraft was then towed to Oban at the end of a string of coal barges.