The Supermarine Sea Otter was an amphibious aircraft designed and built by the British aircraft manufacturer Supermarine. It was the final biplane flying boat to be designed by Supermarine; it was also the last biplane to enter service with both the Royal Navy and the Royal Air Force (RAF).
The Sea Otter was developed as a refinement of the Supermarine Walrus, having been redesigned for longer range operations, to perform dive bombing and to operate from a wider range of vessels than its predecessor. Prior to receiving the name Sea Otter, it was known as Stingray. Due to Supermarine's existing commitments to the Walrus and the Supermarine Spitfire programmes, the aircraft's development was protracted. The maiden flight of the Sea Otter took place on 23 September 1938, while a production order was only issued in 1942 on account of the urgent wartime demands of the Second World War.
Upon its introduction during the latter years of the conflict, the Sea Otter was primarily tasked with maritime patrol and air-sea rescue duties by both the RAF and the Royal Navy. Following the end of the conflict, numerous other operators procured the type for their own purposes; amongst these were the Royal Danish Air Force, Dutch Naval Aviation Service, and the Royal Australian Navy. Supermarine also undertook the conversion of surplus Sea Otters to a civilian configuration, leading to its use by civil operators as well.
Design and development
Background
The origins of the Sea Otter can be traced back to the earlier Supermarine Walrus. Even prior to the Walrus's maiden flight, the company's design team, headed by the aeronautical engineer R. J. Mitchell, were working on an improved version that was powered by either Bristol Aquila and Bristol Perseus radial engines. During February 1936, Mitchell approached the Air Ministry's Director of Technical Development to determine desirable performance attributes in the tentative aircraft prior to the detailed design commencing; from these discussions, it was decided to pursue a dive bombing capability, an elevated loaded weight, longer range, and for it to be fitted with equipment for operating from both aircraft carriers and cruisers.
On 17 April 1936, following Supermarine's submission of technical details, including detailed drawings and costings, the Air Ministry issued instructions to proceed with a pair of prototypes. Progress on these two prototypes was relatively slow, having been impacted by ongoing production commitments associated with both the Walrus and the Supermarine Spitfire programmes. The most visible difference between the Walrus and the Sea Otter was in the mounting of the powerplant; while the Walrus had a rear-facing engine with a pusher propeller, the Sea Otter's engine faced forward with a tractor propeller. In general, the exterior of the Sea Otter was cleaner than that of the Walrus, particularly in its engine arrangement, having disposed of the offset engine alignment to counteract torque by handling this via the vertical stabiliser instead.
Into flight
On 23 September 1938, the first prototype, K8854, performed the type's maiden flight, piloted by Supermarine's chief test pilot George Pickering. During the first flight, it was quickly determined that the original two-blade wooden propeller was inadequate, thus it was replaced by a three-blade counterpart produced by de Havilland, although this also failed to produce entirely satisfactory results. Again, the propeller was changed, this time to a four-bladed unit of which the pairs of blades were unusually set at an angle of 35° instead of the usual 90° so that the aircraft could be more easily moved within shipborne hangars and other enclosed areas.
Prior to the prototype's third flight, the aircraft had been named Stingray, but it was decided to rename it Sea Otter instead. Pickering observed its performance was noticeably better on the third flight, particularly during takeoff. Over following flights, only minor defects were identified and promptly resolved. During February 1939, sea recovery trials commenced from HMS Pegasus, resulting in some deviations being made from the standard practices used for deploying the earlier Walrus. The Admiralty also requested some changes, including that the nose be reprofiled as to reduce its water-spray tendencies as well as the installation of a three-blade Rotol constant speed propeller. Five months later, catapult trials involving HMS Pegasus were conducted, while general seaworthiness trials started during September 1939, although these were performed at Southampton on account of fears of German attacks upon Felixstowe.