Beaufort prototype L4441
The Development and Prototypes of the Bristol Beaufort
The Bristol Beaufort, a British twin-engine torpedo bomber, emerged from the Air Ministry’s desire to modernize its front-line aircraft in the mid-1930s. The Air Staff issued two specifications, M.15/35 and G.24/35, for a land-based twin-engine torpedo bomber and a twin-engine general reconnaissance aircraft respectively. Initially, the intention was to have separate aircraft for these roles, with the Blackburn Botha intended for Coastal Command’s home-based squadrons and the Beaufort earmarked for torpedo-bomber squadrons in the Far East.
- The Bristol Aeroplane Company, led by chief designer Captain Frank Barnwell, responded to Specification M.15/35 with its three-seat Type 150 design, a development of its proposed four-seat general reconnaissance aircraft, which itself was a stretched version of the Type 142M Blenheim bomber.
- In December 1935, the Air Staff suggested that a single design could combine both the general reconnaissance (GR) and torpedo-bomber (TB) roles. This led Bristol to submit its Type 152 design in April 1936, featuring a nose extended by a further 9 inches to accommodate both a navigator’s station and a torpedo. This design was accepted and a production specification, 10/36, was issued, with the aircraft named Beaufort in December 1936.
However, the development of the Beaufort was marked by several challenges and compromises:
- Absence of a Dedicated Prototype: The Air Ministry, eager to see the Beaufort in production, ordered 78 aircraft “off the drawing board” without a dedicated prototype. This decision, while intended to accelerate production, ultimately slowed down the initial production process due to the need to incorporate numerous modifications in early aircraft already taking shape.
- Engine Issues: Initially, the Beaufort was to be powered by Bristol Perseus VI engines. However, weight increases during development led to concerns about performance, and Bristol proposed using its new, but undeveloped, twin-row 1,000 hp sleeve-valve Taurus engine instead. This change, while accepted by the Air Ministry, introduced further delays and challenges, as the Taurus engine proved to be plagued by overheating problems throughout its life.
- Turret Armament Limitations: Bristol’s initial proposal to fit twin Browning .303 in caliber machine guns in the gun turret was rejected due to their allocation being prioritized for fighters. Instead, the Beaufort was equipped with a special-to-type B.IV turret that had only one Vickers K gun, a limitation that was not rectified until well into 1941.
The first Beaufort, L4441, took its maiden flight on October 15, 1938. However, due to the absence of a dedicated prototype, this first aircraft effectively served as the prototype while subsequent aircraft on the production line awaited the Service’s acceptance. L4441 was powered by Taurus III engines and featured rounded lower nose windows. It suffered from several issues, including engine overheating, tailplane vibration, and handling difficulties with the undercarriage. Despite these problems, L4441 achieved a speed of 304 mph at 15,000 ft during trials at the Aeroplane and Armament Experimental Establishment (A&AEE) at Martlesham Heath.
Several modifications were incorporated in subsequent aircraft:
- Modified Cowlings and Oil Coolers: The overheating problems were addressed by fitting Blenheim-type cowlings with circumferential gills and replacing the small oil coolers with larger ones.
- Side-Hinged Undercarriage Doors: To improve handling, the original undercarriage aprons were replaced by side-hinged doors.
- Optically-Flat Nose Windows: From the third aircraft, L4443, onwards, the rounded lower nose windows were replaced with optically-flat windows for better visual clarity.
- Revised Rudder Trim Tab: L4443 also introduced a revised rudder trim tab with an increased chord to improve handling.
The fifth Beaufort, L4446, which flew on October 25, 1939, was the first fully jigged mass-production aircraft. However, even at this stage, the A&AEE refused to clear the Beaufort for operational use due to its “very bad single-engine flight characteristics”. Further modifications, including improved inter-cylinder baffles for better cooling, were necessary before the Beaufort was finally cleared for operational use in mid-April 1940.
The development of the Beaufort was a complex process marked by compromises, delays, and the constant pressure to get the aircraft into service as quickly as possible. The absence of a dedicated prototype, the challenges with the Taurus engine, and the limitations of the initial turret armament all contributed to a somewhat troubled beginning for this important aircraft. However, the Beaufort would eventually prove to be a rugged and dependable workhorse, serving with distinction in various roles throughout World War II.