05/27/2017. Remarks by Johan Visschedijk: "The Hawker Henley's career as a target tug did less than justice to what was undoubtedly an extremely sound design and came about as the result of the Air Ministry's wavering attitude towards the light bomber in 1936. Whether the new attitude, albeit difficult to define, was justified will forever be in doubt, for in the critical days of 1940 when the British and French armies were in retreat it is certain that the one type of aircraft lacking was the close support high performance light bomber. Instead, the Fairey Battle, with a maximum speed almost 50 mph (80 kmh) inferior to that of the Hawker Henley, was used in this role at enormous cost and with relatively little effect.
In February 1934, Air Ministry Specification P.4/34 was issued setting out a requirement for a light bomber capable of tactical support. It was to be fully stressed for dive recovery with full bomb-load and a speed requirement of approximately 300 mph (483 kmh) was mentioned. Among the companies that tendered designs were Faireys (whose prototypes were later used in the development of the Fulmar fleet fighter), Glosters and Hawker Aircraft Ltd.
In order to fulfill the performance requirements it at once appeared obvious to the Hawker Design Staff that the aircraft should be of a similar size to that of the aircraft being currently designed as an interceptor (i.e. the Hurricane) and that it should be similarly powered, since no other engine approached the power/weight ratio of the projected Rolls-Royce Merlin.
Structurally, therefore, the Hurricane and Henley (as Hawkers' P.4/34 design came to be known) were closely related, featuring all-metal construction with fabric-covered rear fuselage, tail and control surfaces. The designs proceeded on parallel lines, in fact the outer wing sections and tailplane were built on identical jigs, though of course the eight-gun battery was absent from the Henley.
Fundamentally, however, the Henley differed in being a two-seater, the pilot and observer/gunner being accommodated in tandem under a somewhat complex canopy superstructure. Internal stowage for up to 550 lb (227 kg) of bombs was proposed while light series racks for up to eight 25 lb (11 kg) bombs could be added under the outer wings. The central stowage for bombs prompted the adoption of a low-mid wing position which in itself demanded a new wing center section, for the undercarriage was thus of wider track than on the Hurricane. Defensive armament was to consist of a wing-mounted Vickers Mark V machinegun and a Lewis gun in the rear cockpit. The Vickers gun's mounting could alternatively accommodate a Bren light machinegun, though the significance of this provision has never been suitably explained.
Construction of the first Henley prototype, serialed K5115, was commenced at Kingston upon Thames, Surrey, in mid-1935, but due to the change of attitude towards the design implied by extensive amendment of the 1934 Specification and the eventual issue of a replacement, work on the aircraft was subordinated to that on the Hurricane. First flight by K5115 took place at nearby Brooklands on March 10, 1937, power being provided by a Merlin "F" (No. 13) driving a Hamilton Hydromatic three-blade propeller. During the first three months some forty hours flying were carried out with this engine but on June 5, K5115 was flown with a Merlin I (No. 235), again driving the Hamilton propeller.
Fabric-covered wings had been fitted on K5115 at first, but on August 20, 1937, the aircraft, at an all-up weight of 8,393 lb (3,807 kg), was flown with metal stressed skin wings. The aircraft then embarked on a program of general handling, spinning and diving with various center of gravity positions, a program which confirmed a viceless design allied with pleasant handling characteristics. Bearing this in mind it is difficult to find justification for subsequent events, for the Henley not only suffered relegation in the attitude of the Air Ministry but an unhappy spate of accidents in service.
A second prototype Henley, serialed K7554, was completed at Brooklands in 1938, this aircraft being flown by Chief Test Pilot P.G. Lucas on May 26 with a Merlin II (No. 25) driving a de Havilland three-blade two-pitch propeller.
In the intervening months, however, the Air Ministry's policy had been made clear that the aircraft was to be relegated to target towing, and a production order – initially for 350 aircraft but now reduced to 200 – was sub-contracted to Gloster Aircraft Company Ltd. at Hucclecote, Gloucester. K7554 remained at Brooklands only long enough for preliminary flight trials before being delivered to Glosters where it was modified to the Target Towing Standard of Preparation for which a wind-driven cable drum winch was installed.
Production got under way at Hucclecote during 1939, the service version being officially termed the Henley Mark.III (the Mk.I being the first bomber prototype, K5115, and the Mk.II, though seldom used as an official designation, being the prototype conversion K7554). The 200 Gloster-built Henleys, powered by an 1,030 hp Merlin II or III driving a de Havilland propeller, were completed in mid-1940, their production space being filled immediately by a further vital Hurricane assembly line.
Though production of the Henley had been transferred to Glosters, development of the type was retained by the Hawker Design Department and early in 1939 came a request that the original prototype K5115 be modified to take the Rolls-Royce Vulture for engine development flying by Rolls-Royce Ltd., and this was delivered to the Rolls-Royce Flight and Test Establishment, Hucknall, near Nottingham, UK, in 1939. Such was the trouble experienced with the Vulture (due principally to fatigue failure of connecting rod components) that a second aircraft, L3302, was similarly modified in 1940. Yet a third Henley test bed, L3414, was adapted (also in 1940), this time for the Rolls-Royce Griffon II, and much of the Fairey Firefly's engine installation was flown in this aircraft.
Production Henleys entered service with the RAF towards the end of 1939, serving as target tugs with Nos. 1, 5 and 10 Bombing and Gunnery Schools, and with the Air Gunnery Schools at Barrow, Squires Gate and Millom. Towing the relatively small Mark III Air Towing Sleeve, the Henley's speed was in the region of 265 mph (426 kmh), though for short periods at full throttle the aircraft could be coaxed another 10 mph (16 kmh) faster. The high rate of engine attrition, however, resulted in a towing speed limit of 220 mph (354 kmh) being imposed, a speed likely to afford little benefit to aspiring fighter pilots and quite unrepresentative of modem enemy aircraft.
Thus the Henley came gradually to be withdrawn as an air-to-air target tug and the survivors (130 remained on charge in October 1940) were distributed amongst the Anti-Aircraft Co-operation Units (AACU) and Nos. 639 and 695 (AA Co-operation) Squadrons.
The Henleys' troubles now started in earnest, for, with the larger drogue target on tow, it was as much as the aircraft could do to achieve 200 mph (322 kmh) at near full throttle, with the result that the Merlin (always an engine requiring a reasonable airspeed for cooling in the best of circumstances), now suffered perpetual overheating, excessive wear and engine failures by the score. How serious the resulting accidents proved, therefore, depended on how quickly the Henleys' crews could release their targets. Reference notes on the 49 aircraft assigned to No. 1 AACU, based at Langham, Norfolk, indicate the extent to which this unit suffered in this context, seven had to make forced landings and four crashed on eleven locations.
Relegation of the Boulton Paul Defiant to target-towing duties and introduction of the Miles Martinet brought the service career of the Henley to an end in 1942, though by June that year fewer than forty aircraft remained on charge.
The pictured aircraft served with the No.10 B.G.S. (Bombing and Gunnery School)."