Flying to the Limit - Testing WW2 single engined fighters - Bulton Paul Defiant.pdf

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CHAPTER THREE
Boulton Paul Defiant
The design concept that led to the Boulton Paul Defiant can be traced back to the Bristol
F.2B Fighter of the First World War, which combined the performance of a single-seat
fighter with the flexibility of a two-seater, with a rear-facing Lewis gun in addition to a
forward-firing Vickers. The idea was perpetuated in the Hawker Demon of the inter-war
years and reached its ultimate expression in the Defiant.
Having already developed a pneumatic turret for its Overstrand bomber, Boulton Paul
were quick to see the advantages of a hydraulic turret developed in France by de Boysson
of the Societé d’Applications des Machines Motrices (SAMM) and acquired licence
manufacturing rights in 1937. The rapid advance in aircraft structures and performance in
the 1930s had led to a reappraisal of aircraft armament, as it was becoming clear that
larger gun batteries would be needed to stand any chance of downing the new monoplane
bombers. The combination of four 0.303-in Browning machine-guns in the de Boysson
turret (instead of the Darne machine-guns used by the French) seemed to be an ideal
solution to the problem of delivering sufficient weight of fire onto a target, and J.D. North,
Boulton Paul’s chief designer, penned a neat low-wing monoplane to accommodate it.
Unlike the Bristol F.2B and Hawker Demon, there was to be no forward-firing armament,
as the aircraft was considered to be a ‘bomber destroyer’, which, it was hoped, would not
have to contend with escort fighters. The resultant P.82 Defiant, designed to meet
Specification F.9/35 was flown for the first time by Cecil Feather on 11 August 1937 at
Wolverhampton Airport. The type was eventually chosen ahead of its main rival, the
Hawker Hotspur.
In its construction, the Defiant was more advanced than the Hurricane, which was the
culmination of the Hart/Fury design stream. At the same time, it was much simpler to
produce than the Spitfire. The front fuselage was built up of four L-section longerons, a
number of bulkheads and contained the pilot’s cockpit, the floor of which was formed by
the upper surface of the wing centre section. The rear fuselage accommodated the turret,
which was streamlined fore and aft by fairings made of spruce and three-ply. These were
raised or lowered by pneumatic jacks actuated by cams in the turret. Much attention was
given to reducing airframe drag, with countersunk rivets being used in the metal skinning
to allow a smooth surface finish. The wing was of two-spar construction and consisted of
a centre section, two outer panels and detachable wing tips. A semi-elliptical planform was
achieved by using a linear taper on the centre section, with a more pronounced taper on
the outer sections. Split flaps ran the full length of the wings from the Frise-type ailerons,
which (like the elevators and rudder) were fabric-covered. A total of 104 gallons of fuel
was accommodated in two self-sealing tanks in the wing centre section. The radiator was
mounted ventrally in a similar position to that adopted for the Hurricane.
The turret was hydraulically operated (the pump being electrically driven) and mounted
four 0.303-in Brownings in vertical pairs, each gun having 600 rounds in ammunition
tanks located under each gun installation. It was able to rotate through 360 degrees and the
guns could be depressed to the horizontal, except when facing forwards, when they were
restricted to an elevation of 17�½ degrees to clear the propeller. The maximum angle the
guns could be raised was 72 degrees as the air gunner could not use his sight at higher
elevations. Electrical cut-outs were provided to prevent the guns hitting any part of the
aircraft.
The prototype Defiant (K8310) was delivered to A&AEE at Martlesham Heath in early
December 1937 for an initial assessment. The turret had not yet been fitted, so a metal
fairing took its place. Powered by a Rolls-Royce Merlin I, performance testing showed a
top speed of 320 mph and the aircraft attained a height of 10,500 ft in 7�½ minutes. The
handling was acceptable in most respects, the only major complaints being that the
ailerons were considered to be too light and the flaps, when fully down, did not produce
the desired nose-down attitude on a glide approach. As the cockpit was set further forward
than either the Hurricane or Spitfire, access on the ground could be made difficult,
particularly if the wing surface was wet. However, the view forwards and downwards was
better than on either of the two aforementioned aircraft. The cockpit also proved to be
rather draughty with the hood closed.
K8310 was fitted with the four-gun turret in February 1938 and returned to Martlesham
Heath for armament trials, with further performance testing taking place the following
October. The figures obtained were slightly disappointing as the maximum speed was
reduced to 303 mph at 15,000 ft and it took 15.1 minutes to reach 20,000 ft. It had been
hoped that the Defiant would have a similar performance to the Hurricane, but its top
speed was 17 mph slower and the climb to 20,000 ft took a full 6 minutes longer. It was
obvious that more power was needed and the first production aircraft (L6950) was fitted
with a Merlin III of 1030 hp and first flown on 30 July 1939. It was delivered to A&AEE
(by now at Boscombe Down) on 19 September 1939, together with the second prototype,
K8620, for further performance and handling trials.
The provision of a non-slip walkway on the wing root made entry to the cockpit slightly
less hazardous, although a spring-loaded step on the fuselage side would have made it
easier still. The cockpit was comfortable, with adequate room for a well-built pilot, but in
flight a cold draught tended to play on the back of the pilot’s neck. It was also rather noisy,
but no more so than contemporary fighters. The gun turret was much more difficult to
enter, access being obtained either by sliding back the segments of the transparent cupola,
or via a hatch in the floor aft of the turret. The latter method could only be used when the
guns were facing forward. The turret was perfectly adequate for an air gunner of small
stature, but anyone who was taller than average found the accommodation rather cramped
when wearing full flying kit.
The control surfaces were operated by a system of cables, pulleys and levers, which
produced little friction in the control circuits and only slight play. The elevator trim tab
control was located on the pilot’s left in the forward upper corner of the cockpit and could
be moved over its full range in about three seconds. Rudder trim could be applied by
moving a quadrant-mounted lever, which was also on the left-hand side of the cockpit
immediately aft of the undercarriage and flap quadrants. The full range of movement
could be obtained in about one second. No aileron bias gear was fitted.
The throttle control quadrant was conveniently mounted on the pilot’s left and showed
no sign of slipping. The propeller pitch could be selected to ‘Coarse’ or ‘Fine’ by a control
immediately below the panel on the left-hand side. It was within easy reach, but could be
easily mistaken for the boost cut-out control, especially when changing pitch after take-
off. The radiator shutter control was located on the right of the cockpit, with the fuel cock
on the left. Some difficulty was experienced with the fuel cock sticking, which was
modified by Boulton Paul. The flap control was mounted in the throttle control box and
could be operated with ease to stop the flaps in any position. An indicator on the left-hand
side of the panel was marked in degrees and covered the full flap range.
The control for the undercarriage was located next to the throttle box and was easy to
operate. The position of the wheels was indicated to the pilot by red and green lights
situated in the centre of the panel–red when the undercarriage was up and green when it
was down. A warning horn sounded if the throttle was closed beyond the last third of its
movement with the wheels still up. There were two methods of operating the
undercarriage in an emergency, consisting of two hand pumps, one operating through a
hydraulic system separate to the normal system. Pilots faced with such an emergency were
then faced with the unenviable task of applying 200 strokes to the pump before the
undercarriage was fully down.
The brakes were pneumatically operated via a lever on the control column and
performed well. A standard blind flying panel was fitted and the layout of the instruments
was good, with no vibration. For night flying, illumination was provided by two lights on
movable arms, one on each side of the cockpit. A dimmer switch was provided for each.
They could be set to illuminate any particular instrument or the complete blind flying
panel and did not cause any unwanted reflections on the windscreen.
In the prototype Defiant no emergency exit was available from the front cockpit. A
sliding hood was fitted, but was found to be almost impossible to open at speeds above
210 mph IAS. Knock-out panels were fitted in the sides of the hood, which were
sufficiently large to enable a small pilot to climb out of the aircraft should it come to rest
inverted on the ground. Later aircraft were fitted with a hood that could be jettisoned. In a
bale-out situation, the air gunner had to leave the turret through the trap door in the floor
of the fuselage, but this could only be done with the guns in the forward position. It was
considered that the chances of the gunner getting out successfully would depend on his
size. A tall man might well have difficulty, particularly if the aircraft was not under control
at the time. The gunner’s exit hatch could be opened from the outside should the aircraft
go over onto its back on the ground, but it was thought that it would have been extremely
difficult for him to extricate himself without assistance.
For a low-wing single-engine monoplane fighter, the view from the cockpit was better
than most other aircraft of the period as the pilot’s cockpit was mounted further forward to
cater for the turret. Although the view ahead when the tail was on the ground was not
good, with the tail up on take-off it was greatly improved, and in flight the view
downwards over the leading edge was excellent. No ‘direct vision’ panel was fitted so
there was virtually no forward visibility when flying in rain. Instead, the pilot had to open
the hood and peer round the windscreen.
Handling trials were carried out at take-off weights of 7220 lb (forward CG), 7390 lb
(normal CG) and 7560 lb (extended aft CG). The aircraft was easy to taxi in winds of up
to 30 mph and showed no tendency to lift its tail even with CG at the forward limit. The

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