No single type of aircraft has made a more spectacular appearance this war than did the Boulton Paul Defiant. In May of 1940 when Allied troops were being bombed and machine-gunned on the Dunkirk beach a squadron of these ships roared over the Channel for its first battle appearance. The squadron accounted for 37 German planes in this initial action, returning to its base without a single casualty.
The Defiants had made themselves world news in a few hours; but they have continued after Dunkirk, to play a vital part in winning the battle of Britain. Today they are utilized mainly to sweep the night skies in search of raiders, and have been responsible for a large number of Luftwaffe's bombers failing to return to Germany. In one night raid, for example, Defiants over London accounted for four Heinkels and one Dornier. On another night Defiants accounted for eight of the raiders, and on a third, during extensive German incursions over Britain, three of the total brought down were accounted for by Defiants. Men from the Dominions, Poland and Czechoslovakia are now numbered among the Boulton Paul Defiant crews.
The designers who produced the Defiant had many problems to overcome. The plane met a need for a two-seat craft heavier than the single-seat type fighter but comparable in performance, particularly in maneuverability, and with mobile guns mounted on a power-operated turret amidships to provide a field of fire covering practically the whole of the upper hemisphere.
In general construction, the Defiant conforms to the now familiar single-engine tractor, low-wing monoplane formula, and the cupola of the four-gun turret mounted on the upper surface of the fuselage just over the wing trailing edge is the only obvious indication that the plane has any unusual characteristics. Owing to the ingenious arrangement of fairings, which play a large part in reducing drag of the turret, the plane has unusually good performance for a ship of its size, with a speed of well over 300 mph.
The actual design detail, although conventional, incorporates many interesting features. Except for the control surfaces and the fairings above the fuselage deck, it is entirely metal-skinned. The front end of the fuselage is built as a unit, on four angle-section longerons of extruded alloy. At the extreme front of this unit is a braced tubular frame carrying the attachments for the engine mountings.
The wings of the Defiant, like the fuselage, comprise a series of unit assemblies; a center section and two outer wings, plus detachable wing tips. Their general form is a close approach to the ideal elliptic plan form, but allows the use of a skin which, except at the wing tips, requires no forming operation. The leading edge for both center and outer wing sections is detachable. It consists of a sheet of light alloy bent to the correct profile, riveted to a series of nose rib formers. The covering sheet is screwed to top and bottom front spar flanges by countersunk steel screws. To accommodate these screws, the spar flanges are provided with spun-in Monel metal bushes, drilled and tapped for the securing screws.
The retractable landing gear comprises two Lockheed "Airdraulic" shock absorber legs hinged to the diagonal beams which are among the few complete "ribs" of the wing center sections. Each leg of the landing gear is retracted or extended by a breakback strut which extends diagonally inward towards the fuselage and rearward towards the rear spar.
The engine oil cooler is contained in a duct-cowling integral with the bottom engine cowling panel. This panel extends aft of the fireproof bulkhead, to form the lower fairing of the front fuselage section up to the wing center section.
The Boulton Paul gun turret which is a removable self-contained unit mounts four .303-in belt-fed guns. It is very economical in its demand for operating power, due mainly to the high overall efficiency of the hydraulic variable gear system incorporated in the control system. The whole of the hydraulic system for turret operation is part of the turret unit, and has no connection with any hydraulic system outside of the turret; a separate system operating the retractable landing gear and landing flaps, Power is normally supplied by an engine-driven pump, but there is also an emergency hand pump by which the landing gear and flap may be operated if the engine driven pump should fail. A pneumatic system, supplied by an engine-driven compressor, operates the collapsible turret fairings and wheel brakes.
The Defiant's power plant is a glycol-cooled Merlin III engine, rated at 1030 bhp at 3,000 rpm at 16,500 ft, driving a constant speed, three-blade propeller of 11' 6" diameter through a reduction gear of .477 ratio. An unusual feature of the power plant is separation of the oil and glycol radiators. Both are of the normal ducted radiator type, but the oil cooler is forward under the engine and is in the same duct as the carburetor air intake, and the radiator proper is considerably further aft. Warm air, after passing the oil cooler, goes to the pilot's cockpit through a controllable flap. Warm air from the main radiator proper can be similarly admitted to the gun turret compartment of the fuselage. Thus both members of the crew may be kept reasonably warm under practically all operating conditions.
The pilot's seat in the Defiant is, in comparison with most modern single-engine fighters, unusually well forward in relation to the wings and consequently the view for landing and take-off is better than in most comparable airplanes. The seat is adjustable for height, with the control column arranged to move with the seat so that raising or lowering it leaves the stick in the same position relative to the pilot.
Specifications of the Boulton Paul Defiant are:
|Length overall||35' 4"|
|Height overall||11' 4"|
|Wing area||250 sq ft|
|Gross weight||7,500 lb|
|Wing loading||80 lb/sq ft|
|Power loading||7.15 lb/hp|
|Engine||Rolls-Royce Merlin III|
1050 bhp at 16,500 ft
This article was originally printed in the September, 1942, issue of Aviation magazine, vol 41, no 9, pp 201, 297-298.
The original article includes 3 photos: Defiants in formation, seen from 4 o'clock, Defiants in formation, seen from 11 o'clock, Defiants on the flight line showing left side of nose.
Photos are not credited.