The Blériot Bl.73, Bl.74, Bl.75 and Bl.76 were large First World War French biplanes designed and built by Blériot. The Bl.73 was built to the BN.3 three-seat night bomber specification, the Bl.74 was to be a bomber-transport, the Bl.75 Aerobus was to be an airliner, while the unbuilt Bl.76 was intended for the BN.4 four-seat night bomber specification. Aside from the Bl.76, just one prototype was built of each type, with both Bl.73 and Bl.74 prototypes being lost in accidents while on test flights.
The Blériot Bl.73 BN.3 was a large equal-span biplane bearing a family resemblance to the slightly smaller Blériot Bl.71, with the exception of the location and shape of the fuselage. The lower wing had considerable dihedral and was attached to the top of the fuselage, while the rear fuselage was curved strongly upwards to a biplane tail unit with twin rudders.[1] Four 300 hp (220 kW) Hispano-Suiza 8Fb water-cooled V-8 engines were used, with two on the upper wing leading edge and two on the lower wing leading edge, directly over each other. The outer wing panels had two wing bays, with the inner bay split with half struts, as on the SPAD fighters. The landing gear consisted of two bogeys, one on each side of the fuselage, with each bogey having four wheels arranged in a rectangle, so that no tailskid was needed. During flight testing on 22 January 1919 the sole Bl.73 crashed on landing at the end of its first flight, killing the pilot.[2]
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After the crash of the Blériot Bl.73, Touillet developed a heavy bomber/airliner with a large double bubble teardrop fuselage mounted on the lower wing. The ngines remained the same as those of the Bl.73 although the wings were enlarged slightly, and rigged so that both outer bays were split with half struts, although the upswept tail was dispensed with. Flight testing at Villacoubly was abruptly ended on 22 January 1920 when oscillations in the tail unit caused a structural failure which resulted in the fatal crash of the sole prototype.[2][3]
The airliner version had a longer rear fuselage and enlarged vertical tail surface but was otherwise similar to the Bl.74. Despite the promising flight-tests of the sole Blériot Bl.75, no orders ensued.[2][4]
Designed for the wartime BN.4 specification, and a beneficiary of the merger between Blériot and SPAD, the Blériot Bl.76 BN.4 bomber more closely resembled a four-engine Blériot-SPAD S.45 repurposed as a bomber than prior designs. Both upper and lower wings were swept-back and rigged with I struts in a single bay outboard of the engines, with the upper 300 hp (220 kW) Hispano-Suiza 8Fb engines mounted further inboard than the lower engines. Positions for gunners were provided in the extreme nose, and on top of the fuselage just behind the wing. A conventional tail with a single fin and rudder was planned, along with a pair of large but conventional main wheels braced to the fuselage and engine, with a tail skid. It remained a design study only, and was not built.[2] The 1921 S.45 differed primarily having the fuselage double bubble reduced in height, straight lower wings, having just two engines mounted on the lower wings without their own wing bay, a more complex undercarriage, and in lacking a gunner's position in the nose.
Data from Aviafrance: Bleriot Bl-73,[1] French aircraft of the First World War[2]
General characteristics
Performance
Armament
