__NOTOC__ The Blériot-SPAD S.91 was a light-weight

fighter aircraft Fighter aircraft (early on also ''pursuit aircraft'') are military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air supremacy, air superiority of the battlespace. Domina ...

by the French aircraft manufacturer Blériot. It was developed in response to a French government program for a light fighter aircraft. The S.91 was a single-seat earlier

single-bay A biplane is a fixed-wing aircraft with two main wings stacked one above the other. The first powered, controlled aeroplane to fly, the Wright Flyer, used a biplane wing arrangement, as did many aircraft in the early years of aviation. While a ...

biplane that resembled pursuit aircraft produced by André Herbermont except for its wing profile.

Duralumin Duralumin (also called duraluminum, duraluminium, duralum, dural(l)ium, or dural) is a trade name for one of the earliest types of age hardening, age-hardenable aluminium–copper alloys. The term is a combination of ''Düren'' and ''aluminium'' ...

and other light metals comprised a substantial amount of the framework; a single girder composed of out of duralumin tube supported the entirety of the fuselage aft of the cockpit. The majority of the covering was

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, and different types of fabric. At first, the word "textiles" only referred to woven fabrics. However, weaving is no ...

. According to Blériot, it was a highly maneuverable aircraft and was also relatively easy to operate. On 23 August 1927, the first prototype performed its

maiden flight The maiden flight, also known as first flight, of an aircraft is the first occasion on which it leaves the ground under its own power. The same term is also used for the first launch of rockets. In the early days of aviation it could be dange ...

; it was lost in an accident during flight testing. The completion of a second prototype permitted the resumption of flight testing. Despite the French government having lost interest in the general concept, the company opted to continue work for a time, promoting the S.91 to various prospective foreign operators. During 1936, at least one of the surviving S.91 prototypes ended up in the

Spanish Republican Air Force The Spanish Republican Air Force was the air arm of the Armed Forces of the Second Spanish Republic, the legally established government of Spain between 1931 and 1939. Initially divided into two branches: Military Aeronautics () and Naval Aeron ...

. While the S.91 would never go into quantity production, it would eventually be developed into the Blériot-SPAD S.510, which would be the last

biplane A biplane is a fixed-wing aircraft with two main wings stacked one above the other. The first powered, controlled aeroplane to fly, the Wright Flyer, used a biplane wing arrangement, as did many aircraft in the early years of aviation. While ...

produced by the French aeronautic industries.

Design and development

The S.91 was a single-seat

biplane developed towards the end of the 1920s by Blériot in order to meet the requirements of a French government program that sought a light fighter aircraft, which was referred to as ''"Jockey"''. The French aircraft manufacturer Blériot was one of several domestic companies to develop a response to this requirement, opting to design a clean-sheet pursuit aircraft that was intended to be both relatively quick and manoeuvrable for the era.NACA 1928, p. 1. In terms of its general configuration, the S.91 bore a close resemblance to the earlier pursuit aircraft produced by André Herbermont (a lead aircraft designer at Blériot), the most prominent exception being the wing profile adopted. Both the upper and lower wings had the same chord; the lower wings were rectangular and staggered aft while the upper wings possessed a clear sweep back. The upper wings were attached to the fuselage via a metal cabane that consisted of two sets of reversed N-shaped

strut A strut is a structural component commonly found in engineering, aeronautics, architecture and anatomy. Struts generally work by resisting longitudinal compression, but they may also serve in tension. A stay is sometimes used as a synonym for ...

s; the lower wings were attached to stubs which constituted integral elements of the fuselage. The wings of each half-cell were joined by a single strut and were braced using

streamlined Streamlines, streaklines and pathlines are field lines in a fluid flow. They differ only when the flow changes with time, that is, when the flow is not steady flow, steady. Considering a velocity vector field in three-dimensional space in the f ...

wires composed of high-resistance

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

. Each of the lower wings were furnished with a relatively large

aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement aroun ...

that was unbalanced. The framework of the wing was entirely composed of

duralumin Duralumin (also called duraluminum, duraluminium, duralum, dural(l)ium, or dural) is a trade name for one of the earliest types of age hardening, age-hardenable aluminium–copper alloys. The term is a combination of ''Düren'' and ''aluminium'' ...

, although the covering was made of

. Each wing had a pair of spars that were made up of open-work rectangular tubing that were free of both

rivet A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylinder (geometry), cylindrical shaft with a head on one end. The end opposite the head is called the ''tail''. On installation, the deformed e ...

s and joints.NACA 1928, p. 2. In the vicinity of the cockpit, the lower wings featured a sight cut away in order to improve the pilot’s downward visibility, which was claimed to be quite good. The pilot could operate a pair of machine guns, which remained entirely accessible throughout the flight.NACA 1928, p. 3. The framework of the fuselage was composed of light metals. The forward section, which included all fuselage elements forward of the pilot’s cockpit, incorporated the aircraft’s power plant and consisted of open-work box girders. Aft of the cockpit, the fuselage formed a single girder that was constructed out of duralumin tubes. All of the longerons, uprights, cross beams and diagonals were assembled using

gusset In sewing, a gusset is a triangular or rhomboidal piece of textiles, fabric inserted into a seam to add breadth or reduce stress from tight-fitting clothing. Gussets were used at the shoulders, underarms, and hems of traditional shirts and chemi ...

s.NACA 1928, pp. 2-3. The horizontal

empennage The empennage ( or ), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow.Crane, Dale: ''Dictionary of Aeronautical Terms, third ed ...

consisted of an adjustable stabilizer, the forward edge of which rested on the

longeron In engineering, a longeron or stringer is a load-bearing component of a framework. The term is commonly used in connection with aircraft fuselages and automobile chassis. Longerons are used in conjunction with stringers to form structural fram ...

s of the upper fuselage; the rear edge of the empennage carried the adjusting apparatus. A two-part

elevator An elevator (American English) or lift (Commonwealth English) is a machine that vertically transports people or freight between levels. They are typically powered by electric motors that drive traction cables and counterweight systems suc ...

formed a continuation of the stabilizer. The vertical empennage consisted of a conventional

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...

and

rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium (usually air or water). On an airplane, the rudder is used primarily to counter adverse yaw ...

. All of the tail planes were composed of duralumin and were covered with fabric. The S.91 was typically powered by a single Hispano-Suiza 12Hb V-12 engine, capable of generating up to 500hp. Cooling was achieved using a Lamblin water-based

radiator A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

s that was mounted on struts and positioned forward of the

landing gear Landing gear is the undercarriage of an aircraft or spacecraft that is used for taxiing, takeoff or landing. For aircraft, it is generally needed for all three of these. It was also formerly called ''alighting gear'' by some manufacturers, s ...

. The Lamblin-supplied oil radiator was located underneath the fuselage. The aircraft was typically equipped with a removable fuel tank, which was positioned inside of the fuselage. The landing gear was formed of two V-shaped struts that consisted of box members in front and tubes aft. It incorporated a hinged axle; each half-axle, which was relatively elevated in respect to the axis of the disk wheel, was likewise a box spar with a streamlined housing. The

shock absorber A shock absorber or damper is a mechanical or hydraulics, hydraulic device designed to absorb and Damping ratio, damp shock (mechanics), shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typic ...

s were located on the inside of the wheels. The tail skid, which consisted of a spring with steel leaves, was hinged to the is sternpost of the fuselage and was attached at its upper end to a removable shock absorber. The skid could be replaced relatively easily by removing the pin that connected it with the stern post.NACA 1928, p. 4.

Prototypes and into flight

On 23 August 1927, the first prototype performed its

. During the flight testing of this initial aircraft, the company announced that it had attained a maximum speed of 270 km (168 miles) per hour while at an altitude of 4Q00 m (13,123 ft.). Furthermore, the prototype was claimed to be both highly manoeuvrable and relatively easy to operate as a result of its compact dimensions, favourable centring and the selected wing profile.NACA 1928, pp. 1-2. This prototype was destroyed in a crash; flight testing resumed following the completion of a second prototype. Even after the French government dropped interest in the program seeking a light fighter for the

French Air Force The French Air and Space Force (, , ) is the air force, air and space force of the French Armed Forces. Formed in 1909 as the ("Aeronautical Service"), a service arm of the French Army, it became an independent military branch in 1934 as the Fr ...

, André Herbemont designed a further prototype with an inverted

sesquiplane A biplane is a fixed-wing aircraft with two main wings stacked one above the other. The first powered, controlled aeroplane to fly, the Wright Flyer, used a biplane wing arrangement, as did many aircraft in the early years of aviation. While a ...

wing configuration. While flight demonstrations were performed in both

Romania Romania is a country located at the crossroads of Central Europe, Central, Eastern Europe, Eastern and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to ...

and

Greece Greece, officially the Hellenic Republic, is a country in Southeast Europe. Located on the southern tip of the Balkan peninsula, it shares land borders with Albania to the northwest, North Macedonia and Bulgaria to the north, and Turkey to th ...

in the hope of garnering sales, no orders for the type followed. André Herbemont would use his experience with the S.91 prototypes in order to develop the S.510, another biplane fighter which would go into production and would be later used by the

. In 1936, at least one of the surviving S.91 prototypes ended up in the

Variants

;S.91 ''Leger'':Powered by a Hispano-Suiza 12Hb V-12 water-cooled engine with twin-leg radiators mounted on the undercarriage, (1 built). ;S.91/1:The S.91 ''Leger'' fitted with a frontal

design. ;S.91/2:The S.91/1 prototype fitted with a Hispano-Suiza 12Gb W-12 water-cooled engine. It was displayed in

and

. ;S.91/3:The S.91/2 was fitted with a

Gnome-Rhône 9A Jupiter The Bristol Jupiter is a British nine-cylinder single-row piston radial engine that was built by the Bristol Aeroplane Company. Originally designed late in World War I and known as the Cosmos Jupiter, a lengthy series of upgrades and developme ...

nine-cylinder air-cooled radial engine, (1 built). ;S.91/4:The S.91 ''Leger'', fitted with a Hispano-Suiza 12Mb V-12 engine, flew again on 4 July 1930 with some modifications, including radiators mounted on top of the upper wing. ;S.91/5:The sole S.91/3, fitted with a Gnome-Rhône 9Ae Jupiter, which crashed on the day of its first flight killing the pilot. ;S.91/6:This S.91/4 conversion flew in November 1930 and differed from the prototype in having rounded wingtips, a lengthened fuselage and a tailplane lowered to the fuselage base. Later the tailplane was restored to its former position. ;S.91/7:A new prototype with inverted

wing configuration in which only the lower wing had

s. It flew on 23 December 1931 and was fitted with a Hispano-Suiza 12Mc V-12 engine. On 2 June 1932 this plane established a record of in a closed-circuit. ;S.91/8:The S.91/7 prototype fitted with a supercharged

Hispano-Suiza 12X The Hispano-Suiza 12X was an aircraft piston engine designed in France by Hispano-Suiza during the early 1930s. A 12-cylinder Vee, liquid-cooled design, the 12X was used on several aircraft types, some of them being used in limited numbers durin ...

brs engine and a variable-pitch

Ratier Ratier-Figeac is an aircraft components manufacturer in Figeac, France. From 1926 until 1930 it also built a car with a 746 cc overhead camshaft engine. From 1959 until 1962 Ratier made motorcycles, having taken over the motorcycle business of t ...

propeller made its maiden flight on 20 August 1932. This version is reported to have achieved a speed of . ;S.91/9:The S.91/8 became a

testbed A testbed (also spelled test bed) is a platform for conducting rigorous, transparent, and replicable testing of scientific theories, computing tools, and new technologies. The term is used across many disciplines to describe experimental research ...

for

Hispano-Suiza Hispano-Suiza () is a Spanish automotive company. It was founded in 1904 by Marc Birkigt and as an automobile manufacturer and eventually had several factories in Spain and France that produced luxury cars, aircraft engines, trucks and weapons. ...

's engine-mounted cannon, after being leased to the company in December 1932. Besides the cannon it was also fitted with a large-diameter Levasseur fixed-pitch propeller.

Operators

; *

Specifications (S.91 ''Leger'')

References

Citations

Bibliography

* *
"Bleriot-Spad 91 Airplane (French) : pursuit single-seater, type "Jockey""
''

National Advisory Committee for Aeronautics The National Advisory Committee for Aeronautics (NACA) was a United States federal agency that was founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its ...

'', 1 May 1928. NACA-AC-74, 93R19948.