Lioré et Olivier LeO 45

Lioré-et-Olivier LeO 45 was a French medium bomber used during World War II. It was a low-wing monoplane, all metal in construction, equipped with a retractable undercarriage and powered by two 1,060 hp Gnome-Rhône 14N engines. The prototype had two 1,100 hp Hispano-Suiza engines. It was a very effective bomber, but it appeared too late to give any substantial contribution to the war effort. Although designed before World War II, it remained in service until September 1957.

The 1934 B4 bomber programme
The LeO 451 was conceived as a second-generation strategic bomber for the new French Air Force. In contrast to its predecessors which relied on machine guns for protection, the emphasis was placed on high-speed high-altitude cruise. The expectation was that high speed would force enemy fighters into tail-chase attacks and to that effect the aircraft was designed with a rear-firing cannon with an unobstructed rear arc of fire thanks to the twin rudders.

The Service Technique Aéronautique released the initial requirements on 17 November 1934, specifying a 5-seat bomber with a top speed of 400 km/h (215 knots, 250 mph) at 4,000 m (13,125 ft), and a combat radius of 700 km (435 mi) carrying a payload of 1,200 kg (2,650 lb). In September 1936, the requirements were revised to account for development of 1,000 hp (746 kW)-class engines, with cruise speed raised to 470 km/h (255 knots, 290 mph) and crew reduced to four. The Air Force's Plan II called for 984 of the resulting B4-class bombers. Numerous manufacturers submitted a proposal, including Latécoère, Amiot with its Amiot 351, and Lioré et Olivier, which was to be soon nationalized as part of the SNCASE.

LeO 45 project
Lioré et Olivier was a long-time purveyor to the Armée de l'air with its LeO 20 and other lesser-known biplane bombers, that had earned a reputation for reliability, but were very traditional in design. The 1934 programme required modern solutions, and consequently the company management put Pierre Mercier, a younger engineer with expertise in cantilever airframes, at the helm of the design team.

Mercier's work resulted in a design, christened LeO 45, of a twin-engined aircraft of all-metal construction with a monocoque fuselage. Because of the speed requirements of the programme, a lot of effort was spent in reducing parasitic drag. Wings were equipped with slotted flaps and small bomb bays in the wing roots in addition to the main fuselage bomb bay, so as to limit the fuselage's cross-section. A specific wing structure was designed and patented by Mercier, where the inner part used two spars, with enough room between them for a 200 kg-class bomb and large self-sealing fuel tanks. However the spars did not continue to the wing-tip, but made way for a box-type structure.

Mercier also used his patented type of fairing for the LeO 45's radial engines. Unlike typical NACA cowlings, flow adjustment was not provided by flaps, but by a frontal ring that moved back and forth to respectively reduce or increase flow, without change in drag. Like many other French twin-engine planes of the era, propellers rotated in the opposite directions to eliminate the undesirable effects of propeller torque. Undercarriage was fully retractable, with an unusually complicated mechanism for the main wheels in order to reduce the size of the engine nacelles.

The fuselage hosted the four-man crew in the following order: the bombardier, who was also the commander as per French tradition, sat in the glazed nose ahead of the pilot. Immediately behind the pilot, the radio operator could man a defensive 7.5 mm M.1934 (500 rounds) machine gun from an underbelly retractable "gondola". A corridor alongside the main bomb bay led to the rear gunner position, that featured a powered mounting for the required 20 mm cannon Hispano-Suiza HS.404, with 120 rounds, and excellent ballistic proprierties (over 800 m/s), well over MG FF and other Oerlikon guns. The turret was retraclable when not needed. The armament was completed with another 7.5 mm machine-gun M.1934, this time fixed in the nose (300 rds). Overall, the Leo's armament was: 120 20 mm rounds, 800 7.5 mm rounds, up to seven 200 kg bombs, or other combinations (up to 1-2 500 kg bombs in the belly, plus the two 200 kg in the wings). The maximum load penalized the fuel, that was reduced to only 1,000 lts. The fuel tanks were: two 880 lts (inner wings), two 330 and two 410 lts (all in the external wings).

The LeO 45-01 prototype, powered by a pair of Hispano-Suiza 14AA-6 / Hispano-Suiza 14AA-7 radial engines producing 1,120 hp (835 kW) each flew for the first time on 16 January 1937. Despite problems with longitudinal instability, and engine reliability and overheating, the aircraft demonstrated excellent performance, reaching 480 km/h (260 knots, 300 mph) at 4000 m, and attaining 624 km/h (337 knots, 388 mph) in a shallow dive. In July 1938, the prototype fitted with the new Mercier cowlings reached 500 km/h (270 knots, 311 mph). Subsequently, the troublesome Hispano-Suiza engines were replaced with Gnome-Rhone 14N 20/21 producing 1,030 hp (768 kW) each, and the aircraft was redesignated LeO 451-01.

As the international situation was worsening, the Armée de l'Air ordered the LeO 451, explicitly asking SNCASE not to delay production with further improvements, even though teething troubles were far from cleared.

Operational history
The first production LeO 451 was built in 1938. The decision to abandon Hispano-Suiza engines and a shortage of propellers resulted in production delays. The latter also caused most aircraft to be fitted with slower Ratier propellers which reduced the top speed from 500 to 480 km/h. As a result, although 749 LeO 451 had been ordered, only 22 were delivered by the start of World War II. Of these, only 10 were formally accepted by the Air Force. They were issued to a frontline unit tasked with experimenting the new type in the field, and flew a few reconnaissance flights over Germany, which resulted in the type's first combat loss.

At the start of the Battle of France on 10 May 1940, only 54 of the 222 LeO 451 that had been delivered were considered ready for combat, the remainder being used for training, spares, undergoing modifications and repairs or having been lost. The first combat sortie of the campaign was flown by 10 aircraft from GB I/12 and GB II/12 on 11 May. Flying at low altitude, the bombers suffered from heavy ground fire with one aircraft shot down and 8 heavily damaged. Within the next 8 days many of them were shot down, like the one piloted by sergent-chef Hervé Bougault near Floyon during a bombing mission over German troops. By the Armistice of 25 June 1940, LeO 451 of the Groupement 6 had flown approximately 400 combat missions, dropping 320 tons of bombs at the expense of 31 aircraft shot down by enemy fire, 40 written off due to damage, and 5 lost in accidents. There are other numbers, stating about 47 bombers lost (26 to fighters, 21 to flak). Although the LeO's were faster than many 1940's fighters and faster than almost all other types of bombers, unfortunately for them the Luftwaffe was equipped with fighters that were even faster (Bf-109 and 110). The cruise speed, up to 420 km/h (7 km/min), was one of the strengths in the LeO's performances and would make them difficult to track down. The diving and climbing speed were very good as well (the Italian SM.79 took 17 minutes to reach 5,000 m, compared to 14 for the LeO), even if not that useful for a bomber. Leo's were optimized for medium-altitude operations (5,000 m), but were forced to go far lower to search and destroy tactical targets, but seldom had an even basic fighter escort (P-75, D.520). The LeO's were not unarmed however and German fighters had to keep a look out for their dorsal turret: on 6 June 1940, a single LeO (gunner Stg. Grandchamp, GBU.II/11) shot down two Bf-110C's with the Hispano gun. German fighters came to avoid this danger by attacking from below, forcing the Leo's to deploy they retractable turret, which slowed them appreciably (not known how much). Another problem had been the Germans' initial strike. The 6th Groupement had 50 Leo's, but they were not dispersed and even lacked AA defence on their airfields. When the Luftwaffe attacked them, they destroyed 40 bombers. In spite of this, 6th Groupement continued the fight, since the LeO's were produced on a fast pace (around 4-5/day, over 200 built within 45 days) allowing them to re-equip. Losses still remained high though, in a single mission 13 LeO's were intercepted and four shot down by Luftwaffe fighters. 6th Groupement totalled around 70 losses both in air and ground, but still continued to fight until the end.

Meanwhile, with Italy joining the war on the axis side, LeO's attacked Livorno, Novi Ligure, Vado, and even Palermo in a 4-ship morning mission. Against Italy the LeO's operated without much difficulty. Turin (Fiat plants) was near the border and thus easy to reach, if needed. Furthermore Italy did not possess an operative radar system and radios were not commonly used on the Italian fighters. On the other theatre however, Flak and Luftwaffe took a heavy toll.

A total of 452 aircraft had then been built, 373 accepted into service (including 13 for the Aéronautique navale), and around 130 lost in action in Europe.

Following the Armistice, LeO 451s continued to fly, now under the Vichy government. The aircraft were fitted with larger rudders and, later, two additional 7.5 mm machine guns in the rear turret. These extra weapons were added because of the limited capacity of the cannon magazines, and the fact that changing them in flight was extremely difficult. Aircraft production had totally stopped with the German occupation, but a 1941 agreement authorized Vichy authorities to have a limited number of military aircraft built. As a result, 109 additional LeOs were manufactured in 1942. The most notable of these was LeO 451-359 which was fitted with an experimental degaussing coil for remotely detonating naval mines (some British Vickers Wellingtons and German Junkers Ju 52s also carried a similar device).

Two bomber units equipped with LeO 451s, GB I/12 and GB I/31 were based in Syria when Allied forces invaded on 8 June 1941, at the start of the Syria-Lebanon Campaign. These were supplemented by GB I/25, which was dispatched from Tunisia. During this campaign, the LeO 451s flew a total of 855 sorties, losing 29 LeO 451s in the process.

After Operation Torch which began on 8 November 1942, surviving French LeO 451 in North Africa were used primarily for freight duties, although they flew a few bombing missions against Axis forces during the Tunisia Campaign. They were ultimately replaced in active service by Handley-Page Halifax and B-26 Marauder bombers.

The Germans were not especially interested in this airplane, but on 21 May 1943, Luftwaffe requested Regia Aeronautica to hand over 39 Lioré et Olivier LeO 451, captured by Italians troops in the SNCASE factory in Ambérieu-en-Bugey (Lyon). The Luftwaffe - that claimed to have previously bought the Lioré - gave in exchange a stock of 30 Dewoitine D.520. Subsequently, the 451s were converted into transport aircraft for fuel and troops. Other Lioré were delivered to the Regia Aeronautica and 12 were put in service with a ground attack unit, although they saw almost no active service.

Following the war, the 67 surviving aircraft were mostly used as trainers and transports. The LeO 451 was finally retired in September 1957, making it the last pre-war French design to leave active duty.

Variants

 * LeO 45.01
 * First prototype, powered by two Hispano-Suiza 14AA-6 / Hispano-Suiza 14AA-7 radial piston engines.


 * LeO 451.01
 * The first LeO 45.01 prototype was redesignated, fitted with two Gnome-Rhone 14R engines.


 * LeO 451
 * Production version variously fitted with Gnome-Rhône 14N-48 / Gnome-Rhône 14N-49 or Gnome-Rhône 14N-38 / Gnome-Rhône 14N-39 or Gnome & Rhône 14N-46 / Gnome-Rhône 14N-47 engines


 * LeO 451C
 * Twelve LeO 451T aircraft were redesignated, used as mail transport aircraft for Air France.


 * LeO 451E
 * Post-war flying laboratory, 11 modified.


 * LeO 451T
 * German-captured bombers modified for freight duty, seating for up to 17 troops. Around about 50 aircraft were modified.


 * LeO 453
 * Post-war conversion to high-speed transports and search-and-rescue aircraft, powered by two 895 kW (1,200 hp) Pratt & Whitney R-1830-67 engines, seating for 6 passengers, range 3500 km (1,890 nm, 2,175 mi) at 400 km/h (215 knots, 250 mph) cruising speed, 40 modified.


 * LeO 454
 * Bristol Hercules II engines, one prototype left unfinished.


 * LeO 455
 * High-altitude version with turbosupercharged Gnome-Rhône 14R engines producing 1,375 hp (1025 kW) each, 400 ordered, one prototype built. The aircraft flew on 12 March 1939 but was later destroyed on the ground.


 * LeO 455Ph
 * Post-war photoreconnaissance variant, powered by two 1,600 hp (1195 kW) SNECMA 14R engines. Five LeO 451s were modified and were used by the Institut Géographique National.


 * LeO 456 (LeO 451M)
 * Naval version for the French Navy, 68 ordered. Also known as the LeO 451M.


 * LeO 458
 * Wright GR-2600-A5Bengines, 10 ordered

Operators

 * French Air Force
 * Aviation Navale
 * Air France
 * 🇫🇷 Vichy France
 * Vichy French Air Force
 * undefined
 * Luftwaffe operated several captured aircraft.
 * undefined
 * Regia Aeronautica
 * 🇺🇸 United States
 * United States Army Air Forces operated several captured aircraft as unit hacks.
 * United States Army Air Forces operated several captured aircraft as unit hacks.