LeO 451 - Partial Success
This aircraft stands out radically from all the other French bombers described in previous articles. This is true in every way: its sleek appearance—it was not for nothing that the LeO 451 was considered one of the most beautiful bombers of World War II—its large production volume, certainly for pre-war France, and its fame—only rivaled by the Dewoitine D 520 fighter. More books and articles have probably been written about the LeO 451 than about all other French bombers combined. It even boasts its own "golden" legend—of an incomparable bomber, far surpassing anything created by humanity up to the time of its introduction. This, of course, was a gross exaggeration—along with its numerous advantages, the aircraft also had serious shortcomings that hindered its successful use.
The most detailed information about the LeO 451 is provided in the two-volume book by Meunier Lioré et Olivier LeO 451, dedicated exclusively to this aircraft, but it has a major drawback - it is not available online. Another detailed source on the LeO 451 is the book by Cuni and Danel "LeO 45, Amiot 350 et outres B4" (Jean Cuni et Raymond Danel "LeO 45, Amiot 350 et outres B4"). This book is available online and is written in fairly simple language, so the automatic translation is quite understandable. Moreover, the authors tried to give an objective assessment of the aircraft, which they defined as a partial success. There are many other sources, including Russian-language ones, but they mostly repeat each other. I am not going to give a full history bomber - only about what is connected with the main theme of the whole cycle, i.e. how the French bomber aviation By May 10, 1940, it found itself in the situation it found itself in. Perhaps I'll have to focus on the aircraft's shortcomings, because much has been written about its advantages before me.
The aircraft's official history began on November 17, 1934, when the General Staff of the Air Force published the program for the four-seat high-speed bomber B4, designated A-21. While many sources cite the B5 program, i.e., a five-seat aircraft, they themselves point out that one crew member was quickly eliminated, so there's little difference. The program called for a maximum speed of 400 km/h at an altitude of 4 to 5 km, prioritizing aerodynamics. Defense against fighters from all directions was no longer required, so the nose turret could be replaced with a pintle or ball mount with a limited field of fire, making the nose section more streamlined. The bomb bays were recommended for placement of the combat load, which had to be at least 1500 kg.
For the first time, the military demanded the installation of 20–25 mm cannons in the upper and lower turrets. Moreover, the turrets were required to be retractable and capable of 360° rotation. However, almost all firms ignored this requirement, and two years later, the A-21/1 program essentially accepted this state of affairs. Now, the STAe (Le Service technique de l'aéronautique or Section technique de l'aéronautique, the French government agency responsible for coordinating aeronautical research) agreed to a dorsal cannon mount without 360° fire and a lower hatch mount with a MAC Mle 1934 7,5 mm machine gun. The requirement for a 20 mm cannon remained, with the long-barreled HS 404 specifically specified, but with limited firing angles—30° from the aircraft's axis. The fact is, the A-21/1 program called for a speed increase to 470 km/h and a twin-fin tail. It was assumed that at that speed, an enemy fighter would only be able to attack from behind and above, and that would be the only way to get what they deserved.
LeO 45 firing sectors according to the original design
Until 1934, Lioré et Olivier focused on seaplanes and bombers—specifically, from 1927 to 1934, it produced a rather successful series of wood-and-fabric biplanes, the LeO 20-206. Moreover, the company was unwilling to abandon the biplane design until almost the mid-30s: in June 1934, the last of these Mohicans, the LeO 208, took to the air for the first time. This, shall we say, design achieved a top speed of 325 km/h during testing, after which even the most die-hard conservatives realized that the era of biplanes was over forever.
The LeO 208 was the last biplane built by Lioré et Olivier.
The future LeO 45 was developed by a team headed by the young, energetic engineer Pierre Mercier. Previously, he had been responsible for the fuel system and chassis of the LeO 208. In other words, he had no experience as a chief engineer. In 1935 (the exact date is unknown), he brought to the design bureau a project that had already been sketched out with great precision. It was sleek, elegant, and seemed the complete opposite of the LeO 208. This new project was designated LeO 45.
A few months later (again, no specific dates were given), construction of the LeO 45-01 prototype began. It left the assembly line in Villacoublay in late 1936 and made its maiden flight on January 16, 1937. This first, and only, prototype was essentially identical to the future production aircraft. Specifically, only the engines, their associated equipment, defensive armament, and tail unit design were different. No one could have imagined back then that problems with the engines, armament, and tail unit would plague the aircraft for years to come.
Upper turret of the LeO 45 as originally designed
Initially, a dorsal turret with 360° rotation and an HS 9 cannon was planned, but these plans likely remained on paper only. In reality, the same hydraulically powered dorsal gun mount with a retractable canopy and the same retractable (hydraulic with manual backup) MAC machine gun pod as on production vehicles were installed. However, there was a significant addition: the designers attempted to ensure fire coverage in the forward hemisphere, at least in limited sectors. To this end, a machine gun was mounted in the forward part of the canopy and pod. Clearly, the design proved unviable in both cases.
Lower turret
The first prototype was equipped with Hispano-Suiza 14Aa-06/07 air-cooled engines (according to other sources, 14Aa-08/09), where the number 14 denoted the number of cylinders. Like some other French radial engines, to eliminate the yaw moment during takeoff and landing, the HS 14Aa was produced in two versions—left-hand and right-hand rotation: an even number denoted left-hand rotation, and an odd number, respectively, right-hand rotation. The following characteristics are given for the 14Aa-06/07: takeoff power - 1080 hp at 2125 rpm, nominal at sea level - 980 hp and 1100 hp at an altitude of 2.85 km at 2125 rpm. The data for other modifications were almost identical.
Since the history of these engines greatly influenced the fate of the LeO 45 bombers, as well as the entire French bomber force, it's worth telling a little about them. And their history is a very sad one. Since World War I, Hispano-Suiza had been known for its water-cooled engines and had no experience designing air-cooled engines. To enter this market, in 1928 the company acquired licenses for the Wright Whirlwind R-540, Wright Whirlwind R-975, and Wright Cyclone R-1820 engines. The latter, with very limited success, was produced under the designation 9V. Based on this engine, in 1933 the company began designing the 14-cylinder 14Aa engine (initially designated 14Ha).
In fact, Wright, based on the R-1820, began developing the famous R-2600 Double Cyclone a little later in a similar fashion. Moreover, the 14Aa even surpassed the American engine in displacement—45 and 42 liters, respectively. But Hispano-Suiza CEO Mark Birkigt took a daring step: he limited the weight and dimensions of the future engine as much as possible. Specifically, while the R-1820's diameter was 139 cm (the same as the R-2600), the 9V's was 137 cm, and the 14Aa's was only 126 cm. This is despite the fact that double-row sprockets traditionally have problems with cooling the rear cylinder bank. But, as we know, miracles don't happen, and the price was paid in engine reliability, or rather, the lack thereof.
Overheating, oil leaks, and component failures, including the crankshaft, were constant problems. While the engines of their competitors from Gnome et Rhône weren't exactly models of reliability, they were simply less reliable. Only by the end of 1938 did the final modifications, the 14Aa-12/13, demonstrate satisfactory reliability, but it was too late. In retrospect, it can be said that abandoning these engines entirely was as wrong as placing excessive hopes on them before.
It's difficult to say whether the company's decision to install HS 14Aa engines on the LeO 45 prototype—in other words, installing untested engines on an untested aircraft—was a mistake. In a similar situation, Félix Amiot demanded testing on the old Amiot 143—and, as expected, the tests ended poorly. Louis Breguet declared from the outset that his Breguet 462 could only be equipped with Gnome et Rhône 14Ns, and until they were ready, with GR 14Ks. Incidentally, it was the second Breguet 462 prototype, which made its maiden flight in February 1937, that was first equipped with GR 14Ns and achieved a 100-hour trouble-free run, a remarkable achievement for the French Air Force.
Marcel Bloch initially designed the MB 131 with the proven, though far from ideal, GR 14K engines, and only later installed the GR 14N in the MB 131 and the HS 14Aa in the MB 132. Undoubtedly, Lioré et Olivier and its chief engineer were under intense pressure from both the Air Force General Staff and the Air Ministry, which were caught up in the premature enthusiasm for the HS 14Aa engines. It is unclear whether the company could afford the luxury of refusing. However, already in February 1937, Lioré et Olivier was nationalized and incorporated into the SNCASE (Société nationale des constructions aéronautiques du Sud-Est – National Aircraft Manufacturing Society of the Southeast Region), which further limited its freedom of choice.
So, on January 16, 1937, the LeO 45-01 prototype took to the skies for the first time. The flight lasted a full five minutes before it had to be aborted due to engine overheating. Already during the maiden flight, insufficient stability was evident due to the overly small vertical stabilizers. The vertical stabilizers were quickly replaced with larger, lower-mounted tailplanes, which also proved inadequate. The designers lowered them to reduce dead zones when firing the cannon, but this led to aerodynamic problems—the tailplanes were located three-quarters below the horizontal stabilizer and were exposed to propeller turbulence during takeoff and landing, especially with the landing gear extended and the doors open, i.e., during takeoff and landing.
But an even more serious problem was the constant failure of the HS 14Aa engines. Cuny and Danel's book states that thirteen engines were "expended" in 66 flight hours. It's unclear how testing could have been carried out under such conditions, but on August 31, 1937, the LeO 45-01 prototype officially entered testing at the CEMA (Centre d'Essais de Matériels Aériens – Aeronautical Testing Center). The aircraft was tested with standard NACA cowlings and three-bladed Hispano-Suiza propellers—licensed Hamilton Standards—which could be adjusted in one of two positions in the air. A new aerodynamic Mercier cowling with pneumatically controlled retractable rings had already been designed but had not yet been tested.
In this configuration, it reached a speed of 390 km/h (240 mph) at sea level and 465 km/h (280 mph) at 4800 m (15,500 ft). On September 15, in a shallow dive, the prototype reached 624 km/h (390 mph). Returning to SNCASE, it underwent various modifications, the most notable of which were Mercier cowlings and the addition of bulky auxiliary oil coolers protruding from the wing's leading edge. Among the most serious problems with the Hispano engines were oil temperature issues, which were mistakenly thought to be solved by enlarging the radiating surfaces. In reality, the problem was an inefficient oil pump, which eliminated the wing growths.
However, engine problems persisted, particularly overheating during takeoff. Mercier cowlings, when used correctly, increased speed by approximately 15–20 km/h, but were unable to significantly improve heat transfer. It even reached the point where a proposal was considered to introduce separate fuel tanks for 100-octane gasoline—for takeoff and landing only (at that time, French aviation flew on 85-octane gasoline, and a little later, 87-octane). Therefore, reluctantly, in September 1938, CEMA agreed to use Gnome et Rhône 14N-20/21 engines with a nominal output of 1030 hp at 4 km.
Mercier hoods on LeO 451
Naturally, the engine change required some modifications, so the prototype, now designated LeO 451-01, resumed flying on October 20, 1938—a year and nine months after its maiden flight. Subsequent testing proceeded successfully and at an accelerated pace, which later turned out to be too accelerated. Of course, there were reasons for this—the situation in Europe was rapidly deteriorating, no one doubted the inevitability of war, and the French Air Force lacked a modern bomber worthy of the name—certainly not the MB 131. Moreover, competitors had arrived, in the form of Amiot: on April 16, 1938, the Amiot 340 was submitted to CEMA for official testing (even though its maiden flight had been on December 6, 1937). While it didn't yet meet the A-21 and A-21/1 requirements, being a three-seater with a single tail, Félix Amiot hoped to secure an order, at least as an interim option. Moreover, no one expected that converting it to the Amiot 351 to meet Air Force requirements would be so complex and time-consuming.
Meanwhile, the LeO 45 program was falling catastrophically behind schedule. Back in May 1937, an order was placed for two pre-production aircraft under contract No. 1439/7: one LeO 450 with 14Aa-06/07 engines and one LeO 452 with 14Aa-12/13. This order was followed by two more, each for 20 aircraft, on November 29, 1937 (No. 51/8) and March 26, 1938 (No. 361/8), again under "Plan II" of 1936. Moreover, it was assumed that the first aircraft from the November order was to be delivered in May 1938. Of course, the decision to use Gnome et Rhône engines effectively cancelled these orders, and a new one, for 100 aircraft, was placed on October 20, 1938 (No. 294/9). Finally, on April 18, 1939, another order was placed (No. 911/9), for 507 aircraft at once, although only 480 of these were produced. There's no point in mentioning the ever-increasing subsequent orders—for obvious reasons, they were never fulfilled.
During official tests at CEMA in January 1939, the LeO 451-01 prototype, powered by GR 14N-20/21 engines and Gnôme-Rhône 2 400/2 401 propellers, achieved a maximum speed of 502 km/h (311 mph) in level flight at an altitude of 5.1 km (17,600 ft). Thus, the LeO 451 became the first French bomber to exceed 500 km/h (311 mph). However, there were some nuances, which will be discussed below. The official performance at CEMA, with a takeoff weight of 10560 kg (i.e., fully fueled but without bombs), was as follows:
Maximum speed is 388 km/h above sea level, 432 km/h at an altitude of 2 km, 476 km/h at an altitude of 4 km, 500 km/h at an altitude of 5 km and 454 km/h at an altitude of 8 km.
Ascent time 4 km - 10 min. 56 sec., 5 km - 13 min. 31 sec., 8 km - 28 min.
Ceiling - 9100 m.
The flight range at a cruising speed of 360 km/h is 2900 km.
Preparations for the bomber's production began long before testing was completed. As early as April 1938, Air Minister Guy La Chambre effectively banned any design changes (obviously, this ban didn't apply to the engines), naturally with good intentions—to speed up serial production. At the same time, he dismissed some concerns raised by the General Staff regarding various features of the LeO 45, which he characterized as "serious defects," to put it mildly. These included a speed drop of 80-100 km/h with the upper cannon mount extended and the lower machine gun mount lowered, a lack of forward protection—the pilot's single fixed MAC was essentially a popgun—the lack of a second emergency control for the navigator, and, of course, engine overheating. Later, the ineffectiveness of the tail unit was added to the list. In other words, a machine that was clearly unfinished was being launched into production.
From the outset, work was divided between several sites: SNCASE handled assembly at the Argenteuil plant, while partial disassembly for transport and completion took place in Villacoublay, where the first flights took place. The complexity of the LeO design forced the company to resort to subcontracting; for example, Breguet was responsible for manufacturing the wing panels. Increasing orders and production rates led to the installation of two more assembly lines at SNCASE's plants in Ambérieu-en-Bugey and Marseille-Marignane, and then at the SNCAO (Société nationale des constructions aéronautiques de l'Ouest—National Aircraft Society of the West) plant in Bouguenais, near Nantes, where the Morane 406 series was completed in the fall of 1939. In other words, production expansion was planned using purely extensive methods. True, in 1940, engineers from the SNCAO association from the design bureau in Saint-Nazaire were given the task of modifying the aircraft to simplify and reduce the cost of production, but it was hopelessly late.
Production was launched with great difficulty—production aircraft numbered 1 and 2 flew on March 24 and April 28, 1939, respectively, and it took four months to complete the first five aircraft. Only 22 aircraft were built before the outbreak of World War II, of which only 10 (or 11) were accepted by the Air Force. There were numerous reasons, both technical—the complex design of the aircraft—and organizational—a shortage of machine tools, personnel, and components. The latter were particularly problematic—private companies producing equipment (propellers, bomb racks, gunsights, oxygen equipment, etc.) were unable to fulfill the ever-increasing orders. The LeO 451 program, however, was far from worse off—since another program, the Amiot 351/354, was even further behind schedule, some components, particularly propellers and gunsights, were taken from its existing inventory.
The quality situation was no better at first: on October 25, 1939, the Inspector General of Bomber Aviation noted:
The aircraft are delivered incomplete in terms of equipment and weapons. They are also prone to numerous mechanical failures, apparently due to a lack of factory adjustments or manufacturing defects.
Now comes the real indictment—first, regarding the airframe: poor-quality paint, poor riveting, poorly designed fairings, poorly covered and crudely assembled... The surface condition of all the aircraft is such that there is concern about a significant reduction in performance. The paint is peeling badly, as filler was not used... Many of the fairing mounting screws have been hammered shut. The situation with the engines is no better. Numerous leaks have been discovered in all seals, forcing mechanics to perform an unacceptable amount of maintenance and inspection work... The reason for this situation was clear to everyone—an overly hasty adoption. I think that anyone who has studied the history of domestic aviation should not be surprised by such a description.
As for the complexity and cost, this was known even before serial production began. As one French aviation blogger put it, the LeO 451 was a masterpiece, but in the medieval sense—like an apprentice, on his way to becoming a full-fledged master, creating a masterpiece to demonstrate his full abilities, sparing neither effort, nor time, nor materials. Pierre Mercier created a design with exceptional performance, while also being extremely robust and maneuverable for its size and weight—the aircraft could perform aerobatics without bombs (though this wasn't the purpose of the bomber). But the price was quite high, including in the literal sense.
In his book "Twenty-five Years of Military Aviation," General Hébrard called the LeO 451 a demonstration tool for a course in aircraft design (and, one might add, in aviation materials science), as it consisted of 40000 different parts machined with 26000 tools. All metals and alloys, colonial woods, fittings of all types, and all manufacturing processes (milling, stamping, planing, electric welding, oxyacetylene welding, etc.) were used in its production.
The difference with competitors from Amiot was particularly noticeable. As early as March 1938, the vice-chairman of the Senate Aviation Committee, Amaury de La Grange, noted the difficulty of manufacturing the Lioré 45 and continued:
On the other hand, the Amiot 340 boasts remarkable design simplicity. It can be mass-produced by unskilled workers thanks to the appropriate tooling, and it would likely require only 24,000 hours of operation (compared to 45,000 for the Léo).
For the latter, this estimate turned out to be underestimated—initially, the labor intensity approached 60,000 man-hours, and even when mass production of the LeO 451 began at several sites, its labor intensity dropped to approximately 50,000 man-hours or slightly less, 30–40% more than that of the serial Amiot 351.
The complexity of production also meant difficulty in field repairs—the Inspector General of the Air Force warned of this as early as 1936: "It is to be feared that repairs may be difficult to carry out by military units alone." And he was right. Besides the common headache of all French bombers—unreliable engines—the overly complex electrical, hydraulic, and pneumatic systems caused numerous problems. Regarding the engines, an additional and very serious problem was the Mercier cowlings. A brilliant solution from an aerodynamic standpoint, they were a disaster for maintenance.
It's unknown how reliably these devices themselves worked, or perhaps how reliably the entire pneumatic drive system worked, but it's well known that the cowlings hindered access to the engines and, consequently, prolonged repair times. In one case, six experienced mechanics were able to replace a single engine in 10 days (not hours). And the icing on the cake: many pilots didn't know how to properly adjust the cowlings and set them to the maximum open position. As a result, the percentage of combat-ready aircraft in these groups was much lower than in groups armed with American Douglas DB 7s and Martin 167Fs.
But this, of course, became clear later, while production, albeit with some difficulty, was still underway. The description below applies specifically to the aircraft in service on May 10, 1940. By the end of 1939, 132 aircraft had been produced, but only 66 of them reached CRAS (Centre de réception des avions de série, or something like the initial acceptance service)—component production was still lagging behind, a situation that persisted until the capitulation. Considering that CRAS was only the second (of four) acceptance stages, after the factory test flight and before the aircraft's stay at EAA 301 (Agency for the Inspection and Maintenance of Aircraft), where the armament was installed, and the DMAM (Department of Aviation and Maintenance), which determined whether the aircraft would remain in storage or be sent to one of the units, the picture becomes quite bleak. By the end of winter, LeO 451 production had stabilized at an average of 50 aircraft per month. By May 5, 1940, five days before the Phoney War became a reality, 360 aircraft had been produced, 230 of which were delivered to the CRAS. In total, approximately 500 LeO 451s were built before the capitulation, of which the Air Force received 365. At this stage, LeO 45 production exceeded 100 aircraft per month.
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