Brief description of the serial aircraft LeO 451 and its flight characteristics
The Lioré et Olivier LeO 451 was a twin-engine, medium-sized horizontal (i.e., non-dive) bomber of all-metal (almost) construction, with a crew of four and all the attributes of a modern aircraft, at that time, of course: closed cockpits, working skin, advanced wing mechanization, landing gear that retracted into the engine nacelles, modern instrumentation, etc.
The low-mounted cantilever wing had an area of 68,0 m² and a NACA 4415 profile (i.e., a relative thickness of 15%) at the root and NACA 4411 (respectively, 11%) at the tips. It consisted of four parts: two central ones, connected along the centerline of the fuselage, and two consoles. The central part was supported by two spars with steel shelves and duralumin webs; they were supplemented by five light alloy ribs, between which additional bomb bays, internal fuel tanks of 880 liters each, engine nacelles and two outer ones - the front ones of 300 (or 330) liters and the rear ones of 410 liters - were successively located; the wing boxes of the consoles were inserted into the outer section. The center sections were connected to each other in the axial plane and to the fuselage by a complex system of diagonal braces and various fittings, which Kuhni and Danel even called "clever" or "ingenious"—they, of course, knew better. The wing panels were box-type, with bevels on the inner side that were inserted into the center section, and wingtips on the outer side.
LeO 451 wing. The pitch and thickness of the corrugations varied depending on the distance from the aircraft's axis.
The skin had two layers—an inner corrugated layer and a smooth outer layer. The leading and trailing edges of the wing were bolted to the wing spars and the outer wing boxes. The trailing edges of the center section had electrically controlled slotted flaps, while the trailing edges of the outer wing boxes had fabric-covered, full-length slotted ailerons. The trailing edge of the left aileron had a ground-adjustable trim tab. The ailerons were lowered during takeoff, acting as flaperons.
The two horizontal surfaces of the tail unit were constructed from two separate halves and had a dihedral angle of 13°; they were bolted to a short central section secured to the top of the fuselage; the fixed surfaces had a metal frame and a corrugated inner covering and a fabric outer covering, the elevators had only a metal frame and a fabric covering; the elevators were provided with fletters.
The vertical stabilizer, in the form of elliptical endplates, and the rudders, equipped with trim tabs, were attached to the outer edges of the tailplane. The endplates consisted of welded steel tubes with a metal covering. In 1940, after the surrender, new, larger tailplates of a more traditional design were adopted—with three spars, ribs, and metal covering.
The flight control system was mixed. The rudder and elevator were controlled by hardwired control lines. The ailerons were controlled by cables and pushrods. The elevator and rudder trim tabs were adjusted in flight from the cockpit using chains and cables. The flaps, in addition to being electrically powered, had a direct emergency control, operated by a handle near the upper gunner.
The fuselage was an all-metal monocoque of oval cross-section, comprising 60 main and secondary frames, connected by longitudinal stringers; all of this was covered with light alloy sheets, riveted securely. Two main frames comprised box beams to which the wing spars were attached. The regular frames, depending on their relative size, had either a box section or a simple Z-shaped sheet metal structure. The glass nose cone structure was made of welded steel tubing.
Fuselage on the assembly cradle, source: J.M. Meunier
The nose section housed the bombardier/navigator's station (who, traditionally for French bombers, also served as the crew commander), located behind the heavily glazed nose. It featured flat sliding side windows, a dedicated observation hatch, and a folding table. And, of course, a bombsight (or sights—it's unclear). The single-seat pilot's cockpit was offset to the left. A passage to the right provided access to the forward and rear cockpits. The instrument panel had to be trimmed accordingly, with additional panels added on the sides (fuel and oil level gauges for each engine) and even on the floor (two pressure gauges). The seat to the right provided temporary accommodation for a mechanic, instructor, or navigator (during takeoff and landing—a necessary precaution due to the dangerous nature of these phases of flight).
Pilot and navigator cockpits
The radio operator/gunner, also known as the flight engineer, was positioned behind the pilot, facing rearward, level with the wing's leading edge. Slightly lower, also offset to the left, was a retractable machine gun nacelle. When the nacelle was lowered, the seat retracted. The main bomb bay occupied the left half of the fuselage between the two wing spars. Above the wing's trailing edge was the gunner/observer's station. A glass, rising fairing/visor provided the necessary space for aiming and firing. All stations were connected by a corridor running along the entire right side of the central and forward fuselage. Entry and exit were typically through a door on the starboard side (for the pilot and navigator) behind the cockpit and a ventral hatch behind the gunner's station (for the gunner and radio operator). Both the door and the hatch, as well as part of the cockpit glass, could be jettisoned for an emergency escape.
The main landing gear (made by Messier) had oleo-pneumatic shock absorbers. They retracted hydraulically and were fully closed by doors (a rarity for the period—usually, to reduce damage to the engine nacelles during belly landings, a small portion of the wheels were left exposed). The landing gear had an unusually complex mechanism for the main wheels to reduce the size of the engine nacelles. The tail wheel was also retractable (an even greater rarity).
aircraft Chassis
The LeO 451 used Gnome et Rhône engines: 14N-38/39 or 14N-48/49. The latter digits denoted the same as those of its competitors from Hispano-Suiza: even digits for left-hand rotation engines and odd digits for right-hand rotation. The engines were mounted on welded steel tubular frames attached to the front spars of the center section, with a ring at the front. The engines were secured using 14 Dynaflex anti-vibration mounts. The engines developed a nominal ground power of 920 hp at 2400 rpm, a takeoff power of 1100 hp (14N-38/39) or 1180 hp (14N-48/49) at 2650 rpm, and a power of 1030 hp (750 hp) at 2,500 rpm. or 1060 hp at optimal altitude, respectively.
The engines were equipped with electric inertia starters from Air Equipment (with a manual back-up). Mercier cowlings were used not only to cool the engine cylinders but also to heat the cabin: air passed through two ducts and a heat exchanger, where it was heated by the hot exhaust gases, after which it was supplied to the crew cabins and instrument heating. The exhaust pipes were individual for each cylinder; it is unclear whether this was the first use of jet pipes in France, but Cuny and Danelia report a speed increase of 10 km/h.
Typically, three-bladed metal Ratier 1634/1635 propellers with a diameter of 3,20 m were used, equipped with variable pitch (automatic or manual) and a feathering system. It is highly likely that Gnome-Rhône 2400/2401 propellers were also sometimes used. Many articles attribute the reduced maximum speed of production aircraft compared to the prototype to the use of Ratier propellers instead of the scarce Gnome-Rhône propellers. This is, at best, inaccurate—there were many reasons, and they will be discussed later.
The total fuel tank capacity was 3225 liters (including a 45-liter service tank). Each engine was fed by its own circuit—the main tank and two additional tanks. In exceptional cases, the circuits could be connected. In the event of failure of both pumps on one engine (that's what the text says, but likely the intended meaning was one circuit), there was an emergency hand pump. On production aircraft, the oil coolers were relocated to the engine nacelles, while their air intakes remained in the leading edges of the center wing. The oil tanks had a capacity of 77 liters each, although 70 was usable. During operation, it became clear that the oil supply was insufficient.
The instrumentation was standard for the time. The navigator's instrument panel, in front of the navigator, included standard instruments such as a clock, speedometer, altimeter, and compass, as well as an automatic radio compass indicator for navigation using ground-based radio beacons. The pilot also had a standard set of instruments—for navigation, control of the propeller-engine group, landing gear, and so on. Special equipment also included a Jaeger-Smith pneumatic autopilot, a Sperry artificial horizon, an LMT radio compass with a retractable frame, an SIR onboard telephone, two cameras—a portable Altiphote and a fixed Planiphote at the gunner's station (in practice, cameras were rarely used), and a gunner-controlled audible warning system. The oxygen system consisted of 8 1000-liter cylinders (as stated in the source, although this is clearly a mistake - for example, the Ju 88A had 16 2-liter cylinders as standard and the same number of additional ones), pipes with fittings and Munerelle inhalers at each workstation.
The SARAM 3-10 radio station operated in telephone and telegraph modes. Its frequency range was 19 to 2170 MHz (i.e., high, medium, and low frequencies) across six bands. The station weighed 50 kg, not including the weight of the shielded cables. The antenna consisted of a cable approximately 100 m long, which was unwound under the aircraft during flight. In service, the station proved to be quite reliable, unlike the internal telephone.
I will simply quote the description of the pneumatic and electrical systems from the book by Cuny-Danel: there was a triple pneumatic circuit with two “Air Equipment” compressors, supplying two cylinders with compressed air necessary for the main equipment (control weapons, various hatches, movable Messier cowling rings, etc.); but two special compressors powered the autopilot, and two vacuum pumps powered some of the instruments (course indicator, horizon, turn indicator). The electrical system was powered by two 100-watt, 24-volt dynamos driven by the engines and by a 24-volt, 90-amp, 10-minute battery (a weaker battery was installed on the first 25 aircraft, but then there was a Lutetia generator, which drove the emergency generator). On-board lighting was conventional, as were flares. There were two landing lights, located on the leading edges of the wings.
There's virtually no information about fuel tank protection, other than the most basic—that it was there, albeit very primitively. Armor protection was most likely limited to a 7mm-thick armor plate for the pilot. The Air International layout diagram, number 97, lists an armor plate for the upper gunner, but the aircraft that entered the war on May 10th almost certainly did not have one. But, again, the design itself was very robust and allowed many crews to return with damaged aircraft.
The first 80 aircraft (i.e., the majority of the aircraft that actually saw combat) were equipped with Thévenot-Alkan TA 35 sights, about which almost nothing is known. In one article, this sight was called a pendulum (viseur pendulaire) - I don't even have a rough idea what that is supposed to mean. If anyone knows, please tell me. The TA 35 could be connected to the TA 105 stabilizer and the TA 103 "guiding device" (this device calculated the necessary corrections, indicated them to the pilot or transmitted them to the autopilot). According to Jean Cuny (French Bomber and Reconnaissance aviation, 1918-1940; Docavia No. 12), the sight and guidance system were rarely, if ever, used together.
For this or some other reason, the Bronzavia B15 or D30 sights (sometimes called the Bronzavia-Dervaud BD 30) were later used. The former is mentioned only in the order listed and was apparently a temporary option. The Bronzavia D30 became the most widely used. In short, it was rated something like this: "better than the Bristol Blenheim sight, but worse than the Norden." The D30 sight was a periscope and could rotate around a vertical axis. It was linked to a target course indicator (a "Type 11 repeater" or "course indicator") mounted on the pilot's instrument panel. All bombing parameters were entered using knobs on the top of the sight.
Field of view: 30 °
Magnification: 1,5
Reticle color: black during the day, red at night.
As it turned out, the D30 sight was practically ineffective at altitudes below 2 km. While very useful for calculating navigational elements, it was replaced during bombing runs with the excellent and simple "backup" Bronzavia D 40 sight (or simply Pritsei 40). It is unknown whether the backup sight was used in conjunction with the TA 35. But, as far as can be judged, the D 40 sight required approaching the attack course from a greater distance, which increased the bomber's vulnerability to anti-aircraft fire.
Placement of sights in the navigator's cabin
The offensive armament, or bomb load, was housed in three bomb bays – the main one in the fuselage and two additional ones in the center section. AL 125 or AL 128 racks were mounted in the wings for carrying a single 100-, 200-, or 500-kilogram bomb. The fuselage contained two D- or V-type racks (each capable of carrying 32x10 kg or 8x50 kg bombs) or one S-type rack (each capable of carrying 5x200 + 1x100 kg or 6x100 kg bombs). Theoretically, up to 2100 kg of bombs could be carried, and in practice, even more, as the actual weight of all bombs except the 10-kilogram ones was greater than the nominal. Specifically - 9,6 kg, 56 kg, 116 kg, 225 kg and from 518 to 540 kg.
Thus, the payload was more than impressive for a 1939-1940 bomber, especially since it was all stored in the bomb bays, without compromising the aircraft's aerodynamics. Another issue is that these bombers primarily used 50 and 100 kg bombs, meaning the payload was only 800 kg. The use of 200 kg bombs was rare, and 500 kg bombs were generally a one-off. I was able to find only one instance of the LeO 451 using a 500 kg bomb—during the attack on the British base in Gibraltar.
During combat, it became clear that 50- and 10-kg bombs were the most effective. Here, too, the LeO 451 lagged significantly behind its German counterparts: the He 111P could carry up to 32 50-kg bombs or up to 128 10-kg bombs on its internal pylon, while the Ju 88A-1 could carry up to 28 and 80, respectively. Moreover, the Luftwaffe used the 100-kg caliber, as an intermediate between light anti-personnel and anti-vehicle bombs and heavy anti-hardened targets, very sparingly and only on external pylons.
Defensive armament included:
A wing-mounted 7,5mm MAC 34 machine gun was mounted on the floor of the forward fuselage, to the pilot's right. It protruded through a removable cowling. The pilot could aim through an external grille and an adjustable sight located at his position. The weapon was fed from a 300-round magazine.
The second 7,5mm MAC 34 machine gun covered the rear lower sector and was housed in a retractable gondola (or turret, carriage, or turret—it's called by various names). It was mounted on a SAMM AB 161 manual mount and was equipped with an Alkan mle1935 collimator sight, i.e., model 1935 (according to other sources, the sight was an OPL 35). Ammunition consisted of five 100-round drums.
The top gunner/observer was armed with a 20mm Hispano-Suiza HS 404 cannon mounted on a De Boysson Type 170 bis mount with a SAMM hydraulic drive. Aiming was accomplished using one of two collimator sights (one on each side). It's now difficult to determine which sights were used—Alkan, OPL, or Baille-Lemaire GH-38—especially since their technological level was roughly the same. They likely used whatever was at hand. Ammunition was 120 rounds—theoretically, drums of 60, 30, or 15 rounds could have been used. The firing arc was ±30° horizontally from the aircraft's axis—exactly what was required to fire between the tail washers—and probably +30° vertically. This meant that the gunners in the group were unable to provide mutual support.
For its time, the HS 404 cannon was a very powerful weapon: its rate of fire was 600–700 rounds per minute, muzzle velocity was 880 m/s, and the high-explosive shell weighed 130 g. The main problem was the drum feed: a loaded 60-round drum weighed 25,7 kg and was sufficient for 6–8 seconds of continuous firing. However, to prevent jamming, it was usually loaded with 50–55 rounds. The drum was located about a meter from the shooter—at arm's length—so only an exceptionally strong person could replace an empty drum with a second 60-round one (without dropping it on one of the sights). More common configurations were 1x60 + 2x30 or 4x30 rounds. While the 30-round drum was also quite heavy, around 14–15 kg, it was a significant difference compared to the first. Since the beginning of 1940, plans had been underway to modernize the installation, again extensively—they were going to add a pair of MAC machine guns with 300 rounds of ammunition to the cannon, but, as always, nothing had been done before May.
A later photograph of the gunner with the cannon and machine guns added
Main flight characteristics of the LeO 451
The LeO 451 bomber had the following geometric dimensions: wingspan: 22,50 m, length: 17,17 m, height: 5,24 m, wing area: 68 m². There are some inconsistencies with the weights: usually, either 7530 kg or 7830 kg are given for the empty weight of the aircraft. It is often specified that the latter figure is the empty weight of the aircraft, loaded with fuel. But then it is unclear what is meant by this definition – usually it refers to the empty weight of the aircraft (including the remaining fuel) + the weight of the crew, oil, oxygen, ammunition, etc. But four pilots with parachutes is already 360 kg. For takeoff weight, all sources give a figure of 11400 kg – it is unclear whether this is the normal or maximum; however, there are no other figures. With a full payload of 2 tons, no more than 1100 liters of fuel were allowed to be poured into the tanks, although it is unlikely that this ever happened.
In any case, the empty weight of the aircraft was too high compared to its counterparts. For example, the LeO's competitor, the Amiot 351/354, had an empty weight of about 6500 kg (its three-seat predecessor, the Amiot 340-01, weighed only 4785 kg), while its German contemporaries, the Dornier Do 17P-1, weighed 4600 kg, the Do 215B-1 5774 kg, the He 111P-2 6200 kg, and so on. Only the Ju 88A-1 was heavier, but it was also designed as a dive bomber. There were several reasons for this: the excessive complexity of the design, additional features that other bombers lacked (Mercier cowlings, additional bomb bays, retractable canopies and nacelles), and, of course, the emphasis on aircraft durability. Of course, durability is good, but everything has its downside. And it's not that a ton or a ton and a half could have been spent on armament or range. The excessive weight, combined with other factors, made takeoff and landing quite dangerous.
The maximum speed of production aircraft varied, varying widely, so to speak. It shouldn't be surprising that production aircraft were at least 7-10 km/h slower than the prototype, but production aircraft also varied by approximately 35 km/h, i.e., 460-495 km/h at 4,8 km. Sometimes an average figure of 480 km/h is cited. When a production aircraft was accepted by CRAS, it was considered suitable if it reached a maximum speed of 465 km/h at altitude and climbed 5 km in 15 minutes. Later, after the surrender, speed measurements of production aircraft in 1941 yielded the following results: 366 km/h at sea level, 403 km/h at 2 km, 440 km/h at 4 km, 459 km/h at 5 km, and 423 km/h at 8 km. There were many reasons—different engine modifications, different propellers, and, most importantly, different manufacturing quality: pilots noticed that the machines assembled in Amberie flew slightly faster than the others.
LeO 451 Top Speed Charts
The maximum cruising speed was 420 km/h at an altitude of 4,8 km, and the actual cruising speed was 360 km/h at an altitude of 2,5 km. At this speed, the maximum range was 2900 km, and with a 500 kg payload, 2300 km. The rate of climb was slightly worse than the prototype—it took 14,0 minutes to reach 5 km, while the ceiling remained virtually the same, at 9000 meters.
Information