Pierre-Georges Latécoère designed and built the Latecoere 521, a French six-engined double deck flying boat.
When completed, it was the largest aircraft in France. It also stood out as one of the first large passenger aircraft for trans-Atlantic routes.
The first flight was on 10 January 1935, and the Laté 521 set several world records for payload and endurance.
Air France introduced it to passenger service, enabling numerous long-distance routes. The Laté 521 offered luxury accommodations for up to 72 passengers.
After World War II began, the French Navy operated it for long distance maritime patrols. Sadly, none survived the war due to sabotage by retreating German forces.
It was the foundation for the single Laté 522 and the three navalized Laté 523 variants.
In the early 1930s, Groupe Latécoère in France began designing a large flying boat, aiming for long-range flights.
This boat would carry passengers and air mail. At the time, nations strived to create large aircraft to surpass predecessors and replace ocean liners for principal travel.
Read More: Blohm & Voss BV 238 – The Flying Boat Beast
Developing such aircraft was seen as a symbol of national prestige, gaining support for Latécoère’s project.
Latecoere 521 Design
The design, called the Laté 520, was a large sesquiplane with a central double-decked hull and stub wings. It resembled the smaller Latécoère 300. Sadly, one latécoère 300 crashed into the atlantic ocean: 5 killed, you can read the report here Bureau of Aircraft Accidents Archives
Weighing around 37,000 kg loaded, it was France’s largest aircraft then, with a max range of 4,500 km. It could reach speeds of 260 km/h with its four inboard water-cooled engines.
Named Lieutenant de vaisseau Paris, it could accommodate 80 passengers in its large double-decked hull. This hull was mainly made of duralumin and featured a longitudinal construction approach.
It was divided into seven water-tight compartments to prevent potential damage.
The stub wings, attached to the hull, added stability and housed fuel tanks. They also assisted during takeoff. To handle stress, the construction included numerous stiffeners, distributing it across the frames and hull spars.
The Laté 520 had a spacious double-decked hull, accommodating 80 passengers and constructed mainly from duralumin. This all-metal structure utilized longitudinal construction, effectively dissipating stress across its frames.
To prevent damage, the design included seven watertight compartments in the hull. Each stub-wing, hinged below, carried a sponson for extra stability on water and housed large fuel tanks, assisting during takeoff. Numerous stiffeners throughout ensured a stress-resistant structure, distributing stresses effectively.
Concerning the wings, they had a center and two fabric-covered outer sections, braced to the stub wings. This conventional two-spar design featured duralumin spars and ribs.
A combination of duralumin and high-tensile steel tubing internally braced the wing structure. Lattice-form spars supported the rounded wing tips.
The ailerons, split into three sections, had lattice-form spars centrally and round tubing spars externally. Each wing was supported by four inclined v-struts, faired with shaped duralumin and held internally by compact plates and U-sections.
Latecoere 521 in Civilian Service
In its civilian service mode, the Laté 521 could accommodate up to 72 passengers in opulent settings.
However, trans-Atlantic services would usually transport around 26 passengers, situated on the lower deck. The design was initially intended for trans-Mediterranean routes, allowing for larger passenger numbers.
The lower level featured a salon with 20 armchairs and tables, six luxurious double cabins with individual bathrooms, additional seating for 22 passengers, a kitchen, a bar, a lavatory, and a baggage hold.
Read More: Short Sunderland – The Flying Porcupine
Compartments for the wireless operator and navigator were also located on this deck.
The upper level, being more compact and narrow, provided seating for 18 passengers and contained a storage compartment, another kitchen and bar, and an additional lavatory.
The front section of the upper deck accommodated the flying officers and the commanding officer, while the flight engineers were stationed behind the pilots, with direct in-flight access to all six engines via compact walkways within the wings.
Initially, the Laté 521 was designed to be equipped with four 890 kW (1,200 hp) Hispano-Suiza 18Sbr W-18 engines, derived from an existing racing engine.
However, these engines were never obtained. Consequently, six Hispano-Suiza 12Ydrs, each delivering 660 kW (890 hp), were chosen for the initial aircraft. This modification necessitated extensive redesigning, subsequently delaying the project’s completion.
Read More: Short Shetland – A Pioneering Flying Boat
Managing these engines was challenging; thus, the throttles for the engines were developed with integral design features, including declutchable tips and stirrups, resolving control issues and improving maneuverability on water.
In civilian service configuration, the Laté 521 could host up to 72 passengers in luxurious conditions. However, for trans-Atlantic routes, it usually carried about 26 passengers, all accommodated on the lower deck.
Initially, the design was conceptualized for trans-Mediterranean services, with the capacity to transport more passengers.
The lower deck was equipped with a salon featuring 20 armchairs and tables, six deluxe double cabins each with private bathrooms, along with additional seating for 22 passengers, a kitchen, a bar, a lavatory, and luggage storage.
Read More: Blohm & Voss BV 238 – The Flying Boat Beast
Specialized compartments for the wireless operator and navigator were also situated on the lower deck. The more confined upper deck had seating for 18 passengers and included a storage compartment, an additional kitchen and bar, and another lavatory.
Hispano-Suiza 12Ydrs Engines
The front of the upper deck was designated for the flying officers and the commanding officer. The flight engineers were placed behind the pilots, with access to all six engines during flight via walkways within the wings.
Originally, the Laté 521 was to be powered by four 890 kW (1,200 hp) Hispano-Suiza 18Sbr W-18 engines, adapted from an existing racing engine model.
However, these never became available. Subsequently, six Hispano-Suiza 12Ydrs engines, each producing 660 kW (890 hp), were installed on the first aircraft.
This change in plans required substantial redesign and led to delays in the project. The management of these engines posed several challenges, leading to the development of an innovative throttle design with declutchable tips and stirrups, mitigating control problems and enhancing the aircraft’s water maneuverability.
With a gross weight of 40 tonnes, the Latécoère 521 achieved a speed of 256 kilometers per hour (159 miles per hour) at an altitude of 3,100 meters (10,171 feet).
It had a cruising speed of 210 kilometers per hour (130 miles per hour) and a service ceiling of 5,800 meters (19,029 feet).
The Latécoère 521 seaplane, on December 27, 1937, in Biscarosse, piloted by Henri Guillaumet along with Messieurs LeClaire, Le Duff, Le Morvan, and Chapaton, established a Fédération Aéronautique Internationale (FAI) World Record for Speed Over 1,000 kilometers (621.37 statute miles), carrying a 15,000-kilogram (33,069 pounds) payload at an average speed of 211.00 kilometers per hour (131.109 miles per hour).
Subsequently, on December 29, 1937, Guillaumet and his crew maneuvered the 521 over a 1,000-kilometer closed circuit between Luçon and Aurelihan, sustaining an average speed of 189.74 kilometers per hour (117.899 miles per hour) with the same payload weight.
The following day, December 30, 1937, two additional FAI world records were set by Guillaumet and his team when they lifted an 18,040-kilogram (39,771 pounds) payload to a height of 2,000 meters (6,562 feet); and ascended with a 15,000-kilogram (33,069 pounds) payload to an altitude of 3,508 meters (11,509 feet).
The 521, registered as F-NORD, undertook multiple transatlantic flights to New York City. During one such journey, it suffered damage due to a storm and had to be disassembled and shipped back to France for repairs.
Post-repair, the Latécoère 521 resumed its role in airline service. However, with the onset of World War II, modifications were made to convert it into a maritime patrol aircraft.
Following France’s surrender to Germany, the flying boat was kept in storage near Marseilles. Unfortunately, during Germany’s withdrawal in 1944, the historic airliner was destroyed.