Short-Mayo Composite Aircraft – Nonstop Transatlantic Flight 1938 | Season 4 – Episode 74
Wings Oct 01, 2024
In the early days of commercial aviation, limited aircraft range was the main obstacle to expanding new passenger routes. Mid-1930s aircraft had to compromise passenger and mail space to carry extra fuel for transatlantic flights and other distant destinations.
Short Brothers and Imperial Airways worked together to devise a unique two-plane piggy-back solution, and on July 21, 1938, the inaugural transatlantic flight of these unique aircraft took off from Foynes, County Limerick to Boucherville, Canada, with the design later copied in the 1980s by NASA for transporting the Space Shuttle.
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AVIATION EXPERIMENTS
Imperial Airways explored two solutions to the challenge of launching heavily loaded aircraft with a short take-off distance: in-flight refuelling and assisted take-off.
In-flight refuelling was proven technically possible on June 27, 1923, when a DH-4B flown by Lieutenants Virgil Hine and Frank W. Seifert of the U.S. Army Air Service transferred petrol via hose to another DH-4B flying below, piloted by Lieutenants Lowell H. Smith and John P. Richter. All four officers received the Distinguished Flying Cross for this achievement.
While in-air refuelling was achievable as early as 1923, it could only be completed at the time over land, where refuelling planes equipped with large capacity tanks could take off and rendezvous with long-distance aircraft. This was demonstrated on October 25th, 1923, when U.S. Army Air Service Lieutenants Smith and Richter flew nonstop from the Canadian to the Mexican border, covering 2,011 kms with three mid-air refuelling operations. Although in-air refuelling was demonstrated to extended flight range, it was practical only over land, not oceans.
ASSISTED TAKE-OFF
Imperial Airways collaborated with Short Brothers, who opened a factory in Belfast, County Antrim, in 1937. This commercial partnership led to the creation of the “Short Mayo Composite,” a piggy-back long-range seaplane and flying boat combination. It featured a large four-engine flying boat named ‘Maia’ with a smaller seaplane, ‘Mercury,’ mounted on top.
The ‘Mercury’ aircraft was designed for long-distance mail transport but required assistance for take-off when fully loaded with fuel and mail. The ‘Maia’ (S.21 G-ADHK) was specifically designed to lift the ‘Mercury’ (S.20 G-ADHJ) mounted on its back, using all engines from both aircraft during take-off. Once they reached a suitable altitude, the two would separate, allowing ‘Maia’ to return to base while ‘Mercury’ continued its journey.
MAJOR ROBERT MAYO
Short Brothers were already constructing the Empire, a medium-range, four-engine monoplane flying boat, heavily influenced by its primary customer, Imperial Airways. Ultimately, they built 42 of these flying boats, which were designed to operate long-range routes within the British Empire. However, to attempt the transatlantic route, they had to sacrifice passenger and mail capacity for additional fuel.
Into our story comes Major Robert Hobart Mayo, the Technical General Manager at Imperial Airways, who was inspired by the established understanding that assisted flight was feasible if aircraft could sustain flight with a greater payload than what was possible during take-off.
In what must have seemed like an audacious proposal in 1937, he suggested mounting a smaller heavily laden long-range seaplane on top of a larger carrier aircraft. By harnessing the combined power of both, the larger aircraft would lift the smaller aircraft to operational altitude. At that point, the two would separate, with the carrier aircraft returning to base while the seaplane continued to its destination.
SHORT-MAYO COMPOSITE
This aircraft configuration was named the “Short-Mayo Composite” and featured the Short Brothers S.21 Maia (G-ADHK), a variant of the four-engine Short “C-Class” Empire flying boat. The Maia aircraft was equipped with a trestle or pylon on top of the fuselage to support the Short Brothers S.20 (G-ADHJ).
MAIA FIRST FLIGHT
Maia first flew (without Mercury attached ) on July 27th 1937, but not before the Short Brothers engineers overcame the take-off weight restrictions. Although the Maia aircraft was generally like the four-engine Empire boat, it featured significant design differences. The engines were mounted further from the wing root to accommodate Mercury’s floats that extended down from above, and the rear fuselage was swept up to elevate the tailplane relative to the wing. Additionally, the hull sides were flared with a “tumblehome” design, making them narrower above the waterline than at the beam to increase the planning surface essential for higher take-off weights. The aircraft also included larger control surfaces, which enhanced the total wing area.
MERCURY FIRST FLIGHT
The upper aircraft, Mercury, first flew on September 5th 1937 was a twin-float, four-engine seaplane powered by four Napier Rapier VI 16-cylinder “H Block” engines. It was crewed by a pilot and a navigator sitting tandem in an enclosed cockpit. With a top speed of 341 km/h, the aircraft had a payload capacity of 450 kg of mail and carried 5,500 litres of fuel.
To ensure Maia (lower aircraft) remained the primary flight control aircraft, the Mercury’s flight controls—except for the elevator and rudder trim tabs—were locked in a neutral position until the two planes separated at a chosen altitude.
To prevent over-stressing the coupling components, the mechanism allowed for slight in-flight movement. Indicator lights showed when the upper aircraft was balanced, enabling trim adjustments before release. A third lock held the two aircraft together, releasing automatically at 13,000 Newtons (3,000 pound-force). During separation, Maia would drop away while Mercury climbed away.
The first successful in-flight separation between Maia and Mercury was carried out from the Shorts works at Borstal, England on February 6th 1938.
FIRST TRANSATLANTIC FLIGHT
After further successful in-flight separation tests, the first transatlantic flight was set for July 21, 1938, taking off from Foynes, County Limerick, to Quebec, Canada.
The day before, on 20th July, both planes were ferried in from Southhampton, England and the Irish Independent reported: “The Mercury, piloted by Capt. Bennett, arrived here [Foynes, Limerick] from Southampton at 12.55 and made a perfect landing. She was just being tied up to her moorings when the carrier ‘plane, Maia, piloted by Capt. A. S. Wilcockson, appeared and settled gracefully in the water beside the Mercury. Maia flew the Irish tricolour on her bows…”
After take-off from Foynes on 21st July, Mercury separated from Maia at 8:00 pm over the Shannon estuary to embark on the first commercial non-stop east-to-west transatlantic flight by a heavier-than-air aircraft. With ten passengers on board and after a flight lasting 20 hours and 21 minutes, with an average ground speed of 232 km/h, having completed a 4,720-kilometer flight, Mercury landed in Boucherville, near Montreal, Canada.
OPERATIONAL USE
The Maia-Mercury composite remained in service with Imperial Airways and, after modifications to extend Mercury’s range, it set a seaplane record of 9,728 km from Dundee, Scotland, to Alexander Bay, South Africa, from October 6th to 8th 1938.
Maia was destroyed by German bombers in Poole Harbour on May 11th 1941. Mercury was returned to Shorts at Rochester on August 9th 1941, and its components dismantled for the war effort.
IN-AIR REFUELLING
Only one Short-Mayo composite was built, and with the outbreak of World War II, the composite approach became obsolete, halting further development. In 1939, Imperial Airways demonstrated the commercial viability of in-air refuelling. Two flying boats, Cabot and Caribou, made eight round trips from Foynes to Newfoundland, refuelled mid-air by a converted Harrow bomber from a grass runway at Rineanna (now Shannon Airport). The Harrow launched a rocket-propelled harpoon with a cable that connected to a hose, allowing fuel to flow by gravity into the flying boat’s tank.
NASA
The Short-Mayo composite concept did not fade into obscurity; in 1976, Jim McLaughlin, Chief Weights Engineer for the Boeing 747 and former Short Brothers employee, advised NASA on the 1938 composite system. They embraced the idea and used a modified Boeing 747 to transport the Space Shuttle.
If you have an idea for a story, please email Kevin Reid [email protected]
Sources of Information and Photo Credits:
Aviation Bloombergguy History
British Movietone – Associated Press (AP)
Bryan R. Swopes
Foynes Flying Boat Museum
Mary Evans Picture Library
National Museum of the United States Airforce
Shannon Aviation Museum
This Day in Aviation
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