By: Norm Goyer
The British Empire in the 1930s was huge, spreat out over many continents where a number of smaller colonies and countries owned and governed by the British Government located in England. The need for quicker travel than what was currently available by steamship was a major problem. The Short four engine Empire flying boats did have transcontinental range, such as from England across the Atlantic to South America. The problem was the passenger load and freight allowance would be mostly consumed by gasoline and oil for the four large radial engines. There had to be paying revenue in order for the flights to be possible if not profitable. Short engineers knew that it took more horsepower to lift an airplane into the air than to keep it in the air once it was at altitude. The plan was to modify the Empire flying boat to act as an aircraft carrier for a smaller Short Mercury four engine flying boat which would have two crew men, and could easily carry payload such as passengers, mail and packages to far off destinations.
The Short-Mayo composite project comprised of the Short S.21 Maia, which was a variant of the Short Empire flying-boat fitted with a trestle or pylon on the top of the fuselage to support the Short S.20 Mercury. Although generally similar to the Empire boat, Maia differed considerably in detail: the hull sides were flared and used an expanded lower fuselage called “tumblehome“. The Empire had vertical sides but the Maia needed the wider bottom to increase the planeing surface (necessary for the higher takeoff weights); larger control surfaces; an increase in total wing area from 1,500 sq ft to 1,750 sq ft. The engines were mounted further from the wing root to clear Mercury’s floats and the rear fuselage was swept up to raise the tail plane relative to the wing. Like the Empire boats, Maia could be equipped to carry 18 passengers. Maia first flew (without Mercury) on July 27, 1937, piloted by Shorts’ Chief Test Pilot, John Lankester Parker.
The upper component, Mercury, was a twin-float, four-engine seaplane crewed by a single pilot and a navigator, who sat in tandem in a fully enclosed cockpit. There was capacity for 1,000 lb of mail. Mercury’s flight controls, except for elevator and rudder trim tabs, were locked in neutral until separation. Mercury’s first flight, also piloted by Parker, was on 5 September 5,1937.
As planned all eight engines were used during combined flight but the controls of Mercury were locked. The airfoil designs of the two aircraft were such that Mercury’s wings were carrying the major part of the air load at the speed and height chosen for separation. Safety locks prevented separation until this speed and height were reached and both pilots had an unlocking handle, both of which had to be pulled to cause release.
The mechanism that held the two aircraft together allowed for a small degree of movement. Lights indicated when the upper component was in fore-aft balance so trim could be adjusted prior to release. The pilots could then release their respective locks. At this point the two aircraft remained held together by a third lock which released automatically at 3,000 lb. The design was such that at separation Maia would tend to drop while Mercury would climb.
The first successful in-flight separation was carried out from the Shorts works at Borstal, on February, 6, 1938, Maia piloted by Parker and Mercury by Harold Piper. Following further successful tests, the first transatlantic flight was made on July, 21, 1938 from the west coast of Ireland, to Montreal, Canada, a flight of 2,930 miles. As well as Mercury’s payload, the launch aircraft Maia was also carrying 10 passengers and luggage. Mercury separated from her carrier at 8 pm to continue what was to become the first commercial non-stop East-to-West transatlantic flight by a heavier-than-air machine. This initial journey took 20 hrs 21 min at an average ground speed of 144 mph. The Maia-Mercury composite continued in use with Imperial Airways, including Mercury flying to Alexandria, Egypt, in December 1938. After modifications to extend Mercury’s range, it subsequently established a record flight for a seaplane of 6,045 miles from Dundee in Scotland to Alexander Bay, in South Africa between 6 and 8 October 6 to 8, 1938.
Specifications (S.20 Mercury)
- Crew: 2 (pilot and navigator/radio operator)
- Payload: 1,000 lb (454 kg)
- Length: 51 ft (15.5)
- Wingspan: 73 ft (22.2 m)
- Height: 20 ft 3 in  (6.17 m)
- Wing area: 611 ft² (56.8 m²)
- Empty weight: 10,163 lb (4,614 kg)
- Loaded weight: 26,800 lb ()
- Max. takeoff weight: 15,500 lb (7,030 kg)
- Engines: 4 Napier Rapier VI 16-cylinder “H-block” piston engines, 365 hp each
- * Normal composite launch weight: 20,800 lb
- Record composite launch weight: 26,800 lb
- Maximum speed: 212 mph
- Cruise speed: 195 mph
- Range: 3,900 mi
- Wing loading: 33.6 lb/sq ft
- Extended range: 6,100 mi (Cape flight)
Specifications (S.21 Maia)
- Crew: 3
- Capacity: 18
- Length: 84 ft 11 in
- Wingspan: 114 ft
- Height: 32 ft 7½ in
- Wing area: 1,750 ft2
- Empty weight: 24,745 lb
- Loaded weight: 38,000 lb
- Engines: 4 Bristol Pegasus XC radial engine, 919 hp each
- *Takeoff weight (composite): 27,700 lb
- Maximum speed: 200 mph
- Range: 850 mi
- Service ceiling: 20,000 ft