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The phrase "Urban Air Mobility" (UAM) seems like it's been with us for quite a while, but really it's only been in widespread use for two or three years. NASA officially recognized UAM in 2017, calling for a market study of remotely piloted or unmanned air passenger and cargo transportation around an urban area. Most people would probably call this the "air taxi" idea—a vision of hundreds of small, unmanned electric multi-copters shuttling two or three passengers from nearby suburbs or city spaces to vertiports at about 100 mph (roughly 161 km/h).

But if things had worked out differently in the late 1950s and early 1960s, we might have a very different understanding of UAM—something more like mass-transit. We might have had a city-center to city-center 55-passenger vertical take-off and landing (VTOL) airliner shuttling between urban heliports at 180 mph (289 km/h).

Actually, we did have that, it's just few people remember. It was called the Fairey Rotodyne.

Born by grounding

Fairey Aviation was born in a western London suburb in 1915. With World War I in full swing, the story goes that Charles Richard Fairey, a gifted young engineer with Short Brothers Aircraft, was prevented from joining the Royal Naval Air Service (RNAS) because Britain's Admiralty Department felt he'd be more useful as an aircraft designer/builder than a (likely short-lived) pilot.

Irked at being barred from joining his peers, the 26-year-old Fairey agreed not to make a fuss if he were given a subcontract for aircraft production that allowed him to form his own company. Airplanes were in high demand as the latest thing in weaponry, so the Admiralty consented and he was given a contract to build 12 Short 827 seaplanes for the RNAS in mid-1915.

Fairey Aviation grew from there, producing its first in-house design, the ship-borne Campania seaplane, in 1917. The company continued to design and produce seaplanes, fighters, and bombers through WWII and the 1950s, including the Firefly, Swordfish, and Gannett.

With peace on the horizon in the closing months of WWII, manufacturers then building military aircraft realized there would shortly be much less demand for their output. Commercial aviation was an obvious new vein of business. Meanwhile, rotary-wing flight development had accelerated dramatically during the War. Fairey competitor Westland Aircraft had started locally license-building the S-51, a helicopter developed by the American company Sikorsky, in 1947.

Like other British airplane makers, Fairey wanted a piece of this new helicopter market.

Tip drive, autogyros, and jets

Most early "helicopters" were multi-rotor machines, not the single main rotor helicopters we think of now. One of the few early single rotor designs, the Brennan Helicopter, began development in England during WWI. It departed from the complexity of most helicopters, which spun their rotors by coupling them directly to an engine via chain or geared drives. Instead, its Irish inventor, Louis Philip Brennan, came up with the idea of rotating the blades using thrust from a small four-blade propeller mounted at the tip of each rotor blade.

The tip-propellers were powered by drive shafts, which ran through a hollow tube (spar) that supported the rotor blade. These connected to an engine below the rotor head via shafts and right-angle gearboxes. Brennan's "tip-drive" helicopter was capable of lifting a pilot, four men, and an hour's worth of fuel, but control issues meant it never flew higher than 10 feet (3m). A crash in 1925 shook confidence in the project, and the emergence of another machine, the autogyro, diverted interest from Brennan's tip-drive concept.

Conceived by Spanish engineer Juan de la Cierva in the early 1920s, the autogyro looked like a helicopter and airplane combined. That's because Cierva wasn't designing a helicopter. He was trying to design an aircraft that could fly at low speed without stalling.

An autogyro sustains flight by pairing small wings with a free-spinning rotor that turns solely because of the passage of air through it in forward flight. A separate engine and propeller (in tractor or pusher configuration) provide forward thrust, and the rush of air across the angled rotor blades causes them to rotate and generate added lift. Autogyros generally aren't capable of vertical takeoff. But they can make short takeoffs and vertical landings by virtue of their "autorotating" blades.

Cierva later partnered with Scottish industrialist James G. Weir to establish the Cierva Autogiro Company in England. Though they never really made the big time, improvements to autogryos by Cierva and American aircraft maker Pitcarin saw them become popular novelties in the 1930s.

Concurrently, Englishman Frank Whittle and German Hans van Ohain were each working separately on the turbojet engine. The world's first jet airplane, the Heinkel 178, first flew in 1939 in Germany. Three years later, Austrian engineer Friedrich von Doblhoff decided to combine Brennan's appealingly simple tip-drive concept with the jet engine.

The Doblhoff WNF 342 was a three-bladed helicopter that used a piston engine driving a compressor to provide compressed air. When mixed with fuel, it was fed up through the rotor hub and out through the three hollow rotor blades to be burnt in tip-mounted jets, generating rotor thrust. The rotor jets were only used for take-off, hovering, and landing.

The war aborted development of the WNF 342, but in the following 15 years Fairey engineers would ultimately bring together the threads of Brennan's, Cierva's, and Doblhoff's work.

Compound interest

While the modern helicopter configuration was well established by 1946, the desire to minimize torque, vibration, and stress on the drivetrain was also well known. Helicopters were amazing, but they weren't particularly reliable. Fairey recognized this as a real commercial limitation. So they set out to design something else—a compound helicopter.

Fairey recruited Dr. J.A.J. Bennett, who previously worked with the Cierva and Weir companies. Leveraging a proponent of designs that blended helicopter and autogyro traits, Dr. Bennett proposed a "Gyrodyne" concept. The aircraft would have a powered rotor like a helicopter but also stub wings and a thrust-producing propeller like an autogyro.

Powered by the same engine that drove the rotor, the starboard wing-mounted prop counteracted rotor torque as well as providing thrust. Most of the engine's power was transferred to the rotor for takeoff, hover, and landing. In forward flight, it went to the propeller. The rotor then autorotated, alleviating stress on the rotor head and transmission while still generating lift along with the stub wings.

Reduced rotor rpm allowed for higher cruise speeds. Seven months after its first flight in 1947, the first of two Gyrodyne prototypes set a world helicopter speed record flying at 108 knots (124mph/200km/h). The aircraft made progress in testing, but a fatal crash in 1949 paused the program.

Among the Fairey staff looking at further development were German engineers recruited from Doblhoff. With their encouragement, the company decided to retrofit the second Gyrodyne as a test-bed for a tip-jet drive system. Its rotor and gearbox were removed, replaced with a two-blade tip-jet driven rotor. The tip jet rotor worked by taking air from two compressors driven by the engine, which flowed through the rotor blades alongside kerosene, which was then mixed and burned by the jet.

Like the Gyrodyne, the rotor was powered for takeoff and landing. In forward flight the jets were shut off and the aircraft flew as a gyrocopter. Forward thrust and yaw control came from engine-driven pusher propellers on the stub wings. The Jet Gyrodyne could only maintain level flight as a gyrocopter for short periods, but its promise convinced Fairey to undertake two other tip-jet designs. The first was a small purely tip-jet powered helicopter called the Fairey Ultralight Helicopter. The second was the ambitious Rotodyne.

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