A triumphant looking pilot

Eccentric Engineer: The painfully visible birth of the world’s first ‘invisible’ bomber

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This edition of Eccentric Engineer tells the story of how the world’s first stealth bomber became the most visible object in the sky.

There are many problems with flying a bomber aircraft. You’re basically flying a cargo plane, albeit one that only takes its goods halfway. The larger the cargo you can carry, the larger the ‘impression’ on your enemy. This favours large, slow aircraft. Aircraft that make easy targets.

It’s hardly surprising that since the First World War, engineers and designers have been trying to find a way to conceal their huge creations in that most revealing of backdrops – the sky. Camouflage has always been the first option, but it presents a number of difficulties. A plane painted in traditional brown and green camouflage can be almost invisible from above, but sticks out like a sore thumb against a blue sky. By the Second World War many planes had two kinds of camouflage: blue underneath and green and brown above. Not that it helped on a cloudy day. For this reason some reconnaissance Spitfires were painted a fetching pink. It was invisible in the morning sky, but not hard to spot on the ground.

So, what if a plane could be made invisible? That was the obsession of engineers of the German Linke-Hofmann company in 1916. It was a bold dream, particularly for a company that had until recently only manufactured railway rolling stock.

Linke-Hofmann had a very particular problem. The German government had awarded it a contract to build one of its ‘R-planes’ – the ‘R’ standing for Riesenflugzeug or ‘giant aircraft’. These were to be the largest planes of the war, with multiple engines capable of being serviceable in flight and carrying heavy payloads for several hours. As such they needed to be big.

And the ‘RI’ was big. Linke-Hofmann designed the fuselage to completely fill the gap between the biplane wings. This fuselage held not only the flight crew, but four 260hp Mercedes engines, which were connected to gearboxes and driveshafts that then transferred power to two tractor propellers mounted between the wings, giving the aircraft a maximum speed of 140km/h. Inside the fuselage, the engines could thus be accessed during flight. It did, however, also mean the engines were always in danger of overheating, while vibration made piloting the plane more akin to using a pneumatic drill. More worrying still, the deck configuration meant that a plane on its way home – with less fuel and no bombs – would be very top-heavy.

Then there was the size. The plane was over 15.5m long with a wingspan of over 33m. The forward section was so large it had three decks. The top deck housed the pilots and wireless station, the middle was the engine compartment, and the lower held the bombardiers, fuel tanks and payload. It was never going to be easy to make this monstrous aircraft disappear, but the makers believed they had a secret weapon. Instead of covering the fuselage behind the cockpit with doped canvas, they intended to use a high-tech material called Cellon, which was transparent and hence would, they thought, make the plane (at least in parts) invisible.

There were three main problems with this stealth technology. First, Cellon, a type of cellulose acetate, was highly flammable, which, combined with the wooden fuselage, made the plane a tinderbox. Secondly, it wasn’t very strong or stable; in dry weather the material shrank, warping the wooden fuselage, while in damp weather it expanded and made the whole structure sag. This had a very unnerving effect in flight, as the control surfaces changed characteristics from moment to moment depending on humidity. Cellon also decayed in ultraviolet light, becoming yellowed, brittle and prone to explosive shattering.

Yet it was the third drawback that really sealed the machine’s fate. Cellon was transparent but very shiny, meaning that in flight the plane almost glowed as sunlight bounced off its sides.

The RI only ever made two flights that we know of, in the spring of 1917. On the first the wheels fell off and, when the machine finally got airborne, the pilot, sitting in his sweltering Cellon cockpit, found the controls so soft that he couldn’t steer. In his elevated position he also couldn’t calculate how far the ground was below him. When he landed, the plane nosed over and was badly damaged.

On the second flight the wings fell off and the plane hit the ground vertically. Amazingly, all but one of the crew survived.

Despite this setback, two more of the aircraft were built, though there is no record of anyone trying to fly them. After that the project was shelved and the world’s first stealth bomber quietly disappeared. 

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