Whirlybirds go green

tauringhuang.

Sep 4th 2008
From The Economist print edition

Transport: Switching to diesel engines could make helicopters, and other aircraft, more efficient and less damaging to the environment


THE ability to take off and land vertically, and to hover, makes helicopters useful machines. Unfortunately, because they lack fixed wings to help provide lift, they are also expensive to operate. Helicopters need powerful engines to drive their rotors, and that means they use a lot of fuel. So could they benefit from one of the developments being used to make car engines more efficient—a switch to diesel fuel? EADS, Europe’s aerospace giant, seems to think they could. One of its subsidiaries, Eurocopter, has begun work on a diesel-powered helicopter. Makers of fixed-wing aircraft are also looking at diesel engines, in small planes at least, where they show promise as a serious alternative to standard piston engines, which run on a high-octane form of petrol.

The first diesel-powered aircraft, a modified Stinson, took to the air in 1928. But even though diesels were tried out in airships and some early Junkers bombers, they never really caught on. Their chief drawback was their weight. Both of a diesel’s big advantages on the ground—its efficiency and its torque (pulling power)—are the result of its high compression of the fuel-air mixture in its cylinders. The resulting high pressure, however, requires a big, heavy engine to contain it. That is why diesels have traditionally been used mostly in heavy machines, such as locomotives and lorries.

Now the weight penalty is starting to diminish. New casting and manufacturing methods can produce lighter, stronger components. Computerised fuel-injection has improved diesels’ performance, and their inherent strength means they can be turbocharged to boost power even further. As a result, many of the least thirsty cars now on the road have diesel engines. And these same advances are being exploited in a new generation of diesel engines for use in small fixed-wing aircraft.

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Diamond Aircraft, an Austrian firm, is making a diesel-powered light aircraft which it says uses just 17 litres of fuel an hour, instead of the 35 litres that a similarly sized plane with a traditional piston engine would consume. General Atomics, a Californian company, has performed a similar trick. It has begun supplying diesel-powered versions of its Predator unmanned aerial vehicle, known as Sky Warrior, to American forces in Iraq and Afghanistan. The Sky Warrior is intended for long-range reconnaissance missions, rather than as a weapons platform.

Eurocopter, meanwhile, has proposed fitting an advanced, turbo-charged diesel engine into one of its EC120 helicopters. Normally this aircraft, like many commercial helicopters, is powered by a jet-turbine engine that runs on kerosene. (Only the smallest helicopters still use piston engines.) Guillaume Faury, Eurocopter’s engineering chief, hopes to have a prototype diesel EC120 flying by 2012. If it works well enough, the company might build a twin-engined version, too.

Eurocopter calculates that although an unloaded diesel EC120 would still be heavier than a jet-turbine version, it would use less fuel per kilogram lifted. This would not bring much benefit on short flights, but on longer ones (say, two or three hours) the weight of the extra fuel needed for the jet turbine would compensate for the inherent heaviness of the diesel engine, meaning that the diesel would offer significant fuel savings. That would make diesel helicopters ideal for missions such as search and rescue, and border patrols. For the whirlybirds at least, this would be less thirsty work.