Faster with Fans

Into service with American Airlines this week went the world's fastest commercial airliner--the Boeing 707-120B, a faster version of the 707. Called by American the Astrojet, the new plane cruises at 640 m.p.h., flies from New York to Los Angeles in 5 hr. 25 min., trimming 25 min. off the old schedule. The new plane will also benefit those living near airports: it takes off and climbs so quickly that it will pass over neighboring communities at higher altitudes, markedly reducing the noise. Since it does not need to mix water with fuel for added thrust, no black smoke spews out of its engines on takeoff.

The Astrojet gets its pep from a new engine: the Pratt & Whitney turbofan, which develops 17,000 Ibs. thrust. Basically, the turbofan sucks in a larger mass of air than regular jet engines to produce greater thrust with less fuel. A fan, set just inside the air intake (see diagram), pulls in the air. then blasts about 60% of it out through openings on the side of the jet pod to provide just under 50% of the engine's total thrust. The rest of the air is directed into the engine's burning chamber. The engine produces 20% more thrust than a regular jet engine of similar capacity, while burning 20% less fuel on a transcontinental flight.

Engineers have long known that the turbofan is the most efficient engine for subsonic aircraft. Rolls-Royce pioneered turbofan development, turned out a prototype of its famed Conway by-pass engine in 1950. But U.S. enginemakers, under pressure from the military to produce regular jet engines that are more efficient at supersonic speeds, lagged behind.

When American began flying Boeing jets just over two years ago, it had to settle for regular jet engines. Later, Pratt & Whitney developed a way to convert the jet engines on American's planes into turbofans. At a cost of $50 million, American is converting 23 Boeing 707s and ten 720s. The Boeings that American still has on order will also be outfitted with turbofans. By next year American expects to be the only U.S. airline to have its entire jet fleet powered by turbofans.

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