P&W highlights philosophical split on 737 Max engine

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Pratt & Whitney has questioned a propulsion philosophy that lies at the heart of Boeing's choice of a competing engine to exclusively power the Boeing 737 Max.

P&W marketing director Jim Speich gave journalists an educational briefing on the geared turbofan at the company's headquarters in East Hartford, Connecticut, yesterday.

One slide was labeled, "The truth behind fan diameters" and noted that "at a given thrust moving a larger flow [of air] slower is more efficient." Speich was asked if that message was a response to claims made in March by Boeing vice president of business strategy Mike Bair, who said the nearly 174cm (68.4in)-diamter fan of the CFM International Leap-1B fits the "sweet spot" of the 737 Max.

"For me, fan diameter makes a big difference," Speich replied.

Speich explained that the propulsion equation dictates that force equals mass multiplied by acceleration. A smaller fan diameter means there is smaller mass, which requires the engine to accelerate the air more quickly, he says. Speeding up the air through the engine core produces more noise and reduces fuel efficiency, he adds.

Speich's remarks reveal a philosophical split between Boeing and P&W on the propulsion architecture for the next generation of narrowbodies.

P&W's geared turbofan architecture is premised upon relying on a larger fan diameter and accelerating a greater volume of air more slowly through the core of an engine, as compared to a conventional engine architecture.

Airbus has selected both P&W's geared turbofan - the PW1500G - and the CFM Leap-1A for the A320neo. Tellingly, the P&W option includes a 210cm fan diameter, or nearly 12cm larger than the Leap engine for the same airframe.

Some prominent aircraft buyers, including Air Lease Corp's chief executive Steven Udvar-Hazy, have called on Boeing to consider offering two engines for the 737 Max. But Udvar-Hazy also has expressed concerns that an engine with a larger fan diameter than the Leap-1B will not fit under the wing of the 737.