The Short, Happy Life of the Prop-fan

Meet the engine that became embroiled in round one of Boeing v. Airbus, a fight fueled by the cost of oil

Boeing’s 150-seat 7J7 concept (left) would meld prop-fan technology and lightweight composite structure to deliver big gains in fuel efficiency. (The Boeing Company)
Air & Space Magazine

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Conventional turboprops hit a wall when the combined forward speed of the airplane and the rotational speed of the propeller tips exceeds Mach 1, resulting in shock waves. The problem was that the rotational speed of the propeller was limited to only a few hundred rpms because the blade tips could exceed Mach 1 by only a small fraction; above that, efficiency plummets. But that speed was uncomfortably low for a turbine. A conventional turbine would have to be very large, with lots of stages, and each rotating stage would be followed by a fixed “stator,” turning the flow so it hit the next turbine wheel at the right angle to convey force.

What was unique in Adamson’s design, which had been refined by engineer K.O. Johnson, was that in profile, the counter-rotating turbine stages were interlaced; the direction in which each row of blades spun was the opposite of the direction of the stages immediately upstream and downstream of it. The design had no stators, and the relative velocity between each stage was doubled. Counter-rotation effectively doubled the turbine’s rpm, so the turbine could be made smaller, simpler, and more efficient.

But think about the mechanics. The turbine blades that drove the aft propeller were attached to a solid shaft in conventional bearings. The turbine stages driving the front propeller were riding outside the aft set and could not reach a central shaft. The turbine blades were attached at the tip to an outer case, which was carried on inter-stage bearings and a ring bearing at the rear of the nacelle. This design had to allow for thermal expansion and the load imbalances that would occur if a propeller blade broke off.

It was vital that the blades be lightweight so that the engine would survive if a blade separated. The UDF would have blades made from carbon fiber composite materials.

The new engine offered enormous potential but presented equally large risks. Rowe decided to fly a full-scale demonstrator in collaboration with Boeing, whose 727 test bed would fly in 1986. “I thought it was an engine of the future,” he says, “something we ought to pursue.” NASA headquarters ordered the Lewis center to support GE’s privately funded efforts even though Lewis was developing its own engine. While GE regarded the UDF as a technology program, Boeing presented it as the engine that would power its newest airplane: the 150-seat 7J7.

The 7J7 represented a changing of the guard at Boeing, the first project to be launched by a new generation of leaders: Program chief Jim Johnson reported to a rising vice president named Phil Condit. The goal, Johnson said in early 1986, was to deliver an airplane that cost less per seat than the 737.

Veteran executives were more cautious. Mulally says today that “anyone who had worked with propellers really wanted to see the concept validated”—they wanted to be convinced that the new engine would be reliable. At Boeing, Mulally recalls, the 737 team proposed an improved, longer-range 737 that would cost far less to develop than a new airplane. But Boeing promoted the 7J7 and its UDF with an enthusiasm that rings in Mulally’s voice almost two decades later. “It was a tremendous improvement,” he says. “We could have delivered that airplane.”

Johnson and Condit sold the 7J7 concept hard. Conboy says he took part in 50 presentations in 1987. In that year, Boeing settled on a larger design that used a little more fuel but offered six-abreast, twin-aisle coach seating, banishing the hated middle seat. The airplane was too large for the Allison-P&W 578-DX engine, and Boeing settled on GE’s planned production UDF-based prop-fan, the GE36-C25.

The UDF made its first flight on August 20, 1986, aboard a Boeing 727 test bed. The tests encouraged Prop-Fan and UDF proponents, demonstrating that noise was a problem but not an insurmountable one. A February 1987 Washington Post headline read: “The aircraft engine of the future has propellers on it.”

But not everyone was convinced. At the 1985 show, Jim Johnson wanted to pitch the 7J7 to Lufthansa’s technical director, Reinhardt Abraham. His endorsement would be a huge blow to the A320; Johnson directed Rudy Hillinga, Boeing’s chief salesman in Germany, to get Abraham to the Boeing 7J7 mockup at Paris.


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