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.
- By Bill Sweetman
- Air & Space magazine, September 2005
The Boeing Company
(Page 2 of 6)
The big difference between the two designs was how the propellers were driven. The core of a turbine spins at tens of thousands of revolutions per minute, and to transfer power to a propeller or fan, traditional design relied on some form of gearing. The Allison-P&W team saw no problem with driving the radical new propellers via a 13-to-1 reduction gearbox, similar to the ones they’d used for years on the venerable T56 turboprop, which powered the Air Force’s Lockheed C-130. But Rowe and GE disliked gearboxes, which were heavy and costly to maintain.
Boeing’s Alan Mulally, now president of the company’s commercial airplanes division, headed engineering on the 7J7 project from start to finish; he calls Adamson’s solution “really, really cool,” and it’s hard to disagree. The UDF blades were powered directly and gearlessly by a turbine, driven by hot gas from the engine. The two rows of propeller blades were each anchored to multiple rows of turbine blades.
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.