Air Transport World

G.E.'s unducted fan spices propfan stew.

Questions about propfan technology most commonly heard at engine and airframe manufacturers these days no longer concern whether it will work, but rather how best to use it.

The current bet at Pratt & Whitney, Rolls-Royce and to some extent within NASA is that a propfan is at its best when driven by a gearbox.

But General Electric has laid down a multi-million-dollar bet that it will be able to prove a concept of using direct turbine drive to spin small, highly loaded contraroating propfan disks. The numbers are convincing enough to entice NASA into spending $27 million on G.E.'s direct drive research. General Electric calls its new concept the Unducted Fan, and has gone so far as to trademark the initials "UDF."

The possibility that airlines can enjoy all the benefits of propfan economics while avoiding gearboxes probably brings large, peaceful smiles from some portions of the industry. In those circle large gearboxes are about as welcome as big, fast moving dark things that go bump in the night by your window, and should be treated in the same way, with a stake through the sun gear, a silver bullet up the output shaft or wolfsbane in the oil cooler.

Others in the industry say this fear of gearboxes borders on the irrational, given the decades of smooth and low-cost performance given by turboprop gearboxes, large and small, around the world. But like a lover once burned, the industry remembers well the late 1940s and 1950s when gearbox worries were at their peak. UDF efficiency questions

The G.E. program will appeal to this group. But in order to achieve universal appeal, UDF must be demonstrated to deliver efficiencies equal to gear-driven propfans, or nearly so. There is a body of opinion that states G.E. will lose too much efficiency in using small, highly loaded props spun by heavy direct drive turbomachinery that must turn too slowly to be efficient.

Although G.E. engineers admit to some efficiency loss, they maintain the losses are small, and are offset by the elimination of gearbox weight, maintenance and drag, and the drag of the accompanying oil cooler.

But at this stage of the game nobody is sure of their answers, hedging their bets with conditional disclaimers in both directions--the gearbox crowd admitting that direct drive just might be the ticket, and G.E. holding its commitment to full product development until late 1986, after completion of a large scale flight test of a UDF demonstrator mounted on a Boeing 727-100. UDF to that point is termed a proof of concept program. G.E. is confident enough, however, to become the first engine manufacturer to launch a hardware development program that includes a time-table for the first availability of a propfan airliner. …

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