This Crazy Looking Engine was Ultra Efficient But Was Never Adopted. What Happened?

The UnDucted Fan developed by General Electric should have been more than a great trivia question.

(Welcome to Avgeekery Jeopardy.)

“Alex, I’ll take Dismissed Aircraft Technology for one hundred.”

Answer: “This fuel-efficient engine type was developed for the proposed Boeing 7J7.”

“What is the UnDucted Fan?”


In 1985 at the Paris Airshow Boeing was spreading the news about a General Electric UnDucted Fan prop-fan engine that would power its new 7J7. The 150-seat aircraft, equipped with the two ground-breaking engines, would use half the fuel that the Airbus 320, which was close to coming on line.

Three years later, the first plane equipped with a UDF flew but by then both the 7J7 and the UnDucted Fan were scrapped projects.

After the 1973 Yom Kippur War an oil embargo was put in place impacting the United States, Europe and Japan. Long gas lines for automobiles was one pressing issue while higher fuel prices were wrecking airlines’ bottom lines.

NASA’s Lewis Research Center in Cleveland conducted research to find the sweet spot between a propeller driven engine that would be fuel efficient but still deliver speeds so that a New York to Los Angeles flight wouldn’t take eight hours.

Originally called a “turboprop” that term had to be changed to “prop-fan.” A survey of potential passengers rejected the first term because apparently it referenced propeller driven aircraft but half of those surveyed were OK with the “prop-fan” term. Go figure.

GE started developing the technology in the late 1970s when there were rumors that its CFM56 turbofan was about to be surpassed by a competitor. The UDF engine nacelle was egg-shaped. At the narrow end was two rows of propeller blades made from carbon fiber composite materials 12-feet in diameter. It was larger and more powerful than the engine that NASA had developed.

The GE engine’s twin propellers spun in opposite directions to reduce losses due to “swirl” – energy wasted in imparting spin to the air behind the airplane. 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.

For the 7J7, the engines would be mounted near the tail to allow clearance for the propellers and to reduce cabin noise.

But what sparked the interest in developing a fuel efficient engine and aircraft became the demise of the UDF and the 7J7. The end of the oil embargo led to cheaper fuel prices. The 737 continued to be a workhorse for short to mid-range flights and the Airbus 320 offered even better performance. Developing and selling another 150-seat with radically different engines didn’t prove financially feasible in the late 80’s. Plus, the radical engine design – those darn propellers – led to scrapping both the 7J7 and the UDF for commercial use.  People loved jet engines and the UDF design along with some noise issues led to a postponement and eventually cancellation of any follow-on test program or adoption.