The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine, a hybrid between a turbofan and a turboprop known as a propfan. The 578-DX was developed as a joint venture between Pratt & Whitney and Allison. Unlike the competing General Electric GE-36 UDF, the 578-DX was fairly conventional, having a reduction gearbox between the LP turbine and the propfan blades. First run in 1986, noise considerations, plus a significant reduction in the real cost of aviation fuel, brought the NASA funded program to a halt
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