Pratt & Whitney/Allison 578-DX

The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine that combined a turbofan and a turboprop, known as a propfan. Allison, a General Motors division, unveiled its Model 578 contra-rotating propfan at the Paris Air Show in 1985, based on the core of its T701 heavy-lift helicopter turboshaft, and in 1986 Allison and Pratt & Whitney agreed to develop it as a joint venture, formally PW-Allison Engines in February 1987. It was meant for airliners of 100 to 160 seats such as the Boeing 7J7 and the McDonnell Douglas MD-91 and MD-92, but Boeing shelved the 7J7 in December 1987. It remains one of only four contra-rotating propfan engines to have flown in service or flight testing.