Now that we've had another visitor to our aero thread, I wanted to address commercial/military large transport aero's as related to flying test bed engines etc, and changing engines. The mounts are usually fairly common, and for most of these projects, the pylon may be modified, but the cowling most always will be the major article of change. Through the years we have had various configurations for testing engines, mostly high wing pylon slung engines, but an occasional pylon built on top of a wing for needed ground clearance. So flying the WS-20 engines on the Y-20 will be no biggie when the time comes, particularly as the Y-20 has been designed to use the WS-20 from the beginning. The airplane should fly fine, and most flying differences can easily be trimmed out or adjusted by power settings???