Cuban_8 ,
You make me laugh......
Try standing behind a jet engine at full thrust at brake release you tell me there is no action on the air behind it.
F=MA
Yes once the airplane is moving at flight speed it is "standing" on it's fans, but that is not what the guy first asked. He wanted to know would you lose assymetry or a lot of thrust if you ducted two engines together. I assumed he meant from our typical definition of a commercial airplane and meant engines coming from each wing ducted together to exhaust behind the tail. I could have been a little more clear when I said that I thought there would be huge thrust loss because of the length of the duct. In my mind, with the experience I have with turbo props, the spinning motion of the engine has an effect on directional control. The King Air has a yaw damper requirement in case you lose an engine at rotate that is removed if you install the Raisbeck Engineering strakes. Without them, the plane will roll over and crash. US military has lost lots of them practicing Vmcg stalls. But, on something the size of a commercial jet I am sure the component of the engine spinning is very small.
