Middle engine placement
Where can a plane have a middle engine?
A plenty of aircraft have a S-duct from inlet down into the rear fuselage where the engine is. So on Trident the original trijet, Boeing 727, Yak-40, Tristar, Tu-154, Yak-42, Dassault Falcon 50, 900 and 7X.
Only DC-10 and MD-11 have the engine higher in tail, aligned with inlet.
Is there any technical reason why such tail engine placement has not been popular with small trijets?