Middle engine placement

Downfalls of this position of engine are that engineers need a cherry picker to carry out maintenance,

Which central engine is harder to access for maintenance or replacement – the DC-10 one (higher up) or Tristar (a bit lower, but a widebody tail is still high, and buried deep in fuselage)?

The No2 engine on both the Tripestar and the 10 were a pain in the A**e to do any maintenance on! Eaven topping the oil up was an epic.
I was involved with a No2 engine change on a Tristar once. It required special docking, a special sling and a crane. It took an entire gang all night just to prepare it to drop! You can have a N01 or 2 down and up in a shift if you put your mind to it! As for doing one down route! They used to two engine ferry them back as a matter of course!
They stripped the normal accessorys (Hyd pump IDG) off the KC-10’s No2 engine and put a second starter and start valve on instead. This improved it’s dispach reliability considerably down route. I think also that the parcel operators have done the same with the DC-10’s.Don’t know for sure though.
It was not for nothing that they were known as “The three headed ulcers”!

Rgds Cking

Also… air doesnt like being turned. Designers have a nightmare designing “S” ducts as you have to get it perfectly right for the flying conditions you will encounter. In an “S” duct, air may separate from one side of an intake and cause turbulence within the intake… which is bad news for the engine!!

The straight through arrangement on the DC-10 is a much easier, simpler way of designing a place for the third engine.

Downfalls of this position of engine are that engineers need a cherry picker to carry out maintenance, there was that turbine failure which slashed all 3 of the hydraulic lines in a DC-10. Its also harder for structural engineers as you have to get the fin spars through the engine.

One good point of having an “S” ducted, tail engine as in the TriStar is that the fan noise of this third engine can be effectively controlled. The fan is buried in the fuselage and noise would have a hard time escaping from the front end of the “S” duct, however… it would still come from the rear of the bypass duct!

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 populat with small trijets?

It is because the weight is placed on the tail is a lot larger.

In the case of the DC10/MD11 the full weight of the engine lies on a structure of perhaps 4 by 0,5 metres. Thats a lot of weight for a small surface. In the case of the other planes the weight is supported by the entire rear fuselage.

Advantage of the DC10/MD11 layout is that it is easier to get to the engine for maintenance. You won’t have the strip half the rear fuselage, you can just open the panel at the tail same as you would for a wing mounted engine.