Engine placement

[PMN1]

When the designers of the DC-10 and the Tristar were looking at engine placement, did they consider putting all three engines at the tail?

In the same way, did the 727 and Trident designers look at 2 wing and 1 tail mounted engine?

 

[KJ_Lesnick]

When you say three engines on the tail, do you mean a 727 like layout? I don't think so, turbofans are kind of big and heavy, and you'd almost certainly require a T-tail. T-tails are often prone to deep-stalls, which are very dangerous to a commercial-airliner with some exceptions, (Il-62, and some of the Learjets, although they have a number of modifications to deal with it including large ventral-fins, and a dog-tooth on the Il-62 IIRC) and require a heavier tail-structure.

I remember looking at a book which showed all sorts of preliminary designs for the 727, and while I don't think any of then showed such a layout, I was told that Boeing had considered such a layout (don't take my word for it)

 

[ysi_maniac]

A have seen a photo of a model by Boeing with a 727 T-tail, one engine in tail fin root (as standard 727) and the other engines under wings, as in 737. in some days I will give you the reference.


EDIT: http://www.amazon.com/History-Passenger-Aircraft-William-Sweetman/dp/0600372480/ref=sr_1_1?ie=UTF8&qid=1381938625&sr=8-1&keywords=history+passenger+aircraft . Here is reference

 

[alfakilo]

When you say three engines on the tail, do you mean a 727 like layout? I don't think so, turbofans are kind of big and heavy, and you'd almost certainly require a T-tail. T-tails are often prone to deep-stalls, which are very dangerous to a commercial-airliner with some exceptions...

Very dangerous?

Given the number of successful t-tail designs, I would advise taking this opinion with a significant chunk of salt. Of course, after only 16 years of airline flying in these designs (B-727, DC-9, MD-80), I could be wrong. Makes me wonder how I survived it all.

 

[Bill Walker]

T-tails on any high speed airplane can be very dangerous, if not done correctly. This usually means preventing stalls or other high alpha maneuvres, with a reliable and/or fail operational stick pusher or equivalent. The prototype CL600 was lost in stall testing, which the manufacturer thought was not needed because of the stick pusher design. Many other t-tail swept wing airplanes get certified without ever stalling. You just have to prove that a stall is virtually impossible.

I think the question of where the engines go is fairly complicated, and is a trade off, like anything else in airplane design. The 727 was optimized for relatively short field performance, so the aerodynamic penalty of engines on the wing was not acceptable. Without the short field requirement, engine mounted wings offer a lighter wing structure because of bending relief from the engine weight. BUT that has to be traded off against the longer undercarriage that wing mounted engines on a low wing may require. There are also c of g issues with rear mounted engines, if you intend to offer long fuselage variants (like the 757 and the A320 families).

The answer to this question is the standard engineering answer to complex problems: "it depends".

 

[Delta Force]

T-tails on any high speed airplane can be very dangerous, if not done correctly. This usually means preventing stalls or other high alpha maneuvres, with a reliable and/or fail operational stick pusher or equivalent. The prototype CL600 was lost in stall testing, which the manufacturer thought was not needed because of the stick pusher design. Many other t-tail swept wing airplanes get certified without ever stalling. You just have to prove that a stall is virtually impossible.

I think the question of where the engines go is fairly complicated, and is a trade off, like anything else in airplane design. The 727 was optimized for relatively short field performance, so the aerodynamic penalty of engines on the wing was not acceptable. Without the short field requirement, engine mounted wings offer a lighter wing structure because of bending relief from the engine weight. BUT that has to be traded off against the longer undercarriage that wing mounted engines on a low wing may require. There are also c of g issues with rear mounted engines, if you intend to offer long fuselage variants (like the 757 and the A320 families).

The answer to this question is the standard engineering answer to complex problems: "it depends".

Rear mounted engines don't seem to result in that much redesign for stretches. The Boeing 727s use the same tail and wings. Every DC-9/MD-80/MD-90 series aircraft uses the same tail and a variant of the DC-9-30 wing (except for the -10). It looks like the Comac ARJ21 even uses a DC-9-10 wing. The Boeing 737 went through much larger changes between its variants. The later McDonnell Douglas aircraft have much higher wing loading than the DC-9s though, so it's possible Boeing could have done more carry over with later variants if they desired. I'm not as sure on this, but it seems (non-trijet) rear engined designs can upgrade engines without needing as much redesign. For example, the Boeing 737 MAX will require new landing gear to maintain engine clearance due to the greater diameter of the engine.

 

[Sundog]

When the designers of the DC-10 and the Tristar were looking at engine placement, did they consider putting all three engines at the tail?

In the same way, did the 727 and Trident designers look at 2 wing and 1 tail mounted engine?

Designers usually do look at those trade-offs. Partly it depends on if they are going to use an existing engine or going to have a new engine developed for the design in combination with the mission requirements. Some of the early 767 design concepts had the engines mounted on the tail like they are on the 727. There is also a variant with the engines mounted like those on the L1011, IIRC, and a four engines variant as well. You can see those designs here at secret projects if you a perform a search on the 767.

It's a matter of testing all of the possibilities and seeing which version ends up meeting the mission requirements the best.