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Inside Rear Mounted Engines  
User currently offlineBaw2198 From United States of America, joined Dec 2003, 637 posts, RR: 4
Posted (7 years 10 months 2 days 2 hours ago) and read 3329 times:

Got to thinking after reading the thread in civil about front mounted engines and started thinking about mounting ideas on the rear of an aircraft.

Lets say you take a 717 and mount the engines inside the tail (like a 727 only two close together) and either have the "S" ducts on top (two independent or one large oval) or off to the sides. Would you gain anything by doing this and would the drag be reduced by not having the engine nacelles out in the wind?

Also, could you do a single engine setup, like with a medium sized wide body and do the same as suggested above using a GE90?


"And remember, Keep your stick on the ice"--->Red Green
12 replies: All unread, jump to last
 
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 10 months 2 days 2 hours ago) and read 3330 times:

The problems I see, is that maintenance would be a nightmare, and having two engines in close proximity (like the F-4 phantom) leads to complex interference between the two engines' exhaust streams.

Otherwise, potential benefits would include low drag and nearly center-line thrust, which is good in the advent of an engine failure


User currently offlineBuckFifty From Canada, joined Oct 2001, 1316 posts, RR: 20
Reply 2, posted (7 years 10 months 2 days 1 hour ago) and read 3303 times:

External engine nacelles also provide lift, and in fact are a significant contributor to that lift at the speeds they fly at, much more so than most people think. In effect, the drag they produce at subsonic speeds are more or less inline with engines that are mounted internally, as the intake size would more or less have to be the same.

User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1608 posts, RR: 52
Reply 3, posted (7 years 10 months 2 days 1 hour ago) and read 3291 times:
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Quoting BuckFifty (Reply 2):
External engine nacelles also provide lift, and in fact are a significant contributor to that lift at the speeds they fly at, much more so than most people think. In effect, the drag they produce at subsonic speeds are more or less inline with engines that are mounted internally, as the intake size would more or less have to be the same.

Having nacelles provide lift is not a good thing. They are very low aspect ratio surfaces and thus produce disproportionate induced drag.

Internally mounted engines would be much lower drag, as the airframe would have lower wetted area. The downsides to internally mounted engines are:

1) Providing them air with a minimum of total pressure losses and flow distortion (much tougher than it sounds).

2) Figuring out a way to carry tail loads through the engine bay area (also tougher than it sounds).

3) Figuring out where to put all the stuff that is carried in the aft fuselage normally (ECS systems, etc.).

Despite the challenges, it has been tried...


User currently offlineBaw2198 From United States of America, joined Dec 2003, 637 posts, RR: 4
Reply 4, posted (7 years 10 months 2 days ago) and read 3289 times:

Quoting BuckFifty (Reply 2):
as the intake size would more or less have to be the same.

could you eliminate the intake cowling all together and go with a grate type setup? example, 727 apu exhaust, but instead of blowing backwards, you're sucking air in from the top of the grate and down into the engines.



"And remember, Keep your stick on the ice"--->Red Green
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 5, posted (7 years 10 months 2 days ago) and read 3266 times:

Quoting Baw2198 (Reply 4):
could you eliminate the intake cowling all together and go with a grate type setup? example, 727 apu exhaust, but instead of blowing backwards, you're sucking air in from the top of the grate and down into the engines.

Because of the boundary layer of air around the fuselage in flight, you could not. That is why the 727 and L1011 do not have the #2 air intake flush with the fuselage. Even so with the DC-9 and 727's #1 & 3 engines, the engines are mounted on pylons away from the fuselage, to prevent ingestion of "gurgled air." Same thing can be said with the photo above, and even fighter-jets. Notice the F/A-18 Hornet's intakes are actually a few inches away from the fuselage.

Note that the 727's engines are mounted away from the fuselage-to keep the engines from ingesting the boundary layer on the fuselage.


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User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 6, posted (7 years 10 months 1 day 19 hours ago) and read 3201 times:

Quoting AeroWeanie (Reply 3):
Despite the challenges, it has been tried...

And the former HQ of Visionaire is now a telecommunications company. It's right across the street from my flight school.



DMI
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1608 posts, RR: 52
Reply 7, posted (7 years 10 months 1 day 7 hours ago) and read 3098 times:
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Quoting N231YE (Reply 5):
to prevent ingestion of "gurgled air."

Good write-up, but the technical description is not "gurgled air". What happens is that the viscous effects in the boundary layer on top of the fuselage produce total pressure losses in that layer of air. What designers do is place the inlet outside of this region, to get the highest total pressure.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (7 years 10 months 1 day 7 hours ago) and read 3082 times:

You want to recover the energy in the ram air in the intake. That you will not do with a flush grill. Sucking the air in is just what you'd do, and you'd use engine power for sucking rather than for providing thrust.

You'd be simulating engine power at zero airspeed, all the time. Anyone who has been in a starting jet aircraft knows that the push increases significantly as airspeed goes up. Very, very bad idea. Boundary layer ingestion is the least of the problems associated with such a setup.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1608 posts, RR: 52
Reply 9, posted (7 years 10 months 1 day 5 hours ago) and read 3060 times:
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Quoting FredT (Reply 8):
You want to recover the energy in the ram air in the intake. That you will not do with a flush grill. Sucking the air in is just what you'd do, and you'd use engine power for sucking rather than for providing thrust.

You'd be simulating engine power at zero airspeed, all the time. Anyone who has been in a starting jet aircraft knows that the push increases significantly as airspeed goes up. Very, very bad idea. Boundary layer ingestion is the least of the problems associated with such a setup.

Total pressure IS the energy you need to recover. The worst losses in total pressure are due to the boundary layer and shock waves.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 10, posted (7 years 10 months 1 day 3 hours ago) and read 3019 times:

Quoting AeroWeanie (Reply 9):
Total pressure IS the energy you need to recover. The worst losses in total pressure are due to the boundary layer and shock waves.

Yes, in a sane design.

What I am hinting at here is, however, that if you as was suggested here design an oversize static port and try to use it as an engine intake, those will not be your worst total pressure losses...  Smile



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBaw2198 From United States of America, joined Dec 2003, 637 posts, RR: 4
Reply 11, posted (7 years 10 months 1 day 2 hours ago) and read 3006 times:

Quoting FredT (Reply 8):
You want to recover the energy in the ram air in the intake. That you will not do with a flush grill. Sucking the air in is just what you'd do, and you'd use engine power for sucking rather than for providing thrust.

Why would the ram air matter in a turboshaft engine? I understand a ramjet, but a turboshaft/fan I would think this to be a bad thing. Its hard for me to word this question being that I don't really understand how the ram air is a positive benefit for a rotating engine. You would think that you would get a certain amount of drag by the ram air pushing the engine to spin vs. letting the engine pull its own in?



"And remember, Keep your stick on the ice"--->Red Green
User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1608 posts, RR: 52
Reply 12, posted (7 years 10 months 20 hours ago) and read 2953 times:
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Quoting Baw2198 (Reply 11):
Why would the ram air matter in a turboshaft engine?

A turbine engine works by compressing air, burning it and then expanding it. If you start out with air that is already compressed to a degree, you have to compress it less.


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