baw2198
Posts: 587
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Inside Rear Mounted Engines

Tue Oct 24, 2006 8:06 am

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
 
N231YE
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 8:14 am

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
 
buckfifty
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 8:51 am

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.
 
aeroweanie
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 9:14 am

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...
 
baw2198
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 9:20 am

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
 
N231YE
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 9:57 am

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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pilotpip
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RE: Inside Rear Mounted Engines

Tue Oct 24, 2006 3:02 pm

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
 
aeroweanie
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Joined: Fri Dec 03, 2004 9:33 pm

RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 2:28 am

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.
 
FredT
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RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 2:52 am

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.
 
aeroweanie
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RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 4:31 am

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.
 
FredT
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RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 6:26 am

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.
 
baw2198
Posts: 587
Joined: Wed Dec 24, 2003 11:20 am

RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 7:29 am

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
 
aeroweanie
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Joined: Fri Dec 03, 2004 9:33 pm

RE: Inside Rear Mounted Engines

Wed Oct 25, 2006 1:38 pm

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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