voiceofgoa
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Engine Mounted On Tail Fin

Thu Sep 22, 2011 5:09 am

The new generation of jetliners is designed with all the engines located underneath the wings. Why has the MD-11 type of design where the 3rd engine was located on the tail fin fallen out of favour? From an engineering perspective what are the merits and demerits of locating the engine on the tail fin?

PS: even the MD-80 type of designs where the twin engines are located near the tail seem to be passé.
 
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Starlionblue
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 7:31 am

Both kinds of trijets, with fin mounted engine (DC10, MD/11) and with fuselage mounted tail engine (727, L-1011) are gone because it is simply not worth the hassle mounting an engine there.

Advantages:
- You can have three engines and don't have to jump from two to four.
- Airliners.net forum members think the aircraft is the coolest thing since sliced bread..

Disadvantages:
- If you a have an uncontained engine failure like in the Sioux City accident, debris is flying around areas with vital control lines.
- Maintenance requires cherry pickers, scissor lifts and so forth.
- Airflow issues to the center engine, for example around rotation. 727s center engines would surge on rotation on a regular basis.
- The engine mount in that location is very heavy compared to an engine under the wing. This was one of the major hurdles for the proposed MD-12 triplet.
- The DC-10/MD-11 fin mount means your rudder is smaller and further from centerline. This means it imparts more roll moment which must then be countered.

The engines of forty years ago when the big triplets went into service were quite small compared to today's mega-high bypass turbofans. Squeezing a GE-90 in there is just not practical.


When it comes to the MD-80 type it has to do with a couple of factors. Airports have better facilities than forty years ago in the form of jetbridges and so forth. So there is no need for airliners that go to minor airports to be very low to the ground. Maintenance on wing mounted engines is way easier. The wing mounted engines provide wing bending and twisting relief, allowing for a lighter wing.

[Edited 2011-09-22 00:32:53]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
474218
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 11:41 am

Quoting voiceofgoa (Thread starter):
Why has the MD-11 type of design where the 3rd engine was located on the tail fin fallen out of favour?

Because the the improvement in engines has allowed two (2) engines to do the job it use the take three (3) engines. Look at the maximum takeoff weights the 777 is 65K lbs higher than the MD-11 with 1/3 the engines.
 
Chamonix
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 12:42 pm

How was it possible to inspect the tail-mounted engine (DC-10,MD-11,L1011 et al.) during walk-around?
How could the crew inspect the blades?

[Edited 2011-09-22 05:53:59]
 
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Starlionblue
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 12:52 pm

Quoting Chamonix (Reply 3):
How was it possible to inspect the fin-mounted engine during walk-around?
How could the crew inspect the blades?

If you're talking FOD inspection I guess it is pretty hard for the tail mounted engine to ingest FOD given its location. So a bit of a moot point I suppose.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 1:17 pm

Quoting Chamonix (Reply 3):
How was it possible to inspect the tail-mounted engine (DC-10,MD-11,L1011 et al.) during walk-around?

Good flashlight, keen eyesight.

Quoting Chamonix (Reply 3):
How could the crew inspect the blades?

You can't from the ground. Even with a lift it can be tricky on anything with an S-duct.

Tom.
 
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tb727
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 3:06 pm

Quoting tdscanuck (Reply 5):
You can't from the ground. Even with a lift it can be tricky on anything with an S-duct.

Except on the 727, it's fairly easy via the Engine Access hatch on the #2, we regularly have maintenance pop it open and check for ice and snow build-up.
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voiceofgoa
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 6:16 pm

Thank you, Starlionblue, for your response.
 
bonusonus
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RE: Engine Mounted On Tail Fin

Thu Sep 22, 2011 7:01 pm

Quoting 474218 (Reply 2):
Because the the improvement in engines has allowed two (2) engines to do the job it use the take three (3) engines. Look at the maximum takeoff weights the 777 is 65K lbs higher than the MD-11 with 1/3 the engines.

This, along with ETOPS are the big reasons. Fuel efficiency is the most important metric for aircraft these days, and any large twin will kill a trijet (let alone a quad) on fuel burn. ETOPS allows 777s and A330s (and soon, 787s and A350s) to fly trans-pacific and trans-polar routes safely, so there is no reason for three-engined aircraft.
 
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Zkpilot
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 5:15 am

Likely the only time you will see a trijet large commercial airliner again is when Blended Wing Bodies (BWB) come in (think like the B2 stealth bomber). They will likely have 3 engines (about the size of 777 engines) rather than 2 huge 170k engines.
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Starlionblue
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 5:26 am

Quoting Zkpilot (Reply 9):
Likely the only time you will see a trijet large commercial airliner again is when Blended Wing Bodies (BWB) come in (think like the B2 stealth bomber). They will likely have 3 engines (about the size of 777 engines) rather than 2 huge 170k engines.

Possibly, but not necessarily. There is no big advantage inherent in a BWB being a trijet. The image has been popularized because Boeing's concept was initially an MD concept and MD had a tradition of trijets.

Since BWBs have no traditional tail, placement of an engine in the center does not carry the disadvantages the position entails in a tube with wings.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
aklrno
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 6:51 am

My 2 cents:

The engines are the heaviest parts on the aircraft. Mounting them close to where the lift is generated saves all the weight of the structure needed to transfer the engines' weight to the wing.

For a tri jet, mounting one at the tail is one of the few reasonable ways to get to mount the third engine. As others pointed out, you can now use two larger engines, so the 3rd engine and all the issues that it brings can be eliminated.
 
thegeek
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 7:18 am

Wouldn't an additional disadvantage of the tail mounted centre engine (DC-10/MD11 style) be that it provides a pitch forward moment which must be countered with negative lift (and therefore drag) from the stabiliser.
 
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Starlionblue
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 7:34 am

Quoting thegeek (Reply 12):
Wouldn't an additional disadvantage of the tail mounted centre engine (DC-10/MD11 style) be that it provides a pitch forward moment which must be countered with negative lift (and therefore drag) from the stabiliser.

I don't think so. Or at least I don't think it's a very big deal. If you look at the DC-10/MD-11 the tail engine is canted backwards quite visibly, perhaps to compensate. Wing mounted engines impart a pitch up moment.

BTW here's an MD-11 with an afterburner fitted to #2.


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"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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HAWK21M
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 11:51 am

The Tail mounted is Maintenance Unfriendly in terms of access.But can be an asset if one has a u/s APU & need to keep an Engine running prior to pushback without danger through FOD.
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tdscanuck
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RE: Engine Mounted On Tail Fin

Fri Sep 23, 2011 1:34 pm

Quoting Starlionblue (Reply 10):
There is no big advantage inherent in a BWB being a trijet.

True but, as you noted, a BWB doesn't have any of the unique disadvantages of the third engine placement, so it's neutral on the topic of number of engines. That means you can go to a third engine if other considerations make sense without a big penalty.

Quoting thegeek (Reply 12):
Wouldn't an additional disadvantage of the tail mounted centre engine (DC-10/MD11 style) be that it provides a pitch forward moment which must be countered with negative lift (and therefore drag) from the stabiliser.

That is partly countered by the weight of the engine sitting back there causing a nose-up pitching moment.

Quoting Starlionblue (Reply 13):
If you look at the DC-10/MD-11 the tail engine is canted backwards quite visibly, perhaps to compensate.

I think that's more to align it with the local airflow than to provide offsetting pitch.

Tom.
 
voiceofgoa
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RE: Engine Mounted On Tail Fin

Sat Sep 24, 2011 5:09 pm

Why not use the third engine only as a back-up - that is, power it on only in an emergency in case of failure of the 2 front engines. Now I know there is a cost trade-off here, and the reliability of modern engines has vastly improved. There is the additional drag and weight that the aircraft will have to bear on every flight, and there are capital costs involved in the initial design and manufacture (and maintenance). But wouldn't the vastly improved factor of safety justify this trade-off? (Apparently not - else it would have been implemented)  

[Edited 2011-09-24 10:10:46]
 
tdscanuck
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RE: Engine Mounted On Tail Fin

Sat Sep 24, 2011 8:38 pm

Quoting voiceofgoa (Reply 16):
But wouldn't the vastly improved factor of safety justify this trade-off?

Not even close...you'd need that extra engine, on average, something like once per 200,000 flight hours. In return, you're talking about something like another $20-30 million dollars in upfront costs, 50% more engine maintenance, several thousand pounds of dead weight, and a big drag hit.

Tom.
 
BMI727
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RE: Engine Mounted On Tail Fin

Sat Sep 24, 2011 10:26 pm

Quoting thegeek (Reply 12):
Wouldn't an additional disadvantage of the tail mounted centre engine (DC-10/MD11 style) be that it provides a pitch forward moment which must be countered with negative lift (and therefore drag) from the stabiliser.

Yes. Piper is going to use a thrust vectoring system on the PiperJet Altair to counter this.
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