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Engines Under The Wing And On The Tail At Once  
User currently offlineYYZALA From Canada, joined Nov 2009, 155 posts, RR: 0
Posted (5 years 9 months 2 weeks 2 days 13 hours ago) and read 8071 times:

I am wondering if there is any advantage in having a 4-engined airplane with an engine hanging on each main wing and two more engines in the back (MD80 style)? Obviously each design has its advantages and disadvantages but would the combination of both yield better results in terms of fuel efficiency, field performance, etc?

23 replies: All unread, jump to last
 
User currently offlineSpacepope From Vatican City, joined Dec 1999, 3180 posts, RR: 1
Reply 1, posted (5 years 9 months 2 weeks 2 days 12 hours ago) and read 8075 times:

The closest thing I could think of that has been built was the Martin XB-51. Photo of a model below.

http://www.scaleworkshop.com/workshop/images/xb51ga_3.jpg

Though the front engines were fuselage mounted, they were kinda under the wing too.



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User currently offlineTdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 2, posted (5 years 9 months 2 weeks 2 days 12 hours ago) and read 8069 times:



Quoting YYZALA (Thread starter):
I am wondering if there is any advantage in having a 4-engined airplane with an engine hanging on each main wing and two more engines in the back (MD80 style)?

The problem I forsee is that any modern airliner that needs 4 engines is big, which means large high-bypass fans. Mounting those on the tail would be a lot trickier than doing it with relatively slim engines like the MD-80.

Tom.


User currently offlineYYZALA From Canada, joined Nov 2009, 155 posts, RR: 0
Reply 3, posted (5 years 9 months 2 weeks 2 days 11 hours ago) and read 8038 times:



Quoting Tdscanuck (Reply 2):

Surely 2 engines of y thrust (big) in the front and 2 engines of x thrust (small) on the tail would solve this problem?


User currently offlineVikkyvik From United States of America, joined Jul 2003, 11318 posts, RR: 25
Reply 4, posted (5 years 9 months 2 weeks 2 days 11 hours ago) and read 8025 times:



Quoting YYZALA (Reply 3):

Surely 2 engines of y thrust (big) in the front and 2 engines of x thrust (small) on the tail would solve this problem?

Then you have twice as many parts, at least, because you have 2 different engines. More training for people to learn both engines, more maintenance, more spare parts to stock, etc.

Also, you'd have to put the engines on the wing rather far out to avoid having the tail-mounted engines breathing in exhaust from the wing-mounted engines (which is not good). Moving the engines farther out on the wing requires more structure, and probably longer landing gear as well, to achieve acceptable bank angles on the ground without scraping an engine.



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User currently offlineDw747400 From United States of America, joined Aug 2001, 1273 posts, RR: 1
Reply 5, posted (5 years 9 months 2 weeks 2 days 11 hours ago) and read 8012 times:



Quoting YYZALA (Reply 3):
Surely 2 engines of y thrust (big) in the front and 2 engines of x thrust (small) on the tail would solve this problem?

The cost of maintaining two types of engines on one aircraft, in addition to the inherent inefficiencies generally associated with smaller, low-bypass engines, would make this a non-starter from an economic standpoint.

Boeing evaluated a "thrusting APU" concept for the 777, where a small tail mounted engine would supplement two larger wing-mounted engines on takeoff. Because of the weight and maintenance issues associated with putting a 30,000 pound thrust engine in the tail of a completely different design than the main engines, Boeing determined two larger wing mounted engines were the best solution. I expect any analysis of your concept would yield the same results.



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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17365 posts, RR: 66
Reply 6, posted (5 years 9 months 2 weeks 2 days 8 hours ago) and read 7957 times:



Quoting Dw747400 (Reply 5):

The cost of maintaining two types of engines on one aircraft, in addition to the inherent inefficiencies generally associated with smaller, low-bypass engines, would make this a non-starter from an economic standpoint.

The B-36 was probably a non-starter economically in many ways, but it illustrates the problem.  Wink



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineOly720man From United Kingdom, joined May 2004, 6999 posts, RR: 11
Reply 7, posted (5 years 9 months 2 weeks 2 days 4 hours ago) and read 7893 times:



Quoting Tdscanuck (Reply 2):
Mounting those on the tail would be a lot trickier than doing it with relatively slim engines like the MD-80.

It was done when the VC10 was used as the RB-211 testbed, though an RB-211 is quite small in comparison to the more recent high bypass engines.

Some interesting photos (and a hair raising story) here...

http://www.vc10.net/History/Individual/XR809.html



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User currently offlineGoldenshield From United States of America, joined Jan 2001, 6149 posts, RR: 13
Reply 8, posted (5 years 9 months 2 weeks 2 days ago) and read 7851 times:



Quoting Vikkyvik (Reply 4):
Quoting YYZALA (Reply 3):

Surely 2 engines of y thrust (big) in the front and 2 engines of x thrust (small) on the tail would solve this problem?

Then you have twice as many parts, at least, because you have 2 different engines. More training for people to learn both engines, more maintenance, more spare parts to stock, etc.

Another problem is that if you lose the two big engines, the two small ones will only get you as far as the site of the crash.



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User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 3
Reply 9, posted (5 years 9 months 2 weeks 2 days ago) and read 7843 times:

And, the more engines, the more chances for an engine to break. One good reason to have two big engines instead of 2+n smaller ones.


Position and hold
User currently offlineNorthwest727 From United States of America, joined Jul 2005, 491 posts, RR: 0
Reply 10, posted (5 years 9 months 2 weeks 2 days ago) and read 7840 times:

Just a little trivia...the Boeing 367-80 (the 707 prototype) had a JT8D attached to the tail with a "dog-leg" pipe to route exhaust gasses over the horizontal stabilizer. This was done back in the late '50s early '60s, when the JT8D was a new engine, and needed to be tested for the 727, which it would power.

I have a book with photos of it, but can't seem to find it.


User currently offlineDalmd88 From United States of America, joined Jul 2000, 2740 posts, RR: 14
Reply 11, posted (5 years 9 months 2 weeks 1 day 23 hours ago) and read 7821 times:

Hanging a large diameter fan on the tail isn't a problem. Just look at the MD90. Those engines are about the same size as what is on the A340. The problem lies with the structure needed. By putting engines on the wing and the tail both areas need more structure.

User currently offlineThebatman From United States of America, joined Aug 2007, 903 posts, RR: 8
Reply 12, posted (5 years 9 months 2 weeks 1 day 21 hours ago) and read 7774 times:
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Here, I believe I've found a picture that basically describes what you're looking for. Of course it's totally fake! I thought it was an interesting photo...  Smile

http://i719.photobucket.com/albums/ww199/xxdarkknight/4engine_CRJ.jpg



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User currently offlineA346Dude From Canada, joined Nov 2004, 1347 posts, RR: 7
Reply 13, posted (5 years 9 months 2 weeks 1 day 21 hours ago) and read 7753 times:



Quoting YYZALA (Reply 3):
Surely 2 engines of y thrust (big) in the front and 2 engines of x thrust (small) on the tail would solve this problem?

Well then why not add the 2 small engines together and make a tri-jet? Oh wait, those are uneconomic too...



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User currently offlineSNAFlyboy From United States of America, joined Oct 2007, 86 posts, RR: 0
Reply 14, posted (5 years 9 months 2 weeks 1 day 20 hours ago) and read 7722 times:



Quoting Thebatman (Reply 12):
Here, I believe I've found a picture that basically describes what you're looking for. Of course it's totally fake! I thought it was an interesting photo...  

Can anybody say...ground clearance?
 scared 

~SNAFlyboy


User currently offlineYYZALA From Canada, joined Nov 2009, 155 posts, RR: 0
Reply 15, posted (5 years 9 months 2 weeks 1 day 19 hours ago) and read 7698 times:

Everyone is posting a lot of negatives, but are there any advantages with such design?

User currently offlineNomadd22 From United States of America, joined Feb 2008, 2028 posts, RR: 0
Reply 16, posted (5 years 9 months 2 weeks 1 day 19 hours ago) and read 7677 times:



Quoting YYZALA (Reply 15):
Everyone is posting a lot of negatives, but are there any advantages with such design?

The mechanics who maintain them would all be able to afford new swimming pools.



Anon
User currently offlineAerdingus From Ireland, joined Dec 2006, 3118 posts, RR: 15
Reply 17, posted (5 years 9 months 2 weeks 1 day 14 hours ago) and read 7579 times:



Quoting Oly720man (Reply 7):


Some interesting photos (and a hair raising story) here...

Very interesting! The size of the RB211 to the whoel VC10 itself is something else, I always thought the old VC was big, but damn, that engine....



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User currently offlineDocLightning From United States of America, joined Nov 2005, 21998 posts, RR: 63
Reply 18, posted (5 years 9 months 2 weeks 1 day 14 hours ago) and read 7571 times:



Quoting SNAFlyboy (Reply 14):

Can anybody say...ground clearance?

That's why it's not on the ground, silly!  duck 

Tail-mounted engines are good for two reasons:

1) they raise the engines off the ground so that the aircraft can land on semi-prepared fields where FOD is a risk. This was more of an issue back in the days when the first tail-mounted planes were designed. The DC-9 and 727 were examples of planes that might have used a semi-prepared field when they were first introduced, but no aircraft of that size would ever use such a field today.

2) They allow the aircraft to sit closer to the ground, which simplifies loading, MX, etc.

Most modern commercial aircraft do not need either consideration, so major jet transports that have been introduced in the last decade or so have under-wing engines. Private jets may need either of the above advantages, so I expect that they will continue to have tail-mounted engines for some time.


User currently offlineViscount724 From Switzerland, joined Oct 2006, 27500 posts, RR: 22
Reply 19, posted (5 years 9 months 2 weeks 1 day 14 hours ago) and read 7552 times:



Quoting DocLightning (Reply 18):
Tail-mounted engines are good for two reasons:

They also significantly increase the aircraft's overall length due to the T-tail design common on most rear-engine designs. That may create some airport gate space issues. For example, the overall length of the shortest DC-9-10 is 6 feet longer than the 737-200, although the DC-9-10 has fewer rows of seats. The 727-200 is also a few inches longer than the longest 707-320.

Aircraft with wing-mounted engines make more effective use of the fuselage length to carry passengers.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 20, posted (5 years 9 months 2 weeks 1 day 12 hours ago) and read 7527 times:



Quoting DocLightning (Reply 18):
The DC-9 and 727 were examples of planes that might have used a semi-prepared field when they were first introduced, but no aircraft of that size would ever use such a field today.

There are still some 737-200's with gravel kits frequenting semi-prepared fields in odd corners of the earth...however, it's not common and, as far as I know, nobody caters to that market with newbuilds anymore.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 21998 posts, RR: 63
Reply 21, posted (5 years 9 months 2 weeks 1 day 8 hours ago) and read 7476 times:



Quoting Viscount724 (Reply 19):

They also significantly increase the aircraft's overall length due to the T-tail design common on most rear-engine designs.

I said they were good for two reasons. There are a lot more reasons why they aren't good. That's one. Worse aerodynamics are another. There's also more difficulty in accessing them for mx.

That's why even the new RJ's have wing-mounted engines.


User currently offlineZkpilot From New Zealand, joined Mar 2006, 5015 posts, RR: 9
Reply 22, posted (5 years 9 months 2 weeks 21 hours ago) and read 7388 times:



Quoting Vikkyvik (Reply 4):
Also, you'd have to put the engines on the wing rather far out to avoid having the tail-mounted engines breathing in exhaust from the wing-mounted engines (which is not good). Moving the engines farther out on the wing requires more structure, and probably longer landing gear as well, to achieve acceptable bank angles on the ground without scraping an engine.

 checkmark  This is what I was also thinking re: the engines breathing in jetwash. Engines like to breath in cold air and are much more efficient when they do so.



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User currently offlineCloudyapple From Hong Kong, joined Jul 2005, 2456 posts, RR: 9
Reply 23, posted (5 years 9 months 1 week 4 days 5 hours ago) and read 6996 times:



Quoting Zkpilot (Reply 22):
  This is what I was also thinking re: the engines breathing in jetwash. Engines like to breath in cold air and are much more efficient when they do so.

And if you noticed, aircraft with tail mounted engines usually come with wings that are mounted lower on the fuselage. MD80/90, CRJs etc. Not sure if this is to provide a clean laminar flow into the engine by avoiding any turbulence coming off the trailing edge of the wing or to save structure/weight or simply you don't need the ground clearance.



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