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Wing-mounted Versus Tail-mounted Engines  
User currently offlineQuokkas From , joined Dec 1969, posts, RR:
Posted (1 year 10 months 1 week 23 hours ago) and read 4520 times:

An obvious one maybe, but I am not an engineer.  

We see aircraft designed to have rear (tail) mounted engines and aircraft that have wing-mounted engines. Compare the E145 with the E170, for example. Both have two engines. The former has the engines mounted at the rear while the latter has them suspended from the wing.

Is there any performance benefit in having one versus the other and at what point does it become impractical to design an aircraft with rear mounted engines (I'm guessing passenger capacity and the weight of the engines and structural strength of supports would be involved)?

Current aircraft tend to have two engines but in the past there were a number with three. What motivated the move to two? Was it just improved efficiency, reducing the weight, or reducing maintenance costs with having fewer engines to maintain?

11 replies: All unread, jump to last
 
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 1, posted (1 year 10 months 1 week 22 hours ago) and read 4494 times:

Big thing with twins is lower maintenance. If you can make it with two engines, you do it. Only if you cant (such as A380), you start thinking 4.

Wings vs. tail mounted is discussed often around here. Short version, plane with engines under wings is lighter, therefore more economical. If there is a recent plane with tail mounted engines, there is probably other design constraint, like it being too small for it to make sense to place the engines under the wings, or it needs fuselage to be low, so that you can benefit from easy access to it (typically bizjets, you want to be able to operate without external stairs and beltloaders and whatnot)



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User currently offlineL-188 From United States of America, joined Jul 1999, 29802 posts, RR: 58
Reply 2, posted (1 year 10 months 1 week 22 hours ago) and read 4476 times:

The big reason that engines got mounted on the tail was for cabin noise, the cabin is just that much quieter with the engines way back there.

Tail mounted engines also are less likely to pick up fod from sloppy runways, are less likely to be damaged by something running into them, and allow for a clean wing leading edge whose while length can be used for lift devices.

Wing mounted engines are easier to service, less likely to be fodded by ice on the wings but more likely to be fodded by runways slop. And the tail structure of those aircraft can be made lighter since they aren't supporting motors. Also wing mounted designs are simpler to offer multiple engines types on since on e pylon really has to be changed. However



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User currently offlineflyingturtle From Switzerland, joined Oct 2011, 2441 posts, RR: 14
Reply 3, posted (1 year 10 months 1 week 21 hours ago) and read 4467 times:

Hi,


yes, this discussion often pops up. And it gives you a good opportunity to learn about aircraft design.  Smile

Some older threads about that topic:

Engines Under Wing Or On Fuselage (by Extra300 Oct 18 2012 in Tech Ops)

Wing Mounted VS. Fuselage Mounted (by Jmacias34 Oct 19 2001 in Civil Aviation)

Under Wing Engines Vs Tail Mounted Engines (by Qatar Feb 27 2002 in Civil Aviation)

Happy reading & your questions are always welcome!



David

[Edited 2012-12-17 10:27:54]


Keeping calm is terrorism against those who want to live in fear.
User currently offlineDocLightning From United States of America, joined Nov 2005, 19927 posts, RR: 59
Reply 4, posted (1 year 10 months 1 week 14 hours ago) and read 4361 times:

Quoting L-188 (Reply 2):
The big reason that engines got mounted on the tail was for cabin noise, the cabin is just that much quieter with the engines way back there.

That and also because at the dawn of the jet era, smaller aircraft like the Caravelle and DC-9/727 might have been operating into semiprepared airfields. Rear-mounted engines are much less likely to ingest FOD.

The 737 Jurassics sat about the same distance off the ground as the 727 did, in spite of having wing-mounted engines, and the engines were the same JT8D's used on the DC-9 and 727, so it probably wasn't necessarily about noise (although Boeing marketed the 727 that way). The original 737 had an optional gravel kit that allowed it to operate out of gravel airstrips with reduced FOD risk.


User currently offlinefanofjets From United States of America, joined Apr 2000, 1995 posts, RR: 3
Reply 5, posted (1 year 10 months 5 days 23 hours ago) and read 4142 times:

...In addition, the wing-mounted engines counterbalance wing flexing that would occur as the airplane gained speed during take-off, allowing for a more economical wing structure.


The aeroplane has unveiled for us the true face of the earth. -Antoine de Saint-Exupery
User currently offlineflyingturtle From Switzerland, joined Oct 2011, 2441 posts, RR: 14
Reply 6, posted (1 year 10 months 5 days 22 hours ago) and read 4129 times:

Quoting fanofjets (Reply 5):
...In addition, the wing-mounted engines counterbalance wing flexing that would occur as the airplane gained speed during take-off, allowing for a more economical wing structure.

That's also the matter during cruise.

For the same reason, the center fuel tank is emptied first, afterwards the wing tanks are.


David



Keeping calm is terrorism against those who want to live in fear.
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25626 posts, RR: 22
Reply 7, posted (1 year 10 months 5 days 16 hours ago) and read 4050 times:

Quoting DocLightning (Reply 4):
Quoting L-188 (Reply 2):
The big reason that engines got mounted on the tail was for cabin noise, the cabin is just that much quieter with the engines way back there.

That and also because at the dawn of the jet era, smaller aircraft like the Caravelle and DC-9/727 might have been operating into semiprepared airfields. Rear-mounted engines are much less likely to ingest FOD.

Some 727s operated from unpaved runways but as far as I know, no Caravelles or DC-9s were certificated for such operations. 737-200s of course did (and still do in Canada) and Boeing offered a gravel runway kit for that model.


User currently offlineworkhorse From France, joined Jul 2005, 219 posts, RR: 0
Reply 8, posted (1 year 10 months ago) and read 3602 times:

Talking about wing mounted engines, I have always wondered who can be reasonably called the inventor of this way to mount engines on aircraft (that is, on pylon under the wing)? This can definitely be called one of major inventions in aviation history: it's still in use after at least 60 years!

It looks like the first passenger aircraft to feature this was the Boeing 707, however, the DC8 had a similar layout and was developed around the same time.

Looking at military designs, it seems that the first one was the B-47 (Boeing again), followed by the B-52, however, I wonder if there wasn't a German WWII design (Arado?..) that had something like that as well...



[Edited 2012-12-25 07:35:30]

User currently offlineflyingturtle From Switzerland, joined Oct 2011, 2441 posts, RR: 14
Reply 9, posted (1 year 9 months 4 weeks 1 day 23 hours ago) and read 3567 times:

Quoting workhorse (Reply 8):
Looking at military designs, it seems that the first one was the B-47 (Boeing again), followed by the B-52, however, I wonder if there wasn't a German WWII design (Arado?..) that had something like that as well...

Yes, the Arado 234 bomber and the Messerschmitt 262 fighter, for example. The British Gloster Meteor had engines like the RB-57, and all other jet fighters of that era had the engines within the fuselage (AFAIK).


David



Keeping calm is terrorism against those who want to live in fear.
User currently offlineU2380 From United Kingdom, joined Dec 2010, 325 posts, RR: 0
Reply 10, posted (1 year 9 months 4 weeks 1 day 17 hours ago) and read 3521 times:

Wing mounted engines have the benefit of increasing the speed before the occurrence of wing flutter (or in other words, gives greater buffet margin) by damping out aeroelastic forces. This allows the wing construction to be lighter. Aft mounted engines also require the tail section of the aircraft to be reinforced.

Rear mounted engines also have a rearward effect on CoG which may require a larger tailplane and control surfaces. One way of counteracting that issue is the T-tail.

As a side note, aft mounted engines tend to produce a pitch down moment upon application of thrust and wing mounted engines produce a pitch up effect ( the thrust line is below the CoG) all other things being equal.

However, aft fuselage mounted engines do have their own advantages, for example they produce a lesser yawing moment in the event of an engine failure and they also allow the wings to be 'cleaner' in comparison to wings with pylons and engines attached.

Well, that's my rather rudimentary ATPL understanding, I'd be more than happy for more knowledgeable members to correct me.

[Edited 2012-12-25 14:44:16]

[Edited 2012-12-25 14:51:21]

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (1 year 9 months 4 weeks 20 hours ago) and read 3365 times:

Quoting workhorse (Reply 8):
I have always wondered who can be reasonably called the inventor of this way to mount engines on aircraft (that is, on pylon under the wing)?

I believe credit goes to the Germans. The British had a love affair with embedded engines for a while, and the B-47 and it's descendants stole basically everything they knew from the Germans after the war.

An aero prof of mine once said that the answer to "Who first thought up X configuration?" is always "The Germans in World War II."

Tom.


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