Why not tail jets?
The Cumulogranite, you mean this might happen?:
Incidents and Accidents
Of course you don't balance engines with removable items (fuel,passengers or cargo), you design the structure to balance when empty. In the incident in the above link, the fin tank was still full, causing the aircraft to tip up.
There is something to be said for this layout (Tupolev TU-22 Blinder), as there is less assymetric thrust in single engined flight, so less rudder drag and therefore less power required to overcome it:
The engine change crew won't thank the designer though...
Incidents and Accidents
Of course you don't balance engines with removable items (fuel,passengers or cargo), you design the structure to balance when empty. In the incident in the above link, the fin tank was still full, causing the aircraft to tip up.
There is something to be said for this layout (Tupolev TU-22 Blinder), as there is less assymetric thrust in single engined flight, so less rudder drag and therefore less power required to overcome it:
The engine change crew won't thank the designer though...
Last edited by Mechta; 30th Jul 2012 at 15:23.
Those closely-spaced engines are all fine and dandy in case of a simple flameout, but also consider that engines have been known to shed debris around on occasion. An engine failure epidemy is quite a known feature of such a design - see the accidents to SP-LAA, a LOT IL-62 at EPWA in 1980 or SP-LBG, a LOT IL-62 M also near EPWA in 1987 for example.
On the other hand, on the Tu-22 You still have the silken way out, should this happen.
A little side note - when it comes to clustered engines, nothing beats the FMA I. A. 36 with its 5 RR Derwent engines all together in the tailcone. It was maybe a smart move to shelve this design:
On the other hand, on the Tu-22 You still have the silken way out, should this happen.
A little side note - when it comes to clustered engines, nothing beats the FMA I. A. 36 with its 5 RR Derwent engines all together in the tailcone. It was maybe a smart move to shelve this design:
Last edited by Tu.114; 30th Jul 2012 at 15:31.
Was the VFW614 an act of misplaced teutonic determination to bolt the jets to the wings?
The jury is still out on the HondaJet, it's competing in a very crowded marketplace so if it doesn't sell well that may not have anything to do with the engines or where they are.
Join Date: May 2002
Location: UK
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
I would agree with all that has been said so far regards the pros of fitting the engine under the wing against fuselage mounted, and yes wing mounted engines help with wing bending, so resulting in a lighter structure. However surely the wing structure has to be strengthened for when the aircraft is on the ground as it now has support the weight of the engine[s]. So perhaps the weight saving is not so great.
To move the topic a bit off subject: Given the structural and maintenance advantages of wing pylon mounted engines and their one principle disadvantage (longer landing gear) why don't we see more large high wing passenger aircraft?
Think about the advantages of a lower fuselage when designing and marketing one airframe for passenger and freight applications. Or to offer a military cargo variant of a commercial model. Boeing tried and failed to sell its 747 for this app. in part because of the handling equipment that would be needed (and unavailable at some sites) to load and unload it.
Think about the advantages of a lower fuselage when designing and marketing one airframe for passenger and freight applications. Or to offer a military cargo variant of a commercial model. Boeing tried and failed to sell its 747 for this app. in part because of the handling equipment that would be needed (and unavailable at some sites) to load and unload it.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
However surely the wing structure has to be strengthened for when the aircraft is on the ground as it now has support the weight of the engine[s]. So perhaps the weight saving is not so great.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
Also, the wings just have to support their own weight on the ground usually and not carry the whole aircraft around.
Join Date: Jul 2012
Location: in the shadows
Age: 48
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
Of course underwing engines would not fit that, but do the technical arguements previously expressed in favour of underwing still apply to smaller airframes in theory?
The main reason why fuselage or above wing mounted engines are used on smaller jets is the length of the landing gear. Imagine how big the landing gear would have to be with underwing engines, and compare that to the size of the plane. That would not only add weight that would outweigh the benefits of the lighter structure, but would also use a lot of space.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Bending moment
Since the MLG legs are so far inboard, the bending moment they induce on the center section is relatively low compared to flight loads.
Join Date: Jan 2011
Location: Poland
Age: 36
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
Wing mounted engines give a lot of advantages.
- First and foremost they reduce bending moments in the wings root. Alowing for a lighter wingbox or a heavier ZFM.
- Second they are mounted on a pylon quite forward of the wing which acts as a mass balance and helps mitigating high speed flutter. This allows for a less stiff (and thus lighter) wing construction. Ever noticed that wing on those airplanes bends quite a lot upwards while the tail engined airplanes have them almost rock solid?
- Third the fuel system can be smaller
- Fourth they are further from the cabin so less noise
- Fifth they are easier to access
- Sixth they are further apart so a failure of one engine is less dangerous to the other
- Seventh they don't have a wing forward of them so less problems with wing induced turbulence or with ice breaking from it during take-off.
- Eighth they sit below Center of pressure which in turn reduces the amount of downforce the elevator has to create during flight
- Ninth they sit closer to center of gravity which eases the loading process, reduces airplane inertia and reduces size of the elevator.
Soooo why bother with the tail mounted engine design?
Because it gives some operational advantages. (I would say they are very third world like)
- First the whole airframe can sit low, so you don't need stairs. Perfect for airports without any passenger infrastructure.
- Second the engines are mounted high so you don't need a sqeaky clean runway. (even a grass or water runway will do)
- Third the gear struts can be much shorter, so you can make a lighter landing gear (or make it much sturdier for the same mass)
- Fourth the rudder can be smaller along with lighter control lines. Although the vertical fin will still stay big.
Those are all of the advantages I can think of.
- First and foremost they reduce bending moments in the wings root. Alowing for a lighter wingbox or a heavier ZFM.
- Second they are mounted on a pylon quite forward of the wing which acts as a mass balance and helps mitigating high speed flutter. This allows for a less stiff (and thus lighter) wing construction. Ever noticed that wing on those airplanes bends quite a lot upwards while the tail engined airplanes have them almost rock solid?
- Third the fuel system can be smaller
- Fourth they are further from the cabin so less noise
- Fifth they are easier to access
- Sixth they are further apart so a failure of one engine is less dangerous to the other
- Seventh they don't have a wing forward of them so less problems with wing induced turbulence or with ice breaking from it during take-off.
- Eighth they sit below Center of pressure which in turn reduces the amount of downforce the elevator has to create during flight
- Ninth they sit closer to center of gravity which eases the loading process, reduces airplane inertia and reduces size of the elevator.
Soooo why bother with the tail mounted engine design?
Because it gives some operational advantages. (I would say they are very third world like)
- First the whole airframe can sit low, so you don't need stairs. Perfect for airports without any passenger infrastructure.
- Second the engines are mounted high so you don't need a sqeaky clean runway. (even a grass or water runway will do)
- Third the gear struts can be much shorter, so you can make a lighter landing gear (or make it much sturdier for the same mass)
- Fourth the rudder can be smaller along with lighter control lines. Although the vertical fin will still stay big.
Those are all of the advantages I can think of.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
Since the MLG legs are so far inboard, the bending moment they induce on the center section is relatively low compared to flight loads.
Seems to me a high-wing design could give you the benefits of wing-mounted engines while keeping the fuselage close to the ground. But no doubt introduce its own disadvantages.
Moderator
Some high wing problems -
(a) maintainer access
(b) wing carrythrough structure - not a popular marketing consideration
As with every design consideration, compromise outcomes are the name of the game ..
(a) maintainer access
(b) wing carrythrough structure - not a popular marketing consideration
As with every design consideration, compromise outcomes are the name of the game ..
Popular with smaller jets (think bizjets) because the structural issues aren't so great with smaller engines and fuel savings not as large an issue in bizjets ops compared with airlines. Also putting the engines at the back leaves a clean wing with more of the leading and trailing edges available for high lift devices - which is a big issue for bizjets because they often operate into smaller airports.
The philosophy of putting the VC10's engines at the back was to leave the whole wing available for the flaps and slats. The aircraft was designed to operate into BOAC ''Empire route'' airfields such as short, high and hot Nairobi. The VC10 had approach speeds some 20 knots slower that the B707. Boeing got round this by getting the US government to fund longer runways at the places thier potential customers would need to operate. This made the VC10 look heavy and expensive which is one of the reasons why it wasn't as successful as the 707.
The philosophy of putting the VC10's engines at the back was to leave the whole wing available for the flaps and slats. The aircraft was designed to operate into BOAC ''Empire route'' airfields such as short, high and hot Nairobi. The VC10 had approach speeds some 20 knots slower that the B707. Boeing got round this by getting the US government to fund longer runways at the places thier potential customers would need to operate. This made the VC10 look heavy and expensive which is one of the reasons why it wasn't as successful as the 707.
Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes
on
0 Posts
Having the weight of the engines being supported by the wing, not the wing root makes sense. Any engine weight put on the fuselage would require stronger wing to fuselage strength.
Join Date: Feb 2006
Location: Hostage to geographical fortune.
Posts: 89
Likes: 0
Received 0 Likes
on
0 Posts
As I look at the new A350/B787 developments I can't help but wonder why they don't stick the engines in the back a la DC-9/CRJ? Benefits would be:
1. Lesser noise in cabin.
2. You can have bigger fans, which equals better economy without risking having them scrape on the ground and/or have to make tall landing gears.
3. Better view for passengers.
4. Lesser structure to support engines, thus lighter.
5. Less asymmetrical thrust problems.
Why is this design getting abandoned?
1. Lesser noise in cabin.
2. You can have bigger fans, which equals better economy without risking having them scrape on the ground and/or have to make tall landing gears.
3. Better view for passengers.
4. Lesser structure to support engines, thus lighter.
5. Less asymmetrical thrust problems.
Why is this design getting abandoned?
In other words: an over-sized rudder for engine out controllability looses out to a smaller rudder for reduced specific fuel consumption.
From the perspective of asymetrical thrust handling, tail mounted engines have no advantage because the designer has built to aerodynamic specs which are good enough with anything better being detrimental to economic performance.
I've flown both the DC9 and DC8 - on a V1 cut the bootfull of rudder required to keep said aircraft going straight with the blue side up is about the same. Different engine configuration but a different size of rudder so the amount of applied boot is rendered equal.
That said High Wing designs might become a serious design consideration with ducted fans or very high bybass GTF somewhere down the road...
Last edited by henra; 1st Aug 2012 at 20:26.
