Why not tail jets?

[AdamFrisch]

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?

[Dan Winterland]

Some answers to some of your questions.

1. Less noise? I used to fly the VC10. The compressor noise in the rear of the cabin was horrendous.

4. This isn't correct. As wing mounted engines are hung from where the lift is being generated, there is less supporting structure required than if you mopunted them on the fuselage. The loads paths will have to be beefed up to transfer the weight to the wings. The VC10 weighed 7 tonnes more than the equivalant size B707 - most of this was the added structure to support the engines.

[DutchOne]

Try to put two 777 engines on the tail. Firstly, way to heavy. Will need to be compensated for balance and secondly, the structure needed to support such a weight and force generated is insane asfor DANs reasons.

[pattern_is_full]

See also: http://www.pprune.org/tech-log/491109-what-happened-aft-engine-mounted-t-tail-design.html

[AdamFrisch]

I don't buy that. Having an engine closer to the fuselage will require less structural support than hanging it 10ft out on a wing. The moment you have to carry is much bigger the further out it gets.

[Tu.114]

Adam,
consider the bending moment at the wing roots. With tail-mounted engines, the wing roots carry the fuselage, the tail and also the rather heavy engines; consequently the structure has to be rather strong and heavy.

Now if the engines are installed on the wings, the wing roots only have to bear the fuselages and tails weight - the weight of the engines is carried by the wings themselves internally, so the structure can be much lighter.

Additionally, the wings themselves will be lighter with wing-mounted engines. Consider that lift is generated all over their surface and they are hinged to the fuselage only at one side - there is a substantial bending moment involved. Now, one can either slap the engines to the tail and reinforce the wing structure or simply use the weight of the engines. By attaching them as far outboard as possible on the wings, much of that bending moment will be countered. Of course, placing them too far out will cause large yaw in case of an engine failure, requiring a large, heavy and drag-inducing rudder as compensation. Compare the DC-10 with its small vertical stabilizer and #1 and #3 rather close to the fuselage and the L-1011 with the way larger fin and the wing engines further out:

Photos: Boeing MD-10-10F Aircraft Pictures | Airliners.net (http://www.airliners.net/photo/FedEx-Express/Boeing-MD-10-10F/2128930/M/)

Photos: Lockheed L-1011-385-3 TriStar KC1 (500) Aircraft Pictures | Airliners.net (http://www.airliners.net/photo/UK---Air/Lockheed-L-1011-385-3-TriStar/2107315/M/)

[TURIN]

Compare the DC-10 with its small vertical stabilizer

It does have a double acting rudder though to compensate for it's size.


Of course the L1011 had a better engine too so less chance of an assymetric condition developing in the first place. :E:E;)





Head down, incoming...:}

[juliet]

Adam, have a think about wing relief and the secondary role that the engine plays by being mounted on the wing.

[anotheruser]

Having an engine closer to the fuselage will require less structural support than hanging it 10ft out on a wing.You are aware that the wing is lifting the fuselage, not the other way round?

[AdamFrisch]

OK, I suppose you might get slight economics-of-scale structural advantages by hanging them on the wing.

You are aware that the wing is lifting the fuselage, not the other way round?

This is the old "you can lift yourself by pulling your hair upwards"-gag in a new form. The weight the wing has to carry is the total weight of the aircraft, no matter where the engines are and it has to be structurally accounted for. Or are you telling me that if you put fuel in the wings, or engines, that weight doesn't count?

The only reason fuel is carried in the wings, is because it's a good place to hide it. A 1000lbs of fuel in the wing is a 1000lbs of weight, no matter where you put it.

[Owain Glyndwr]

OK, I suppose you might get slight economics-of-scale structural advantages by hanging them on the wing.

Not economics of scale - real structural advantages because the design bending moments are reduced allowing lighter structures.

This is the old "you can lift yourself by pulling your hair upwards"-gag in a new form.The weight the wing has to carry is the total weight of the aircraft, no matter where the engines are and it has to be structurally accounted for. Or are you telling me that if you put fuel in the wings, or engines, that weight doesn't count?

Of course the weight counts, but if you put the engines on the wing or keep the fuel outboard as long as you can the static and fatigue loads are reduced.

[TyroPicard]

a 1000 lbs of fuel in the wing is a 1000 lbs weight

Strictly speaking it's 1000lbs of mass - the weight will vary with g ....

[Intruder]

The only reason fuel is carried in the wings, is because it's a good place to hide it. A 1000lbs of fuel in the wing is a 1000lbs of weight, no matter where you put it.
Maybe you better [re]read Tu.114's post #6. Bending moment is a significant limitation in wing design, which is carried forward as the "Maximum Zero-Fuel Weight" limitation.

[barit1]

The heaviest part of the wing is the wing root, which is designed around the bending moment from carrying the fuselage and all it contains. Look up Zero Fuel Weight - this determines the size of the spar etc. at the wing root.

If the engines are tail-mounted, then their mass must be counted as part of this load. In addition, there is heavier aft fuselage structure, and weight & balance issues to consider.

OTOH, if the engines are hung on the wing, the load on the center section is reduced, all else being equal.

[AerocatS2A]

I don't buy that. Having an engine closer to the fuselage will require less structural support than hanging it 10ft out on a wing. The moment you have to carry is much bigger the further out it gets.

If you put the weight of the engines in line with the fuselage then the weight of the engines plus the fuselage must be born by the wings via the wing root. The wing root is the area of the wing that has the most stress, note how thick it is on an airliner? If you put the engines under the wing then only the weight of the fuselage is carried by the wings via the wing root, the weight of the engines is spread out across the wing itself. This doesn't mean you are somehow getting something for free, it just means the structure of the wing doesn't need to be as strong at the root. It is a more efficient way of utilising the ability of the wings to carry a certain amount of weight.

[Cumulogranite]

just to add a slightly less technical note here, can you imagine trying to carry out the loading plan and weight & balance calculations with around 14 tonnes of jet engine aft of the centre of gravity?? Before taking fuel and passengers into account you will need to have 14 tonnes of freight in the front hold!!! Unless the wings are mounted further aft and that creates its own problems.

[Junkjet]

Can't help but notice that "tail jets" work for all the exec jets from the little Cessna 510 up to the Bombardier Global Express (nee CRJ), they all have rear mounted engines.

:confused:

[DaveReidUK]

Can't help but notice that "tail jets" work for all the exec jets from the little Cessna 510 up to the Bombardier Global Express (nee CRJ), they all have rear mounted engines.

This might give you a clue why Cessna thought that it wouldn't be a good idea to put the engines under the wing:

http://www.abpic.co.uk/images/images/1358547M.jpg

[Junkjet]

Very Funny David.
:O
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?
Was the VFW614 an act of misplaced teutonic determination to bolt the jets to the wings?

[Denti]

Well, Honda uses the same idea.

[Mechta]

The Cumulogranite, you mean this might happen?:

Incidents and Accidents (http://www.vc10.net/History/incidents_and_accidents.html#XR806%20Brize%20Norton%2018%20D ecember%201997)

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:

http://www.enemyforces.net/aircraft/tu22_2.jpg

The engine change crew won't thank the designer though...

[Tu.114]

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:

https://sites.google.com/site/stingraysaeroarchive/_/rsrc/1314236092529/fma-ia36-condor-ii/Condor2_planos%20originales.jpg?height=201&width=400

[Mechta]

Strewth! :eek:

Thanks for posting that Tu-114. Its the sort of thing EE Lightning engine technicians probably saw in their nightmares.

[DaveReidUK]

Was the VFW614 an act of misplaced teutonic determination to bolt the jets to the wings?

The VFW614 was doomed to failure as soon as they selected the M45 to power it, regardless of where the engines were positioned.

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.

[Brit312]

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.

[EEngr]

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.

[Owain Glyndwr]

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.

Except that on the ground the wing just has to support the MTOW. In flight it has to support 3.75*MTOW.

[Tu.114]

Also, the wings just have to support their own weight on the ground usually and not carry the whole aircraft around.

[Owain Glyndwr]

Also, the wings just have to support their own weight on the ground usually and not carry the whole aircraft around.

Well OK, about 5% of the weight is carried by the nose landing gear, and in some cases a bit more by a centreline gear, but if the main gear is mounted on the wings then the rest of the weight is supported by the main gear legs and that bending moment has to be carried by the wing root structure. Still a lot less than 3.75g acting out at 45% semispan though.

[anotheruser]

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? Yes they do, but they are of course smaller in scale.

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.

[barit1]

Since the MLG legs are so far inboard, the bending moment they induce on the center section is relatively low compared to flight loads.

[wlaziu]

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.

[Owain Glyndwr]

Since the MLG legs are so far inboard, the bending moment they induce on the center section is relatively low compared to flight loads.

Sure! Just part of my point really - the wing loads on the ground are far less than those in flight so the structure is sized by the latter.

[Chu Chu]

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.

[john_tullamarine]

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 ..

[Dan Winterland]

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.

[bubbers44]

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.

[cvg2iln]

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?

Ref #5. You'd think so, but it doesn't actually work out that way. It's true that there's less turning moment produced by asymmetrical thrust closer to the centreline but the designer, not missing a trick, will enhance operating economics (less drag) with a commensurate reduction in tail fin/rudder size.

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.

[henra]

Some high wing problems -

(a) maintainer access

(b) wing carrythrough structure - not a popular marketing consideration


(c) MLG requiring additional structure adding to weight&drag and competing with fuselage fairings containing CWT.

That said High Wing designs might become a serious design consideration with ducted fans or very high bybass GTF somewhere down the road...

[barit1]

You mean, like someday... http://www.af.mil/shared/media/photodb/photos/990522-F-5891C-008.jpg

[Mechta]

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?From a fuel systems point of view, in the case of multiple pump failure, gravity feed uses up most of the fuel in the case of a low wing with dihedral and underslung engines, whereas a high wing with anhedral could leave a fair amount of unused fuel in the tips.

In addition, wingtip surge tanks are self draining on a wing with dihedral.

[3 Point]

If you don't buy the concept of spreading the mass laterally to reduce the wing root bending moment consider this ...

Tune in and watch an Olympic gymnast. I bet you could hold your entire body weight by hanging with both hands from one ring directly overhead but I bet you can't do a crucifix. Watch an Olympic champion doing a crucifix and consider the additional structure (muscle) he has in his shoulders to withstand the wing root bending moment!

Works the same way for an aeroplane!

Happy landings!

3 Point