Why not tail jets?
 

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?

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.

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.

See also: http://www.pprune.org/tech-log/49110...il-design.html

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.

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

Photos: Lockheed L-1011-385-3 TriStar KC1 (500) Aircraft Pictures | Airliners.net

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...:}

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

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

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

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.

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

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.

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

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.

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.

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.

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.

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:

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

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?

Well, Honda uses the same idea.