Engine configuration - why?

[ja203]

I searched the internet and forums before posting this, but couldn't find anything, which surprised me.

I was wondering why the engines on all new aircraft are under the wing, as supposed to at the rear of the fuselage. I would have thought that there were advantages (cabin noise, wing design?), otherwise the MD80 wouldn't have chosen this configuration....

In addition, why has the three-engine idea fallen out of favour? Is it because a twin is more efficient, and new engines can supply enough thrust/reliability to quite safely power an aircraft?

[411A]

Generally speaking, wing mounted engines in pods, provide for a lighter wing structure, than would otherwise be required...they provide wing bending relief.
In addition, fuel, electrical, hydraulic and bleed air installations are somewhat simpler/lighter with wing mounted engines.

Aft mounted engines have advantages of course...one of the main considerations being shorter/lighter landing gear.
Also, the wing design allows for a smoother airflow distribution.

Compromise...the watchword in aircraft design.

[davethelimey]

Must agree with reduced cabin noise in tail-mounted aircraft, unless you're unlucky enough to be in the back five rows.

And not quite all new aircraft either: the 717 is a new plane with rear engines.

[panda-k-bear]

Sorry Dave, I beg to differ. The 717 is a joy to fly in, but a new aircraft it certainly is not. The only reason it exists is because it is a continuation of the DC9/MD80/MD90 line and so used it's heritage. It's a mix of DC9 and MD80 with some other bits and pieces thrown in.

The true new rear engined aircraft are the Embraer 135/145 and (to a lesser extent because of their lineage) the CRJs. Interesting that as soon as it became practical to do so, Embraer moved the engines under the wings in pods a la 170/175/190/195. It seems Boeing got it right first time 50 years ago!

[TURIN]

The Tri-jets (L1011, DC10 etc) were a compromise as at the time legislation did not allow twin engined a/c to fly transatlantic.

Better engine/system reliability and maintenance/operational experience created the ETOPS system.

The rest is history.

Podded, wing mounted engines are a lot easier to maintain and replace. They also tend to be closer to the ground so inspection access is easier and requires less ground support equipment (steps).

[kansasw]

"Generally speaking, wing mounted engines in pods, provide for a lighter wing structure, than would otherwise be required...they provide wing bending relief."

Hello 411, first off I am simple SLF so have no position on this issue except curiosity. I am however technical by profession and not entirely ignorant. Just positioning myself.

That said, it seems to me that wing -mounted engines would require a heavier wing structure than fuselage-mounted engines.

Why is this not the case?

[Notso Fantastic]

It's right- the wing would have to support the weight of the aeroplane and engines and would have a stronger 'snapping upwards' tendency at the wing root. If you mount the engines on the wing, their weight is supported on the wing and not carried at the wing root. Remember you have to design it for several 'g' as well, so the effect is enhanced.

[capt.sparrow]

This is the same reason fuel is preferebly stored in the wings, as bending stresses are reduced - lift is trying to fold the wing up, so adding fuel and engines on/under the wing counter this. Also mounting of the engines forward on pylons 'mass balance' the wing to help prevent flutter.

[Groundloop]

Turin, The DC-10 and TriStar were NOT a compromise because twins were not allowed to fly transatlantic. ALL early customers were domestic airlines eg American, United, TWA, BEA (ie Europe only) etc.

What the airlines wanted was an aircraft smaller than the 747 so it only needed 3 engines instead of 4. 2 engines at the time would not have provided enough power.

[Mud Clubber]

Chaps,

When the engines are at the back they require a large amount of extra structure there to support them. This incurs a massive weight penalty and causes balance problems. It is also difficult to attach a new, decent High By Pass Ratio engine to the back of an airliner. Having them at the back also causes servicing/access problems.

The bending moment relief thing is right. The disadvantages are that the engines suck up more FOD under the wing and they cause problems with assymetry in the single engine case. There is also an issue with the design against catastrophic disc burst of the engine, it being right by the main fuel tank and spar (and passengers).

The overlap between the position of the engines seems to take place around the 80/100 seater mark, with most new aircraft upwards of that favouring the underwing design.

MC

[ja203]

Cheers, that all makes sense. I reckon that if I designed a/c, I'd think of somewhere more interesting to stick 'em - how about putting the passengers outside under the wings, and have a bloody great engine in the centre of the fuselage??? Make it more exciting to travel, at least...

[TURIN]

Groundloop

I stand by my statement as that is what I was told as an apprentice by numerous college/airline instructors.

You may well be right. :ok:

Back to the subject

Shame they didn't pursue the big fan idea on a VC10 layout.
Can't remember the name BAC 3-11 or something.

From a pax point of view the rear engine design is ideal. Very quiet and, in my experience smooth flight. However from a maint point rear engines are a pain (in the rear)!

ja203

It worked for Richard Noble in Thrust2!:\\