View Full Version : Why the end of rear engine-mounted planes?
22nd Aug 2008, 19:13
Obviously a question that has arisen out of the Spanair tragedy, but I was pondering the reason for the disappearance of tail / rear fuselage mounted engines, and the move towards an almost exclusively wing mounted engines on newer planes?
But what was the reason for this shift?
Was it safety? Or due to better fuel consumption? Or more simply the ending of McDonnell-Douglas designed planes by Boeing? I saw a (newspaper) comment that rear-mounted engines 'push' the plane when taking off, whilst wing mounted engines 'pull' them into the air so I presume there are big fundamental differences. Have their ever been differing and competing 'camps' of airplane design that fundamentally favoured one design over the other?
I'm presuming it's a simple answer, but i'm unsure! Ta for your time!
22nd Aug 2008, 19:20
I'm no engineer mate, but, to me it is, as you suggest, a question of "Push-Vs-Pull"
Modern cars are mostly front wheel drive, the stabilty is far better.
I suspect that engines on the wing, mid-body, are far better balanced than rear end placed jobs, pushing the ar$e end around.
Might be wrong though!:\
22nd Aug 2008, 20:24
I always thought it was for maintenance purposes - easier to get at under the wing.
Mad (Flt) Scientist
22nd Aug 2008, 20:37
There's still plenty of rear-engined planes around, and people building new ones - look at the business aircraft sector. The "push versus pull" thing is somewhat bogus. There are engineering reasons to prefer either design, and the specifics of a given aircraft design requirement will dictate - along with, to be frank, corporate history and accumulated knowledge - the specific configuration.
22nd Aug 2008, 21:10
Put the engines on the wings and you get load bearing relief........so you don't have to make the wings as strong ( heavy ) as with tail mounted engines.
Just a spotter
23rd Aug 2008, 20:18
Right, first a caveat, I'm no expert! But I seem to recall seeing a TV programme suggesting it had something to do with airflow over a high T-Tail design and problems that design had in a stall.
24th Aug 2008, 13:56
Yes T-tails can go into a deep, unrecoverable stall.
In an engine failure the rear mounted engines produce less yaw due to them being closer to the thrust line, and in theory should be easier to control.
24th Aug 2008, 15:45
The bit about "push vs. pull" (relative stability) sounds like a journo's "theory" - with no basis in engineering fact. The important thing in engine-out handling is the lateral arm (engine c/l to fuselage c/l) compared to the size and arm of the vertical tail. :ok:
24th Aug 2008, 16:51
As has been mentioned hanging the engines on the wing allows a designer to build lighter wings as the engines them selves provide wing bending relief.
If you go back to the late 50s and early 60s when ac like the VC10 and the 707 were being designed you can see the divergence in design. Boeing went for simplicity and a design type that was well understood having built the B47/B52 ac with engines on the wings. Vickers wanted an ac that had a clean wing so they could add high lift devices so that the ac could be flown from hot/high airfields with short runways that the 707 could not use.
The trouble was that by not having the engines on the wing the wing had to be stronger and stiffer and thus heavier than the 707... likewise, the engine mounting at the rear of the ac was heavy as well, thus increasing the cost/mile for each passenger.
There are some great advantages to putting the engines at the back... the cabin is quieter, and if an engine fails it is not so much of a drama as the asymmetric effect is not so bad as the engines are close to the centre-line.
The question of super-stall is one that often gets asked, and all T-tailed ac have the possibility of entering one if miss handled... remember, not all T-tailed ac have rear-mounted engines (eg C-17, BAe 146 etc)...
As for pushing or pulling an ac into the air... well, I kinda see what they may be thinking, but it is all down to where the thrust acts upon the ac and I am not sure that is really the best way to describe it.
The fashion has certainly gone away from rear-mounted engines, particularly in med-long haul, though there are plenty of short haul types around...
Sadly I don't think we will see an ac like the VC10 ever built again, and no-matter who won on the cost front the VC10 beat the 707 and its like in the beauty stakes hands down!!!! (not that I am biased)
24th Aug 2008, 18:13
It is my understanding, if I'm wrong I'm sure someone will correct me, that you are correct in saying that a VC-10 style aircraft will not be designed to built again, is because of the four engines mounted on the tail. The FAA will not issue an airworthiness certificate to any aircraft with two engines mounted side by side due to fear of un-contained massive engine failure taking out two engines at the same time. Therefore, other than I think some Russian designed aircraft, the only aircraft with four engines tail mounted are the VC-10 and the Lockheed Jetstar. An aircraft designed like the Comet would have the same problem.
Military aircraft of course are exempt.
24th Aug 2008, 19:05
Ah, the VC10! I fondly remember my first flight on one, in 1966. What a magnificent airliner! I seem to recall that another factor might be that rear engined airliners can have less usable space towards the back of the cabin. I don't know where/when I came across this idea. And what about centre-of-gravity considerations? Are these relevant?
25th Aug 2008, 00:27
Some of the advantages and problems associated with tail-mounted engines have been listed here above by some of you. Not everything is against the tail-mounted engines.
There are advantages to have rear-mounted engines, such as reduced asymmetric thrust, and wing profiles favoring better aerodynamics, and noise levels being lower in the cabin... Despite wing-mounted engines, the 747 engines still require a tall ladder for oil servicing. So... maybe only the 737 and A-320 have convenient engine location for line maintenance.
While tail-mounted engines benefit less damage on airports with foreign object ingestion, it could be the opposite in flight. When I flew the 727 (or the Learjets), we could not consider "deicing" the wing in flight after ice accumulation. It had to be "anti-icing" prior reaching icing conditions, in fear of engines becoming ice blenders for your next "Margarita" or "Caipirinha" cocktails... Was so critical on the 727, that the VHF antenna (a "blade antenna") mounted on top of the fuselage was anti iced, in fear of ice ingestion into nš 2 engine.
The other problem is bringing the fuel from wing tanks, to the engines. This requires long pipes (which may leak, therefore they are shrouded), and all this is extra weight. The T-tail associated with rear engines requires extra structural weight of the tail... And bad stall characteristics of T-tail airplanes required "stick pushers" for certification.
You say that tail mounted engines make it easy to control an "engine-out" due to reduced control inputs... not true... In the Lear 24B, VmcG is 108 KIAS and requires full rudder input to continue the takeoff. The "multi-engine rating" obtained in a Learjet is not "center-line thrust only"... But the 727 is rather mild-mannered, and is "center-line-thrust"...
Further, in the 727, I remember that on some landings, the use of reversers from nš 1 and 3 engines often blanked (or disturbed) the air over the rudder surface, making the rudder controls somewhat ineffective, for crosswind corrections. Finally, if an engine blows-up to pieces, I do not feel too good about the fact that it is very close to the tail surfaces...
And to those of you telling us that the VC-10 is a nice looking airplane, we all have our preferences... So, my favorite airliner is the SE-210 Caravelle for the looks.
25th Aug 2008, 01:03
Was so critical on the 727, that the VHF antenna (a "blade antenna") mounted on top of the fuselage was anti iced, in fear of ice ingestion into nš 2 engine.
Wasn't this also true of the DC-10?
And bad stall characteristics of T-tail airplanes required "stick pushers" for certification.
The 727 - and most other T-tails - did not require stickpushers originally per the FAA. It was when the CAA was accepting the type that the pusher was added. But that was after a couple high-sink-rate 727 accidents elsewhere.
26th Aug 2008, 22:22
It will be more of a structural problem
The engines at the back needs additional reinforcement to stop the fuselage from buckling. when the center of gravity is moved back from the additional weight of the reinforcement and the engines the wings come back too leaving the forward fuselage much longer making it necessary for reinforcement adding weight to the airframe.
27th Aug 2008, 01:22
The noise profile is significantly different for pax between wing-mount and rear-fuselage-mount. The latter is a lot quieter.
But generally there are the stall problems of a high-T tail (the blanking effect of the engines on the tailplane in a high AOA), strengthening of the wings (and a leading edge design that smooths out the air flow to the engines and the tail in all configurations) that make it less attractive.
27th Aug 2008, 17:57
Just about all the recent generation of regional jets have rear engines and T-tails, so won't be the end for a long time.
I always felt uncomfortable sitting at the back on the F-100 though, knowing that in the event of an accident, I only had one way out, and that was forward. Not a big fan of sitting at the back when there are no rear e/exits, the overwings being the nearest :eek: