f.pier
Posts: 1405
Joined: Fri Aug 25, 2000 2:50 am

Position Of Engines

Tue Jan 06, 2004 11:56 pm

The emergency landing of an Austrian Fokker 70 showed that the rear mounted engines are much safer in case of an emergency belly landing.

Why the most of new airliners have wing mounted engines?

Which are advantaged and disadvantages of the solution "rear mounted" and "wing mounted"?
 
traveler_7
Posts: 448
Joined: Tue May 23, 2000 12:45 am

RE: Position Of Engines

Wed Jan 07, 2004 1:32 am


It was discussed a couple of times
for example here

http://www.airliners.net/discussions/general_aviation/read.main/1270363/4/

some time ago I posted a question but it did not attract much attention  Crying

http://www.airliners.net/discussions/general_aviation/read.main/765412/6/

In spite of all advantages of wing engened aircrafts listed in the recent discussion I still think (hope ) that there is still place for rear engened aircraft.
Perhaps one with deeper knowledge of aerodynamics and aircraft engineering could list all advantages of rear engend construction.



 
Trident
Posts: 477
Joined: Sun Feb 27, 2000 4:49 am

RE: Position Of Engines

Wed Jan 07, 2004 1:44 am

Rear engines have some advantages but not that many.

Advantages -

Quieter cabin for passengers.
A clean and, theoretically, a more aerodynamic and efficient wing.

Disadvantages -
Access is more difficult for maintenance.
A fin mounted T-Tail arrangement is usually required which which needs a stronger (and heavier) tailfin.
More danger of foreign body ingestion from debris being thrown up from mainwheels.
Although the wing is "clean", it also needs to be stiffer (therfore heavier) to counteract upward bending in flight. Wing mounted engines are used to help prevent this wing bending tendency without the need for stiff, heavier wing spars.

 
sovietjet
Posts: 2547
Joined: Sat Mar 15, 2003 12:32 am

RE: Position Of Engines

Wed Jan 07, 2004 5:29 am

I believe less FOD danger is actually and advantage of tail mounted engines.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Position Of Engines

Wed Jan 07, 2004 5:37 am

All designs are a bundle of compromises.

I've seen video of the nosewheels throwing spray (and presumably small debris) right into the wing-mounted engines.

I have also known of shredded tire caps winding up in the intakes on tail-mounted engines. The 727 had to have the No.2 VHF antenna anti-iced with bleed air because of its position in relation to the No. 2 engine intake.

I've seen uncontained disk ruptures so violent that they would have taken the wing off if the engines had been buried in the wing like the Comet.

One consideration (can't really call it a disadvantage - not enough info) for tail mounted engines is that the fuel lines must be routed through the fuselage (if not actually within the pressure vessel) from wing tanks to the engines. Likewise, hot bleed air for wing anti-ice purposes.

A bundle of compromises.


Happiness is not seeing another trite Ste. Maarten photo all week long.
 
MITaero
Posts: 485
Joined: Mon Jul 28, 2003 8:00 am

RE: Position Of Engines

Wed Jan 07, 2004 5:43 am

Yeah, there are a bunch of compromises. I just wanted to mention that wing-mounted engines help cancel some of the bending moment created by lift, and this can help the wing structurally, allowing more aerodynamically efficient wings, i.e. higher aspect ratio.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Position Of Engines

Wed Jan 07, 2004 7:41 pm

I e, the fuselage of the aircraft is essentially standing on the wings in flight. Take a metal tube, suspend it in the middle. Now, how much structural weight do you have to add to allow you to mount two heavy engines in one end of the tube? It is better to have the weight suspended where the lift is generated. That way, the load transfer paths are shorter.

Add plumbing to get fuel to the engines and the risk of disturbed airflow and ice off the wings to the other points mentioned.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
paulc
Posts: 1440
Joined: Mon Mar 05, 2001 10:42 pm

RE: Position Of Engines

Wed Jan 07, 2004 11:38 pm

Don't forget the deep stall - a potential problem of any t-tail / rear mounted engine aircraft regardless of manufacturer
English First, British Second, european Never!
 
KGAI
Posts: 118
Joined: Tue Dec 23, 2003 5:35 am

RE: Position Of Engines

Thu Jan 08, 2004 5:47 am

Seems easier to service wing mounted engines too.
 
POSITIVE RATE
Posts: 2121
Joined: Sun Sep 16, 2001 11:31 am

RE: Position Of Engines

Thu Jan 08, 2004 11:37 am

Don't forget the deep stall - a potential problem of any t-tail / rear mounted engine aircraft regardless of manufacturer

That's really not that big a problem since all T-tailed jets are equipped with stick pushers. Seriously how many T-tailed jets have been sestroyed by deep stalls recently? The answer is none.
I believe the advantages of rear-mounted engines outweigh the disadvantages.
*quieter cabin
*more aerodynamically efficeint wing
*less chance of Foreign object ingestion by the engines
*ability to do powered pushbacks safely
*plus it looks way cooler Smile
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Position Of Engines

Thu Jan 08, 2004 7:28 pm

Weight, efficiency, weight, weight, efficiency, weight.

BTW, having underslung pylons don't significantly detract from the efficiency of the wing. Junk on top of the wing is bad, junk under it doesn't matter nearly as much.

Is the cabin quieter with rear-mounted engines? Perhaps if you are sitting up front...

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Kay
Posts: 1797
Joined: Fri Mar 01, 2002 3:41 pm

RE: Position Of Engines

Fri Jan 16, 2004 6:47 pm

One thing seems rather sure. Rear mounted engines are safer.

However, people will buy tickets even if planes have wing-mounted engines. If people were afraid to buy tickets because of this or because of the bi-monthly or so crashes that occur around the world, then the whole industry would have been revamped for a higher level of safety. Unfortunately, people don't need more and are happy with it the way it is (since they are still buying, and they are satisfied by the statistics, compared to other things they do, like riding a motorcycle or driving a car), therefore no investment will be made for improvement.

The wide use of wing mounted engines over tail engines is another demonstration of this phenomena (at least it explains it partially).

Rentability over technology/safety.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Position Of Engines

Fri Jan 16, 2004 7:40 pm

Well, looking at the cases where fatalities have been caused by having the engines rear-mounted (ice ingestion etc) vs those where the cause has been wing-mounted engines (Ethiopian ditching), especially in the light of the flight hour produced by either design, I'm not so sure that rear-mounted is safer at all.

Perhaps in a controlled crash... but it is always better to avoid the crash as you're basically screwed anyway once you're aimed for that piece of terrain, statistically speaking.

And, like it or not, safety and economy do have something to do with each other. How much more are people willing to pay for any perceived safety benefits?

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Kay
Posts: 1797
Joined: Fri Mar 01, 2002 3:41 pm

RE: Position Of Engines

Fri Jan 16, 2004 9:03 pm

Rear mounted engines are safer only (or mostly) when crash landing. If crash landings don't have a significant contribution to the statistics (them being in most cases doomed, or constitute a small percentage of accidents), then I guess rear-engined planes aren't safer in the absolute sense.

But the rest of my post still stands, as, I guess, you've agreed.

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