Trent_800
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Why Do Modern Times Call For 2 Or 4 Engines

Fri Apr 12, 2002 2:58 pm

Is it me or has the 3 engine design died a death? Is there any good reason why aircraft companys choose 2 or 4 engines and never 3 for new aircraft.
 
bsergonomics
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Fri Apr 12, 2002 9:16 pm

Four words: Weight, Balance, Commonality and Maintenance. Oh, and aerodynamics.

The three engine design essentially means one engine at the nose or tail end of the fuselage. This causes stress on the fuselage (thus reducing lifespan) in a manner that it different to mounting the engines symmetrically on the wings.

You want all engines on the a/c to be the same, in order to minimise spares, crew training etc. When fuselage internal volume is at a premium, locating an engine in there as well is undesirable.

A fuselage-mounted engine is more difficult to access than one slung under the wing. This increases maintenance costs and turnaround times. This is a critical factor.

An engine (ideally) wants a "clean" airflow to suck in and push out. Therefore, (ideally), a fuselage-mounted engine would have air intakes at the front (or high) and exhaust at the rear, with the engine somewhere in between. This has structural, weight and accessibility implications.

Finally, why use three engines when you can use two? And if you need a larger MTOW, longer range etc., why use three engines when four will do the trick, at lower power per engine (thus increasing engine life), using variations on existing wing/fuselage designs (rather than new developments).

Not all of these arguments are applicable to all a/c and situations, but a combination of them means that the 2/4 engines, symmetrically mounted on the wings, is the preferred option.
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ThirtyEcho
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Sat Apr 13, 2002 4:43 am

Personally, I am partial to the Dornier DO X with ten engines. Or was it 15? It was the precursor to the B-36, which also had 10 engines or so. ETOPS, who needs it?
 
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Sun Apr 14, 2002 5:36 am


The DO X had 12 engines (Curtiss Conqueror rated each at 600 hp),with 6 of the props being pushers.

 
BWIrwy4
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Sun Apr 14, 2002 7:11 am

And the B-36 had 6 engines. P&W Wasp Majors, which I believe were the largest piston engines produced.
 
ThirtyEcho
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Sun Apr 14, 2002 10:24 am

Well, the later B-36s had 4 jets in addition to the 6 recips. That's 10.
 
tbar220
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Tue Apr 16, 2002 4:31 pm

Aerodynamically, the L-1011 "looks" like its quite an aerodynamic aircraft. Is this true? And if it is, why can't other airplanes be designed to be aerodynamic with three engines?
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sllevin
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Wed Apr 17, 2002 4:03 am

B-36's were commonly referred to as having "six turning and four burning."  Smile

Steve
 
airplay
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Wed Apr 17, 2002 11:25 am

Over the years the requirements for trans-oceanic flight have evolved. 3 and 4 engine aircraft were developed for certain longer routes for safety reasons. Obviously, if a pre-determined engine failure probabilty was too low, adding engines would reduce the probability that the airplane could not make it's destination.

Some Newer modern twin engine aircraft can now be eligible for ETOPS certification (Extended Range Twin Engine Operations). This enables an operator to bypass regulations requiring 3 or 4 engines only if they pass the eligibility criteria. Not only does the aircraft need to conform, but the operations and maintenance procedures of the airline are put under scrutiny.

 
NDSchu777
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RE: Why Do Modern Times Call For 2 Or 4 Engines

Fri Apr 19, 2002 5:00 am

and i believe that tail-mounted engines, like on the DC-10 or L1011, are pretty expensive to design and build, plus they making maintenance more difficult since unlike having 2 or four similar wing-mounted engines, your trijet has two wing mounted (or in the case of the 727, rear fusalage-mounted) engines, with the tail-mounted engine different than the other two requiring some different maintanance.

--Nick

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