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Why Do Modern Times Call For 2 Or 4 Engines  
User currently offlineTrent_800 From United Kingdom, joined Jan 2002, 136 posts, RR: 0
Posted (14 years 1 month 2 weeks 6 days 21 hours ago) and read 3567 times:

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.

9 replies: All unread, jump to last
User currently offlineBsergonomics From United Kingdom, joined Jan 2002, 462 posts, RR: 0
Reply 1, posted (14 years 1 month 2 weeks 6 days 14 hours ago) and read 3476 times:

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.

The definition of a 'Pessimist': an Optimist with experience...
User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1741 posts, RR: 1
Reply 2, posted (14 years 1 month 2 weeks 6 days 7 hours ago) and read 3418 times:

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?

User currently offlineGanymed From , joined Dec 1969, posts, RR:
Reply 3, posted (14 years 1 month 2 weeks 5 days 6 hours ago) and read 3357 times:

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

User currently offlineBWIrwy4 From United States of America, joined Nov 2001, 940 posts, RR: 1
Reply 4, posted (14 years 1 month 2 weeks 5 days 5 hours ago) and read 3331 times:

And the B-36 had 6 engines. P&W Wasp Majors, which I believe were the largest piston engines produced.

User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1741 posts, RR: 1
Reply 5, posted (14 years 1 month 2 weeks 5 days 1 hour ago) and read 3304 times:

Well, the later B-36s had 4 jets in addition to the 6 recips. That's 10.

User currently offlineTbar220 From United States of America, joined Feb 2000, 7013 posts, RR: 23
Reply 6, posted (14 years 1 month 2 weeks 2 days 19 hours ago) and read 3233 times:

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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User currently offlineSllevin From United States of America, joined Jan 2002, 3376 posts, RR: 5
Reply 7, posted (14 years 1 month 2 weeks 2 days 8 hours ago) and read 3202 times:

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


User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 8, posted (14 years 1 month 2 weeks 2 days ago) and read 3184 times:

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.

User currently offlineNDSchu777 From United States of America, joined Oct 2001, 419 posts, RR: 3
Reply 9, posted (14 years 1 month 2 weeks 7 hours ago) and read 3125 times:

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.


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