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Power/weight Ration On Different Aircraft  
User currently offlineMegatop From Denmark, joined May 1999, 350 posts, RR: 0
Posted (13 years 8 months 1 week 1 day 5 hours ago) and read 4754 times:

There was no reply on the Civil forum, there for I try the Tech forum:

Here on this forum, there has always been talked about what a "rocket" the B757 is, and that the A340 was fitted with 4 APS's.

Here are some facts. Data is from Boeing.com and Airbus.com.

col 1: Type of aircraft
col 2: Lb trust one engine
col 3: No of engines
col 4: Total lb trust
col 5: MTO in kilos
col 6: Every lb of trust must lift x kilos
col 7: Total lb trust with one engine out.
col 8: Every lb of trust mus lift x kilos with one engine out.

B737-900 .......27.300.....2......54.600......78.240.....1,43.....27.300...2,87
B757-200 .......40.200.....2......80.400.....115.680....1,44.....40.200...2,88

This shows, that the A300 and A310 has a bette power/weight ration than the B757. And with one engine out, even the A340-300 is more powerfull than the B757.

This agian shows, that power/weight ration, is not the only thing to talk about, when judging a plane.


5 replies: All unread, jump to last
User currently offlineCovert From Ghana, joined Oct 2001, 1586 posts, RR: 2
Reply 1, posted (13 years 8 months 1 week 20 hours ago) and read 4718 times:

The reason why you did not elicit a response, is because the above matter is a technical topic.
The aircraft are not proportional to each other in terms of size and weight, and airframe design and wing design differ. This can cause various operational differences between airframes. You can't say a bus can outaccelerate a lotus just because the bus engine has ten times more cylinder volume. The bus is heavier and looks like a box, while the small sports car may have a little engine that even seems too small for its size, but the car has many streamlined aerodynamic properties that present very little drag brought about structural design which enable the little engine to shoot the car 0-100 in 3 seconds.
I don't think that just the fact that the A340 has more power with one engine out means that it could take a mountain faster than the 757. They aren't the same size, nor do they have the same wing.
Besides thrust to weight ratio is the proportion of engine thrust to MTOW, power to weight means different.

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 2, posted (13 years 8 months 5 days 11 hours ago) and read 4678 times:

Dear Megatop -
Airplanes with 2 engines will have (generally) better power/weight ratio than airplanes having 3 and 4 engines...
Airplanes with 3 engines will have (generally) better power/weight ratio than airplanes with 4 engines...
This just because a "2 engine airplane" one engine failure during takeoff as consequence of a loss of 50% of power available...
In a "3 engine airplane" the loss of that one engine is 33.3% power loss...
In a "4 engine airplane" the loss is 25%...
Generally speaking, 2 engine airplanes have outstanding initial climb capability and performance, with a "normal" all engine takeoff...
Other consideration... you are in one of those 767 or 777 "2 engine wonders", and you just lost one of them close to ETP equal time point, over the ocean...
and the pilot says, "Folks, we are turning back to... San Francisco, due to some problems" (... with the toilets flush system...), or you fly an oldie but goodie 747, one engine goes bad, same place, but continue to destination, perfectly legal in 4 engine planes... my only concern is to find an excuse why we will arrive 1 hour later at destination...
To tell you the truth, 4 engines is an "overkill" (the more engines, the more potential troubles), 2 engines is... "nothing much left" when one fails, and you drag yourself at 10,000 feet above the waves... the BEST is, 3 engines, as in DC-10, MD-11 or L-1011s... is a "good trade", on long oceanic flights...
Megatop, you dont have the answer to "all numbers" with your table...
As an example, there are 747 classics, with 4 x 46,000 lbs thrust... total is 184,000 lbs of thrust...
Then there are 747 classics with 4 x 56,000 lbs thrust, total 224,000 lbs...
Almost like an extra engine if we take off at same weight...
Friend, there is, the "little picture" and the "big picture" in all considerations...
 Wink/being sarcastic
(s) Skipper

User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 5
Reply 3, posted (13 years 8 months 5 days ago) and read 4666 times:


I do perfectly agree that for long oceanic flights, three engines would be the optimal choice (less than four, cheaper to operate and less fuel consumption, but you can still continue to your destination usually), however from an engineering point of view, three engines is not a nice choice.

With two or four engines, you can mount them on the wings, under the wings, on the tail, even two on the tail and two under the wing is a feasible configuration, but with three engines, you always have a 'bastard' engine you do not really know where to put. Of course, you can put it on the V-Stab, but maintenance people really dislike that configuration, engine #2 exchange on a DC-10 is supposed to be hell. And if you put it IN the tail? well, you have to redirect the airflow into the engine, which is not nice, it decreases performace, and increases vibration. Also, when designing a plane (not an engine), an uncontained engine failure should be taken into consideration, and we all know of UA232, don't we?

However, if you have a blended body design, mounting three engine on top of the wing would be a very good configuration, and indeed, several studies show this engine setup.

Happy Landings,

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 4, posted (13 years 8 months 4 days 13 hours ago) and read 4656 times:

Dear SailorOrion -
Fully agree with your engineering remarks about tri-motors...
Where in hell do we put that "odd" engine...
With my 4 motors on a 747, doubles the chances of a bird "visiting" it... compared to a twin... then some guys in my last classroom started to say that the total "surface" of the engine inlet(s) will indeed invite more birds...
I once "swallowed" a BIG (meaning it) bird in my #4 engine in Lagos, Nigeria, causing engine failure, engine fire warning and... severe vibrations when we exceeded 200-250 KIAS speed to get flaps retraction, all this due to the fact that many fan blades failed (out of balance) and engine pod shook bad, as well as the outer part of the wing, forcing us to dump some fuel, then elect to land again despite my total attraction for a few nights in that part of the world, the plane could not be flown at "normal engine out speeds" and flaps retracted, so what can I say...
I suppose my name is on the list of bird lovers as a criminal... Trust me, I love beef and hate chicken (chicken meals are for first officers)...
Technically what I would see, for 3 engines, is a configuration like Concorde or TU-144, with 3 engines under the belly-wing area, maybe... And no matter how I trust technology, ETOPS is still "engines thrusting, or people swimming" and I know one day, one day, one day... a twin will ditch in the ocean... That Canadian Airbus almost had to do it, if they weren't lucky enough to be near the Azores... God forbid...
I dont feel "superior" to be in 747s, and I am jealous at DC-10 and L-1011 flight crews, they got a nicer, wider flight deck than we have in the "whale", and yes, again fully agree with you about the Sioux City DC-10... that location of the odd engine... quite a critical area...
As I write this, I remember having an engine failure in a 727-200, we took off and a main wheel tire, number 3, blew up at rotation speed, part of the tire entered in #3 engine, hit the fan, stopped the engine at once, and the whole engine pod fell off the aircraft... we did not know it sheared off the aircraft, we thought it was a regular engine failure... we reported to the tower that we had "lost #3", requesting to come back for immediate landing, the tower answered, "you - lost - the engine, correct... the engine is on the runway"...
Unrelated story, but thought I would pass that one along here...
 Wink/being sarcastic
(s) Skipper

User currently offlineFlight152 From United States of America, joined Nov 2000, 3508 posts, RR: 6
Reply 5, posted (13 years 8 months 4 days 11 hours ago) and read 4643 times:


Just for the correction, comparing 2 numbers like you did is a RATIO, a ration is something much different.

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