Re: 757 highest thrust to weight ratio ?

From: (Gregory R. Travis)
Organization: Data Parallel Systems, Inc
Date:         10 Dec 92 16:07:07 PST
References:   1 2 3 4
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Karl makes some excellent points regarding the reasons twins tend to be
overpowered compared to 3 and 4 engined jets.

I would just like to add that FAA certification requires not only that an
airplane be able to finish takeoff, return, and land with AN engine out,
but that it also meet certain climb performance criteria during a
single-engine climb.

Karl is absolutely right that, on a 747 that loses one, the remaining three
need to make up, individually, much less of the lost thrust.  Whereas on
a 757 the remaining engine must make up ALL of the lost thrust.

However, I am confused by Karl's statement that Lufthansa chose the A340
over the A330 because of concerns that the twin-engined A330 would place more
stress on its two engines whereas the A340 would enjoy higher engine
reliability as its four engines loafed along.

Since a twin engine jet is nominally overpowered compared to a four engine
jet, it should be able to operate, on aggregate, at a lower thrust setting
during takeoff or be operated at high thrust for a shorter overall climb.

Since a four-engine jet has all four operating closer to the margin, in
normal operation, the engines should suffer from higher demands all around.

Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated (For MX mailers only!)
Bloomington, IN (For the others)