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A turbo fan engine cannot use all of its available thrust in cruise any longer that it normally does at today's technologies, otherwise it would risk a meltdown of the internal hot engine parts and or operate inefficiently because the fan blades would be above supersonic speed. It however needs its full thrust for take off.

The Extra disused thrust however represents extra engine weight relative to the total weight of the engine i.e weight whose thrust does not contribute to the engine at cruise. That weight is weight that the aircraft has to carry around for the thousands of hours of flying the aircraft throughout and through its entire lifespan, representing a loss in total efficiency.

Given the fans in a modern bypass turbofan engine produces 54% of the thrust Could the aircraft be a lot more efficient if an extra fan was added to the side of the engine though with no engine, to be or powered by the disused torque of the engine at cruise. I know there are problems with adding gears or a prop shaft to spin the extra fan for some engines, particularly large engines but some engine designs can accommodate gearing. The extra fan would then provide thrust that mitigates the relative dis used weight

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    $\begingroup$ If an engine was as simple as just a fan (no combustion chamber, no cowling, no plumbing, ...), then why have all the extra cruft? I get the distinct feeling that you're missing something in your analysis which results in a major error in your final sentence conclusion. $\endgroup$ – a CVn Apr 10 '18 at 14:11
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    $\begingroup$ You have a key misconception in the premise of the question. Full thrust isn't used at cruising altitude not because it would damage the engine, but because you don't want to overspeed the aircraft. Remember your "what happens if you apply full thrust for five minutes"-question? $\endgroup$ – Sanchises Apr 10 '18 at 16:55
  • $\begingroup$ If you appy full thrust at cruising altitude for a long time, the fan blades will melt $\endgroup$ – securitydude5 Apr 10 '18 at 17:18
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    $\begingroup$ no, that's wrong, fan blades don't melt. and if they do, then you have other more pressing problems. $\endgroup$ – Federico Apr 10 '18 at 17:26
  • $\begingroup$ read the answer to this question aviation.stackexchange.com/questions/45455/… $\endgroup$ – securitydude5 Apr 10 '18 at 17:37
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Having a fan with no engine to the rear of the combustion chamber is quite simply the best method... of reducing the efficiency and thrust output of the engine. As a whole the addition of a source of drag without adding more energy to the system reduces power output and you might as well add some bricks to the intake. I have a feeling you are grasping at the idea of increasing bypass, but then if you want a thorough answer I would recommend a focused question such as Why is the bypass ratio of a turboprop higher than turbofan?

I recommend you do some additional study into the concepts of turbofan engines before making a question again, but if you have a conceptual issue then feel free to ask; this is the place to learn all about subjects such as this.

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