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I spent years trying to find out that when an aircraft flies or anything works it consumes energy more than it delivers back, means no perpetual motion exists. My query is that when an aircraft is at a constant speed then thrust=drag and lift=weight but why is there no relation ever found b/w thrust and weight, for 747-8

(https://en.wikipedia.org/wiki/Boeing_747-8)

thrust 296 KN is lesser than the max takeoff 448 tons weight hence the thrust to weight ratio is too less.

so isn't the theory of energy conservation being defied.

for an object to be sustained at a height on a pulley system the other side wieght has to be the same.

Please give me a good response thanks in advance.

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closed as off-topic by Carl Witthoft, Gert, Sebastian Riese, user36790, Frédéric Grosshans Dec 17 '15 at 10:41

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    $\begingroup$ "Spent years" ? How about starting with a couple textbooks on aerodynamics? $\endgroup$ – Carl Witthoft Dec 16 '15 at 14:59
  • $\begingroup$ Drag is a function mainly of speed and angle of attack, and other variables. I can't recommend a better source of information than this. Also, the 747 is very efficient aerodynamically, as are all jet transports. This can be seen in their glide ratio, the angle of their descent under no power. A typical glide ratio for a jet transport is around 25:1, while for a Cessna 172 it is only around 9:1, in a clean configuration. If landing gear, flaps, or spoilers are out, it is a lot less. $\endgroup$ – Mike Dunlavey Dec 16 '15 at 21:45
  • $\begingroup$ Thanks for the link. But consider a helicopter and an aircraft, for helicopter weight=lift made by the engine thrust and for aircraft engine thrust is lesser than aircraft's weight which we call as thrust to lift ratio hence thrust force lesser than lift force. All cases for constant altitude. so for the aircraft isn't input lesser than output. $\endgroup$ – Joe Dec 17 '15 at 18:08
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Nothing in this world would give you anything more than what you gave to it.

The 296KN thrust produced by the jet engine is a huge force and your argument that the 448 tons of the takeoff load is not the same is not correct. Force is something else and mass is something completely different.

You forgot to take into account the pressure lift produced by the aircraft wings. probably the wing span would be so large that it is more than enough to lift the aircraft. The aircraft is not at rest in the air it is in motion.

It is even more important to know that the aircraft jet engines do not provide the lift, they just give the forward force for motion. The air that passes on the wings at that high speeds give the lift.

And according to the second law of thermodynamics you can not make a system that will give you more than 100% efficiency.

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  • $\begingroup$ well look mass has a force and work done =F.D and so is potential energy as (mg).D hence, F=mg so there is a work relation with mass. Hence as mass has a force we are good to compare it with lift. consider a helicopter and an aircraft, for helicopter weight=lift made by the engine thrust and for aircraft engine thrust is lesser than aircraft's weight which we call as thrust to lift ratio hence thrust force lesser than lift force. All cases for constant altitude. so for the aircraft isn't input lesser than the output. $\endgroup$ – Joe Dec 17 '15 at 18:11

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