# Aircraft lift theory vs energy conservation [closed]

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

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.

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

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• "Spent years" ? How about starting with a couple textbooks on aerodynamics? – Carl Witthoft Dec 16 '15 at 14:59
• 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. – Mike Dunlavey Dec 16 '15 at 21:45
• 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. – Joe Dec 17 '15 at 18:08