Problem involving the 4 forces that maintain an airplane in level flight The 4 forces affecting an airplane in level flight are gravity, lift, thrust, and drag. By
altering one, the others are affected. My question is could a wing (retractable) be installed on top of a vehicle with a computer controlling the configuration of the wing to 
maintain a constant lift of 50% when speed allows. Reduction in weight results in reduction in thrust. How does the increase in drag on the wing compare with the pros of lift regarding fuel burn. 
 A: A nice idea, but it actually sorts itself out in flight.
As fuel is burned and weight is reduced the aircraft trim is changed to reduce both the lift and the drag of the main wing. The drag reduction reduces the fuel burn.
A: What you propose is an adaptable aircraft which continuously reconfigures itself according to the actual mass and, therefore, lift requirement. Something like this actually exists as a glider: The Akaflieg Stuttgart fs29 (picture source).

fs29 multiple exposure with wing span changing between 13.3 and 19 m.
This makes sense, because by reducing wing area with increasing speed, the airfoil will stay at a similar operating point over a range of speeds.
But if you are not bound to a particular flight altitude, there is a much simpler way to keep the airfoil at the same operating point regardless of lift required. By flying higher and thus in less dense air, airliners do exactly this (mostly over the open sea; over land only as far as air traffic control allows). A long range airliner carries fuel weighing as much as 40% of its take-off mass and adapts to the loss of mass over time by steadily (or stepwise) climbing higher.
