THE 4 FORCES OF FLIGHT
As a plane goes faster, it's lift increases. As a plane moves forward, it's lift force increases until it equals it's weight. In level flight, lift equals weight as the plane flies at constant velocity.
Weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity. Excess weight requires more lift, and therefore more thrust. Planes with less weight require less thrust.
Thrust opposes drag. The engine creates thrust and moves the plane forward. The engines push air back with the same force that the air moves the plane forward. When the plane flies level at constant velocity, thrust equals drag.
Imagine sticking your hand out the window of a moving car and flying your hand. As your hand pushes on the wind, the wind also pushes against your hand. When the engines of a plane quit, drag slows the plane down