I know that we are taught that all forces are in equilibrium at unaccelerated straight and level flight. lift equals weight and thrust equals drag But I don't understand, I get the lift and weight are equal, but isn't thrust not equal and greater then drag if the airplane is moving forward. That idea doesn't make sense to me. If someone could explain that would be great, maybe i'm interpreting it wrong

Force equals mass times acceleration. In cruise you have no acceleration. So the force that is acting on moving the plane forward has to equal the force that is preventing the plane from accelerating since acceleration is 0. Therefore thrust and drag must be equal.

Classic physics question! And well answered by the two previous posters Think about it this way. Stand on a scale on an escalator. Will the scale show you weighing more? No Even in space, where there is no "drag" (okay, there is some, but let's not get into that) the forces would still "cancel" thanks to Newton's third law - the ship would push back on the engines with equal force that the engines are pushing forward.. however in this case you would accelerate. Or on a parachute with a skydiver, he(or she) hangs down from the chute with the same force as the parachute is pulling up on him(or her)

parasitic drag increases as the airplane accelerates. At some point the thrust available is unable to overcome any further drag and can accelerate no more. At this point thrust and drag are in equilibrium. If you double the airspeed you square the drag.

An object in motion stays in motion at a constant speed unless acted upon by an unbalanced force. That's Newton's first law of motion. If thrust exceeded drag the plane would keep going faster and faster (accelerate). Same way with lift and gravity. If there were, say, 15 pounds less lift than weightâ€”it would fall the same as a bowling ball. With 15 pounds less weight than lift, on the other hand, it would pop up to the top of the atmosphere like a cork from the bottom of a lake. When drag increases enough or the wing makes less lift, the acceleration stops and the speed, rate of climb or descent stay constant at the new equilibrium.

Think about the definition of acceleration...the rate of change of velocity. The change in velocity per unit time. If your airspeed and vertical speed is not changing, then there is no acceleration.

Thanks everyone for the answers, I can wrap my head around it better now. The key I guess was the unaccelerated part. Also it helped to understand the roll of drag.