simple aerodynamics: thrust, lift, weight, drag

The simple answer: wings and engines.

If enough air moves under a pair of wings, those wings will lift the airplane up. But to get enough air to generate lift, the plane has to move forward fast. Strong engines are what get the aircraft enough speed to stay airborne.

Think of a paper airplane. Throwing the paper airplane simulates what the engines do on a real airplane——it moves the plane forward. That lets your paper airplane’s wings do their job and keep the plane lifted. A paper airplane doesn’t fly long because there isn’t a force that keeps it moving forward, which means there isn’t enough air under the wings to keep the airplane flying.

Also, consider a kite. Regardless of how big it is——how big its “wings” are——it needs wind to fly. A kite can only stay airborne if there’s enough air under it, and this typically comes in the form of a strong breeze or you sprinting across a field while your child yells that you’re slow AF and need to pick up the pace.

aerodynamics, thrust, lift, weight, drag

The flight school answer: lift and thrust.

Equilibrium flight exists when the sum of all forces and the sum of all moments around the center of gravity are equal to zero. An airplane in straight and level flight at a constant velocity is acted upon by four forces: thrust, drag, lift, and weight. When these forces exactly cancel each other out, the aircraft is in equilibrium. Of note, an aircraft does not have to be in straight and level flight to be in equilibrium; thrust must simply overcome drag plus the parallel component of its weight. Lift must overcome the perpendicular component of weight.