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There’s a whole lot of science behind what makes an aircraft fly, but it turns out you can break down the primary forces acting on an aircraft into four types: lift, weight, thrust, and drag.

To keep your aircraft safely airborne, you must have all of these forces in balance. If you are flying a straight and level path, keeping you that way requires that the lift you generate be equal to the weight you’re carrying, and the drag produced by moving through the air must be equal to the thrust generated to propel you through it.

Change any of these factors, and the trajectory of your aircraft will change. If lift is greater than your weight, the plane shifts upward. If thrust is greater than drag, the aircraft will speed up, and subsequently, if drag is greater than the thrust, you’ll slow down.

Lift

Lift is the force that counteracts the weight of an object in order to keep it in the air. As the aircraft goes faster, lift increases until its force is equal to weight. Once this equilibrium is reached – the aircraft is able to leave the earth and head off into the sky.

Weight

To keep an aircraft flying level requires both lift and weight to reach balance. The more weight an aircraft has, the more lift is needed, the more thrust must be generated to produce lift. This is why heavier aircraft require more power and longer runways in order to take flight.

Thrust

This force is generated by the aircraft’s propeller, powered by the aircraft engine, and it’s what’s acting against the drag generated by the aircraft pushing through the air. This moves the aircraft forward. It’s the job of the propeller to push air back with equal force, keeping your aircraft balanced in the air.

Drag

This is the force that works in opposition of thrust. Newton’s Third Law states that the force of an aircraft pushing on the air is always equal to the force of the air pushing against the aircraft. If the aircraft engine is turned off, the drag on the aircraft will slow and then drag down the aircraft. Lifting up landing gear in flight reduces drag, as does making the aircraft body design more streamlined.