Tie, Principles of Flight, Silk

while an airplane is in flight there are four forces acting upon it they are lift weight thrust and drag lift is the upward force created by the wings as air flows around them and keeps the airplane in the air weight is the downward force toward the center of the earth opposite lift which exists due to gravity next we have thrust this is the forward force generally created by the aircraft's propellers or turbine engines which pulls or pushes the aircraft through the air finally there is drag drag is the force acting in the direction opposite of thrust which fundamentally limits the performance of the airplane when an aircraft is maintaining its heading altitude and airspeed it is said to be in straight and level unaccelerated flight in unaccelerated flight lift equals weight and thrust equals drag let's look at these four forces in a little more detail the key to an aircraft being able