Two pairs of opposing forces act on an aircraft in flight, thrust the forward driving force provided by a propeller or jet is opposed by the friction of air known as drag. Lift is the upward force developed as air passes by an aircraft's wings weight. The downward force resulting from the Earth's gravity is the opposing force that must be overcome by lift for flight to occur. Planes are designed around wings, and wings are designed according to the rules of aerodynamics and the vertical plate is subjected to fast moving air. A turbulent and chaotic air flow will occur as air passes by, but if this plate is rotated towards the direction of the air flow and turned into an air foil with a curved upper surface and a flat lower surface lift will result from a reduced air pressure that occurs above the wind. The faster air moves, the lower its pressure air flow over a curved surface is faster than a flat one, so the reduced air pressure above the wing results in lift by decreasing air friction and turbulence. With smooth design, drag is minimised, making an aircraft airworthier. The angle of the wings affects lift increasingly angle at which the wind meets the air can increase lift to a point, but beyond the critical angle, air flow will become turbulent, increasing drag and decreasing lift.