One of the first things that is likely to be noticed during a visit to the local airport is the wide variety of airplane styles and designs. No matter what each looks like they all depend on the same four factors which are lift, weight, thrust, and drag. The direction in which the force of weight acts is constant. It always acts straight down toward the center of the earth (Four Forces on an airplane 1). Lift is the upward force produced by the effect of airflow as it passes over and under the wings. It helps maintain the airplane in flight. Weight opposes lift, it is caused by the downward pull of gravity. Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power being used. Opposing thrust is drag, which is a backward force that decreases the speed of the airplane (What is Thrust 1). Lift is the key aerodynamic force. It is the force that opposes weight. In straight and level flight when weight and lift are equivalent, an airplane is said to be in a state of equilibrium. If the other aerodynamic factors remain constant, then that airplane neither gains nor loses altitude. Movement of air on the airplane, particularly the wing, is necessary in order for the aerodynamic force of lift to become effective. During flight, however, pressures on the upper and lower surfaces of the wing are not the equal. Although several factors contribute to this difference, the shape of the wing plays an important role. The wing is designed to divide the airflow into areas of high pressure below the wing and areas of comparatively lower pressure above the wing (Four Forces on an airplane). This pressure differential, which is created by movement of air about the wing, is the primary source of lift. The weight of the airplane is not a constant. It varies with the equipment installed, passengers, cargo, and fuel load. During the course of a flight, the total weight of the airplane decreases as fuel is consumed (Four Forces on an airplane 1). Thrust is the forward-acting force which opposes drag and propels the airplane (What’s Thrust 1). In most airplanes, this force is produced when the engine turns the propellers. Each propeller blade is cambered like the airfoil shape of a wing. This shape and the angle of the blades creates a reduced pressure in front of the propeller and increased pressure behind it. As is the case with the wing, this produces a reaction force in the direction of the lesser pressure. This is how a propeller produces thrust, the force which moves the airplane forward. To increase thrust by using the throttle to increase power, thrust must exceed drag, causing the airplane to accelerate faster. This acceleration, however, is accompanied by a corresponding increase in drag (Four Forces on an airplane). The airplane continues to accelerate only while the force of thrust exceeds the force of drag. When drag and thrust gain equilibrium at some point again, the airplane ceases to accelerate and maintains in a constant airspeed. However, the new airspeed is higher than the previous one. When the thrust becomes a reduced thrust, the force of drag causes the airplane to decelerate. But as the airplane slows, drag diminishes. When drag has decreased enough to equal thrust, the airplane no longer decelerates. Once again, it maintains a constant airspeed. Now, however, it is slower than the one previously flown. As it has been seen, drag is associated with lift. It is caused by any aircraft surface that interferes with the smooth airflow around the airplane. A highly cambered, large surface area wing creates more drag and lift than a small, moderately cambered wing. If the airspeed is increased, or angle of attack, the drag and lift increases. Drag acts in opposition to the direction of flight, opposes the forward-acting force of thrust, and limits the forward speed of the airplane. Drag is broadly classified as either parasite or induced.