The Forces of Aeronautics

A force is defined most simply as a push or a pull.  There are four primary forces that act on an airplane in flight: thrust, weight, drag and lift.  The interactions between these 4 forces result in flight.

The amount of force that gravity exerts on an object is weight.  The weight of an object depends on it’s mass.  The Earth's gravitational pull weakens as objects move farther away from it. Thus we say that objects that are far from the Earth "weigh less" than when they are on the Earth. For objects "on" and "close" to the Earth (we will assume that airplanes fly at altitudes "close" to the Earth) the weight of an object is constant.

In order for an airplane to fly, a force must be generated that is stronger than the weight force (gravity). That force is called lift. The lift force is created by air flowing over an object. The differences in air pressure above and below the wing cause lift.  The direction of the lift force will always be perpendicular to the direction that the air is flowing. As an airplane is flying, air is flowing over its wing, from the front (or leading edge) to the back (or trailing edge). This generates a lift force perpendicular to the direction of the airflow.

				Lift
	Wind

The shape of the wing determines how air flows around it.   The way that air flows from the front of the wing to the back of the wing determines how much lift will be generated.  An object that is shaped to generate maximum lift is called an "airfoil".

When an airplane is on the ground not moving, there is not enough air flowing around it to create lift. Another force is needed to get the airplane moving through the air, so that the airflow can do its job of creating lift. This force is called thrust. Thrust propels an object in a particular direction. The arm of a baseball pitcher generates thrust and applies it to a baseball (that is, throws it) towards a batter. Likewise, a jet engine generates thrust and, because it is attached to the wing of an airplane, its thrust will be applied to the airplane. So, as the engines thrust the airplane in the direction that they are pointed, air flows over and under the wings which creates the lift force. If enough lift is generated, the airplane will fly.

The fourth primary force is drag - and the drag force does a great job of living up to its name. Drag is the force that resists any object trying to move through a fluid (air).   It takes energy to move the air out of the way of an airplane.  The drag is the force air is exerting.  Obviously drag is hard at work when a massive object, like an airplane, tries to fly through a fluid like air.  A drag force will always resist any motion or movement by the airplane. The direction of the drag force is opposite to the direction of flight.

Reducing drag is one of the main concerns of aeronautical engineers when designing aircraft. Drag can stress different parts of an aircraft which can lead to structural failure during certain maneuvers. The less drag an aircraft has, the faster an aircraft can go, or the less power is needed from the engine. Less powerful engines are generally lighter (that is, have less weight) and need less fuel (that is, cost less to fly). A lighter aircraft means that less lift is needed to fly and the airplane can be more maneuverable. If less lift is needed, a smaller wing is required which decreases weight and drag. All of this, taken together, reduces the cost of building and flying the airplane.

The Four Forces in Balance

Let us look more closely at the interactions between the four forces. Recall that in our model, the four forces work in opposite pairs: lift versus weight and thrust versus drag.

When forces are in balance, that is their strengths are the same and their directions are opposite, the speed and direction of the object will not change. Imagine an airplane, flying along at its cruising speed and its cruising altitude. The wings are creating enough lift to counteract the weight of the aircraft and keep it at its cruising altitude. The engines are creating enough thrust to counteract the drag of the aircraft and keep it at its cruising speed.

Let's say that the lift force is increased or the weight of the aircraft is decreased (it's using up fuel, for instance). Now there is an imbalance between the lift force and the weight force and the airplane will ascend (rise).  Conversely if the lift force is decreased the lift force and the weight force will not be balanced and the airplane will descend.

In the same way, if the thrust force is increased, an imbalance is created, and the airplane will increase its speed in the direction the thrust is directed. Similarly, if the thrust is decreased, or the drag increased (say the flaps on the wings are extended), the airplane's speed will decrease.

Thus, the task of a pilot is to manage the balance between these four forces - we call this flying!