Aeronautics

The Forces of Aeronautics

Four Forces

Four Forces

A force is defined most simply as a push or a pull. There are two components to a force: magnitude (or the amount of force applied) and direction. Thus we say that "a force of a certain magnitude is applied in a particular direction.

Four Forces
There are four primary forces that act on an airplane in flight: thrust, weight, drag and lift. It is the interplay between these four forces that result in an airplane's motion.

Weight

Weight

All objects in the universe exert an attractive force on each other that is called gravity. The magnitude of this force is dependent on the mass of the object. In our day-to-day lives this attractive force is recognizable only for objects with enormous mass, such as the Earth. Gravity is the word we use to define the attractive force specifically between the Earth and all the objects that are within its influence. Included in a list of these objects would be people, aircraft - even the moon!

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 can be considered constant. Weight is the force that measures the effects of gravity.

 

Lift

Lift

In order for an airplane to fly, a force must be generated that is stronger than the weight force. That force is called lift. The lift force is generated by air flowing over an object. 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.

It is easy to understand that the shape of the wing will have a direct influence on "how" the air flows from front to back. "How" the air flows will have a direct influence on how much lift the wing can generate. An object that is shaped to generate maximum lift is called an "airfoil".

90 Degree Thrust

90 Degree Thrust

 

The inflated balloon has compressed air pressing equally against all the sides.

The air rushes out the open hole at the bottom. The action is that the air is pushed out in one direction. The reaction is that the balloon flies in the other direction

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.

Thrust

Thrust

 

Balloon FullBalloon Open

Drag

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. The drag on an airplane is the result of, among other things, the energy needed to move the air out of the way of the airplane. Obviously drag is hard at work when a massive object, like an airplane, tries to fly through a fluid like air. Any motion or movement by the airplane will always be resisted by a drag force. The direction of the drag force is opposite to the direction of flight. The thrust force is aligned to counter the drag force.

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. Further, reduction of drag has a "domino" effect on other important aspects of flight. 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.

Lift and Weight

Thrust and Drag

Lift and Weight

The Four Forces in Balance
Let us look more closely at the interplay between the four forces. Recall that in our model, the four forces work in oppositional pairs: lift versus weight and thrust versus drag.

When forces are in balance, that is their magnitudes 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. 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!