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Atmospheric Flight

Four Forces

Force

Can you think of examples of force? If you push a door closed, your push is a force. If you pull a drawer open, your pull is a force. A force actually has two parts. One part is the strength of the force. When you push a door closed, do you push it gently or does it slam shut? How hard the door is pushed is called strength or magnitude of the force. The other part is the direction of the force. Let's say you are pulling on a rope. Pull it to the right, then to the left. Pull it upwards and then pull it downwards. Where you pulled the rope is the direction of the force. A force has both magnitude and direction.

In aeronautics, there are four important forces. These four forces are called lift, weight, thrust and drag. Each force works in a specific direction. The magnitude of each force can vary from weak to strong. All of these four forces are hard at work when an airplane is in flight. They often work in opposite directions from each other, but together they make flight possible.

a drawing of an
airplane with arrows extending in the four directions of the four forces,
straight up for lift, behind the airplane for drag, straight down for
weight, straight ahead for thrust

Weight

Weight is a force that you probably already know about. When you step on a scale, you are checking to see what your weight is. An airplane has weight too, just like every other object that exists around Earth. All objects whether big or small, exert a gravity force on other objects. As you are sitting in your chair, you are exerting a gravity force on the person next to you, even if you are not touching them!

If an object is very large it can exert a lot of force on objects around it. One example of a very large object is Earth. We know that Earth exerts a gravity force on us. We call that force weight. The Earth's gravity force pulls on all objects that are within a very wide area. The moon reacts to the Earth's gravity force by revolving around the Earth.

When you weigh yourself, you are actually measuring the force of Earth's gravity on your body. Remember that there are two parts to a force: magnitude and direction. The direction of your weight force is toward the Earth. The magnitude of your weight force is how heavy you are.

Airplanes have a weight force, too. Even as they fly many feet above the Earth, their weight force is pulling them toward the Earth. Since they are so heavy, the magnitude of their weight force is very great.

a drawing of an
airplane with an arrow pointing straight down labled weight

Lift

You are probably familiar with the lift force as well. You lift books off a desk. When a strong wind blows pieces of paper and other lightweight objects are lifted off the ground. When those objects lift off the ground it means that the lift force is stronger than the weight force. If the weight force were stronger, then the object would not lift into the air.

a drawing of an
airplane with an arrow pointing straight up labeled lift

The lift force works the same way on an airplane. If an airplane can create enough lift force to overcome the weight force, then the airplane will fly. Since the weight force of an airplane is so great, the airplane needs to be able to generate a lot of lift. Such a great lift force is generated by the air flowing over the wings of the airplane.

As the air flows over the wing, the lift force is created in an interesting direction. Can you guess what that direction is? Most of the time the direction is "up". But actually it is at a 90 degree angle to the airflow. Most of the time the airflow is going over the wing from the front to back of the airplane. The lift force is always acting 90o to the direction of flight.

An Animation about lift An Animation about the angle of attack
drawing lift a picture of an airfoil at a 45 degree angle

a drawing
of an airplane with a forward arrow labeled thrust, an upward arrow
labeled lift and an arrow underneath from the front of the
airplane to the back labled airflow, and the same drawing
showing the plane flying at a steep angle of attack

Thrust

In order for the lift force to be created, air has to be flowing over the wings. We make air flow over the wings by moving the wings through the air. As the wing moves through the air, the air molecules flow over and under the wing which generates lift.

Somehow we need to get the airplane moving forward so that the air can begin flowing over the wings. To do this we need thrust. Thrust is provided by the propulsion system. Engines can be mounted on the wings or fuselage. They propel the airplane along the runway and forward through the air.

a picture of a plane 
with an arrow pointing straight ahead labled thrust and arrow
pointing straight back labeled airflow

The force that is created by the engines is called the thrust force. The direction of the thrust force is based on where the engines are pointing. Since the engines are usually mounted facing forward, the direction of thrust is usually towards the front of the airplane. An exception is the the Av-8B Harrier which can create vertical lift.

A very strong engine can create lots of thrust while smaller engines will create less thurst. The magnitude of the thrust force is determined by the size and power of the engines.

Drag

As an airplane is thrust through the air, it must push aside all those air molecules that are moving around in the space in front of it. As a wing moves through the air, the air separates and some of the molecules follow along the top of the wing wile others flow underneath the wing. The same thing happens with the rest of the airplane. The air molecules must separate so that the airplane can move through the air. Air molecules resist being separated by the airplane. This resistance is called drag. Drag can slow the forward motion of the airplane. To overcome this drag, the thrust force needs to be greater than the drag force.

a picture of a plane
describing the
drag force

There are different types of drag. One type of drag has to do with how smoothly the airflow moves around the airplane. Another type of drag concerns what happens when smooth airflow becomes turbulent airflow. Turbulent airflow changes the air pressure at certain points on the airplane. These changes in air pressure increase drag. A third type of drag occurs naturally with the design of any wing. Energy is lost in the process of generating lift because some of the air flows out around the edge of the wingtip. This creates drag. All wings have this type of drag. Some wing designs create less drag than others.

The amount of drag depends on these things:


  1. size of the aircraft
  2. details of the shape and smoothness of the aircraft
  3. lifting efficiency of the wing
  4. dynamic pressure (density and speed)


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