Drag is a force that acts on an object in the opposite direction than that object is moving. An object must be moving through some kind of fluid for drag to occur. A fluid is a substance in which the particles can move past each other freely. The most obvious fluid is water, but gases, including air, are fluids as well.

Air is the fluid through which airplanes move. When an airplane flies through the air, it runs into air molecules which cause the airplane to slow down. Energy from the moving plane is transferred to the air molecules. In other words, some of the kinetic energy (energy it possesses because of its motion) from the airplane is given to the air molecules, slowing down the airplane and speeding up the surrounding air. The amount of energy lost by the airplane is exactly the amount of energy transferred to the air. This is an example of the First Law of Thermodynamics that states energy can neither be created nor destroyed. Engineers often refer to the First Law of Thermodynamics as the Conservation of Energy principle, which means that energy is always conserved. Energy has to go somewhere and in the case of drag, the energy in the movement of an object is transferred into moving the gas in the path of the object (in the case of an airplane, moving the air around it). This transfer of energy results in three types of drag: friction drag, form drag and induced drag.

Friction drag comes from air moving across the surface of the airplane. On a very small scale, the surface of an airplane is rough, like sandpaper. If you run your hand over sandpaper, the sandpaper catches your skin and slows down, or stops, the movement of your hand. The same thing happens with an airplane. The "skin" of the airplane catches the air particles next to it and tries to pull the air particles along. This slows the airplane down as air particles speed up.

Form drag is caused by the airplane pushing air molecules to the side so the airplane can pass by them. A streamlined shape (like an airplane wing) will have very little form drag. A non-streamlined shape, like a parachute, will have a lot of form drag.

Figure 1. Induced drag at the tip of an airplane wing. click for copyright

Induced drag (see Figure 1) is created at the tips of the airplane wings. In order to achieve lift, the wings create a low-pressure region above the wings and a high-pressure region below the wings. At the end of the wing, high-pressure air underneath the wing tries to move around the end of the wing to the low-pressure air on top of the wing. This creates a swirling vortex of air at the wingtip. The energy needed to move the air in the vortex is taken from the movement of the plane creating induced drag.

Engineers' challenge is to find creative ways to reduce drag so that airplanes can go faster and fly more efficiently. The less drag an airplane experiences, the less fuel it will need to fly at the same speed. Friction drag increases as the surface area of the wing increases and as the roughness of the wing increases. Form drag increases as the cross-sectional area of the plane increases, and the shape becomes less streamlined. Engineers reduce form and friction drag by making the body of the plane more streamlined, the wings more narrow, or by using new materials and manufacturing processes to make the skin of the plane smoother. Engineers reduce induced drag by making the ends of the wings oval shaped or by adding wing tips that stick up from the end of the wing. Through the efforts of engineers, airplanes are continually changing shape to improve their efficiency and performance.

Discussion Question/Answer Review: Solicit, integrate, and summarize student responses.

• What are the four forces affecting airplane flight? (Answer: Lift, weight, thrust and drag.)
• What is lift? (Answer: When the air pressure below a wing is greater than the air pressure above the wing, there is a net upward force called lift.)
• What is weight? (Answer: The force with which a body is attracted to Earth or another celestial body, equal to the product of the object's mass and the acceleration of gravity.)
• How would you define drag? (Answer: Drag is a force that acts on an object in the direction opposite the direction that the object is moving.)

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the number of true and false and write the number on the board. Give the right answer.

• True or False: The purpose of engines on an airplane is essentially to overcome the force of drag so that the pilot decides when to land the plane instead of having gravity take over. (True)
• True or False: Drag is always a bad thing. (False: Parachutes need drag to effectively slow a parachuter down in order to avoid injury when landing.)
• True or False: As the speed of an object increases the drag decreases. (False: The drag increases as speed increases.)
• True or False: There is no drag in space. (True: There is no air in space, and therefore, there is no drag.)

Figure Drawing: Have students sketch an airplane and label the forces of flight on their drawing. An alternative is to draw an airplane on the board and have the students generate the placement of the four forces as a class.

Group Flashcards: Each student on a team creates a flashcard with a question on one side and the answer on the other about something they have learned about airplane flight so far. If the team cannot agree on the answers they should consult the teacher. Pass the flashcards to the next team. Each member of the team reads a flashcard and everyone attempts to answer it. If they are right, they can pass the card on. If they feel they have another correct answer, they should write their answer on the back of the flashcard as an alternative answer. Once all teams have done all the flashcards, discuss and clarify any questions as a class.