Lesson: May the Force Be With You: Drag

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

An aircraft showing the four forces acting on it: thrust, drag, lift, weight.
Forces on an aircraft.

Summary

This lesson explores the drag force on airplanes. The students will be introduced to the concept of conservation of energy and how it relates to drag. Students will explore the relationship between drag and the shape, speed and size of an object.

Engineering Connection

When designing airplanes, engineers keep in mind the force of drag and the principle of energy conservation. Since drag slows down an airplane and makes it less efficient (requiring more fuel), an engineer's goal is to design a plane that reduces drag. Minimizing the amount of drag acting on an aircraft may require modifying the wing or fuselage shapes.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standard Network (ASN), a project of JES & Co. (www.jesandco.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Students will develop an understanding of and be able to select and use transportation technologies. (Grades K - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use tools to gather, view, analyze, and report results for scientific investigations about the relationships among mass, weight, volume, and density (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?

Learning Objectives

After this lesson, students should be able to:

  • Know that drag is one of the four main forces acting on an airplane.
  • Describe how the shape, area and speed of an object affects drag.
  • Identify which of the four forces of flight opposes the force of drag
  • Recognize why understanding drag is important to engineers designing airplanes

Introduction/Motivation

How would you define drag? (Answer: For our purposes, drag is defined as something that slows you down.) You can feel drag when you walk in a swimming pool. A fisherman feels the drag on his lure as he pulls it through the water. Drag is a force that acts in the opposite direction than an object is moving.

What happens to your arm/hand when you stick it out the window of a moving car? (Answer: The arm/hand gets pushed backwards.) This is because drag is acting on your hand. Now, what can you do to increase the amount of drag on your hand? (Answer: Turn the hand so that the palm is facing into the wind or have the driver speed up.) Drag depends on characteristics such as the size, shape and speed of an object.

Can you think of a situation where drag is a good thing to have occur? (Possible answers: to help cars stop after a drag race, to slow down a plane when it lands on a runway, etc.) An excellent example of drag being a useful force is during skydiving: a skydiver relies on drag to slow down their fall so that a safe landing can take place — hence, the reason for a parachute.

Like any other object that moves through the air, an airplane also experiences drag. This is not always beneficial, however, since an airplane wants to move forward very quickly, and drag slows planes down. Overcoming drag has always been a primary design challenge for aerospace engineers. Airplanes overcome drag by generating thrust.

Engineers calculate the drag on an airplane by using information about the size, shape and speed of the airplane. Using this information, they can then decide how much thrust will be needed to overcome drag and keep the airplane aloft.

Lesson Background and Concepts for Teachers

What is Drag?

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.

How does Drag Slow an Airplane?

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.

A colorful drawing shows a computer simulation of a wing moving through the air.  A vortex is extending off the edge of the wing, which demonstrates the induced drag at the tip of a aircraft wing.
Figure 1. Induced drag at the tip of an airplane wing.
copyright
Copyright © http://people.nas.nasa.gov/~rogers/images/

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.

What do Engineers Do About 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.

Vocabulary/Definitions

cross-sectional area: The projected area of a 3-dimensional object onto a 2-dimensional plane.

drag: The phenomenon of resistance to motion through a fluid.

fluid: A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.

gas: The state of matter distinguished from the solid and liquid states by relatively low density and viscosity, relatively great expansion and contraction with changes in pressure and temperature, the ability to diffuse readily, and the spontaneous tendency to become distributed uniformly throughout any container.

kinetic energy: The energy possessed by a body because of its motion, equal to one half the mass of the body times the square of its speed.

liquid: The state of matter in which a substance exhibits a characteristic readiness to flow, little or no tendency to disperse, and relatively high incompressibility.

molecule: The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.

surface area: The extent of a 2-dimensional surface enclosed within a boundary.

Associated Activities

  • What a Drag! - This activity focuses on the way shape and size effect drag. Students will also observe the speed with which various objects of different shapes and cross-sectional areas fall to the ground when dropped.

Lesson Closure

Review the four forces that affect flight and discuss how they cause an airplane to fall, rise, slow down, or speed up. Discuss how drag can slow an airplane down. Ask students if the drag on an object will increase if the surface area is increased or the object speeds up. (The answer to both of these questions is: yes, the drag does increase.) Discuss how an airplane with less drag can go faster and will be more efficient because more power from the engines is used in pushing the airplane forwards instead of moving the air molecules out of the path of the plane.

Assessment

Pre-Lesson Assessment

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.)

Post-Introduction Assessment

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.)

Lesson Summary Assessment

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.

Lesson Extension Activities

Have students research and learn more about how drag affects airplanes, rockets, boats and even people.

There are many websites for airplanes and the four forces affecting flight. It is recommended to start with a keyword search for "4 forces of flight" and "airplanes." Following is a good website with which to start a search: http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html

References

Campbell, Neil A., Reece, Jane B., and Mitchell, Lawrence G. Biology, Addison Wesley Longman, Inc., 1987.
Guyford, Stever H., Haggerty, James J. Flight, Time Inc., 1969.
http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html
http://people.nas.nasa.gov/~rogers/images/
http://www.allstar.fiu.edu/aero/fltmidfly.htm
Jones, Charlotte Foltz. Mistakes That Worked, Doubleday, 1991.
World Book Encyclopedia. Polymers, 2002.

Contributors

Tom Rutkowski; Alex Conner; Geoffrey Hill; Malinda Schaefer Zarske; Janet Yowell

Copyright

© 2004 by Regents of the University of Colorado.

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

Acknowledgements

The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.