Four Forces of Flight
By John Cowens

The Wright Flyer (above) was the first successfully manned airplane.

As students arrive in your classroom, there may be times when someone wads up a sheet of used paper into a ball and tosses it towards the nearest recycle container. As the wad is launched, it must contend with four forces to keep it airborne – gravity, lift, drag and thrust. If given the correct amount of force and proper angle of trajectory, the wad will "fly" into its target and save you a lot of grief from directing the potential basketball star to pick up the missed shot and walk it to the container.

Heavier-than-air flight did not get off the ground until the late 1800s. The most recognized men in the early years of airplane flight were Orville and Wilbur Wright. Their first powered flight of a manned aircraft, the "Wright Flyer," took place on December 17, 1903, ushering in the "Air Age."

There are four forces that act upon an aircraft in flight:

Gravity is the natural force the Earth produces as a result of its mass. Gravity keeps an airplane on the ground or pulls it back to Earth unless there's an equal or greater force to overcome the effect of gravity (Newton's First Law of Motion).

Lift is created in a fixed-wing aircraft by changes in air pressure around the wing as the airplane moves through the air (Bernoulli's Principle).

Drag is the force that tries to hold the airplane from moving forward. Drag is produced by air molecules, or particles hitting against the airplane surfaces as it moves forward, including the friction of the tires on the runway until the plane becomes airborne.

Thrust is the opposing force great enough to overcome the force of drag as well as the weight of the airplane. This is achieved by using a propeller or jet engine (Newton's second and third Laws of Motion).

The following experiments explore the four forces that enable aircraft to fly:

Gravity experiment
Which substance is acted upon the most and least by gravity?

Materials:

• pail
  
• wooden tripod (or equivalent)
  
• hook
  
• marking pen
  
• pie pan
  
• yardstick (or meter stick)
  
• shelf paper
  
• weight scale
  
• two cups of sand
  
• two cups of birdseed
  
• two cups of soil
  
• two cups of cotton balls
  
• measuring cup

Procedures:

1. Hang a pail from a door spring suspended on a hook.

2. Fill the pail with a material, such as sand, and make a mark on the shelf paper to show how far the pail falls.

3. With the yardstick, measure the "fall" distance to the nearest 1/16th of an inch (2 mm).

4. Next, empty this material into the pie pan. Weigh the material on the scale. Record on the chart provided below.

5. Repeat Steps 1-4 for each of the other materials.

Lift experiment
Materials: a strip of paper 1" x 12" (2.5 cm x 30 cm)

Procedures:

1. Hold the end of a strip of paper with your index finger and thumb so that it curls over the front of your hand.

2. Illustrate lift by blowing over the top surface of the paper strip.

3. Answer the following questions:
   • Airflow is fastest (a.) over the top surface of the paper, or (b.) over the bottom of the paper strip

   • Faster airflow means less air pressure. If the top surface of the paper strip has a faster airflow passing over it, where is the air pressure greatest? (a.) on top surface of the paper strip, or (b.) on the bottom of the paper strip

   • The greater air pressure pushes upward to the area where there is less air pressure. How did your strip of paper react that showed that greater air pressure pushes toward an area of less air pressure? (a.) The strip of paper was lifted up, (b.) The strip of paper curled down

Extensions:
Make a paper plate face with a moveable tongue. Use your strip of paper for the tongue. Cut openings on the paper plate for eyes and mouth. Tape one end of the paper strip to the inside of the mouth hole. It will form a "tongue" sticking out. Students can blow air through the hole and make the paper tongue wave.

Attach a long piece of string to two styrofoam balls. Hang these balls in the doorway about face-high. Be sure the balls are very close together but not touching. Take a straw and blow air between the balls. Can you blow them apart? Repeat this experiment several times to see if the same results occur.

Thrust experiment
Isaac Newton's Third Law of Motion states that "For every action, there is an opposite but equal reaction." Here is a simple experiment that demonstrates this particular motion.

Materials:

• balloon
  
• clothespin
  
• drinking straw
  
• fishing line
  
• masking tape

1. Inflate the balloon, twist the end and attach the clothespin. Do not tie it off.

2. Tape the inflated balloon to a straw.

3. Thread fishing line through the straw.

4. Have two students hold each end of the line so that it is taut.

5. Release the air from the balloon and observe its movement. (If the balloon spirals around the fishing line as it travels, tape a nickel to underside of the balloon and repeat the experiment.)

6. Ask your students to record, in picture form, before launching the balloon, launching the balloon and after launching the balloon.

7. Answer the following questions:
   
   • What did you have to do in order to get action?

   • What was the reaction?

   • What does a plane use to put itself into action?

   • What's the reaction then to an airplane?

Drag experiment
How does the surface of a material effect the distance an object travels across it?

Materials:

• incline plane
  
• washer
  
• ruler/tape measure
  
• wooden blocks with applied surfaces: notebook paper, coarse sand- paper, safety glass, grained wood

Procedures:

1. Place one of the sample surface material blocks at the bottom of the incline plane. Be certain that the sample touches the base of the incline plane.

2. Put the round washer at the top of the incline plane.

3. Push the washer so that it slides down the incline plane.

4. When the disk comes to a complete stop, measure the distance it traveled.
   Use the measure on the side of the sample.

5. Record the distance traveled on the table provided.

6. Repeat Steps 1-5 for each of the surface block samples.

7. Test each surface block sample at least three times.

John Cowens teaches sixth grade at Fleming Middle School in Grants Pass, OR.