The four forces of flight are lift, weight, thrust and drag. Lift and weight are opposing forces, which means they act in the opposite direction. Likewise, thrust and drag are opposing forces. All airplanes are subject to these four forces (see Figure 1). Thrust is what moves the aircraft forward and also creates air speed, which we will see later is part of what creates lift. Lift is what pushes the airplane up, while gravity is the force that pulls the airplane down. Drag is a force that acts against thrust and slows the airplane down. When the thrust is greater than the drag, the plane moves forward. When weight is greater than lift, the plane descends.

Figure 1. The four forces of flight: lift, weight, thrust and drag. click for copyright

The wings are the parts of an airplane that create lift. If we look at a wing from the side, as in Figure 2, we can see that it is shaped somewhat like a teardrop, with a thick, rounded front end and a thin, pointed back end. The curve on the top of the wing is longer than the bottom, which means air traveling across the top of the wing has to move faster to keep up with the air moving under the wing. According to Bernoulli's Principle, there must be less pressure on the top of the wing than on the bottom of the wing.

Figure 2. Bernoulli's Principle demonstrated on an airplane wing. click for copyright

The result of this difference in air pressure is a net upward force called lift. As illustrated in Figure 3, the air moving under the wing moves slower and exerts more pressure/force on the wing than does the air moving over the wing. Since there is more force under the wing than above it, the net result is that the wing rises up; hence, lift. This principle forms the basis of winged flight.

Figure 3. How a wing produces lift. click for copyright

There are flaps on the front and back edges of the wing. During takeoff and landing, pilots extend the flaps on the back edge of the wing. The flaps increase the camber (curve) of the wing, which maintains the lift at slower speeds. After takeoff, the pilot retracts the flaps for normal flight. Engineers use wind tunnels and computers to continuously test wing designs to determine their lift.

Question/Answer Review: Ask students if they remember Bernoulli's Principle from Lesson 1 of the Airplanes unit. Ask for explanations/descriptions to the following:

• Can you remember how much air pressure is pushing on you at all times? (Answer: 14.7 pounds per square inch at sea level; 12 pounds per square inch in Denver.)
• What does Bernoulli's Principle tell us about air pressure? (Answer: The faster air moves the lower its pressure.)
• Have the students brainstorm what Bernoulli's Principle might have to do with flight. Get them to realize if there is high pressure below the airplane and low pressure above the airplane it will move up, which is where the lift force comes from. Then get them to use Bernoulli's Principle to determine that somehow the air must be moving faster over the top of the airplane to cause lift.

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.

• All airplanes are subject to three forces during flight. (Answer: False, there are four forces of flight: lift, weight, thrust and drag.)
• Bernoulli's Principle causes thrust to happen? (Answer: False, lift is the correct force.)
• When weight is greater than lift, an airplane descends? (Answer: True)

Numbered Heads: Have the students on each team pick numbers (or number off) so each member has a different number. Ask the students a question (give them a time frame for solving it, if desired). The members of each team should work together on the question. Everyone on the team must know the answer. Call a number at random. Students with that number should raise their hands to answer the question. If not all the students with that number raise their hands, allow the teams to work a little longer. Ask the students:

• What are the four forces of flight? (Answer: Lift, weight, thrust and drag.)
• How does Bernoulli's Principle create lift? (Answer: Because the top of a wing is longer than the bottom, and air traveling across the top of the wing moves faster and exerts less pressure than air beneath the wing. The result is a net force up; hence, lift.)