Lesson Plan: The Concept of Lift Force on Wind Turbines

Subject Area(s): Natural Sciences, Physics, and Math

Grade Level: High School

Lesson Title: The Concept of Lift on Wind Turbines

Lesson Summary: This lesson is an interactive and physical activity to show students the concept of lift force and its implications to wind turbines. The activity can be performed outside or in an indoor gym.

Time Required: 40 Minutes

Materials Required:

20 Cones

36 in ruler or longer

Stopwatch

Paper and pencil

Calculator

Whiteboard

Marker or chalk

Figure 1. Airbus A380 (left), Wind Turbine (right). Source: Wikipedia

Lesson

Motivation: Lift is the force that maintains airplanes and even birds in the air. Moreover, lift force makes wind turbines rotate, as the flow passes through its blades. However, lift is a somewhat complex engineering concept that can be difficult for high‐school students to understand. For instance, air is invisible to the naked eye, and the idea that an invisible medium is capable of exerting enough force to maintain an A380 aircraft, such as the one shown in figure 1 (left) in the air, is quite difficult to visualize. Also, lift force can be strong enough to make a wind turbine the size of a football field, such as the one presented in figure 1 (right), to rotate. This activity provides a hands‐on experience so that high school students can understand and remember the concept of lift force.

Background: Lift force results from a difference in pressure between the lower and upper halves of the wing of an aircraft, to give an example. Because the cross‐sectional area of the wing is asymmetric, the perimeter of the upper halve is greater than that of the lower. Since the flow over the wing has to travel a greater distance, it is forced to move faster. This is because the air in the upper half has to get across the wing in the same amount of time as the air on the lower half (see figure below). As the airflow accelerates over the wing, the pressure drops accordingly. Then, there is a region of lower pressure over the wing than there is beneath it. Because pressure is defined as the force per unit area, the higher the pressure difference and the area, the stronger the lift force. The figure below shows the lift force equation.

Figure 2. The Lift Equation. Source: www.grc.nasa.gov Activity:

*Note: The students should perform all steps in order to keep them engaged in the activity.

Step 1: Move the class outside or to the indoor gym

Step 2: Arrange the 20 cones as shown in figure 3, to form the cross‐section of an aircraft wing (i.e., airfoil). It is import to make the airfoil as big as possible to increase the level of excitement of the activity by challenging the students.

Step 3: Using the ruler, measure and record the perimeter of the lower and upper sides of the airfoil.

Step 4: Let the students organize in two lines. One of the lines will run around the upper side of the airfoil, while the other will run around the lower side.

Step 5: Give one student of each line the signal to start running around the airfoil. Remember that both students have to arrive at the other side of the airfoil at the same time, as it occurs in real airfoils. Since the airfoil is asymmetric, the student running around the upper side of the airfoil will have to run considerably faster, just as it happens in real airfoils. Encourage all students to run; this will ensure a wide range of times. See figure 4.

Step 6: Use the stopwatch to measure the time that it takes each student duet to reach the other side of the airfoil. Use pencil and paper to record the times for each student duet. Repeat this step until all students have walk or run to the other side of the airfoil.

Step 7: Go back to the classroom. Using equation 1 and the calculator, calculate the generated lift by each student duet. In the equation, set the coefficient, density and area equal to 1. Input the average velocity of each duet. To calculate the velocity, divide the time by the lower perimeter.

Step 8: Post the lift force generated by each duet on the whiteboard for further discussion.

Closure: Have the students do some reasoning about the results and the implications to real life examples such as wind turbines. Ask students the following questions:

1. Why did the students that ran around the upper side of the airfoil do more physical effort? Answer: The perimeter of the upper side is greater.

2. What determined the highest and lowest values of lift force? Answer: The running speed.

3. What can be done to increase the lift force? Answer: Run faster.

4. Why are wind turbine blades so long (i.e., 50 meters each)? Answer: To increase the area and the lift force, and, therefore, the rotational speed and torque. (Torque is the rotational force exerted on the shaft of the wind turbine.)

5. What can be done to make wind turbines rotate faster while generating a greater torque? Answer: Increase the speed of the wind and increase the length of the blades. 6. How can the speed of the wind be increased? Answer: By placing the wind turbines far from obstacles such as buildings, trees, and mountains. Also, by putting them close by or on bodies of water, such as the sea or big lakes.

Figure 3. Wing cross‐section, or airfoil, formed using cones

Figure 4. Students running around the airfoil

Appendix A: Data Sheet

Lower Perimeter: ______meters

Upper Perimeter: ______meters

Duet Time Lower Perimeter Velocity = Lower Perimeter/Time (m/s) Lift Generated

1

2

3

4

5