What do a teeter totter and a wing have in common?
Learning Objectives
After completing this lesson, the student will be able to describe how centers influence the balance of an airfoil.
Standards
- NGSS HS-ETS1-3
- CCSS.Math.Practice.MP1
- CCSS.Math.Practice.MP2
- CCSS.Math.Practice.MP4
Supplies
- Three BLIMP hulls
- Helium
- String
- Tape
Units Used
- None – this is a math-free lesson!
Center of Pressure and Aerodynamic Center
When an airfoil (or any body for that matter) moves through a fluid (air is a fluid), lift and drag are produced. The air is flowing faster over the top of the foil than the bottom, so there is lower pressure on top than on the bottom. This results in lift. Drag opposes the motion of the body through the fluid. Lift and drag are both vector quantities, so they have magnitudes and directions. Lift is the component of the aerodynamic force that acts perpendicular to the fluid flow and drag acts parallel to the fluid flow. On the airfoil below, sketch vectors for lift and drag.
These forces act at a location called the center of pressure. The center of pressure is to lift what center of gravity is to weight. The center of pressure though will change as we change the angle of the airfoil relative to the flow direction, known as the angle of attack. We can visualize the angle of attack as being the angle the airfoil makes with the airflow. To determine this angle, it is helpful to be able to visualize the chord for the airfoil. The chord is the line that connects the front (leading edge) and back (trailing edge) of the wing. Indicate on your drawing the chord for this airfoil and angle of attack of the airfoil relative to the flow direction.
Because the location of center of pressure changes with angle of attack, that’s a confusing location to use in analysis since it’s always changing. So, we often use instead the aerodynamic center, which is the point where the pitching moment due to these pressure forces (the moment causing the airfoil to rotate up or down) does not change with angle of attack. For most traditional low speed airfoils, the aerodynamic center is at about the quarter chord point – or 25% of the way from the leading edge to the trailing edge. Because this is a constant regardless of angle of attack, it is a much more convenient location to do analysis about!
Any shape moving through a fluid produces lift and drag, but some shapes are more efficient than others. Pull out your three BLIMP hulls and consider their shapes. Which do you think will move most efficiently through a fluid?
Experiment
Let’s test out your hypothesis. Fill each hull with helium (be sure to tie a string to the hulls so you don’t have to retrieve them off the ceiling). Stand on one side of the room and give each balloon a gentle push. Which one goes the furthest? You may want to repeat this experiment a few times to make sure you are consistent with how hard your little push is. Was your hypothesis correct? Why or why not?
Next Steps
From here, learn more about how wing geometry influences performance in this lesson. If you’re ready to apply math to these concepts, hop to the lesson on moments of inertia. Or, if you particularly enjoyed the experimental piece, start checking out the lessons related to movement.
Last updated: November 23, 2022.
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