Answered by David Chandler, Pilot, United Airlines
While the shape of the top of the wing does contribute to lift because of Bernoulli’s equation it is not the only way wings create lift. When an airfoil is inverted it will still create lift because it is using Newton’s 3rd law of motion The wing exerts a force on the air below it, and the air exerts a force back on the wing, so that the wing is constantly deflecting air downward. This is the reason why a water skier or wake boarder floats above the lake and does not sink into it. The faster an aircraft goes the more air is deflected downward creating more lift.
As an aside, something interesting about creating lift is scientists still do not have a complete understanding of it: https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air
Airplanes, birds, kites, insects and anything else that can fly through the air, have a physical force working on them that allows them to fly. That force is called lift.
Lift is the force that lifts things into the air. The lift generated by an airplane depends on the size and shape of the wings, but also on the “angle of attack” of the wings. The picture on the left will help you visualize this better.
As air flows over the surface of a wing, it sticks slightly to the surface it is flowing past and follows the shape. If the wing is angled correctly, the air is deflected downwards.
The action of the wing on the air is to force the air downwards while the reaction is the air pushing the wing upwards. This is Newton’s Third Law: for every action there is an equal and opposite reaction.
Make an Airfoil
The cross-sectional shape of the wing is called an airfoil.
Materials: 4”x6” index card, duct tape, drinking straw, fishing line, hair dryer, pen, table
- Fold the index card in two as in the diagram.
- Push the overlapping ends together. One side of the folded index car with curve up.
- Use a pen to punch 2 holes through the middle of the airfoil, one hole on the top and one on the bottom.
- Carefully push a straw through the holes.
- Pull a piece of fishing line through the straw. Cut the fishing line long enough to fit between the underside of a table and the floor. Hold the fishing line in place, making sure it is perpendicular to the floor and table.
- Tape the fishing line in place between the table and floor. The airfoil should be able to slide freely up and down the line.
- Lift the wind up slightly and air the hair dryer at the folded edge.
- Turn the dryer on. The wing should lift. Point the dryer straight for the best lift.
- Experiment with the angle of attack by moving the string to an 80° angle rather than a 90° angle. Turn the air dryer on. Observe.
- Move string to 70° and 60°. Observe.
- Turn the airfoil upside down and test it again.
- What will happen if we increase the angle too much? This is called Stall Angle, and the bird or airplane will lose its lift and fall!