Swiss scientist Daniel Bernoulli is often considered the father of fluid mechanics, and developed a mathematical relationship between pressure and fluid flow in the 18th century. Both liquids and gases are considered fluids, and the Bernoulli Principle is a mathematical explanation of why things can fly. The faster a fluid flows, the lower pressure it has. Flying objects (and animals) move very quickly relative to their size. Wings, called airfoils when being designed by engineers, are typically rounded on top and flat on the bottom so the pressure on the top of the wing is lower than the pressure of the bottom, creating lift. Lift is a force that is perpendicular to the direction of fluid flow— when air is flowing towards an object, lift is in the upward direction, opposite of gravity.
Planes, cars, bikes, and many other objects are used in wind tunnels to test aerodynamics, or how an object moves through air at many speeds. For bikes and cars, we want them to be pushed lower into the ground (no lift), whereas with airplanes we want them can to create lift so they can fly through the sky. Wind tunnels can be whole buildings which use powerful fans to test life size objects, or small models, like the one you will build and test today.
- Two sheets of paper
- Cardboard box
- Box cutter
- Clear plastic
- Clean air filter
- Duct tape
- Digital scale
- Several different lightweight model airplanes of different shapes (foam works well)
- Get a large, stiff cardboard box that is open at both ends.
- On one side of the box, have an adult cut out a viewing window.
- Duct tape the clear plastic sheet over the viewing window. Make sure it is pulled taut.
- Place the fan at one end of the box facing inward.
- Duct tape the air filter in front of the fan at a distance of about one-third of the box. The filter will help even out the airflow.
- Place a scale in the center of the box so the digital read out is facing the viewing window.
- Tape a small cube of balsa wood to the scale to elevate the plane; this will allow airflow both above and below the airfoil.
- If you think the plane is light enough to fly off the scale, glue piece of string to the nose and tape it to the filter. You can also use adhesive to stick the plane to the wood cube.
- Set the plane on the scale with the nose facing the fan to gather lift data.
- Record the weight of the plane in grams before turning the fan on.
- Turn on the fan, let it run for a few moments, and then record the weight of the plane again. What happens?
- Repeat for different fan speeds and different plane models. Record your data.
The scale will read a lighter weight as the plane is experiences lift. The plane will achieve more lift at faster fan speeds.
The original reading on the scale, the weight, is due to the force of gravity. The plane appears lighter as wind is blown over and under the wings and creates lift. The lift force acts in the opposite direction of the downward gravitational force so the plane puts less pressure on the scale. The shape of the object is one of the determining factors in how much lift an object experiences. This is why wings on planes and birds are similarly shaped.
Air flows over and under the wings. Due to the shape of the wing, the pressure on the top is lower than the pressure on the bottom, creating lift.
The shape of objects can also push them closer to the ground rather than lift them up, like the airfoil—also called a spoiler—on a car. The way the air travels over the car and airfoil force air over the car, adding additional pressure (and thus force) downward. Racing cars use this to make tight turns without flipping or skidding.