Daniel Bernoulli (1700-1782) was a Swiss scientist who studied fluid flows. From his work emerged “Bernoulli’s
Principle.” This law states that for a steady flow of a fluid, the total energy (combination of kinetic energy from the
velocity of the flow, the potential energy from elevation and gravity, and the pressure energy exerted by the fluid)
remains constant along its flow route. Therefore, when velocity increases, the pressure energy of the fluid must
decrease to maintain a constant level of total energy. In simpler terms, as velocity increases, pressure decreases.
Bernoulli’s Principle is used throughout the aviation industry. From the production of lift to the workings of a
carburetor, the relationship between velocity and changes in pressure is paramount to the operation of aircraft.
Bernoulli on a Straw
Bernoulli’s Principle may be illustrated using a simple device
manufactured from common materials. To construct a Bernoulli on a
Straw demonstrator, a straw, segment of cellophane tape, and strip of
newspaper are needed. A milkshake straw works well. Milkshake
straws may be distinguished from common soda straws by their inside
diameters. The former use a larger inside diameter than the latter.
Place a strip of newspaper that approximately measures 1¼” (3.17 cm)
by 6" (15.24 cm) on a flat surface. Cut the tip of one end of the straw at a
45/ angle to provide a bevel. Center the beveled tip of the straw so that
the bevel is downward and around 1" (2.5 cm) from one end of the
newspaper. The remaining 5" (12.7 cm) of newspaper should be free of
the straw. Carefully attach the tip of the straw to the newspaper using
cellophane tape. Ensure that the tape runs perpendicular to the straw
and even with the tip of the straw without blocking the orifice. Lift the unit from the surface. The newspaper should
droop away from the tip of the straw.
To fly the newspaper, raise the device to your lips (be certain
that the newspaper is hanging beneath the straw) and blow through
the free end of the straw. Depending on relative positions of the
straw and the newspaper, the strip of paper should lift and become
parallel with the length of the straw. Blowing excessively hard may
produce a fluttering action. If necessary, begin the flight by blowing
hard and lessen the airflow to achieve a steady flight.
Another, and more telling, demonstration of Bernoulli’s Principle
is shown when raising the straw to a vertical position and blowing.
In this example, the strip of newspaper is shaped like an inverted letter “J.” By vigorously blowing through the straw,
a low pressure of significant magnitude is produced. Such a pressure causes the newspaper to lift until it is parallel
with the straw. At this point, the newspaper is vertical.
Place the unit in your mouth, walk forward, and blow through the straw until the paper rises. Try it again at a
faster speed. For a real challenge, run forward while trying to lift the paper. Two forces are working against the
lifting of the newspaper. First, drag is acting on the paper as it dangles from the end of the straw. The lifting force
must overcome this measure of drag. Also, as you move forward, some of the velocity of the accelerated air flow is
negated by the forward motion of the unit. This lessens the effect of the exhaled airflow
The force producing the action associated with this demonstrator is Bernoulli’s Principle and the pressure
differential created. The air that passes over the newspaper is traveling at an accelerated rate. This generates a
low pressure on that side of the newspaper. Because the other side of the paper has higher pressure, stationary air,
a pressure differential is produced. When the force acting across the surface of the newspaper is sufficient to lift the
paper, the newspaper moves in the direction of the low pressure.
Hot Wings
Another demonstration of Bernoulli’s Principle may be performed using simple wings, Mattel Hot Wheels, and a segment of track.
Construct the wings from a manila folder. After trimming off the folder’s tab, cut it in half so that
the cut is perpendicular to the folded end. Shove one side of the folder towards the folded end to give the wing its
shape. Staple the trailing edge of the wing so that it retains its airfoil shape. Glue the wing to the top of one of the cars.
Hot glue or epoxy work well for this purpose. Ensure that the cambered (curved) surface points towards the
front of the car.
Construct the second wing and attach it to the second
car ensuring that the cambered surface is towards the front of the car.
Place both cars on the track so that they are facing each other. Refer to
the figure showing the top view of the Hot Wings.
Space the cars apart to allow for forward movement of each car.
Generate a low pressure between the wings by blowing in between
them. You may either vigorously blow in between the two wings or use a
blow dryer. As the air is accelerated, an area of low pressure is
produced between the airfoils. A pressure differential is created between
the outboard surfaces of the wings where the air is stationary and the
inboard surfaces where the air is rapidly moving. When this pressure
differential is distributed across the area of the wing, enough force is
generated to propel the vehicles towards each other.
Where these cars
move in a horizontal direction, the force generated is similar to the lift
developed by a wing in flight or a rotor of a helicopter as it revolves in its
circular path. Examine the illustration depicting the action of the blow
dryer.
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