Hi All!  In our last class visit, we discussed what makes an airplane fly.  How can a machine that weighs over 145,000 lbs, or 72.5 tons, glide through the air with the greatest of ease while most of us can't even jump 18 inches high? 

You will remember that we diagrammed the four basic forces that act on an airplane in flight: Gravity, Thrust, Drag, and Lift (see the attached diagram). 

Drag is the sum of all forces that tend to slow the airplane down.  This is easily demonstrated by carefully sticking your hand out the car window (ask your parents first).  With your palm parallel to the ground, your hand easily slices through the air with only a small amount of drag pushing your hand back.  However, when you turn your hand perpendicular to the ground, the drag increases significantly and it takes a lot of strength to hold your hand still.  The shape of an airplane and it's wings are effected by the same principles of Drag. Thrust counteracts Drag.  It is the sum of the forces moving the aircraft forward.  Thrust can be generated by many types of engines: propellers, jet engines, rockets, etc.  When Thrust equals Drag, the airplane is neither accelerating nor slowing down; it is moving at a steady speed. 

Gravity is the natural force on earth that pulls all things straight down toward the ground, including you and me!  Lift is the force that counteracts Gravity.  It is generated by the wing as it moves through the air.  This is where Daniel Bernoulli comes in. 

Bernoulli was a Swiss mathematician who was born in the Netherlands in 1700.  He discovered that as the speed of a moving fluid increases, the pressure within the fluid decreases.  This became known as the Bernoulli Principle.  We demonstrated this effect by blowing between two parallel sheets of paper, and seeing that the sheets actually move closer together, not further apart as most of us expected.  A second demonstration was to blow a strong stream of air (hair dryer or industrial blower works great) into a hole cut into the top of a large box.  The paper plate was then lifted into the downward blowing airstream.  As expected, the air blew the plate downward...until the plate was within 1/2 inch of the top.  At that point, the plate actually lifted upward toward the blowing airstream! The air above the plate was moving fast enough to generate a low pressure area that was stronger than the force of the airflow and gravity! 

Similarly, as an airplane wing moves through the air, the shape of the wing causes the air moving over the wing to move faster than the air passing under the wing.  According to Bernoulli, the faster moving air on top of the wing will have less pressure than the air under the wing.  The difference in the low pressure on top of the wing and the high pressure on the bottom of the wing becomes greater at the airplane's speed increases.  So more speed = more Lift. 

When the speed of the aircraft is fast enough to cause the wing to generate Lift that is stronger that the force of Gravity, the airplane rises into the air!  It is Bernoulli's discovery about fluid flow and pressure that makes flight possible.

So on your next airplane flight, remember that you owe your speedy trip to an 18th century Swiss math teacher!

Check these links for more information:

Forces of flight: http://www.aeromuseum.org/Education/Lessons/HowPlaneFly/HowPlaneFly.html 

Bernoulli: http://home.earthlink.net/~mmc1919/venturi.html, http://pass.maths.org.uk/issue1/bern/