# Bernoulli's principle

Objects that fly using wings get their lift by taking advantage of Bernoulli's principle. Bernoulli's principle says that as a fluid's velocity increases its pressure decreases.

Airplanes and birds have an airfoil shape to each of their wings to produce lift. The airfoil shape produces unequal lengths across the top and bottom of the wing. Air splitting at the front of the wing must rejoin at the back of the wing so as not to create a vacuum. Since the top surface is curved upward and is longer it forces the air to move faster across the top than the bottom. Faster moving fluids create less pressure, so the bottom of the wing creates greater pressure producing lift.

` `

# Upside Down Airfoils

Racing cars use this principle to produce better traction. They place an airfoil on the back of the car but with the curved surface on the bottom. This is commonly called a spoiler. With the curved surface underneath, greater pressure is generated on the top forcing the back of the car down onto the road.

# Forces in Flight

There are four major forces involved in flight.
1. Lift
2. Weight (gravity)
3. Thrust
4. Drag

Lift
In order to fly an object must be able to generate enough lift to overcome its weight. To accomplish this, air must be moving across the wing. This can be done by pushing the wing through the air such as with a jet engine or by moving the air acroos the wing such as with wind. The opposing force of ligt is gravity.

Weight
Weight is the force of gravity. It is a continous downward force that flying objects must constantly overcome to stay aloft. Objects can glide for a very long period of time but will eventually fall back to earth if they are don't generate enough lift to overcome gravity. The opposing forve of gravity is lift.

Thrust
Thrust is the force that moves the object forward. This is generated by a jet engine, propellar or the backward push of a bird wing. To move forward the flying object must overcome drag. To help reduce drag, airplanes are streamlined and most bird shapes have evolved to be streamlined. The opposing force to thrust is drag.

Drag
Drag is actually friction between the moving object and the air. It is also referred to as air resistence. The more
streamlined, or aerodynamic, an object is the less air resistennce the object generates. The opposite force of drag is thrust.