Why Do Planes Fly? Daniel Bernoulli - Theory Of Flight Science

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Why Do Planes Fly? The science of flight is a fascinating subject!
The sketches and text below explain briefly the science of why a plane flies. This is not meant to be a fully scientific or flawless depiction of the principals involved. It is meant merely to help the modeler understand the forces acting on the model plane and to assist in building a better plane after these forces are understood. Daniel Bernoulli's theory briefly discussed. Model aviation is easier if the basic theory of flight is understood and the parts of the aircraft which make flight possible.

01 4 Forces
The Four Forces Acting On An Airplane
Lift The energy of air on the top and bottom of the wing create LIFT which keeps the plane in the air.
Thrust The pulling power of the propellor and the strength of the engine create THRUST which pulls the plane forward.
Gravity The earth's GRAVITY acts to try to pull the airplane down to the earth's surface
Drag Friction between the plane's body and surrounding air creates the DRAG which holds the plane back .

02 4 Forces
DRAG, GRAVITY, and THRUST are easy to understand how to deal with .
There will be less DRAG on a plane with a streamlined design - the air will flow over the curved surfaces rather than impeding it.
GRAVITY will have less effect on a plane built from the lightest materials available.
A properly designed propellor in combination with a properly placed engine will provide the THRUST necessary to pull the plane forward.
But how is LIFT created? And how does it affect the plane?

03 Lift Illustration
A wing is constructed using a design called an airfoil. This wing design, when pulled through the air by the airplanes motor and propellor, causes LIFT. This force lifts the plane off the runway and into the air.
This LIFT occurs because the wings design causes air to flow faster over the top of the wing than it does under the wing. The air pressure is lower on top of the wing than it is below the wing. The High Pressure air under the wing lifts the plane into the air utilizing Bernoulli's Principal.

04 Air Foil Design
The airfoil shape has many designs, depending on the type of airplane it is to be used on.

05 Controlled Movements
Controlled Movements
The terms 'up' and 'down', so clear in meaning on the ground, are meaningless if flying in heavy cloud cover, inclement weather or at night. These special terms were developed to allow the pilot to describe his airplanes movements.
Yaw - movements to the right or left
Pitch - movements up or down relative to the pilot.

06 Vertical Stabilizer
The rudder consists of two parts - one (rudder) movable and one fixed (vertical stabilize or fin). The rudder can move to the right or left and controls the direction of the airplane.
If you move the rudder to the right, the tail of the plane moves to the left and the plane's nose moves to the right.
Move the rudder left and the tail moves to the right, the nose to the left.

07 Horizontal Stabilizer
The horizontal stabilizer has the elevator attatched to it. The elevator is a movable fin, like the rudder and it controls the up and down movement of the plane. Move the elevator up and the tail moves down while the nose of the plane moves up.
Move the elevator down to raise the tail and move the nose of the plane down.

08 Wing
The wing has a movable surface which is called an aileron. The aileron controlls the roll or bank, of the aircraft.
Raise the right ailron to cause a clockwise roll
Raise the left ailron to cause a counterclockwise roll
Lower the right to cause a counterclockwise roll
Lower the left aileron to cause a clockwise roll
The right and left ailerons are used together to achieve banks and rolls at the pilot's discretion.

09 Stability
Stability in flight is obviously very important. Compare the airplane to an arrow. The feathers at the back of the shaft create stability for the arrow to flit straight through the air toward its target.
The horizontal and vertical control surfaces on the plane provide the same sort of control. Rudder and stabilizer work together to keep the nose of the craft pointed forward during flight.

10 Dihedral
Another way to increase the stability of a plane in flight is to design a wing with a dihedral angle. The dihedral is a wing with an upward angle. There are various types of dihedrals used, each with an advantage depending on the type of plane for which it is intended .

11 Center Of Gravity
The center of gravity, or CG, is the point on the airplane where the axis of the pitch, roll, and yaw lines intersect. A model airplane will balance perfectly if suspended from this point. It is usually a point near the front of the plane. A properly adjusted CG will achieve level, straight flight if rudder and stabilizer are straight.
If the CG is too close to the tail, the plane will stall. To correct, add weight to the nose of the plane.
If the CG is too close to the nose, the plane will dive. Add weight to the tail to correct. On model airplanes modeling clay is commonly used to add weight.

Daniel Bernoulli
January 29, 1700 - March 17, 1782
Swiss scientist who did research in both fluids and medicine. While working in Russia, he and an assistant came up with a method for measuring blood pressure by piercing a blood vessel with a glass pipe, a method used for over 200 years after Bernoulli's discovery. The principal is still used to determine the airspeed of airplanes. Further research with fluids led to the discovery of an important

law of physics - the fluid equation, now called Bernoulli's Principal. He discovered that the pressure of a fluid decreases as its speed increases. This principle applies to air pressure, as well, and it explains why planes fly.