LIFT

• Principles of Lift
  The structure of the wing best demonstrates the principle of airfoil lift. In the 19th century a scientist named Bernoulli discovered that the intenal pressure of a fluid (liquid or gas) reduces the faster the fluid flows. If you take a tube, and make the tube smaller in diameter in the middle, this creates a "necked-down" section called a venturi. When air is forced through the pipe, as much air has to come out the exit as goes in the tube entrance. The air in the venturi section must travel faster to get through. Bernoulli found that the pressure at the venturi section was less than at the two ends of the pipe. This is because the speed of the air through the venturi section is traveling faster than at the ends of the tube.

• The Airfoil
  The shape of a wing is called an AIRFOIL. Usually the bottom of the wing is flat or nearly flat. The top of the wing is curved, with the wing being thicker at the front edge of the wing, and tapering to a thin surface at the trailing edge of the wing. When a wing airfoil surface passes through the atmosphere, the atoms of the air on the top of the airfoil (shown as minus) must travel faster than their cousins (shown as plus) passing along the lower and flater surface. This occurs because the distance the air must pass over the curved top of the wing is longer than the distance along the lower surface. According to the Bernoulli Principle, the pressure above the wing is less than the pressure of air below it. Consequently, a pressure difference between the lower and upper surfaces exist. This results in LIFT being produced. The amount of lift depends on the airfoil design and the speed of the air over its surfaces.

• Camber
  The curved surface of an airfoil is called Camber. It can be both Positive and Negative. The curved upper surface of a wing is called Positive Camber. If the lower surface of the airfoil is curved downward, this would constitute a negative camber.

• Chord
  The chord of a wing is an imaginary line from the leading edge to the trailing edge of the wing. The term is used in the definition of "Angle of Incidence" and "Angle of Attack" (defined later).

• Angle Of Incidence
  The angle which the chord of the wing makes with the longitudinal centerline of the aircraft is known as the Angle of Incidence. This angle in a given aircraft never changes. It is fixed by the construc tion of the aircraft.

• Angle Of Attack
  As the aircraft passes through the air it traverses a particular line of flight. The air passing by the surfaces of the aircraft in the opposite direction of travel is called the Relative Wind. The angle which the wing chord makes with this Relative Wind is called Angle of Attack. An increase in angle of attack increases both lift and drag. If the angle becomes to great, it will pass the Critical Angle of Attack. This is a point where the airflow over the wing becomes so disturbed that the wing ceases to produce lift. The wing then enters into a Stalled condition. Stalls will be described more fully later in this chapter.

• Center of Pressure
  Even though the lift of an airfoil is distributed along its surface, the resultant force of all the lift forces can be considered to be at single point along the wing known as the Center of Pressure.

• Center of Lift
  The Center of Lift( shown as CL in the diagram) is the same point as the Center of Pressure. You can think of all the lift of the wing as being a single force concentrated at this point on the wing.

• Dihedral
  When you stand in front of an aircraft, looking toward the tail, the wings are usually higher at the wing tips than at the wing root (where the wing attaches to the fuselage). This upward angle from wing root to tip is called DIHEDRAL.
  
  On an aircraft with dihedral, when one wing drops, it will produce slightly greater lift than the other wing. The aircraft tends to return to a level status providing lateral stability to the aircraft.

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