Production of lift
Lift can be described by the "Laws of Motion" formulated by Sir Isaac Newton in the 17th century. Two laws in particular can be used to describe lift. As you might know, lift acts at right angles to the relative air flow and should overcome weight if the pilot wishes to take-off and gain altitude or balance eachother out in straight and level flight.

Newton's second law of motion states that a force results whenever a mass is accelerated. His third law of motion basically says that for every action there is an equal and opposite reaction. Due to the shape of an airfoil and angle relative to the free air flow, air is forced to curve downward, creating a downwash behind the wing. By applying the third law of motion it is easy to understand that this downwash force will cause an opposite reaction force pushing the wing up.

The above mentioned laws describe what happens when air is deflected downward, but what is the technical mechanism that actually accomplishes this change of direction? Lets have a look at a thin metal plate oriented parallel to a streamlined air flow (figure 1.1). As you can see it causes viritually no alteration to that particular air flow thus experiencing no reaction force. If the plate is somewhat inclined with respect to the streamlined air flow, it experiences a force that both lifts the plate upward and drag it back (figure 1.2). Due to this inclination the streamlined flow of air is disturbed and an upwash is created in front of the plate causing the air to flow through a more constricted area. Passing of air through this constricted area causes it to speed up thus increasing velocity while decreasing static pressure. This can be related to "Bernoulli's Principle" which states that as the velocity of a fluid increases its pressure decreases.

Figure 1.1 - Air flow around a flat plate Figure 1.2 - Air flow around an inclined plate

Static pressure above the plate is now lower than below causing a net upward reaction. The aerodynamic force created by disturbing the air flow around the plate has two components: one at right angles to the relative air flow, called lift, and one parallel to the airflow opposing the direction of movement called drag (figure 1.3).

Figure 1.1 Component of lift

As you might understand, a flat plate is normally not used as a lift generator simply because it breaks up the air flow causing eddying (turbulence) thus decreasing lift while increasing drag. Todays modern aircraft have highly efficient air foils specifically designed to meet performance demands.

Relative air flow, or wind, directly opposses the direction of the airplanes flightpath. Three of the four forces, lift, drag and thrust, are associated with the relative air flow. Unlike the forces previously mentioned, weight acts through the "Center of Gravity" toward the center of the earth. The angle of attack is defined as the angle between the relative air flow and the chord line of the wing (figure 1.4).

 

Newton's Laws
  • Newton's First Law states that an object at rest tends to stay at rest and that an object in uniform motion tends to stay in uniform motion unless acted upon by a net external force.
  • Newton's Second Law states that an applied force, F , on an object equals the time rate of change of its momentum, p .
  • Newton's Third Law states that for every action there is an equal and opposite reaction.
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