Flight is the act of moving through the air, with wings. Gravity is the force generated by the mass of the earth that tugs objects toward its centre. Drag is the resistance of the aircraft to movement through the air. Air is viscous, this means that it sticks to things. The amount of stickiness is measured as the drag of the aircraft. Lift is the translation of the forward motion of the aircraft into forces that act roughly at right angles to the lifting surfaces. Thrust is whatever the aircraft uses to pull or push itself through the air. With gliders, lowering the nose a bit causing the aircraft to ‘fall down’ provides the thrust. In essence the glider always descends. Soaring is the result of finding rising air masses, such that a greater upward movement of the airmass compensates for the natural sink rate of the glider.
Lift is primarily generated by movement of a wing through the air. A wing meets the air at an angle, known as the angle of attack, forcing air to be separated and flow above and below it. The consequence is that air is deflected downward and the wing moves upward. The curve of the wing enhances this process. If the wing meets the air at a greater angle of attack, lift increases. At the same time, the more we angle the wing, the more we increase drag. If we do not increase thrust, that is force pulling the wing through the air, then the wing, and whatever it is attached to (the rest of the aircraft) will slow down. The opposite is true if we decrease the angle of attack. All things being equal, lift and drag will decrease and the wing will go faster for the same amount of thrust. For reference the centre of pressure is the point where a line perpendicular to the relative wind bisects the chord to which the lifting forces have been normalized.
V is the velocity of the air moving over the aircraft and S is the area of the wing and ρ is the density of the air. CD is the coefficient of drag and has two major components. Drag caused by the stickiness of the air to the surfaces is known as parasitic drag and is represented by the coefficient of parasitic drag CDo. Parasitic drag increases with the square of the true airspeed. Drag due to lifting surfaces of the aircraft is represented by the coefficient of induced drag CDi.
In the basic lift equation, CL is the Coefficient of Lift and is a reflection of how the particular geometry of a wing produces lift. It is also affected by the angle the surface of the wing meets the air flowing about it. This angle that the wing is dragged through the air is known as the angle of attack. The angle of attack is further described as the angle with which the wind meets the chord of the wing.
The density of the air is represented by the symbol ρ. We can visualize that if we are displacing the same amount of thicker air that we can better support the weight of the aircraft. Thus as density of the air increases so does the wings ability to generate lift. The density of air is further dependant on the pressure and the temperature. Higher pressures and lower temperatures contribute to increased density, thus better generation of lift. V represents the velocity of the wing through the air. As the wing moves faster through the air, lift is generated proportional to the velocity squared. Finally, S represents the surface area of the wing.