Lift and Drag

Drag = C D 1/2pV*VS

V is the velocity of the air moving over the aircraft and S is the area of the wing and r is the density of the air. C D is the coefficient of drag and has two major components. Drag cause by the stickiness of the air to the surfaces of the aircraft alone. This is known as parasitic drag and is represented by the coefficient of parasitic drag C Do . Parasitic drag increases with the square of the true airspeed. The second component of drag has to do with the lift surfaces of the aircraft alone. Lift is generated by displacing air. When the wing does this certain inefficiencies are produced in the form of drag. This drag due to the lifting of the wing is represented by the coefficient of induced drag C Di . Drag is therefore characterized by:

C D = C Di + C_Do

Delving a bit more deeply into induced drag we find that:

C Di = C L*C L / pAHere C L is the coefficient of lift for the wing, and A is the aspect ratio. The aspect ratio is the span, or the wingtip to wingtip distance squared, divided by the area of the wing. 

AspectRatio = Span /Mean Chord.

On the other side of the coin we have lift. The wing translates the horizontal flow of air over its surfaces to a vertical lifting force. The basic equation that describes the generation of the lifting force is: 

Lift = C L 1/2 rV *V S

C L 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 that the surface of the wing meets the air flowing about it. The angle that the wing is dragged through the air is known as the angle of attack and is described as the angle that the wind meets the chord of the wing. The chord of the wing is an imaginary line drawn from the leading edge (front edge) to the trailing edge (back edge) of the wing.

We often encounter the term relative wind, which is used to describe this airflow. Motion of air over the earth’s surface is called wind. The motion of the wing through still air would still feel like a wind to someone in the aircraft, however it is really motion felt due to the movement of the aircraft alone, hence the term relative wind, relative to the motion of the aircraft. The density of the air is represented by the symbol r. 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. It stands to reason that larger wings will generate more lift than smaller ones. In summary lift depends on the coefficient of lift reflected in the shape of the aerofoil and the angle that it meets the airflow, the speed that the wing moves through the air, and the surface area. Maximum Lift versus Drag (L/D) occurs at the point where the effects of induced and parasitic drag are near minimum and lift is near maximum. Longer spans with higher aspect ratios promote more lift with less drag. Minimum sink is the airspeed at which there is minimum rate of loss of altitude in still air.

High efficiency airfoils have shapes that promote the smooth or laminar flow of air around their curved or cambered surfaces over as wide a range of flight angles as is possible. Often these airfoils are more affected by dirt, moisture or other imperfections in their surfaces than the older turbulent airfoil designs. For most airfoils, as the angle of attack is increased, the lift increases, the drag increases, and the centre of pressure where the lift acts perpedicular to the chord of the wing moves forward. This continues until the critical or stalling angle is reached when the lift drops dramatically, drag increases further, and the centre of pressure usually moves aft.