Drag
Another aerodynamic force which acts on a plane in flight is drag, also known as air resistance. Aerodynamic drag is a mechanical force which arises from the interaction between a solid and a fluid, in this case all parts of the plane and the air. For drag to be generated the solid must be moving through the fluid . There must be a difference in velocity. No motion means no drag. On a plane this means either the plane moves through the air or the air moves over the plane.
Parasitic Drag is the drag created when a solid is moved through a fluid and is combination of skin friction, form drag and interference drag. Parasitic drags is one source of total drag. One type of parasitic drag is skin friction. This is the friction or resistance that arises when the molecules in the air (fluid) interact with the body of the plane (solid) and vice versa as seen in figure 3. Factors which effect skin friction are the smoothness of the surface, the smother the surface the less friction, and the viscosity of the air. Another type of drag is form drag which arise from the shape of the plane. The larger the cross section presented, the greater the drag and the more streamlined and smaller the cross section, the less drag produced as seen in figure 3 below.
Parasitic Drag is the drag created when a solid is moved through a fluid and is combination of skin friction, form drag and interference drag. Parasitic drags is one source of total drag. One type of parasitic drag is skin friction. This is the friction or resistance that arises when the molecules in the air (fluid) interact with the body of the plane (solid) and vice versa as seen in figure 3. Factors which effect skin friction are the smoothness of the surface, the smother the surface the less friction, and the viscosity of the air. Another type of drag is form drag which arise from the shape of the plane. The larger the cross section presented, the greater the drag and the more streamlined and smaller the cross section, the less drag produced as seen in figure 3 below.
Figure 3- Skin Friction and Form Drag
The third type of drag which contributes to parasitic drag is interference drag which is generated when two airflow streamlines interfere with each other, for example after moving over the wings of the plane. When these two airflow streamlines mix they are occupying the same area. Subsequently, the airflow streamlines move at a higher velocity, and generate a shock wave, which in turn generats interference drag.
Another type of drag which contributes to total drag is induced drag. As explained in the lift section of this website, the air moving underneath the wing is at higher pressure than the air at the top of the wing. It is this differential pressure which allows a plane to generate lift and fly. In induced drag the high pressure air from underneath the wing curls around the wingtip of the plane into lower pressure area as seen figure 4 below. The air pushes down on the upper part of the wing, generating drag. This is known as wingtip vortex.
Figure 3- Wingtip Vortice
To counter this airlines use winglets. Winglets are tiny wings which redirect the air which curls around the wing. The winglets redirect the air perpendicularly to the airflow so that the air now moves towards the fuselage instead of the wing as seen in figure 5. This means there is less drag as the vortex is weaker as seen in figure 6 and therefore saves airlines money on fuel.