Aerodynamics of Flight

The airflow outside of the boundary layer reacts to the shape of the edge of the boundary layer just as it would to the physical surface of an object. The boundary layer gives any object an “effective” shape that is usually slightly different from the physical shape. The boundary layer may also separate from the body, thus creating an effective shape much different from the physical shape of the object. This change in the physical shape of the boundary layer causes a dramatic decrease in lift and an increase in drag. When this happens, the airfoil has stalled. In order to reduce the effect of skin friction drag, aircraft designers utilize flush mount rivets and remove any irregularities that may protrude above the wing surface. In addition, a smooth and glossy finish aids in transition of air across the surface of the wing. Since dirt on an aircraft Figure 5-9. Wingtip vortex from a crop duster. disrupts the free flow of air and increases drag, keep the surfaces of an aircraft clean and waxed. altitude versus near the ground. Bearing in mind the direction of rotation of these vortices, it can be seen that they induce Induced Drag an upward flow of air beyond the tip and a downwash flow The second basic type of drag is induced drag. It is an behind the wing’s trailing edge. This induced downwash has established physical fact that no system that does work in the nothing in common with the downwash that is necessary to mechanical sense can be 100 percent efficient. This means produce lift. It is, in fact, the source of induced drag. that whatever the nature of the system, the required work is obtained at the expense of certain additional work that is Downwash points the relative wind downward, so the more dissipated or lost in the system. The more efficient the system, downwash you have, the more your relative wind points the smaller this loss. downward. That's important for one very good reason: lift is always perpendicular to the relative wind. In Figure 5-11, you In level flight, the aerodynamic properties of a wing or rotor can see that when you have less downwash, your lift vector produce a required lift, but this can be obtained only at the is more vertical, opposing gravity. And when you have more expense of a certain penalty. The name given to this penalty downwash, your lift vector points back more, causing induced is induced drag. Induced drag is inherent whenever an airfoil drag. On top of that, it takes energy for your wings to create is producing lift and, in fact, this type of drag is inseparable downwash and vortices, and that energy creates drag. from the production of lift. Consequently, it is always present if lift is produced. An airfoil (wing or rotor blade) produces the lift force by making use of the energy of the free airstream. Whenever an airfoil is producing lift, the pressure on the lower surface of it is greater than that on the upper surface (Bernoulli’s Principle). As a result, the air tends to flow from the high pressure area below the tip upward to the low pressure area on the upper surface. In the vicinity of the tips, there is a tendency for these pressures to equalize, resulting in a lateral flow outward from the underside to the upper surface. This lateral flow imparts a rotational velocity to the air at the tips, creating vortices that trail behind the airfoil. When the aircraft is viewed from the tail, these vortices circulate counterclockwise about the right tip and clockwise about the left tip. [Figure 5-9] As the air (and vortices) roll off the back of your wing, they angle down, which is known as Figure 5-10. The difference in wingtip vortex size at altitude versus near the ground. downwash. Figure 5-10 shows the difference in downwash at 5-7