PARASITE DRAG

If an aircraft were flying at zero lift angle of attack the only drag present would be parasite drag.

Parasite drag is made-up of ‘Skin Friction’,’Form’ and ‘Interference’ Drag.

SKIN FRICTION DRAG: Particles of air in direct contact with the surface are accelerated to the speed of the aircraft and are carried along with it. Adjacent particles will be accelerated by contact with the lower particles, but their velocity will be slightly less than the aircraft because the viscosity of air is low. As distance from the surface increases less and less acceleration of the layers of air takes place. Therefore, over the entire surface there will exist a layer of air whose relative velocity ranges from zero at the surface to a maximum at the boundary of the air affected by the presence of the aircraft. The layer of air extending from the surface to the point where no viscous effect is detectable is known as the boundary layer. In flight, the nature of the boundary layer will determine the maximum lift coefficient, the stalling characteristics, the value of form drag, and to some extent the high speed characteristics of an aircraft.

TRANSITION

Consider the flow of air across a flat surface, as in Figure 6.2. The boundary layer will exist in two forms, either laminar or turbulent. In general, the flow at the front will be laminar and become turbulent some distance back, known as the transition point. The increased rate of change in velocity at the surface in the turbulent flow will give more skin friction than the laminar flow. A turbulent boundary layer also has a higher level of kinetic energy than a laminar layer.

Forward movement of the transition point will increase skin friction because there will be a greater area of turbulent flow. The position of the transition point is dependent upon:

 Surface condition - The thin laminar layer is extremely sensitive to surface irregularities.

Any roughness on the skin of a leading portion of an aircraft will cause transition to turbulence at that point and the thickening, turbulent boundary layer will spread out fanwise down-stream causing a marked increase in skin friction drag.

 Adverse pressure gradient (Figure 6.3) - A laminar layer cannot exist when pressure is rising in the direction of flow. On a curved surface, such as an aerofoil, the transition point is usually at, or near to the point of maximum thickness. Because of the adverse pressure gradient existing on a curved surface the transition point will be further forward than if the surface was flat.

TURBULENT FLOW

NOTE: The vertical scale of the boundary layer in the above sketch is greatly exaggerated. Typically, boundary layer thickness is from 2 millimetres at the leading edge, increasing to about 20 millimetres at the trailing edge.

FORM (PRESSURE) DRAG: Results from the pressure at the leading edge of a body being greater than the pressure at the trailing edge. Overall, skin friction causes a continual reduction of boundary layer kinetic energy as flow continues back along the surface. The adverse pressure gradient behind the transition point will cause an additional reduction in kinetic energy of the boundary layer. If the boundary layer does not have sufficient kinetic energy in the presence of the adverse pressure gradient, the lower levels of the boundary layer stop moving (stagnate).

The upper levels of the boundary layer will overrun at this point (separation point) and the boundary layer will separate from the surface at the separation point. See Figure 6.3. Also, surface flow aft of the separation point will be forward, toward the separation point - a flow reversal. Because of separation there will be a lower pressure at the trailing edge than the leading edge. An aerodynamic force will act in the direction of the lower pressure - form drag.

Separation will occur when the boundary layer does not have sufficient kinetic energy in the presence of a given adverse pressure gradient.

Loss of kinetic energy in the boundary layer can be caused by various factors.

 As angle of attack increases, the transition point moves closer to the leading edge and the adverse pressure gradient becomes stronger. This causes the separation point to move forward. Eventually, boundary layer separation will occur so close to the leading edge that there will be insufficient wing area to provide the required lift force, C _L will decrease and stall occurs.

 When a shock wave forms on the upper surface, the increase of static pressure through the shock wave will create an extreme adverse pressure gradient. If the shock wave is sufficiently strong, separation will occur immediately behind the shock wave. This will be explained fully in Chapter 13 - High Speed Flight.

Laminar and Turbulent Separation: Separation has been shown to be caused by the airflow meeting an adverse pressure gradient, but it is found that a turbulent boundary layer is more resistant to separation than a laminar one when meeting the same pressure gradient. In this respect the turbulent boundary layer is preferable to the laminar one, but from the point of view of drag the laminar flow is preferable.

Streamlining: Each part of an aircraft will be subject to form (pressure) drag. To reduce form drag it is necessary to delay separation to a point as close to the trailing edge as possible.

Streamlining increases the ratio between the length and depth of a body, reducing the curvature of the surfaces and thus the adverse pressure gradient. Fineness ratio is the measure of streamlining. It has been found that the ideal fineness ratio is 3:1, as illustrated in Figure 6.4.

NOTE: The addition of fairings and fillets (see Glossary, Page 1-10) at the junction of components exposed to the airflow is also referred to as “Streamlining”.

Length Depth

Figure 6.4 Figure 6.4

Profile Drag: The combination of skin friction and form drag is known as profile drag. It can be considered that these drags result from the “profile” (or cross-sectional area) of the aircraft presented to the relative airflow.

INTERFERENCE DRAG: When considering a complete aircraft, parasite drag will be greater than the sum of the parts. Additional drag results from boundary layer ‘interference’ at wing/

fuselage, wing/engine nacelle and other such junctions. Filleting is necessary to minimise interference drag.