Equivalent Drag Area

Hoerner use the drag area as a convenient means to represent the relative drag of the various airplane components. These are summed at the end of the analysis to determine overall drag. It is necessary to understand that this sum must include corrections to account for the interference drag created by the influence of one element on another. An important source of such drag is the interference between the fuselage and the wing. The smooth fairing, or fillet, at the juncture of the fuselage and wing of the Bf-109 was intended to reduce this drag contribution to a practical minimum. Drag area is defined as:

(drag areaX = D = (CdS )i, (9.1)

where q is the dynamic pressure, Di is the drag of the component i, and S is an appro­priate reference area. Notice that depending on the component, different reference areas may be used. When working with aerodynamic surfaces such as the wing and empennage, the planform area almost always is used. For other parts, the projected frontal area often is used. Thus, use of this drag area combines the area and the related drag coefficient in a way that prevents errors in applying the results in a sum­mation to determine the complete vehicle drag.

In defining the overall drag, the usual convention is to use the projected wing area, S, as the reference. S is the area found by extending the wing planform to the fuselage centerline. Hoerner stated that the maximum speed of the 1944 Bf-109G was 610 km/hr (380 mph) at an altitude of about 22,000 feet. This corresponds to an effective dynamic pressure of:

The thrust of the Daimler Benz DB605 (i. e., 12-cylinder, inverted “V” engine) plus the jet thrust from the ejector exhaust stacks was about 1,140 lbf. Therefore, because the drag is equal to the thrust in unaccelerated level flight, the overall drag area of the Bf-109G was:

(drag area)total =£ (QS); = 6.2 ft2.

i

Because the projected wing area (S = 172 ft2 for the Bf-109G) usually is used in describing the overall aerodynamic performance, the resultant overall drag coefficient is CD = 0.036 at the stated flight condition. We follow Hoerner in com­piling a breakdown of all drag sources that contribute to this total. It is important to follow the analysis, because many of the concepts developed in previous chapters are reviewed and an understanding of the capabilities as well as the limitations of the analysis are demonstrated. Will careful application of all that has been learned give a total drag area in fair agreement with the one from experimental flight data? We shall now proceed to find out.