DRAG COUNTS 9
As a measure of an airplane’s drag, in practice one will frequently hear the term “drag count” used. Usually, it is used in an incremental or decre – mental sense, such as “fairing the landing gear reduced the drag by 20 counts.” One drag count is defined simply as a change in the total airplane Co, based on the wing planform area of 0.0001. Hence a reduction in drag of 20 counts could mean a reduction in the CD from, say, 0.0065 down to 0.0045.
average skin friction coefficients
In examples to follow, one will see that several uncertainties arise in attempting to estimate, the absolute parasite drag coefficient (as opposed to incremental effects) of an airplane. These generally involve questions of interference drag and surface irregularities. In view of these difficulties, it is sometimes better4 to estimate the total drag of a new airplane on the basis of the known drag of existing airplanes having a similar appearance, that is, the same degree of streamlining and surface finish.
The most rational basis for such a comparison is the total wetted area and not the wing area, since Q depends only on the degree of streamlining and surface finish, whereas CD depends on the size of the wing in relation to the rest of the airplane. In terms of an average CF, the parasite drag at zero CL for the total airplane can be written as
D = qCpSw (4.36)
where Sw is the total wetted area of the airplane. Since
it follows that the ratio of the equivalent flat-plate area to the wetted area is
In order to provide a basis for estimating CF, Table 4.2 presents a tabulation of this quantity for 23 different airplanes having widely varying configurations. These range all the way from Piper’s popular light plane, the Cherokee, to Lockheed’s jumbo jet, the C-5A.
The data in this table were obtained from several sources and include results obtained by students taking a course in techniques of flight testing. Thus, the absolute value of CF for a given airplane may be in error by a few percent. For purposes of preliminary design, the CF ranges given in Table 4.3 are suggested for various types of airplanes. Where a particular airplane falls in the range of CF values for its type will depend on the attention given to surface finish, sealing (around cabin doors, wheel wells, etc.), external protuberances, and other drag-producing items.
Additional drag data on a number of airplanes, including supersonic airplanes, are presented in Appendix A.3 as a function of Mach number and altitude.
Finally, with regard to average CF values, Figure 4.26 (taken from Ref. 4.11) is presented. Although only a few individual points are identified on this figure, its results agree generally with Table 4.2. This figure graphically depicts the dramatic improvement in aerodynamic cleanliness of airplanes that has been accomplished since the first flight of the Wright Brothers.
Table 4.2 Typical Overall Skin Friction Coefficients for a Number of Airplanes Built from Approximately 1940 to 1976. Data Taken from Several Sources
|
Table 4.3 Typical Total Skin Friction Coefficient Values for Different Airplane Configurations
Airplane Configuration CF Range at Low Mach Numbers
Propeller driven, fixed gear 0.008-0.010
Propeller driven, retractable gear 0.0045-0.007
Jet propelled, engines pod-mounted щ 0.0035-0.0045
Jet propelled, engines internal 0.0030-0.0035