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 Airplane Cf Designation Description 0.0100 Cessna 150 Single prop, high wing, fixed gear 0.0095 PA-28 Single prop, low wing, fixed gear 0.0070 B-17 Four props, World War II bomber 0.0067 PA-28R Single prop, low wing, retractable gear 0.0066 C-47 Twin props, low wing, retractable gear 0.0060 P-40 Single prop, World War II fighter 0.0060 " F-4C Jet fighter, engines internal 0.0059 B-29 Four props, World War II bomber 0.0054 P-38 Twin props, twin-tail booms, World War II fighter 0.0050 Cessna 310 Twin props, low wing, retractable gear 0.0049 Beech V35 Single prop, low wing, retractable gear 0.0046 C-46 Twin props, low wing, retractable gear 0.0046 C-54 Four props, low wing, retractable gear 0.0042 Learjet 25 Twin jets, pod-mounted on fuselage, tip tanks 0.0044 CV 880 Four jets, pod-mounted under wing 0.0041 NT-33A Training version of P-80 (see below) 0.0038 P-5 IF Single prop, World War II fighter 0.0038 f).0037 C-5A Four jets, pod-mounted under wing, jumbo jet Jetstar Four jets, pod-mounted on fuselage 0.0036 747 Four jets, pod-mounted under wing, jumbo jet 0.0033 P-80 Jet fighter, engines internal, tip tanks, low-wing 0.0032 F-104 Jet fighter, engines internal, midwing 0.0031 A-7A Jet fighter, engines internal, high wing

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