EXAMPLE ESTIMATES OF DRAG BREAKDOWN

The use of Equation 4.35 is illustrated in Table 4.4, where I have performed an estimate of the drag breakdown for the Cherokee (Figure 3.62). Armed with a tape measure, I made a visual inspection of the airplane, noting the dimensions of all drag-producing appendages. What you see here is a first estimate without any iteration. The total value for / of 0.36 m2 (3.9 ft2) is obviously too low and should be approximately 50% higher. This airplane has a total wetted area of approximately 58.06 m2 (625 ft2).

Undoubtedly, an aerodynamicist working continuously with this type of aircraft would be able to make a drag breakdown more accurate than the one shown in Table 4.4. Based on one’s experience with his or her company’s aircraft, the aerodynamicist can make more certain allowances for surface roughness, interferences and leakage. For example, in the case of the Cherokee’s landing gear, the oleo struts have a bar linkage immediately behind them. The linkage, struts, wheel pants, and brake fittings produce a total drag for the entire landing gear that is probably significantly higher than the total / of 0.065 m2 (0.7 ft2) estimated for the wheels, wheel pants, and struts. The cylindrical oleo struts, in particular, being close to the wheel pants probably produce separation over the pants so that Cd for this item could be

Reference Area	cd	Basis for cd	F = CdA
160 (plan)	0.0093	Figure 4.12 + 50% for roughness	1.49
15.2 (front)	0.058	Figure 4.13 11 de = 5	0.88
25 (plan)	0.0084	Figure 4.12 + 50% for roughness	0.21
11.5 (plan)	0.0084	Figure 4.12 + 50% for roughness	0.10
0.63 (front)	0.3	Figure 4.6 supercritical	0.19
1.75 (front)	0.04	Figure 4.11	0.07
0.63 (front)	0.70	Figure 4.10 corrected to three-dimensional	0.44
0.02 (front)	1.0	Figure 4.7	0.02
0.09 (front)	1.0	Figure 4.7a	0.09

Gas drain cocks

Rotating beacon

Tail tie-down Wing tie-downs

Five whip antennas

OAT gage Antenna fairing

Antenna supports Interference Fuse vertical tail Fuse horizontal tail Fuse wing

1 in. x I in. blunt (2 total)

4 in. D x 5 in.

semispherical 3| in. x g in. blunt 1 in. x I in. blunt (2)

з in. D x 20 in. each

3 in. D x 2 in. each 12 in. long, I in. thick, 2 in. chord 3 in. x I in. D blunt 3 in. deep x 6 in. width stream­lined

5 in. x I in. D (two)

Leakage?

Cooling?

0.01 (front)	1.0	Figure 4.7	0.01
0.14 (front)	0.15	Figure 4.6	0.02
0.01 (front)	1.0	Figure 4.7	0.01
0.01 (front)	1.0	Figure 4.7	0.01
0.17 (front)	1.0	Figure 4.7 ^	0.17
		subcritical
0.08 (front)	0.2	Figures 4.6 and 4.7b	_ 0.02
0.06 (front)	0.06	Figure 4.11	0.04
		+50% for roughness
0.02	1.0	Figure 4.7	0.02
0.13	0.06	Figure 4.11	0.01
		+50% for roughness
0.03	1.0	Figure 4.7	0.03
0.11 (Г2)	0.05	Ref. 4.4
0.06 (t2)	0.05		0.07
0.62 (t2)	0.1

Total

Table 4.5 Parasite Drag Breakdown for Gates Learjet Model 25 (from Ref. 4.10) Item ’ Cd (based on Wing Planform Area) Percent of Total Wing л 0.0053 23.45 Fuselage 0.0063 27.88 Tip tanks 0.0021 9.29 Tip tank fins 0.0001 0.44 Nacelles 0.0012 5.31 Pylons 0.0003 1.33 Horizontal tail 0.0016 7.08 Vertical tail 0.0011 4.86 Interference 0.0031 13.72 Roughness and gap 0.0015 6.64 Total 0.0226 100.00

more like 0.4 or even higher instead of 0.04, as listed in Table 4.4. This would add another 0.06 m2 (0.63 ft2) to /.

Another example of a drag breakdown is provided by Reference 4.10. In this case, the airplane is the Gates Learjet Model 25 pictured in Figure 4.27. Table 4.5 was prepared on the basis of Reference 4.10. The authors of the reference chose to base Cd for each item on the wing area. This is therefore the case for Table 4.5, since dimensions and areas for each item were not available. Also, the reference did not include interference or roughness and gap drag in the parasite drag. Why this was done is not clear, and these two items are included in Table 4.5. These two somewhat elusive drag items are estimated to account for 20% of the parasite drag. Although not related to its parasite drag, according to Reference 4.10, this airplane has an Oswald’s efficiency factor of 0.66.