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94 Aeronautical Engineer’s Data Book
z
q R
qθ
qϕ

ϕ R

y
θ r
x
Fig. 5.12 Spherical co-ordinates for axisymmetric flows

into a fluid flow. Figure 5.12 shows the layout
of spherical co-ordinates used to analyse these
types of flow.
Relationships between the velocity compo­
nents and potential are given by:
∂
1
∂
1 ∂
= q = q =
q R
∂R R sin ∂ r ∂
5.9 Drag coefficients
Figures 5.13(a) and (b) show drag types and
‘rule of thumb’ coefficient values.

Shape Pressure drag Friction drag
(%) (%)
D P D f
U
0 100
U
≈ 10 ≈ 90

U
≈ 90 ≈ 10
U
100 0

Fig. 5.13(a) Relationship between pressure and fraction
drag: ‘rule of thumb’