Page 32 - Aeronautical Engineer Data Book

Fundamental dimensions and units 19
2.3.14 Length and area
Comparative lengths in USCS and SI units are:
1 ft = 0.3048 m
1 in = 25.4 mm
1 statute mile = 1609.3 m
1 nautical mile = 1853.2 m
2
The basic unit of area is square feet (ft ) or
2
2
square inches (in or sq.in). In SI it is m . See
Table 2.9.
Small dimensions are measured in ‘micro­
measurements’ (see Figure 2.8).
The microinch (µin) is the commonly used unit
for small measures of distance:
–6
1 microinch = 10 inches = 25.4 micrometers (micron )

Diameter of a
Smoke hair: 2000µin
particle
120µin Oil filter
mesh
450µin
1 micron (µm) = 39.37µin
A smooth-machined ‘mating’
surface with peaks 16 –32µin

A fine ‘lapped’ surface
with peaks within 1µin

Fig. 2.8 Micromeasurements

2.3.15 Viscosity
2
Dynamic viscosity (µ) is measured in lbf.s/ft or,
2
in the SI system, in N s/m or pascal seconds
(Pa s).
2
2
1 lbf.s/ft = 4.882 kgf.s/m = 4.882 Pa s
2
1Pas = 1Ns/m = 1 kg/m s
A common unit of viscosity is the centipoise
(cP). See Table 2.10.