Page 33 - Aeronautical Engineer Data Book

20 Aeronautical Engineer’s Data Book
Table 2.10 Dynamic viscosity ( )

Unit lbf-s/ft 2 Centipoise Poise kgf/m s
1 lb (force)-s 1 4.788 4.788 4.882
per ft 2 2 10 4 2 10 2
1 centipoise 2.089 1 10 –2 1.020
2 10 –5 2 10 –4
1 poise 2.089 100 1 1.020
2 10 –3 2 10 –2
1 N-s per m 2 0.2048 9.807 98.07 1
2 10 3

Kinematic viscosity ( ) is a function of dynamic
viscosity.
Kinematic viscosity = dynamic viscosity/
density, i.e. = µ/#
2
The basic unit is ft /s. Other units such as
Saybolt Seconds Universal (SSU) are also used.
2
2
2
6
1 m /s = 10.7639 ft /s = 5.58001 2 10 in /h
–4
2
1 stoke (St) = 100 centistokes (cSt) = 10 m /s
1 St >� 0.00226 (SSU) – 1.95/(SSU) for 32
< SSU < 100 seconds
1 St � 0.00220 (SSU) – 1.35/(SSU) for SSU
> 100 seconds
2.4 Consistency of units
Within any system of units, the consistency of
units forms a ‘quick check’ of the validity of
equations. The units must match on both sides.
Example:
To check kinematic viscosity ( ) =
dynamic viscosity (µ)
333 = µ 2 1/#
density (#)
ft 2 lbf.s ft 4
3 = 3 2 3
s ft 2 lbf.s 2
ft 2 s.ft 4 ft 2
Cancelling gives 3 = 3 = 3
2
s s .ft 2 s
OK, units match.