Page 17 - Aeronautical Engineer Data Book

Fundamental dimensions and units 7
2.2 Units systems
The most commonly used system of units in the
aeronautics industry in the United States is the
United States Customary System (USCS). The
‘MKS system’ is a metric system still used in
some European countries but is gradually being
superseded by the expanded Système Interna­
tional (SI) system.

2.2.1 The USCS system
Countries outside the USA often refer to this
as the ‘inch-pound’ system. The base units are:
Length: foot (ft) = 12 inches (in)
Force: pound force or thrust (lbf)
Time: second (s)
Temperature: degrees Fahrenheit (°F)

2.2.2 The SI system
The strength of the SI system is its coherence.
There are four mechanical and two electrical
base units from which all other quantities are
derived. The mechanical ones are:

Length: metre (m)
Mass: kilogram (kg)
Time: second (s)
Temperature: Kelvin (K) or, more
commonly, degrees Celsius or
Centigrade (°C)
Other units are derived from these: e.g. the newton
2
(N) is defined as N = kg m/s . Formal SI conver­
sion factors are listed in ASTM Standard E380.
2.2.3 SI prefixes
As a rule, prefixes are generally applied to the
basic SI unit, except for weight, where the prefix
is used with the unit gram (g), not the basic SI
unit kilogram (kg). Prefixes are not used for units
of angular measurement (degrees, radians), time
(seconds) or temperature (°C or K).
Prefixes are generally chosen in such a way
that the numerical value of a unit lies between
0.1 and 1000 (see Table 2.2). For example: