A study in any science must include measurement and calculation, which presupposes an agreed system of units in terms of which quantities can be measured and expressed.There is one system that has come to be accepted for most branches of science and engineering, and for aerodynamics in particular, in most parts of the world. That system is the Systeme International d’Unitks, commonly abbreviated to SI units, and it
is used throughout this book, except in a very few places as specially noted. It is essential to distinguish between the terms ‘dimension’ and ‘unit’. For example, the dimension ‘length’ expresses the qualitative concept of linear displacement, or distance between two points, as an abstract idea, without reference to actual quantitative measurement. The term ‘unit’ indicates a specified amount of the quantity. Thus a metre is a unit of length, being an actual ‘amount’ of linear displacement, and so also is a mile. The metre and mile are different units, since each contains a different
mount of length, but both describe length and therefore are identical dimensions