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Fluid pressure
In the mechanics of flight we shall be chiefly concerned with fluid pressure, that is, the pressure in a liquid or gas, and the force that it produces. The reason why a fluid exerts a force is because its molecules are in rapid motion and bombard any surface that is placed in the fluid; each molecule exerts only a tiny force on the surface but the combined effect of the bombardment of millions upon millions of molecules results in an evenly distributed force on the surface.
Pressure is a scalar quantity. That means that it has a magnitude, but unlike a vector, there is no direction involved. When a fluid at rest is in contact with
a surface, the pressure produces a force which acts at right angles to the surface (Fig. 1.8). Note that the force does have a direction whereas the pressure that causes it does not. It is not surprising that people often confuse cause and effect when talking about pressure. You will find many old books that say that pressure acts equally in all directions. It does not. Pressure does not act in any direction; it is the force due to pressure that acts in a direction. The direction of the force is always at right angles to the surface that the pressure is exposed to. We measure pressure in terms of the force that it will produce on an area, so pressure has the units of newtons per square metre (N/m2).
A pressure of 1 N/m2 is also called a pascal (Pa). Another common metric unit of pressure is the millibar (mb), which is 1/1000 th of a bar: a bar being 105 N/m2. This seemingly odd unit comes about, because 1 bar is very close to the standard atmospheric pressure at sea level. It had been adopted by meteorologists many years before the metric SI units were introduced, and the reader may often encounter atmospheric pressure given in millibars, particularly in flying manuals. In this book, we will use millibars for atmospheric pressure when appropriate, as in dealing with the effects of altitude. However, for most straightforward aerodynamic calculations we will use N/m2 since this is normal practice in European educational institutions.
