The speed can easily be converted to metric:ġ mph = 1.15 Kts. The cinematic viscosity is to a certain extent dependent on the density of the air, but for our aircraft flying below 12,000 ft., it can be assumed constant (equivalent to 15 x 10 6 sec/m 2 in metric). Re is a dimensionless number, which makes it independent of the measuring systems. V = kinematic viscosity of the air (sec/m 2) For our smaller and slower aircraft, the only "number" which really needs to be considered is the "Reynolds Number" and it is defined as: The important thing is the relative speed of airfoil and air.Įarly investigations into the theory of fluid dynamics have predicted a certain number of constants to which similar disturbances (and an airfoil in the air is a disturbance) produce similar effects - in hydrodynamics, these are referred to as 'Froude Numbers" (hulls of boats) in high speed aerodynamics the "Mach Number' are other examples. This was not the case in scientific circles some 120 years ago, but now is common knowledge, and justifies the wind tunnel tests where true air flows over an airfoil and from which we can predict characteristics of an airplane moving through still air. Today it is universally accepted that an airfoil in motion through still air and air blowing over a stationary airfoil have the same effects. Scientists look at the parts of a corpse and decide they know something about a living body!īut, let us go back to something less serious (!?!) and look at the airfoil or wing section of our airplane in such a way that we will have a little better understanding of how our aircraft flies. It is a bit like somebody trying to understand the human body by studying the skeleton only, or the chemicals of the body only, etc.: the failure of modern medicine comes from this fact. Analysis is necessary but only a synthetic view will give us the whole picture.
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