For air, the kinematic viscosity ν is small and Re is large unless Vl is small. 4) becomes: where l2 has been replaced by the plan area S, a proportional number of the same dimensions, and f(M, Re) is a function, whose form is not determined by the present method, with values which are independent of physical units. 5) is called the (dimensionless) coefficient of the aerodynamic force F ad. 3, and the flow is termed incompressible. Thus, for an incompressible flow, the aerodynamic force coefficient is a function of Reynolds number only.

He is a fellow of many professional societies, including the Royal Aeronautical Society. Professor Rathakrishnan serves as editor-in-chief of the International Review of Aerospace Engineering (IREASE) Journal. He has authored nine other books: Gas Dynamics, 4th ed. (PHI Learning, New Delhi, 2012); Fundamentals of Engineering Thermodynamics, 2nd ed. (PHI Learning, New Delhi, 2005); Fluid Mechanics: An Introduction, 3rd ed. (PHI Learning, New Delhi, 2012); Gas Tables, 3rd ed. , Tokyo, Japan, 2008); Gas Dynamics Work Book (Praise Worthy Prize, Napoli, Italy, 2010); Applied Gas Dynamics (John Wiley, New Jersey, USA, 2010); and Elements of Heat Transfer, (CRC Press, Taylor & Francis Group, Boca Raton, USA, 2012).

In this definition of aerodynamic force, the aircraft is considered to be moving with constant velocity V in stagnant air. Instead, we may imagine that the aircraft is at rest with the air streaming past it. In this case, the air velocity over the aircraft will be −V. It is important to note that the aerodynamic force is theoretically the same in both cases; therefore we may adopt whichever point of view is convenient for us. In the measurement of forces on an aircraft using wind tunnels, this principle is adopted, that is, the aircraft model is fixed in the wind tunnel test-section and the air is made to flow over the model.