The effort here has been to compile and present collectively the concepts of Low Speed Wind Tunnel. I have tried to make it explanatory with help of large number of figures and pictures whose sources have been mentioned under them. I have tried to incorporate as much latest information as available via Internet although the backbone of the work is ‘Wind Tunnel Technique’ by R.C. Pankhurst, National Physical Laboratory, Teddington, England. It includes a very descriptive documentation of the Wind Tunnel and in a very organized manner.This work was done as an assignment for one of the undergrad courses. Information may not be very current but the general principles remain the same nevertheless.
Wind tunnel testing is the technical support of any major development process involving aerodynamics. It is used for aircraft, helicopters,cars, trains, and laboratory research.It is estimated that it took the Wright Brothers less than 20 hours of wind tunnel testing to produce their successful Flyer (although their empirical research was a life time achievement). The Douglas DC-3, perhaps the most successful commercial aircraft ever built, required about 100 hours of wind tunnel testing. Wind tunnel time has been steadily increasing (and so have the costs) since: the Boeing 747 required over 1000 tunnel hours; the Space Shuttle nearly 10 years of time. The wind tunnel provides the engineer with valuable data on scale models. The wind tunnel is the most lasting contribution of the Wright brothers to the science of aerodynamics.
The would-be aeronauts of the nineteenth century closely studied the flight of birds and began building flying machines patterned after avian structures. Their birdlike craft failed miserably. They quickly realized that in reality they knew nothing about the lift and drag forces acting on surfaces cutting through the atmosphere. To fly, man first had to understand the flow of air over aircraft surfaces. This meant that he had to build instrumented laboratories in which wings, fuselages, and control surfaces could be tested under controlled conditions. Thus it is not surprising that the first wind tunnel was built a full 30 years before the Wrights' success at Kitty Hawk. Although the theory of the potential flow of an incompressible fluid has been highly developed, it has a very limited application due to assumption of working fluid being inviscid. Thus it could not account for the profile drag and the study in the boundary - layer was complex and hefty. Moreover, the flow study in the transonic and supersonic regime of flight could not be solved with these potential theories. In general theory was essentially incomplete and some experimental backing was required for more purposeful and correct outcomes to emerge.
These experiments are conveniently conducted on a scale model in a wind tunnel. From the point of view of aircraft design, such experiments have two principal uses: Firstly, they make it possible to determine the influence of various features of design and modifications to them, in a manner which is safe, quick, direct and relatively cheap. In a wind tunnel a component can be individually tested and then its performance with the complete model can be studied. This leads to progressive testing, correction and re-test to establish the configuration. Secondly, this helps to understand the basic flow of air over the surfaces leading to better understanding of basic phenomenon and laying the foundation for future design improvements of the fundamental nature. Thus, the wind tunnel is indispensable to the development of modern aircraft.