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Pressure plotting

 Pressure plotting

  • As the angle of attack is altered the lift and drag change very rapidly, and experiments show that this is due to changes in the distribution of pressure over the aerofoil. These experiments are carried out by the method known as ‘pressure plotting’, in which a number of small holes in the aerofoil  surface (a, b, c, d, etc.) are connected to a number of glass manometer tubes (a, b, c, d, etc.) containing water or other liquid and connected to a common reservoir.
  • Where there is a suction on the aerofoil the liquid in the corresponding tubes is sucked up; where there is an increased pressure the liquid is depressed. This is really several U-tube manometers connected to a common
  • reservoir ,Such experiments have been made both on models in wind tunnels and on aeroplanes in flight, and the results are most interesting and instructive.
  • The reader is advised to work through Example No. 94 in Appendix 3. In this example the results of an actual experiment are given, together with a full explanation of how to interpret the results.
  • In order to follow through to the
  • end of this example it is necessary to have a knowledge of the lift formula given later in this chapter, but the actual ‘pressure plotting’ can be done without this. Multiple manometers provide a good visual indication of the
  • form of a pressure distribution and are frequently used for teaching. If results are to be recorded, it is more convenient to use pressure transducers coupled to a computer ,

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