Design, construction and testing of an aerodynamic balance for use on the University's wind-tunnel

Abstract

The scope of this project was to design, construct and test an aerodynamic balance for use in the University's wind-tunnel so as to enable the measurement of at least the lift and drag of a body under test in the wind-tunnel. Two main types of balances were considered: 1. that utilizing a strain bridge and strain gauges ; 2. simple "levers and weights" type. After doing preliminary design of the first type it was concluded that the strain bridge that was made available for this project did not meet the required accuracy of measurement. In the case of the "levers and weights" type of balance, the aerodynamic forces were balanced by means of simple spring balances. Hence the required accuracy was achieved by using more sensitive spring balances which were readily available at the laboratory. Furthermore, the latter type of balance was found to be less cumbersome than the former type. These two main reasons were the deciding factors in constructing a mechanical balance rather than the one utilizing a strain bridge and strain gauges. Construction of the balance was done with the aid of the University technician at the Fluids Laboratory. In order to aid testing of the balance, a cylinder, a sphere and an aerofoil were constructed from Mahagony wood. All three test-bodies were appropriately polished. After observing significant vibrations of the measuring system due to the buffetting effect of the test-bodies, oil dashpots were used to dampen the vibrations in the drag- and lift-force planes. The damping effect was satisfactory. Four main wind-tunnel corrections were considered, for the treatment of balance readings. 1. Solid-blockage, 2. Wake-blockage, 3. Horizontal buoyancy, 4. Turbulence. Static calibration of the balance was carried out in order to investigate the static error in balance readings. It was found that the error involved with both oil dashpots assembled, was 2.5% (over-reading) for both drag and lift balance. In order to test the drag balance, a 0.07m diameter polished cylinder was used as a two-dimensional body and a 0. 115m diameter polished sphere was used as a body of revolution. Since the wind-tunnel corrections could only be made approximate and in order to check the "repeatability of results", a large number of readings were taken at different times and ambient conditions. The graphical results obtained compared well with those found in relevant textbooks giving approximately constant CD values over the Re range considered as follows: For Cylinder (2-D) CD = 1 For Sphere (3-D) CD = 0.5 . The lift-balance was tested by using an N60 aerofoil section spanning across the test-section (i.e. as a 2-D wing). The reasons for choosing the N60 aerofoil section was because it was the simplest aerofoil to make from the few aerofoil sections for which CL curves are available at low Reynolds’ numbers. Again, the graphical results obtained at four different values of Reynolds’ numbers were quite satisfactory. To assist in the calculations involved in the treatment of the balance readings a computer programme was written for the H.P. 85 microcomputer available at the Mechanical Engineering Department of the University.