The pressure distribution around an Ellipto Zhukovsky aerofoil with a chord of 254 mm at a range of angles of attack (-4? , 7? and 15? ) was determined and pressure contributions to lift were evaluated in a T3 wind tunnel at City University. This was carried out at a chord Reynolds number of 3. 9 x 105. Graphs for lift and pitching moment coefficients were plotted against angles of attack. A graph for Cm and Cl was also plotted from which the aerodynamic centre was determined to be 23. 7%. The value of lift curve slope was determined to be 4. 4759.
Hence the value of k (the ratio of the actual lift curve slope to the theoretical one) for this aerofoil was determined to be 0. 917. The value of Cmo was also found to be 0. 0172. Specimen calculations for 15 degrees angle of attack can be found in the appendix section. LIST OF SYMBOLS Cp Pressure Coefficient Cpu Pressure Coefficient of upper surface Cpl Pressure Coefficient of lower surface Cl Lift Coefficient Cm Moment Coefficient x/c Position of pressure tapping on aerofoil divided by chord length Px Pressure at tapping x (Pa) Patm Atmospheric Pressure (Pa) ? Density of air (kg/m3) i?? Dynamic viscosity ? Kinematics viscosity (m/s2)
h Digital manometer reading ? angle of which manometer is inclined D or t Diameter of cylinder (mm) h tunnel height (mm) V Velocity of air flow (m/s) R Molar gas constant (J/kg. K) T Temperature (K) Re Reynolds Number INTRODUCTION An airfoil is any part of an airplane that is designed to produce lift. Those parts of the airplane specifically designed to produce lift include the wing and the tail surface. In modern aircraft, the designers usually provide an airfoil shape to even the fuselage. A fuselage may not produce much lift, and this lift may not be produced until the aircraft is flying relatively fast, but every bit of lift helps.
The first successful aerofoil theory was developed by Zhukov sky and was based on transforming a circle onto an aerofoil-shaped contour. This transformation gave a cusped trailing edge, and so the transformation was modified to obtain a slender semi-eclipse trailing edge, which gave rise to the name Ellipto Zhukovsky. When a stream of air flows past an aerofoil, there are local changes in velocity around the aerofoil, and consequently changes in static pressure in accordance with Bernoulli’s theorem. The distribution of pressure determines the lift, pitching moment, form drag, and centre of pressure of the aerofoil.
In our experiment we are concerned with the effect of pressure distribution on lift, pitching moment coefficient (Cm), and centre of pressure. The centre of pressure can be defined as the point on the aerofoil where Cm is zero, and therefore the aerodynamic effects at that point may be represented by the lift and drag alone. A positive pressure coefficient implies a pressure greater than the free stream value, and a negative pressure coefficient implies a pressure less than the free stream value (and is often referred to as suction).
Also, at the stagnation point, Cp has its maximum value of 1 (which can be observed by plotting Cp against x/c). Zhucovsky claimed that the aerofoil generates sufficient circulation to depress the rear stagnation point from its position, in the absence of circulation, down to the (sharp) trailing edge. There is sufficient evidence of a physical nature to justify this hypothesis and the following brief description of the Experiment on an aerofoil may serve helpful. The experiment focuses on the pressure distribution around the Zhucovsky airfoil at a low speed and the characteristics associated with an airfoil: coefficient of lift, coefficient of pitching moment and centre of pressure.
The airfoil is secured to both sides of the wind tunnel with pressure tappings made as small as possible not to affect the flow,(appendix- photo 1 . The pressure difference around the airfoil is measured with twenty-five manometer readings which are recorded for each angle of attack. The manometer fluid is alcohol and has a specific gravity of 0. 83 and inclined at an angle of 30 degrees. Tube 1 is left open to atmospheric pressure, while tubes 2-13 are the lower surface of the airfoil and tubes 14-24 are the upper surface of the airfoil.
The pressure tapings are positioned on the airfoil at a distance x/c, noted in the results table and tube 35 is the static pressure of the wind tunnel. The dynamic pressure is given by a digital manometer. The digital readout results were used for all calculations because they are more precise. Results Raw data and calculated values for x/c, Cp and Cp*(x/c) can be found in the appendix. Graphs of Cp against x/c for angles of attack -4, 7, and 15 degrees can be also be found in the appendix. These graphs determine the lift coefficient.