Lift and drag are the two most commonly thought of terms when it comes to forces acting on an airfoil. Pressure acting on the surface is what causes these two forces to occur. Two key components of lift and drag is the coefficient of drag and the coefficient of lift. These coefficients can be used to calculate the lift and drag of an airfoil. Although to calculate these coefficients, the pressure coefficient is needed, which can be calculated by measuring and non-dimensionalizing pressure differences across an airfoil. For this experiment a NACA 0012 airfoil was placed in a 2-foot by 2-foot open circuit wind tunnel. 18 ports were placed at different locations on the airfoil to record the pressure readings. Pressure readings were taken at 9 different angles of attack, -20°, -15°, -10°, -5°, 0°, 5°, 10°, 15°, and 20°, and at three different velocities, 30 fps, 60 fps, and 90 fps. A computer program was used to calculate the coefficients of pressure at each different scenario. Those coefficients were then used to calculate the coefficients of lift.
Discussion of Relevant Theory Lift and drag are two of the most important forces in aerospace engineering. Both of these forces are caused by pressure acting over the surface of an airfoil. A favorable pressure gradient is caused by a negative pressure change of an airfoil that enable and helps flow not separate . The opposite, a positive pressure change, creates what is called an Adverse Pressure Gradient . This Adverse Pressure Gradient causes flow separation over the airfoil. This gradient also causes a higher pressure on the upper surface of an airfoil, thus reducing lift . One very useful method to solve for different forces acting on an airfoil is to use different aerodynamic coefficients, such as lift, drag, and pressure. Using these coefficients, the forces on an airfoil can be calculated at different flow conditions. To find the coefficient of lift, one equation that can be used involves integrating over the chord length for the following equation: . C l = cos ( α ) ∙ ∫ x c = 0 x c = 1 [ C p,l ( x c ) − C p,u ( x c ) ] d ( x c ) (1) The coefficient of drag can be calculated by a similar equation, by integrating over the thickness of the airfoil by the following equation: . C d = 2sin ( α ) ∙ ∫ Y Y max = 0 Y Y max = 1 [ C p ( Y Y max ) ] d ( Y Y max ) (2) To be able to use either of these equations, the coefficient of pressure must be known. The equation for the coefficient of pressure is as follows: C p = p − p ∞ q ∞ (3)
Here p is the local pressure and p ∞ is the free-stream pressure. The difference of those two values are non-dimensionalized by diving by the free-stream density.
Description of Test Equipment and Procedure
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