Airfoil Lift Calculation Using Wind Tunnel Wall Pressures

Lift in subsonic wind tunnels is generally calculated using load-balances or static pressure sensors on the airfoil surface. While the load-balance method is relatively cheaper compared to the airfoil surface pressure method, the set-up requires to be constantly calibrated to ensure accuracy of results. On the other hand, lift measurements using airfoil surface pressure ports require expensive models and do not provide a cheap and effective way to test multiple airfoil sections. Additionally, literature has shown that the pressure ports on the airfoils can cause premature flow transition, thereby leading to inaccuracy in the measurement of data. In this paper, a method to calculate lift using static pressure ports on the wind tunnel walls has been explored. The wall-pressure measurement (WPM) technique for lift calculation was validated for two airfoils: a custom Natural Laminar Flow (NLF) airfoil with surface pressure ports and the NACA 0012 from literature, for a chord Reynolds number range of 100,000 to 500,000. Furthermore, a study was conducted to explore the variation of the lift predictions using the WPM technique with varying airfoil chord to better understand the limitations of the method. Lift predictions from WPM approach compared well with lift data from the surface pressure (NLF airfoil) and force balance (NACA 0012) techniques. Results also showed that the airfoil chord plays an important role in the accuracy of the measurements.