I completed a wind tunnel test on a NACA 23012 aerofoil last week and am now writing a report on my findings! I need some help with working out the lift Coefficient as i seem to be going wrong somewhere. Cl= L/.5 ρ Vw² S To find the lift force i am subtracting the lift force which was necessary to zero the wind tunnel balance by the measured lift force so: L=Lm-Lz For ρ(air density) i am using 1.2kgm as this was not measured at the time of the tests. This is where i belive i could be wrong for Vw(air speed within the working section) i am using the wind tunnel speed .075M water or 75mm water which was the speed we performed the test at. Would this be the speed in the working area if the projection manometer is connected differentially to static pressure tappings in the wall of the wind tunnel upstream of the contraction zone and within the working section. I am gusing it is but my calculations are well out of what i would believe to be normal Lift coeffients at the angles used! Help and advice with many thanks Rob Ps My lift force Lz is 685 lb does this sound normall considering the lift force Lm was 76.3 lb at 0 degrees angle of attack??