On Tuesday, I found a paper (Models of Lift and Drag Coefficients of Stalled and Unstalled Airfoils in Wind Turbines and Wind Tunnels), in which I found this figure. The lower weight plots show lift and drag coefficients for an infinite aspect ratio airfoil (otherwise known as an infinite airfoil), while the heavier weight show the same for an aspect ratio of 9.04.
As discussed earlier, our airfoil actually isn’t infinite, but we’re attempting to model it as such. So our values should fall somewhere in between the two plots above. For the lift coefficient, our data showed a stall point near that of the finite aspect ratio and a maximum lift value near that of the infinite aspect ratio. Again, this corroborates that our calculations so far are correct.
The interesting part comes in when we compare drag coefficients. At whatever angle of attack the lift coefficient sharply decreases due to stall, the drag coefficient begins to rapidly increase. In both the infinite AR and 9.04 AR plots, remains below 0.1 until the airfoil stalls. After that, the infinite AR shoots up to 0.4 at 20° angle of attack and the 9.04 AR rises to about 0.3. Beyond 20° angle of attack, both airfoils experience maximum drag around 90°, with the infinite airfoil nearly reaching 2.0.
This pattern of having a drag coefficient below 0.1 until the stall point and sharply increasing thereafter is seen quite clearly in each of our data sets. Just before separation at 15° angle of attack all three of our test cases experience a drag coefficient of approximately 0.1. In our limited post stall testing (from 16-20°) we see that the drag coefficient increases to approximately 0.35. This value lies directly between the two drag coefficients found in the NASA report above.
So in the end, it actually appears that our drag coefficients are quite reasonable for the angles of attack we measured at. In future testing it could be interesting to continue investigating these post stall regimes at various Reynolds numbers.
