# How do viscous drag and form drag individually change as a function of angle of attack?

Typically, when dealing with airfoils, the drag coefficient $$C_d$$ is given as a function of angle of attack. If we limit the discussion to 2D subsonic incompressible flows, then the drag is the sum of the viscous drag and the form drag (pressure drag), equivalently:

$$C_d = C_{d,\text{visc}} + C_{d_,\text{form}}$$

Where: $$C_{d,\text{visc}}$$ is the viscous drag coefficient, and $$C_{d_,\text{form}}$$ is the form drag coefficient.

I've never seen, these coefficient plotted as function of angle of attack individually. That is, $$C_{d,\text{visc}}=f(\alpha)$$, or $$C_{d_,\text{form}} = f(\alpha)$$.

The reason I want to see them plotted individually, is to emphasize the contribution of each type of drag (qualitatively, I know that for angles of attack below stall, the viscous drag is higher than form drag, but for angles beyond the stall, (boundary layer separation), the form drag dominates).

Are there any graphs showing these coefficients individually, say for NACA airfoils?

• do u want a graph all the way up to 90 degrees or just stall? – Abdullah Jun 19 at 10:12
• Viscous drag probably won't change much if you don't change configuration, but considering form drag is very good. This would be part of induced drag (making lift). Interestingly, form drag also comes into play when one uses excess lift as a propeller. – Robert DiGiovanni Jun 19 at 11:05
• @Abdullah: I think up to 30 degrees will be sufficient. – IamNotaMathematician Jun 19 at 13:19

Wind tunnels are stingy with the data they reveal. They only give the total value but not neatly split into its contributions. Therefore, any such table needs to be calculated.

Since XFOIL, which is now around for more that 30 years, we have such a tool and it does output the single contributions. However, I know of no publicly available collection which lists those drag components for a range of airfoils. And since XFOIL becomes unreliable with larger flow separation, the data is only valid for attached flow and maybe a bit beyond. But certainly not up to 30°.

In Xfoil, load an airfoil and enter OPER. Compute the desired point and switch into VPLO. There you can plot the skin friction coefficient with the CF command. In order to extract its numerical value, toggle the PACC switch in OPER and save a range of points to file. In the file you will find to the right of the CD column one labeled CDp. This is the pressure part of the drag. Use a tabulation software of your choice to calculate the difference between CD and CDp: This should be the friction part of the drag coefficient.

• Actually, for angles below stall, one can compute both components. Using pressure integration to compute the form drag, and the wake rake technique or force balance to compute the total drag. – Navaro Jun 19 at 19:17
• Could you please elaborate a little bit on how to get each contribution from XFOIL, I can't find about that. Thanks – IamNotaMathematician Jun 19 at 19:24
• @Navaro Using pressure integration is highly unreliable for computing form drag. You're much better of computing the shear stresses for viscous drag, then computing the total drag from far field momentum defect, then computing the difference for pressure/form drag. – JZYL Jun 19 at 19:30
• @IamNotaMathematician: I added a new paragraph to the answer. Comments are poor for explaining this. – Peter Kämpf Jun 19 at 20:03