I have been asked by Don Stegall to provide a little interpretation of the graphs that I recently posted. I can’t make you all aeronautical engineers in a few short paragraphs, but perhaps I can help you make sense of these charts.
There are two attachments. Each attachment has two graphs on the page. The first attachment is an analysis of the various airfoils at a Reynolds number (Rn) corresponding to Q-25 speeds. The second attachment is an analysis of the airfoils at a Rn corresponding to 428 speeds. There is no “mystery” about Reynolds number. It is simply a way to express the relative importance of kinematic (inertial) forces to viscous forces in a fluid. At low Rn’s ( i.e. low speed and small chord) viscosity becomes more important that at high Rn. That is, the air acts more like maple syrup at low speeds than at high speeds. That’s why a bumble bee can’t “fly”… actually it’s “swimming”. It’s necessary to take the effect of viscosity into account.
In each attachment, the “hook-topped” graph on the left is called a lift curve and is simply a graph of how much lift the airfoil section is capable of producing plotted against the angle of attack. You can see that the symmetrical NACA 66-012 produces zero lift at zero degrees angle of attack, which is what you would expect from a symmetrical section, and that the cambered MH 17 and S8064 each produce some lift at zero degrees. As you can see, the slope of all three lines is essentially the same, this is typical. All reasonably good airfoils are pretty much the same here. When then line “bends over” at the top, that is the stall. Some airfoils stall with a sharp drop off, while others are pretty mild. All three of these are typical. The higher the line goes before it drops off, the higher the lift the airfoil is capable of producing before it stalls. In both cases, the S8064 is capable of producing more lift before it stalls.
In each attachment, the “U-shaped” graph on the right is called a drag polar and is a plot of how much drag the airfoil produces for the amount of lift being produced. The “bottom” of the “U” is the minimum drag that the airfoil has. It can’t go any lower. As you can see, all three airfoils are pretty much the same. The difference is in the “noise level”. Moving to the right (we don’t care much about flying inverted or negative “g” in racing), the “flatter” that the “U” stays, i.e. the closer it stays to the lower right corner, the better the airfoil is for any particular amount of lift being produced. They are all pretty similar. At Q-25 speeds, the MH 17 and the N66-012 are pretty much equal, and the S8064 is a little better. At 428 speeds, the MH 17 and the N66-012 are pretty much equal up to about 11 degrees angle of attack and then the N66-012 is better. But the S8064 is still lower and closer to the bottom right corner, which means that it produces even less drag for the same amount of lift. Prof Selig did a real good job!.
Caveats: This is TWO DIMENSIONAL data, it’s not a wing. There is a LOT more involved in the real world than a simple 2-dimensional analysis. Also, it doesn’t take trim drag into account. The MH 17 has lower trim requirements than the S8064 which will affect the drag of an actual 3-dimensional wing mounted on an airplane in flight. So what is the “bottom line” about what all this means? I means that I need to practice more and that I’m going to buy a Viper replacement wing from Don…
I hope this helped a bit.