Xfoil Polar Plots

[HELModels]

I just made an airfoil on Xfoil by putting in some rough coordinates and fumbling through the geometric design menu. I then compared my foil to the SA7035 and E387 using the pplo feature. It makes very interesting output but I am not sure how to interpret polar plots. I can see that the vertical scale on all 3 plots is coefficient of lift, but middle plot and last plot are a mystery.
The middle plot shows angle of attack and I think moment coefficient, but where is lowest drag and highest lift? I am guessing that moment coefficient and lift line intersect is an important point, but I dont know why. How do I determine the best operating range from all this? The last plot is baffling. It is relating chord location to lift somehow. Again, guessing, is one line top surface and one bottom? Is the intersection of these 2 lines an important point?
http://lister3.home.mindspring.com/HLESA.jpg

[Ollie]

The laft hand polar is for a wing of infinite aspect ratio operating at a constant reynolds number. To convert it to the polar of a real wing the induced drag has to be added and the change in reynolds number due to changes in airspeed has to be taken into account. To find the polar of a real model, the parasitic drag of tail surfaces, fuselage, etc. have to be added too. The best lift to drag ratio for any of these polars can be found graphically by drawing a line from the origin that is tangent to the polar Cl vs, Cd curve. Where this line touches the polar is the operating point for best L/D. A computer program like PC Soar can do all the necessary calculations for you and display the sinking speed vs. airspeed and the L/D vs. airspeed polars. See:
http://my.athenet.net/~atkron95/pcsoar.htm

The middle graph is really two seperate polars, each with its own vertical scale. The only thing they have in common is the angle of attack on the horizontal axis. The slope of the almost horizontal line indicates that the aerodynamic center is located a little away from the 25% chord point. The aerodynamic center is defined as the point about which the coefficient of pitching moment is constant with variations in angle of attack. Cm is conventionally taken as the pitching moment coefficient about the 25% chord point.

The right most graph depicts the place on the top and bottom of the airfoil where the flow transitions to turbulent, I think.

[markdrela]

Originally posted by Ollie
The laft hand polar is for a wing of infinite aspect ratio operating at a constant reynolds number.

These particular polars are not at a constant Re, but rather at a constant Re*sqrt(CL). This means that the Re variation due to airspeed is already accounted for (assuming the Re*sqrt(CL) = 125000 value is correct for the actual wing loading and average chord). The only necessary corrections which are still needed are for induced drag and for tail+fuse parasite drag.

[Ollie]

Thanks for setting me straight, Mark!

[HELModels]

Thanks for the explanations. It looks to me like the slope of the alpha vs. CL line if shifted over to the first plot is also the tangent line suggested for determining best L/D. My foil and E387 seem to have a higher design point because they both touch that tangent at a higher CL.

IS that hook on My foil and E387 the "Drag Bucket"? Fly in that region and you get best glide?

[Ollie]

Yes, that bump in the Cl vs Cd polar represents the best L/D for that airfoil at that chord and airspeed. It will be slightly different when the effects of induced and parasitic drag are included in the polar of the real model. The L/D versus airspeed polar will have a very sharp peak (because the advantage is over a very narrow range of Cl) and you will find it very hard to take advantage of that peak because of having to maintain a very precise air speed. It would be a practical advantage if you had an airspeed indicator to help you pilot the plane at the nesessary airspeed.