I'm new to this website so I'll introduce myself first. My name is Matt and I live in a remote area of Alaska. I've owned, flown and restored a variety of planes over the last 40 years but recently decided to look into creating my own design.
Most of the process I'm fairly comfortable with, but the one area I'm a bit weak in is airfoil selection so I've been studying that lately. I have found two websites that have lots or airfoil data: airfoiltools.com and www.bigfoil.com. The problem I am having is the polar graphs are often completely contradictory. Bigfoil says it uses JavaFoil and XFOIL to generate its graphs. I don't know what airfoiltools uses. To give you an example, I've been comparing Cm of several different foils at Re=1e+6. and 3e+6. One airfoil I've looked at is GA 30U-618 (just a random one I clicked on on bigfoil). When plotting Cm vs α, the graph looks like an X with JavaFoil giving the exact opposite info as XFOIL. Unfortunately, this airfoil isn't available on airfoiltools so I can't check it out there but I have checked other airfoils like a NACA M8 (again another random selection) against both and I get a third set of graphs that often don't agree. I expected a little bit of discrepancy, but on some airfoils they are wildly different. In the case of the M8, (Bigfoil) Javafoil lists Cm at 15 α as -0.066 and XFOIL as ? and airfoiltools says -0.024. Cl vs Cd shows similar variations.
I figure since I'm new at this I must be comparing apples to oranges somehow, but at this point I'm not sure where I'm going wrong.
How does a person know what to trust? If any of you have any thoughts on this, I'd be interested in your opinions. Or if I'm going wrong someplace, please let me know.
Thanks
M@
Most of the process I'm fairly comfortable with, but the one area I'm a bit weak in is airfoil selection so I've been studying that lately. I have found two websites that have lots or airfoil data: airfoiltools.com and www.bigfoil.com. The problem I am having is the polar graphs are often completely contradictory. Bigfoil says it uses JavaFoil and XFOIL to generate its graphs. I don't know what airfoiltools uses. To give you an example, I've been comparing Cm of several different foils at Re=1e+6. and 3e+6. One airfoil I've looked at is GA 30U-618 (just a random one I clicked on on bigfoil). When plotting Cm vs α, the graph looks like an X with JavaFoil giving the exact opposite info as XFOIL. Unfortunately, this airfoil isn't available on airfoiltools so I can't check it out there but I have checked other airfoils like a NACA M8 (again another random selection) against both and I get a third set of graphs that often don't agree. I expected a little bit of discrepancy, but on some airfoils they are wildly different. In the case of the M8, (Bigfoil) Javafoil lists Cm at 15 α as -0.066 and XFOIL as ? and airfoiltools says -0.024. Cl vs Cd shows similar variations.
I figure since I'm new at this I must be comparing apples to oranges somehow, but at this point I'm not sure where I'm going wrong.
How does a person know what to trust? If any of you have any thoughts on this, I'd be interested in your opinions. Or if I'm going wrong someplace, please let me know.
Thanks
M@
