DaveD
Well-Known Member
I'm planning on using a GA37A315 (Riblett) Airfoil for my wing and have been running some analysis on XFLR5 to generate some design data. The Cl and Cd plots come out pretty much as expected but the Cm does not.
Between -5 and +5 aoa I get a fairly constant value of -0.05 but by 15deg aoa the pitching moment has roughly halved. Both the original Riblett data and XFLR5 give the Cm about the 1/4 Chord rather than the aerodynamic centre so I wasn't expecting straight lines but my understanding is that the centre of pressure only moves away from the 1/4 chord at lower angles of attack so I expected any dodgy values of Cm to be around there on the graphs which doesn't appear to be the case.
As for the difference between the Riblett and XFLR5 curves, my initial thoughts were that Riblett used Eppler code to get his results whereas XFLR5 is based on Mark Drela's code. but the Cl and Cd are certainly still in fairly close agreement.
digging out my copy of T.O.W.S. shows the similar NACA 737A315 as having an almost dead strainght line at about -.012, but presumably that is Cm about the aerodynamic centre.
Can I just go with -0.05 or do I need to build a wind tunnel??!!!!
Between -5 and +5 aoa I get a fairly constant value of -0.05 but by 15deg aoa the pitching moment has roughly halved. Both the original Riblett data and XFLR5 give the Cm about the 1/4 Chord rather than the aerodynamic centre so I wasn't expecting straight lines but my understanding is that the centre of pressure only moves away from the 1/4 chord at lower angles of attack so I expected any dodgy values of Cm to be around there on the graphs which doesn't appear to be the case.
As for the difference between the Riblett and XFLR5 curves, my initial thoughts were that Riblett used Eppler code to get his results whereas XFLR5 is based on Mark Drela's code. but the Cl and Cd are certainly still in fairly close agreement.
digging out my copy of T.O.W.S. shows the similar NACA 737A315 as having an almost dead strainght line at about -.012, but presumably that is Cm about the aerodynamic centre.
Can I just go with -0.05 or do I need to build a wind tunnel??!!!!
