Hi Ron, 2011/8/16 Ron Jensen <w...@jentronics.com>: > > I took a look at the YASim solver today, first comparing the Inertial tensor > with the inertias coming out of aeromatic. Not too far apart, but the 2 yasim > aircraft I looked at were both 50% higher than the aeromatic numbers. > > Then I took a look at the -g switch and was rather shocked at the curves yasim > generates. It generates CL, CD and L/D. My plot software computes force as > (lift^2+drag^2)^0.5. Lift and drag are perpendicular by definition so this > gives the total force in the lift/drag plane. > > I made 3 plots with Tat's A6M2 and Helijah's Katana and Gloster-Meteor models: > http://www.jentronics.com/fgfs/temp/yasim/yasim-A6M2.png > http://www.jentronics.com/fgfs/temp/yasim/yasim-katana.png > http://www.jentronics.com/fgfs/temp/yasim/yasim-glostermeteor.png > > And then I loaded some data I had from a NACA 4 digit 0015 airfoil into the > same plot definition: > http://www.jentronics.com/fgfs/temp/yasim/naca0015.png > > I was pretty shocked to see the YASim charts drag numbers all seem to fall off > at stall. This is counterintuitive to me, I expect drag to increase at stall, > that is what keeps the speed low during the stall. Also, the lift/drag (L/D) > ratio is actually the tangent of the angle the aerodynamic force is acting > in. On the real airfoil it rapidly approaches 0 after the stall because the > force is acting nearly parallel to the free stream velocity and drag > dominates the ratio. This seems not to be the case with the YASim models.
That is because you are comparing apples and oranges. What YASim outputs as a drag is actually the total drag including the induced drag. I guess that for the NACA 0015 you used the classical CD curve which is basically the friction drag only. Hence the differences. However YASim uses the well known formula CDi = k * CL^2 to assess the induced drag (line 224 of src/FDM/YASim/Surface.cpp). The problem is that this formula is obtained with Prandtl lifting line theory and is therefore only applicable in the linear domain (small AoAs) while YASim uses it for any AoA including those close to and beyond stall. The drop you see in the drag is the direct consequence of that : induced drag drops after the stall because the lift itself drops. Bertrand. ------------------------------------------------------------------------------ Get a FREE DOWNLOAD! and learn more about uberSVN rich system, user administration capabilities and model configuration. Take the hassle out of deploying and managing Subversion and the tools developers use with it. http://p.sf.net/sfu/wandisco-d2d-2 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
