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Old Mar 23, 2012, 11:56 AM
Ian
Joined Dec 2008
31 Posts
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XFLR5 for Low Aspect Ratio Wings

So, I'm running XFLR for a project and getting an answer I don't believe, and was wondering if someone could answer some questions. I have a wing using MH78 (which is reflexed), running at Re=1000000. Using the direct analysis for a 2D wing I get numbers that I believe, with a CLmax of 1.6 stalling around 15*. However, when I switch over to trying to do an airplane analysis, things get weird. The wing has AR=3, with a span of 3 meters, root chord of 1.2 meters, tip chord of .75 meters and a straight trailing edge. Basically, the wing doesn't stall until really really high alpha (around 40*). I know that with low aspect ratio wings you get vortices along the leading edge that contribute to lift, but I was under the impression XFLR just used 2D Xfoil at multiple points along the wing? Is it something to do with having to turn the viscous effects off to get it converge?

If it is that Xfoil is just really bad at predicting stall, is it an okay assumption to say that the characteristics in the linear region are okay to use, and just say it stalls at the same alpha as the 2D analysis?
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Old Mar 24, 2012, 04:18 AM
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Joined Jul 2007
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I think XFLR5 is not an appropriate tool for analysys of truly 3D flow: as you said, the flow has to be quasi 2D and XFLR uses 2D viscous polars for each section.
I think it cannot also predict vortices at LE of highly swept wings: ordinary vortex lattice method (VLM) is not fitted for that, I think that XFLR uses ordinary VLM, not the special one (although I am not completely sure).

But in your case wing is not highly swept and airfoil is thick, so the lift enchancing LE vortices are not an issue. So it seems that you can use lift data when viscous effects are weak, but without correct prediction of the stall angle. Be careful with panelling.
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Last edited by truffaldino; Mar 24, 2012 at 06:02 AM.
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Old Mar 24, 2012, 08:48 AM
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Joined Jan 2009
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I did a quick run on one of the easiest software to use I have (MultiSurfaces). Unfortunately it doesn't have the MH78 in the library, so I replaced it by a NACA 25114. I get the stall at 24 degrees for a Cl of 1.23 at 15 m/s @ sea level ISA.

The wing won't stall at the same AoA and Cl than for a 2D analysis (I confirm your values of Cl value around 1.5 for an AoA around 15 degrees). It is not a delta wing but could be assimilated to it. Delta wing are well known for achieving very high AoA before stall...

I hope it helps.
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