VSP Aero Max Lift Coefficient

[Arthur Gevorgyan]

Hi,

When I'm trying to approximate default symmetric wing Cl max estimation getting the max on 90 AOA, Is it a correct behavior. Is there a way to determine the Cl max ?

Regards,

Arthur

[Brandon Litherland]

I'm not sure why you're looking at 90 alpha but you can try using the new Carlson Pressure Correlation on the wing as long as the model is set up properly to get decent results.  Or, you can find the expected local max CL of the airfoil using XFOIL or one of the databases and enforce that max in VSPAERO.  This will give you the notional max CL for the wing.  Keep in mind that RANS CFD with DES/DDES can have a hard time with true CL max even at the best of times because stall is a tricky thing to model well.  See the NASA LaRC High-Lift Prediction Workshop to get an idea of just how hard a problem this is.  However, just because it's hard to get the "right" answer doesn't mean that models can't be useful.  Can you provide some more information about your specific goal?  Maybe there is a different way to get what you need.

- Brandon

[Arthur Gevorgyan]

Hi Brandon, 

Sorry for the pure explanation I gave. 

Let me clearly define what problem I'm trying to solve. I have designed an RC airplane (delta-wing) and trying to do an analysis of that airplane. One of the characteristics I'm trying to find is the stall speed and for that, some online calculators (like https://www.rcplanes.online/calc_stallspeed.htm) require the "Cl max". 

To get experienced in VSP Aero analysis I added a default wing and tried to find the maximum Cl value. The Cl max I found for the default wing was at 90 AOA which I found strange and currently trying to figure out what I'm doing wrong.

Carlson Pressure Correlation option is also giving the same strange CL max

 What I should put in the expected local max CL for RC planes with mixed airfoils?

[Rob McDonald]

Predicting CLmax is very complex and is beyond the theory of pure potential flow methods (which VSPAERO is one).

Consequently, without additional models, VSPAERO will predict flow that does not separate and you will see unreasonable values at unreasonably high alpha.

VSPAERO has two other models that can be applied.

1) a 2D CLmax can be applied.  To use this, look up your airfoil in Theory of Wing Sections or a similar reference, look up the 2D CLmax at an appropriate Reynolds number, and put that value into the VSPAERO GUI.

2) an empirical Cp limit can be applied.  To use this, choose the Carlson Pressure Correlation, but then also set the Mach number and Reynolds number for your model appropriately.  The default Mach=0 will not work with this model.

RC Aircraft operate at very low Reynolds number -- many of the data sets might not extend down to such low Re.  However, XFoil is pretty well suited to low Re flow, you can always set up and run some 2D analyses there yourself.  If you are using the 2D CLmax approach, VSPAERO does not allow you to put in multiple airfoils worth of data -- if you choose the lowest CLmax, you will obtain a conservative result for CLmax, but you will want to think about what this means for tip stall if you have a root and tip with dramatically different CLmax values.

Rob

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[Arthur Gevorgyan]

Rob, Appreciate the detailed explanation.

Arthur