Stall Prediction_Workshop 2023

[Edouardo AL ALAM]

Hello, 

First I would like to thank you for your replies on my previous conversations, they were of great help (and also apologize for not replying)... 

In the workshop of August 2023 in the last slides, it has been mentioned that a post processing method is implemented to predict Stall and a comparison of CL vs Alpha is shown where the linearity is broken.

I am currently trying to add viscous effects to propeller analysis by generating Viscous and Inviscid polars (Xfoil), getting a delta angle and adding this angle to my geometry (Twist angle). The reason being, at Take Off VLM diverges from the CFD

Can my method be replaced by your post processing technique? If yes is it already implemented in a version of OpenVSP?

Thank you in advance!

Best regards,

Edouardo

[Arnaud]

Hi everyone,

I am working on propellers and recently decided to test OpenVSP to better predict cruise and T/O efficiencies.

The tool is great to predict cruise efficiency but I am facing similar issues when it comes to evaluating highly loaded blades during T/O (eta overpredicted with inviscid flow). 

I have been trying to post-process my results introducing compressibility and viscous effects (not very successful so far) and checked for recent VSPaero developments that could bring me closer to my reference propeller map.

Browsing through the 2023 workshop material, I found an interesting CLmax method described on slide 40 of 2023_vspaero_-_what_is_new.pdf

I was about to ask you about this new feature when I came across this post (same topic I believe, if not please correct me and I will create a separate thread).  

Will this new feature be part of the upcoming release (post 3.36) ? It looks very promising !

Thanks for your help

Best

Arnaud

[Brandon Litherland]

VSPAERO has a couple of ways to mimic stall.  Both of these methods have been in the releases for a while. Note that you are only able to set a single 2D Cl_max value which implies that the same max is applied to an entire wing regardless of airfoil distribution, or wings and tails, etc.  Essentially all lifting surfaces have this one limit (for now).  Another method is to attempt using the Carlson Pressure Correlation model which attempts to limit local, sectional Cl by local Mach number and Reynolds number.  This is the approach shown in the linked PDF.  However, I would take the time to do your own validation cases against tunnel data before going forward with it.

[Arnaud]

Thanks a lot Brandon.

I didn't realize that the Valarezo-Chin method + Carlson's Cp limitations were already implemented in VSPAero.

I guess that imposing a single CLmax value would lead to an abrupt efficiency drop when increasing the loading up to a certain point, this CLmax value being reached almost simultaneously on all the blades' sections. The trends I observe on my reference data are much more gradual, I will investigate a bit more.

Have a nice day,

Arnaud