A few possible issues

[Saad Mirza]

1) The Stability function under advanced in VSPAero only works with XZ symmetry turned off.

2) In the results tab, is Cd_tott different than Cd_tot? It seems that they are the same.

[Saad Mirza]

Update: I found another question answered that explained Cd_tott. It would be nice if this was hidden from the user or noted in a manual.

Under Advanced -> Stability Analysis, I noticed Pitch analysis only outputs x_ac aerodynamic center in a separate file than .stab.

[Rob McDonald]

On Sat, Nov 19, 2022 at 12:37 AM Saad Mirza <saadz...@gmail.com> wrote:

1) The Stability function under advanced in VSPAero only works with XZ symmetry turned off.

The stability mode needs to run asymmetrical flow cases, so it needs a full model (not just a half model).

For now, OpenVSP and VSPAERO only send a half model of the geometry when you are working with a symmetrical geometry and a symmetrical flow. 

 

2) In the results tab, is Cd_tott different than Cd_tot? It seems that they are the same.

They should be similar in most situations.

Cd_tott uses the induced drag from a Trefftz plane like calculation where Cd_tot uses pressure integration. 

Rob

[Rob McDonald]

On Sat, Nov 19, 2022 at 2:51 AM Saad Mirza <saadz...@gmail.com> wrote:

Update: I found another question answered that explained Cd_tott. It would be nice if this was hidden from the user or noted in a manual.

In some situations, they can be different.  Hiding it from the user would not be a good solution.

 

Under Advanced -> Stability Analysis, I noticed Pitch analysis only outputs x_ac aerodynamic center in a separate file than .stab.

Yes, some people wanted a quicker way to get the aerodynamic center -- only two cases are needed instead of the seven used for a full stab run.

Rob 

 

On Saturday, November 19, 2022 at 3:37:53 AM UTC-5 Saad Mirza wrote:

1) The Stability function under advanced in VSPAero only works with XZ symmetry turned off.

2) In the results tab, is Cd_tott different than Cd_tot? It seems that they are the same.

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[Franco Staub]

Rob,

I have a quick follow up to the second point above:

Can the pitch stab analysis be run from the command prompt? I only see vspaero commands for -stab, -pstab, -qstab, and -rstab.

A personal wish would be that if I run an alpha sweep independently of the stab analyses, that the ac position pops up in one of the output files. 

Best,

Franco

[Rob McDonald]

OpenVSP always runs VSPAERO through the command line.  So all options are either communicated in the setup file *.vspaero or through the command line itself.

When OpenVSP runs VSPAERO (or other external codes), it first echo's the command line to the output console.  So, if you run a pitch stability case through the GUI and then go to the console and scroll to the top, you will see that it put the '-acstab' option on the command line.

The code for the online help is here.  The -acstab option is not mentioned there (an oversight).  However, the very next subroutine actually parses the options, the -acstab option is parsed here.

Things like the online help have a tendency to get left behind and frequently are out of date.  Sorry about that.

I understand what you're saying about running an alpha sweep and getting a free stab analysis.  Would you only want it if beta=0?  Do you want the deltas calculated between every pair of alpha, or just the two closest to zero - or the first/last, or what?  You can of course 

It might just be as easy for you to calculate these things yourself as you parse the data from your alpha sweep.  The calculations are here...  Where the CMm_alpha and CL_alpha are simply calculated by finite-differencing two runs (typically with delta alpha=1.0).

Rob

[Franco Staub]

Rob,

Thank you for the help! And I see your point. It’s hard to set it up so that everyone is satisfied. 

Personally, I think the most relevant value would correspond to a typical cruise CL - like 0.5, but I know that takes extra steps to find CL. I’m sure finite-differencing alpha at 0 to 1 will give effectively the same answer in 99% of use cases. I’d also imagine 1 ac value per full alpha sweep analysis is probably fine for most users, and that it would only calculate it for beta = 0 runs.

A couple extra points:

-Is it worth putting in a printf() for -acstab?

-I’ve been calculating the ac in the same way as the code, just from the .pol output file values. Nice to know the -acstab option exists though.

-I saw the printf() from the -wtf option and had a chuckle.. 

Long live OpenVSP!

Franco

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