Stability derivatives

[jason]

Good Day All,

I am trying to get the stability derivatives for the wing model.

I have run the stab model in OpenVSP and exporting the Stab file to an excel file.

Using Matlab to extract the information from the excel to compute the eigenvalues and eigenvectors. 

The result came out pretty ugly, not what I was expecting.

The phugoid graph is a decaying curve with no oscillation.

The short period graph was blank.

Some are blank and some are straight lines for the lateral-directional.

For the lateral-directional, could it be due to the fact that there is no tail component involved in the wing model, that's why it became like that?

The mass properties of the wing are as shown.

0.151400             Total Mass
0.874394 -0.000000 0.000000       Center of Gravity
0.060403 0.019707 0.079903       Ixx, Iyy, Izz
-0.000004 -0.000000 0.000006       Ixy, Ixz, Iyz

The SM result in the stab file is the k value or the percentage of the MAC?

Attached are the results, the matlab file and the stab file.

Regard

Jason

[Brandon Litherland]

Without a VSP model, we can't really say one way or the other what is going on.  It could be a number of things.  The static margin is fraction of Cref. (0.8817 - 0.4)/0.7523 = 0.64.

There could be an issue in the code, a modeling problem, etc.  It looks like you're trying to run Panel method which I advise against for this exercise.  Use VLM mode first to convince yourself you have the correct information and the scripts are running properly then (only if you need to) run panel mode.

[jason]

Good Day Brandon,

I have run it on VLM and made some changes to the model. 

The result of my model is unable to get the short period and dutch roll mode.

Attached is the model.

Appreciated your help :)

Regards

Jason

[Rob McDonald]

It is hard to know what you are trying.  Did you save your *.vsp3 file exactly as you ran your analysis -- i.e. with all the VSPAERO settings as you had them?

When I opened the file, it was set to sweep Alpha over ten points, performing stability analysis at all of them, in panel mode, and with the Sref set based on one of the tail surfaces -- there were other questionable things, but that is enough for it to be a real mess.

Next, the CG was set at x=1.6, the entire aircraft is shifted aft (the origin is well in front of the aircraft) and when I ran the VLM analysis, the NP came out at 2.32.  These three positions are depicted below.

The NP looks very reasonable -- your CG location is very far forward for this NP, it works out to a SM (with the Sref based on the Wing) of 0.966 -- which is ridiculously stable.

If your Matlab S&C code is correct (which we have no way to know), then I would start by reducing the static margin (moving the CG aft).

The other likely place for things to be wrong is with your units.  You may think everything is non-dimensional and it doesn't matter -- which is true for some things -- but not for others.  First, where are you getting your inertias from?  Are they in a consistent set of units with everything else?

Rob

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[jason]

Good Day Rob,

Apologies for the messy parameter setting, I did not save the .vsp3 file as per the VSPAERO setting. 

I have saved the file as per what the VSPAERO setting should be. 

The inertias are gotten from the mass properties file and yes, they are consistent sets of units.

I have shifted the origin to the front of the wing and shifted the CG to aft, now the CG is set at x=0.8 with the SM at  0.373465.

From the result, there is no oscillation for short period and dutch roll mode.

Does it mean that the model is so stable that it can go back to level flight after experiencing a disturbance?

Regards

Jason

[Rob McDonald]

A 37% SM is still ridiculously stable.

The file you attached still references the horizontal tail for the reference quantities -- but when I switched it to wing, I got numbers to match.  So I think we're talking the same language, but you need to be careful about saving what you are actually working with.  Otherwise it will become very frustrating for you down the road when you can not replicate your own results.

Move your CG to 1.02 or 1.03 and see how it does.

Rob

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[jason]

Good Day Rob,

Copied, will move the CG further aft to see how it performs.

Thanks for the help. 

Regards

Jason