VSPAERO - basic questions

[Francesco Brunetti]

Hello everyone,

I am new to this fantastic tool and I am eager to dive in and trying to understand as much as possible. I graduated in Aerospace Eng. few years ago and then I transitioned to aircraft manufacturing so I might be a little rusty on some topics (my apologies if questions could look a bit "silly").

I am trying to get an aerodynamic dataset for a flight simulator of a low wing 2 seat trainer jet (fairly simple almost-straight wing, conventional configuration). I thought it might be good to start with a VLM method and exploring the linear region and the steady parameters of that flight regime first. So being new to this type of thing (never used VLM software, nor CFD) I decided to start from VSP and VSPAERO which seemed to me very user friendly. 

I started up building my plane and the switched to the VSPAERO function, below some questions (be sure, more to come :) ).

1. Can the input file be modified manually in any moment also after having clicked on "Create new Setup" ?

2. My A/C show up some touching and intersecting volumes (e.g. vertical tail and fuselage), are those interecting volumes a problem for the analysis ? (I saw some other model with interecting volumes as well...)

3. When modeling the a/c, the surfaces to be created have to respect a hierarchy from each others ? Would this influence the analysis ? if yes in what terms ?

4. Which is the relationships of Vinf , Mach , ReCref in VSPAERO ? When doing a sweep analysis between alpha and Mach, do I need to also specifying each reynolds at that flying conditions ? (M1 - Re1 , M2 - Re2 ecc...). Do I need to set Vinf in some way or just leave it alone (I guess it is already within the Mach, which also includes temperature and density and thus altitude) ? and what about the density values ?

5. I red a presentation on VSPAERO which was mentioning that the .hystory file includes the CL and CLt (treftz) analysis. I checked and my history files only include CL. Is the treftz analysis already in there ? if not how can this function be activated ?

6. Which is the influence on setting the "CLmax" within the input File ?

Thank you and keep up the outstanding work.

Francesco from Italy.

[Rob McDonald]

Francesco,

Welcome to OpenVSP -- I'm glad you're finding it useful for your purposes.

On Wed, Feb 15, 2017 at 2:31 AM, Francesco Brunetti
<francesco...@gmail.com> wrote:

> 1. Can the input file be modified manually in any moment also after having
> clicked on "Create new Setup" ?

Yes. Don't forget to click 'Save Setup' when you are done making changes.

However, this situation is a little confusing... 'Create new Setup'
generates a template setup file with the parameters currently set in
the GUI. You can then edit that setup file and save your changes --
those changes will not be reflected in the GUI.

When you run, VSPAERO will use your setup file.

However (here is the confusing part), some settings are over-ridden by
OpenVSP when running VSPAERO. OpenVSP passes some parameters on the
command line -- thereby overriding what appears in the setup file.
Angle of attack, Mach, Beta, and some others fall in this category.

When OpenVSP runs VSPAERO, it prints out the full command line as the
first line that goes to the solver console. If you see that, you will
see what parameters are overridden.

The main reason to edit the setup file manually is to control things
not yet available from the GUI -- mainly propellers and control
surfaces. If you aren't using those yet, you can safely do everything
through the GUI.

Soon an update to OpenVSP will place full control of everything in the
setup file in the GUI. So, the user won't ever see the setup file at
all.

> 2. My A/C show up some touching and intersecting volumes (e.g. vertical tail
> and fuselage), are those interecting volumes a problem for the analysis ? (I
> saw some other model with interecting volumes as well...)

No, that is the correct way to model things in OpenVSP. You usually
want to bring wings together to the center line (rather than try to
stop them at the edge of the fuselage).

That said, if you are running in Vortex Lattice Mode, there can be
some undesirable interaction effects between the panels representing
the fuselage and a wing -- if the wing comes in at the fuselage center
line.

Start simple, build up your models from there and you'll gain
experience in how to get good results.

> 3. When modeling the a/c, the surfaces to be created have to respect a
> hierarchy from each others ? Would this influence the analysis ? if yes in
> what terms ?

The order of construction of the model won't matter. The spatial
relationships of the components will matter.

> 4. Which is the relationships of Vinf , Mach , ReCref in VSPAERO ? When
> doing a sweep analysis between alpha and Mach, do I need to also specifying
> each reynolds at that flying conditions ? (M1 - Re1 , M2 - Re2 ecc...). Do I
> need to set Vinf in some way or just leave it alone (I guess it is already
> within the Mach, which also includes temperature and density and thus
> altitude) ? and what about the density values ?

This is a little confusing.

Mach is only used to set the compressibility term. Think of it like a
Prandtl Glauert correction.

Re is only used in the skin friction drag buildup for CD0. VSPAERO's
CD0 buildup is extremely simple. I would recommend doing your own
component wise buildup rather than using it.

Vinf and rho are only used for converting propeller coefficients CT
and CP into a dimensional form that makes sense with the rest of the
aerodynamic solver.

So, if you are doing a low-Mach (incompressible) solution, with no
propellers, and ignoring the CD0 term, then none of those matter.

> 5. I red a presentation on VSPAERO which was mentioning that the .hystory
> file includes the CL and CLt (treftz) analysis. I checked and my history
> files only include CL. Is the treftz analysis already in there ? if not how
> can this function be activated ?

OpenVSP still reports the Trefftz lift and drag, I believe a bunch of
things have changed since those videos were put together.

> 6. Which is the influence on setting the "CLmax" within the input File ?

It effects how VSPAERO integrates the forces -- it is a very empirical
adjustment. If you want to play with it, you should start with a
known data set that you want to match and try tweaking the value to
obtain the kind of results you need.

Best of luck,

Rob