Unit issues about VSPAERO Advanced Flow Conditions Module

[Eric Cheng]

Hi everyone!

I have some questions when using VSPAERO.

My software version is 3.24.

 I would like to ask what are the units of velocity（Vinf、VRef） and density（Rho)  in the advanced flow conditions in the screenshot below? Is it inches per second or miles per hour or meters per second? What about the unit of density? Is it the International Standard System of Units?

I'll wait an answer for you guys thanks!

[Rob McDonald]

OpenVSP itself is dimensionless -- whatever you use for length units is your own interpretation.

VSPAERO needs these quantities in a consistent system.  So, if your length units are feet, then velocity should be ft/sec.  If your length units are meters, then m/s, etc.

Density should be in a system (mass/len^3) that works with F=m*a -- force=mass*length/time^2 consistently.  For ft, this is slug/ft^3.  For m, this is kg/m^3.

If you're using inches or anything else, it gets messy.

Rob

--
You received this message because you are subscribed to the Google Groups "OpenVSP" group.
To unsubscribe from this group and stop receiving emails from it, send an email to openvsp+u...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/openvsp/8620d60f-fbf1-4c37-bd9f-1aa5c87058f5n%40googlegroups.com.

[Eric Cheng]

Rob, first of all， thank you very much for your patient answer, what I still want to know is:

How can I set or change my length unit?

 Because I want to know what type of my length unit is. Is it feet, inches or meters?

I'll wait an answer for you guys thanks!

[Rob McDonald]

There is no setting, OpenVSP sets units through mind reading.  Think about your model.  Lets say your wingspan is 10.  If you think your span is 10 m, then the length units are in m.

Once you've thought about your model in meters, then you should set the Velocity to m/s and the density in kg/m^3.

Of course, I would never think in meters.  My mind just couldn't take it.  So instead I would think my model had a span of 10 ft.  OpenVSP would read my mind and I would then set my velocity in ft/s and my density in slug/ft^3.

Rob

To view this discussion on the web visit https://groups.google.com/d/msgid/openvsp/963badca-39a1-4c9a-9480-bce101ecae03n%40googlegroups.com.

[Eric Cheng]

Oh, I finally understand, thank you very much, Rob