Propeller Calculations

[Ash]

What does the term " Vref " mean in VSPaero used for analysis of propeller.

[Rob McDonald]

VSPAERO uses Vref separate from Vinf.

Vinf is the freestream velocity.  If you are doing a hover or static thrust case, this will be zero.

Vref is the reference velocity.  It is used anywhere you need to normalize something by velocity.  Think about force coefficients -- CL = L/(0.5*rho*Vref^2).

In most situations, Vref=Vinf is a good choice -- the OpenVSP GUI makes it the default.  However, when you are more interested in the rotor - or you are doing a Vinf=0 case, then you may want to use something else.  You can use anything you want, but the rotor tip speed is a pretty good choice.

Minf should match Vinf and Mref should match Vref.

Rob

On Wed, Feb 9, 2022 at 6:48 AM Ash <ashish...@gmail.com> wrote:

What does the term " Vref " mean in VSPaero used for analysis of propeller.

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[Ash]

1. In what case is the Aircraft in when Vinf = Vref.

2. At what position is the Vref acting for an aircraft.

[Rob McDonald]

On Wed, Feb 9, 2022 at 9:13 PM Ash <ashish...@gmail.com> wrote:

1. In what case is the Aircraft in when Vinf = Vref.

Whenever you so desire.

Do not think of it as Vinf is equal to Vref.  Think of it as sometimes Vref is equal to Vinf.

 

2. At what position is the Vref acting for an aircraft.

Vref has no direction.  Vref is only a magnitude.

Rob

 

 

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[Ash]

Thank you for your quick response.

The above questions are in regards to the project I am currently working on, based on a dual motor fixed wing electric aircraft. I am actually trying to understand the aft stream propeller wake interactions with the wing and how it affects the aerodynamic parameters such as lift, drag, etc. Also, if Vref is considered to be the rotor tip speed (as mentioned above), how can it be calculated (Vref) to be fed into the advanced flow conditions table in AEROVSP when analyzing the propeller, if given that we know the propeller rpm or the thrust. 

[Rob McDonald]

What you use for Vref will not change the solution to the problem.  It will only change how some of the outputs are scaled.  As long as you dimensionalize the coefficients using a consistent Vref, it won't matter.

If you are mostly interested in the aerodynamics of the airplane in forward flight -- then use Vref=Vinf.  When you are looking at a forward flight case, this is usually the case.

If you are mostly interested in the aerodynamics of the blades themselves -- then use Vref=Vtip.  When you are looking at a hover or static thrust case, this is usually the case.

Rob

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[Ash]

I would like to know what does the  term V/Vref, V_Vref, and V in the term V/Vref denote in the Degenerate geometry csv file of VSPaero analysis under two cases:

1. Forward flight case when Vinf = 18m/s and Vref = 18m/s.

2. Hover case when Vinf = 0 and Vref( rotor tip speed ) = 79.796m/s. And also wanted to know how to locate the Vref value in the csv file.

In the above mentioned hover case, how can we calculate the thrust.

If the propeller rpm(4000) is input in the propeller section of VSPAERO, and the software calculates the rotor tip velocity, why is there an option available to input Vref(which is same as the rotor tip speed as mentioned before).

[Rob McDonald]

The DegenGeom file does not have those items in it.  However, DegenGeom gets placed in the file name -- which VSPAERO then writes many files with that as a base name.

I do not think V, V_inf, or V_ref appear in the DegenGeom file as I would describe it.  Can you post the file you are looking at?

The other place that has a *.csv file that I can think of is when you write out a summary of results from the Results Manager GUI.  That is a secondary file, not an authoritative source.

Helicopter and Propeller force coefficients are defined in terms of tip speed or RPM.  They do not use Vref.

However, the pressure coefficient -- if you run vspviewer still needs Vref.

VSPAERO has a simple 2D drag model based on NACA airfoil data.  It is sensitive to the local Mach and Reynolds numbers.

The user inputs Vref, Mref, Cref, ReCref, and rho.  These all need to match your desired flight condition.

When calculating the drag on a particular section of a wing or rotating blade, VSPAERO will calculate the local velocity and chord.

From the local V, it will calculate V/Vref.  It will then use that to calculate local Mach and (also using local chord) local Reynolds number.  It also calculates local lift coefficient from the flow.  Given the 2D sectional cl, Re, M, it calculates a local sectional drag coefficient.

Rob

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[Ash]

I am currently working on propeller related calculations. Can I know how I can calculate the propwash velocity from the csv file obtained from VSPaero or any parameter related to propwash velocity. I am considering for Forward Flight ( Vref = Vinf ).

[Brandon Litherland]

For unsteady blades or actuator disks, yes you can do this but keep in mind that it will only give you the result for the last step in the run (I think).

See this presentation from the 2020 Workshop on Advanced Modeling Techniques.  Toward the end on slide 40 you'll see how to append a set of sample points to the *.vspaero input file to get XYZ velocities out.  There is also some work on VSPAERO coming down the line from Dave that will display the velocity fields as vectors.

http://openvsp.org/wiki/lib/exe/fetch.php?media=workshop20:vsp-ws20_cad_cfd_interaction_and_advanced_modeling_techniques.pdf