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Simulation CFD

Active Member
Posts: 6
Registered: ‎06-03-2014

Re: Problems with extracting aerodynamic data for a flight simulator

08-19-2014 06:35 AM in reply to: RagnarDa

Thanks for trying to help me, but first of all my model isn't a individual airfoil but a whole airframe. Secondly, my reference wind tunnel data describes the formulas and all the different measurements (reference chord, wing area and wing span) that are used to standardise the coefficients. My model is scaled to the real version. I did try lower angles, but only went so far as 10 alpha or 10 beta and they are still off by orders of magnitude. Changing the axis position doesn't do anything and changing the torque direction only changes the output of the torque line and is the same as the others, or maybe I don't understand something. My reference contains data for up to 60 degrees and my aim is to complete it with wind data for angles all around the design.

Product Support
Posts: 636
Registered: ‎08-24-2011

Re: Problems with extracting aerodynamic data for a flight simulator

08-20-2014 06:13 AM in reply to: RagnarDa

How did the results look for 0 alpha 0 beta?  Why not start there first?

Best regards,

Royce Abel
Active Member
Posts: 6
Registered: ‎06-03-2014

Re: Problems with extracting aerodynamic data for a flight simulator

08-21-2014 01:01 AM in reply to: RagnarDa

It looks like this at M0.3:

Total area 224.353

TOTAL FX -21508.7

TOTAL FY 17295.7

TOTAL FZ 13924.8

Center of Force about X-Axis (Y-Z) 5.53313

Center of Force about Y-Axis (X-Z) 0.146436

Center of Force about Z-Axis (X-Y) -1.53812

TOTAL MX -56248.5

TOTAL MY -149756

TOTAL MZ 96001.1

Torque -86640.4

Velocity: 102.963

Density: 1.20473

Dynamic pressure 6385.8998836077

Which gives

C_m -0.0258761279  (should be around -0.01)

C_n? -0.0509805239 (should be 0)

C_l? 0.0326810704 (should be 0)

I guess these could be attributed to errors in my CFD-model but usually the difference is orders of magnitude wrong.

Valued Contributor
Posts: 92
Registered: ‎01-18-2006

Re: Problems with extracting aerodynamic data for a flight simulator

08-21-2014 12:39 PM in reply to: RagnarDa

Glad to hear it's coming within an order of magintude... but it's really hard to diagnose without more detail. Can you upload a picture, preferably one that illustrates your meshing, and showing the torque axis you're using?

I would highly recommend doing a very simple model with only a few surfaces and a well documented result, and getting the coefficients to match for that setup first. Start at a low angle of attack and low mach, and try to match CL within 10% and CD within 30%, and CM should follow if the reference axes match. Then see if pitch changes (alpha) produce the expected results first, and then yaw changes (beta). Only then go to higher mach (again starting at low alpha and no beta) and make sure compressibility is enabled and variable air material used if needed. Then increase alpha, etc.

Active Member
Posts: 6
Registered: ‎06-03-2014

Re: Problems with extracting aerodynamic data for a flight simulator

08-23-2014 03:25 AM in reply to: RagnarDa

Here's my model. I know it isn't super-accurate but I did try to get the wing form as close as I could but still trying to have the general shape as simple as possible. Even though the model might be inaccurate, I figure the aerodynamics shouldn't deviate more than in the range of something like 30% and not 1000% or more as it is now. Btw, I know the wind tunnel data is accurate as I have tested the flight performance in the flight simulator against real-life performance.

Product Support
Posts: 636
Registered: ‎08-24-2011

Re: Problems with extracting aerodynamic data for a flight simulator

08-25-2014 06:19 PM in reply to: RagnarDa

Hi!

You might want to review some of the setup options discussed in this forum discussion:

http://forums.autodesk.com/t5/simulation-cfd/autodesk-simulation-cfd-fails-benchmark-test/td-p/52281...

Best regards,

Royce Abel