stability analysis

[Kun Drew]

Hi，

I am learning how to do stability analysis for aircraft in openvsp，When I got the.stab and.pstab files after the "steady" stability analysis and the "p" analysis, I couldn't fully understand what the results meant.

I have attached the screenshots of these two result files. How can I understand and use the parameters and values in them to analyze the maneuvability and stability of the aircraft.

I learned from books and some literature that I could construct the nonlinear dynamic model of the aircraft through these parameters, and further obtain the linear model to analyze the maneuverability and stability of the aircraft.

I am now stuck on how to make sense of these stability parameters and how to tell if my model is not accurate enough to cause too much error in the stability parameters.

Best regards

[Brandon Litherland]

There have been many similar posts on this forum asking exactly this question.  Please use the search function and read prior responses.  If you still have questions after that, post again and we can help you out.

[Kun Drew]

Thank you as always for your replies!

 I've read the vast majority of the content here on stability analysis in the last few days, which has helped me understand more about OpenVSP's stability analysis capabilities, and I've figured out the meaning of most of the parameters, but there are still a number of parameters in the output files of the stability analysis (e.g., the .P/q/rstab files, the .aerocenter.stab, and the .stab files) that I don't understand. 

 please help me to understand them!

Below:

(1)The .stab file contains the derivatives of the angular velocities p, q, r, etc. However, we usually think of the derivatives of the angular velocities as dynamic derivatives, but the “steady” mode is a static stability analysis.

(2)In the.stab file, the derivative of the aerodynamic parameter with respect to U, what does U refer to, what does it relate to and how is it calculated?

(3)What do SM and X_np refer to?

(4)In the.qstab and.rstab files, the dynamic derivative is the derivative of the sum of q and alpha_dot and the difference between r and beta_dot. Some equations from the literature show: (CL_q * q + CL_alpha_dot * alpha_dot), and the same goes for r and beta_dot. Can I simply add or subtract the values of the aerodynamic derivatives in the.stab file to split it?

(5)In these output files, most of the values are small values, but there are very few values that are hundreds, thousands or even larger. I think there should be some errors in the results. I have learned that the accuracy of the results of OpenVSP stability analysis is strongly related to the accuracy of the model.How can I tell if some parameters are wrong, and how can I modify my model to correct it?

I would be grateful if you could give me a detailed answer.

Best Regards!

[Rob McDonald]

On Thursday, August 29, 2024 at 4:41:28 AM UTC-7 Kun Drew wrote:

Thank you as always for your replies!

 I've read the vast majority of the content here on stability analysis in the last few days, which has helped me understand more about OpenVSP's stability analysis capabilities, and I've figured out the meaning of most of the parameters, but there are still a number of parameters in the output files of the stability analysis (e.g., the .P/q/rstab files, the .aerocenter.stab, and the .stab files) that I don't understand. 

 please help me to understand them!

Below:

(1)The .stab file contains the derivatives of the angular velocities p, q, r, etc. However, we usually think of the derivatives of the angular velocities as dynamic derivatives, but the “steady” mode is a static stability analysis.

An aircraft can operate in a steady roll.  The steady derivative wrt p represents the change in force/moment coefficients between rolling at 1 rad/sec vs. not rolling at all.  Similar for pitching and yawing.

This calculation is done in VSPAERO by adding a rolling component of velocity to the freestream term -- essentially, you hold the aircraft stationary and rotate the world about it.  This enables simulation of a steady roll.

 

(2)In the.stab file, the derivative of the aerodynamic parameter with respect to U, what does U refer to, what does it relate to and how is it calculated?

U is the magnitude of the freestream velocity.

 

(3)What do SM and X_np refer to?

Static margin.

X coordinate of the neutral point.

 

(4)In the.qstab and.rstab files, the dynamic derivative is the derivative of the sum of q and alpha_dot and the difference between r and beta_dot. Some equations from the literature show: (CL_q * q + CL_alpha_dot * alpha_dot), and the same goes for r and beta_dot. Can I simply add or subtract the values of the aerodynamic derivatives in the.stab file to split it?

Yes you can.

We left them as a combined term because in some forms of writing the equations, they appear together as a sum.  However, if you need them as separate terms, just subtract.

 

(5)In these output files, most of the values are small values, but there are very few values that are hundreds, thousands or even larger. I think there should be some errors in the results. I have learned that the accuracy of the results of OpenVSP stability analysis is strongly related to the accuracy of the model.How can I tell if some parameters are wrong, and how can I modify my model to correct it?

I would look at the visualization of the pressures and wakes for each case.  For the unsteady cases, you should be able to click through and see the change in the solution with time.

I would also look at the load distributions.

Everything should look reasonable and make sense. 

Also, make sure that your reference conditions are set to reasonable values.  A zero for cbar would do terrible things...

Rob