PQR Analysis

[Augustine Bageanis]

I've been looking in VSPAero and I see there is a stability analysis option to use P, Q or R. I've looked around on the internet and I haven't been able to find anything about it. It seems to show the moments about the CG in time using an initial disturbance. I'd like learn more about this. Any tips?

[Rob McDonald]

The 'normal' stab analysis performs various perturbations of the
flight conditions to calculate steady derivatives. A perturbed steady
solution is found and a finite difference is taken from the
un-perturbed solution to estimate the derivative.

The P, Q, R analysis options calculate unsteady derivatives about
roll, pitch, and yaw (respectively) axes by executing a full period
sine oscillation of 1deg amplitude. This is an unsteady analysis, the
derivatives are post-processed from those results.

If you are interested in a full plant, then you'll need to run all
four stab modes.

Rob

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[Augustine Bageanis]

Thanks, that helps.

[Vincent Ren]

Hello,

I was wondering how the coefficients Cx_p, Cx_q, and Cx_r (were x is any parameter) were non dimensionalized ? What are their definition ? In the normal stab analysis, it seems that their unit is rad-1 were time is non dimensionalized with 2b/VTAS for p and q and 2c/VTAS for r. What about those from pqr analysis ?

Also, I've found very different values from these two approach, wich one is more reliable ?

Thanks!

Vincent

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[Rob McDonald]

Forces and moment coefficients are conventionally defined.  Nondimensionalization to coefficients occurs here.

The 'normal' stability analysis runs seven (or more for control deflections) steady state cases and finite-differences them to get steady aerodynamic derivatives.  This includes steady cases in rolling, pitching, and yawing flow.  So, CM_q is calculated - abbreviated CMm_q.

The denominator for these derivatives is calculated here.

The pqr stability analysis runs an unsteady solution for the aircraft through an oscillation about the appropriate axis.  It then does an analysis on the force/moment history to obtain the derivatives.  So, in a pitch analysis, the sum CM_q+CM_alpha_dot is calculated.  In the file, this is abbreviated CMm_(q + alpha_dot).

These derivatives should be calculated in a similar way.

Rob

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[Vincent Ren]

Thank you for your answer! It's very helpful.

But I'm still a bit confused with the coefficients with pqr derivatives. I'm trying to compare results from Open VSP with analytical results from Megginson database for a Cessna182 (http://www.megginson.com/Aviation/roskam-coefficients.html). Should I a do a pqr stability run to get the CX_q coefficients (for eg. CMm_q : pitching moment due to pitch rate) or take the one from the 'normal' stability analysis ?

Thank you 

Vincent Ren

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[Vincent Ren]

Also, why is the sum Cm_q + Cm_alpha_dot calculated? What is the difference between both coefficients?

[Rob McDonald]

In the reference you link, there are two columns...

Cmadot and Cmq

These are the same quantities we are looking at.

In the unsteady analysis, these two can not be independently calculated -- they come out as a combined sum.  Whereas, in the steady analysis, only Cmq can be calculated.

You can subtract them to isolate Cmadot -- however, for VSPAERO to do that, it would mean we need to do both an unsteady and two steady calculations in one run.  Also, since they are calculated in different ways, there may be some accuracy issues.

In the equations of motion, these terms often show up as the sum -- so you end up putting them back together anyway.

For the balance of these reasons, we decided to just report the sum.

Rob

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[Mohammad Bahmani]

Hello Rob

i have a question about this matter. I did both steady analysis and Q analysis to get the value of Cmq for my model. and i got the same results everytime. does it make sense? from what i got from previous conversation , steady analysis should only give CMq value and Q analysis should give the sum of CMq and CMaDot . also am i reading the correct value for Cmq from the picture 

below? or should i use (CMm_ Pitch rate) instead of (CMm_q) ?

[Rob McDonald]

I think you're looking at the wrong file.

The steady stability analysis writes out a *.stab file.

The unsteady Q stability analysis writes out a *.qstab file.

I think you're getting the same thing because you're looking at the same file...

Here are two examples for a simple wing I just ran (VLM mode).

Rob

On Sun, Jun 12, 2022 at 1:58 PM Mohammad Bahmani <bahma...@gmail.com> wrote:

Hello Rob

i have a question about this matter. I did both steady analysis and Q analysis to get the value of Cmq for my model. and i got the same results everytime. Is that right? from what i got from previous conversation , steady analysis should only give CMq value and Q analysis should give the sum of CMq and CMaDot . also am i reading the correct value for Cmq from the picture 

below? or should i use (CMm_ Pitch rate) instead of (CMm_q) ?

On Tuesday, March 24, 2020 at 11:02:18 PM UTC+4:30 Rob McDonald wrote:

To view this discussion on the web visit https://groups.google.com/d/msgid/openvsp/c0452308-f24d-409c-9c8b-568f05e7eb7en%40googlegroups.com.

[Mohammad Bahmani]

Ahh you're right.i had no idea that there is a .qstab file. thank you for you'r answer. one more thing... in my example ,the value of CMq is -34.6435 right ? 

[Rob McDonald]

You are reading the output correctly.

The value will depend on the reference quantities Sref, bref, cref and the moment reference location xref, yref, zref.

Further, if the aero solutions don't look reasonable, then there is no reason this result would be reasonable.  So, build confidence on the base case first (check the load and pressure distributions, wakes, forces, moments, etc) and then build from there.

Rob

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[Mohammad Bahmani]

Many thanks Rob. Ypu and your team has done a great job with this software. I wish you the best