Question about Cnb/Clb behavior and vertical surface modeling in Flow5

[Sorendgy]

Dear Techwinder,

I hope you are doing well.

I am currently working with Flow5 on a Three Surface Aircraft (TSA) configuration. The model is based on the default aircraft template provided in Flow5, with modifications applied only to the geometry. For this analysis, I am not including a fuselage.

I would like to ask about a behavior I have observed regarding the Cnb and Clb plots.

It appears that the signs of Cnb and Clb may be inverted in the way they are displayed. My reasoning is based on the following observations:

1. When I increase the lateral surface area, Cnb becomes more negative while Clb becomes more positive. This trend would be physically consistent if the plotted signs were reversed.
2. When I relocate the vertical surface to a position forward and below the center of gravity, both Cnb and Clb plots appear to invert. In this configuration, the aircraft seems statically stable according to the plots.
3. However, in this second case, the spiral mode pole becomes positive, indicating dynamic instability, which was not present in the initial configuration.

This leads me to suspect either a sign convention issue or a possible misunderstanding on my side regarding how these coefficients are defined and presented in Flow5.

Additionally, I would like to better understand how Flow5 defines and handles multiple vertical surfaces. This question comes in part from my experience with XFLR5, where it is possible to define a single vertical tail entity, whether it consists of one or two physical surfaces. In contrast, it seems that Flow5 may treat lateral areas differently, which could be influencing the results.

I am particularly interested in:

1. H-tail and U-tail configurations
2. The use of dorsal fins
3. Vertical surfaces located outside the main tail assembly
4. Whether it is possible to model endplates in Flow5, and how they are interpreted aerodynamically

It seems that adding or modifying these surfaces produces unexpected effects on both Cnb and Clb, and I would like to better understand how their contributions are modeled.

Finally, regarding fuselage modeling: in configurations where vertical surfaces are not connected through a central fuselage extension, but instead mounted on twin booms, would it be appropriate to represent this by adding two slender fuselages? I would appreciate any guidance on how to best approach this type of configuration in Flow5.

Thank you very much for your time and for developing Flow5—it has been a very valuable tool in my work.

Best regards,
Sorendgy

[Flying-Geek]

Hi,
have a look at the documentation regarding the frames and axes.
I don't have time to double check the results right now, but I know that not all of the derivatives are always intuitive since the sideslip angle definition can be reversed depending on the literature.

Regarding your questions of modelling tails:
The usual limitations apply, so if the lifting surface has a very small aspect ratio don't expect the results to be great.
Other than that, modelling endplates might be a bit difficult. You can give a horizontal tail "winglets" and modelling the vertical tail surface that way, but simply moving the vertical tail to the sides of the horizontal will likely not work.
For a model with twin booms I would ignore the booms entirely.

Cheers,
Stefan

[techwinder]

In addition to Stefan's answer:
Signs of moments: It depends on the convention, and I've struggled historically to determine which convention to use in xflr5 and flow5. It is now aligned with AVL and the moments are evaluated in stability axes with z pointing downwards. See "Axes and frames of reference" in the linked page.

Vertical surfaces: Not sure that I understand the question. All surfaces in flow5 are handled in the same way, whether horizontal, vertical or oblique. Once the mesh has been built on the surfaces, their orientation does not matter anymore.

Twin lateral booms: As Stefan said, you can just ignore them. If you choose to go ahead and insert twin bodies, note that flow5 only handles the connection of the first body to the wings, so in this case
1. make sure that the bodies do not intersect the wings, i.e. leave gaps
2. make sure that the fins are closed at their inner side - this is done in the wing editor
3. do not assemble fuselage and wings in the plane editor

André

[Sorendgy]

Thank you for your replies, they helped clarify the issue.

I will go through the documentation regarding frames and axes more carefully, as you suggested.

I also understand your points regarding vertical surface modeling and solver limitations, as well as the recommendations for twin booms, ignoring then might be my best option for now.

Thanks again for your time and for the clarification,
Sorendgy