Fuselages - Part I: Preliminary considerations

Summary

This presentation series explains the analysis of planes with fuselages using flow5, highlighting the significant impact of fuselage inclusion on performance. It covers the necessary steps, including geometry construction, mesh generation, and critical review of results, emphasizing the importance of correctly connecting wing and fuselage meshes and the use of boundary element methods.

Takeaways

• 🛫 The presentation is a 5-part series focusing on analyzing the aerodynamics of a plane with a fuselage using Flow5.
• 🛠 Unlike xflr5, Flow5 has all the necessary tools for analyzing a plane with a fuselage, both theoretically and numerically.
• 📊 Including the fuselage in the analysis can significantly affect performance, as shown by the difference in polar graphs between the blue (without fuselage) and red (with fuselage) curves.
• ⚠️ The inclusion of the fuselage requires careful construction of geometries and surface meshes, and a critical review of results to avoid errors.
• 🔍 The main challenge is ensuring the union of surface mesh elements forms closed, non-intersecting volumes, which is crucial for the boundary element methods used in Flow5.
• 🛑 It's important to connect wing meshes to the fuselage mesh without leaving panels inside the wing thickness and avoiding volume intersections.
• 🔨 Flow5 uses the open cascade API to build closed volumes, which requires closed trailing edges for wings and careful handling of half-wings like fins.
• 💻 The presenter implemented a custom mesher using the advancing front method due to issues with existing open-source meshers.
• 📏 Mesh quality is crucial, and special attention should be given to avoiding small edges, which can cause high local triangle counts and mesh sizes.
• 🔗 When connecting wing and fuselage meshes, Flow5 aligns nodes, but manual adjustment may be needed for edges crossing the wing root section.
• 🌐 After connecting meshes, the triangle mesh needs to be connected, which can be done automatically or manually, with free edges highlighted for review.
• 📉 The analysis results are derived from doublet densities, so it's essential to ensure these densities are calculated correctly and exhibit no unusual patterns.

Q & A

What is the main purpose of the 5-part presentation?

The main purpose of the presentation is to explain how to run the analysis of a plane with a fuselage using flow5, emphasizing the importance of including the fuselage in the analysis for accurate results.

What is the difference between the results shown by the blue and red curves in the polar graphs?

The blue curve corresponds to the case without a fuselage, while the red curve includes the fuselage. The difference highlights the significant drop in performance when the fuselage is considered, such as in glide ratio and drag polar.

Why is it important to include the fuselage in the analysis?

Including the fuselage is important because it has a significant influence on the plane's performance and can affect results like glide ratio and drag polar, as shown in the polar graphs.

What are the challenges in including the fuselage in the analysis?

The challenges include building and constructing the geometries and surface mesh carefully, ensuring that the union of surface mesh elements defines one or more closed, non-intersecting volumes, and avoiding intersections between volumes like the tail end.

What is the role of the open cascade API in flow5?

The open cascade API is used by flow5 to build closed volumes, which is essential for connecting the wings to the fuselage mesh and ensuring that individual volumes form closed solids.

Why is it necessary to close the trailing edges of the wings in the analysis?

Closing the trailing edges of the wings is necessary to form closed volumes that can be connected to the fuselage, which is a requirement for the boundary element methods used in flow5.

What issues did the presenter encounter with open-source meshers, leading to the development of a custom mesher?

The presenter encountered various issues with existing open-source meshers, which led to the development of a custom mesher using the advancing front method for better performance and quality in most cases.

What are the considerations to ensure a good quality mesh in the custom mesher?

The considerations include defining the geometry as a union of faces with closed contours, avoiding free edges, and being cautious about small edges that can lead to high mesh sizes and numerical issues.

How does flow5 handle the connection between the wing mesh and the fuselage mesh?

Flow5 ensures that the fuselage nodes fall opposite the wing nodes, with the exception of edges in the fuselage that cross the wing root section, which may require manual adjustment.

What is the significance of the wake panels or vortices extending from the wing trailing edges in flow5?

The wake panels or vortices are significant because they need to be checked for intersection with downstream parts, ensuring that elements like the elevator are not located in the same horizontal plane as the main wing.

How does the presenter suggest analyzing a plane with its fuselage for the best results?

The presenter suggests using the thick triangular panel method with linear doublet densities, simplifying and cleaning the geometry before import, and ensuring a good mesh quality for accurate analysis.

Outlines

00:00

🛫 Introduction to Fuselage Analysis in Flow5

The speaker introduces a 5-part presentation focused on analyzing the impact of a fuselage on an aircraft's performance using Flow5 software. Unlike XFLR5, Flow5 has integrated tools for accurate analysis from both theoretical and numerical perspectives. The presentation will cover guidelines for including the fuselage in analysis, the importance of constructing geometries and surface meshes carefully, and reviewing results critically to avoid errors. The speaker will summarize considerations for fuselage analysis, discuss two types of fuselage geometries available in XFLR5, and demonstrate importing and processing STL and STEP files in Flow5. The main challenge is ensuring the surface mesh elements form closed, non-intersecting volumes, which is crucial for the boundary element methods used in Flow5.

05:03

📐 Meshing and Geometric Considerations

This paragraph delves into the specifics of meshing and geometric considerations when analyzing aircraft with a fuselage. The speaker explains the importance of defining the geometry as a union of faces with closed contours and avoiding free edges, especially when importing from CAD software. The issue of small edges, which can lead to high mesh sizes and numerical inaccuracies, is highlighted. The process of connecting the fuselage mesh to the wing mesh in Flow5 is discussed, including handling exceptions where fuselage edges intersect with the wing root. The speaker also touches on the importance of ensuring that wake panels do not intersect downstream parts and the significance of checking doublet densities for quality assurance in panel analysis.

10:03

🔍 Analysis Techniques and Design Options

The speaker discusses various analysis techniques and design options for analyzing an aircraft with its fuselage. The options include modeling wings as thin surfaces, which is simpler but may have numerical issues at mesh connections, and modeling wings as thick surfaces, which requires more complex meshing but is preferred for accuracy. The importance of using triangular panel methods for mesh construction when wings connect to the fuselage is emphasized. The speaker also explains the calculation of pressure coefficients from the surface gradient of doublet densities and the challenges associated with this process. Finally, the design options are summarized, recommending the use of thick triangular panel methods for accurate analysis.

15:05

🛠 Recommendations and Test Case Preview

In this final paragraph, the speaker provides recommendations for conducting a safe and accurate analysis of an aircraft with its fuselage. The advice includes using the thick triangular panel method with linear doublet densities, simplifying and cleaning the geometry before importing it into Flow5, and defining the fuselage with flat faces for robust meshing. The speaker also mentions the implementation of two drag models in Flow5 and invites suggestions for additional models. The paragraph concludes with a preview of the first test case, which will involve a quad face type fuselage, to be covered in part two of the presentation.

Keywords

💡Analysis

Analysis in this context refers to the process of examining and studying the properties and performance of an object, specifically a plane with a fuselage. It is central to the video's theme as it discusses how to perform this analysis using tools like flow5. The script mentions that 'the inclusion of the fuselage is not a click-run process' and requires careful consideration and guidelines to follow.

💡Fuselage

A fuselage is the main body or central structure of an aircraft, which houses the crew and passengers. In the script, the fuselage is a critical component in the analysis of a plane's aerodynamic performance, with the video emphasizing its significant influence on results such as glide ratio and drag polar.

💡Polar Graphs

Polar graphs are a type of chart used to display data in a circular pattern, with the radius representing the magnitude of a value and the angle representing the variable. The script uses polar graphs to illustrate the performance of a plane with and without a fuselage, showing a 'huge drop in performance' in certain cases.

💡Boundary Element Methods

Boundary Element Methods are numerical techniques used in engineering to solve problems involving boundaries, such as in fluid dynamics. In the script, these methods, including panel methods, are required to ensure that the surface mesh elements form closed, non-intersecting volumes, which is a main challenge in analyzing a plane with a fuselage.

💡Mesh

In the context of the video, a mesh refers to a network of connected points that form a surface or volume, used in computational modeling. The script discusses the importance of constructing a high-quality mesh for the fuselage and wings, noting that 'flow5 uses under the hood, the open cascade API' to build closed volumes.

💡Open Cascade API

The Open Cascade API is a set of libraries used for 3D modeling and data exchange in CAD/CAM/CAE applications. The script mentions that flow5 uses this API 'under the hood' to ensure that individual volumes, like wings, form closed solids that can be connected to the fuselage.

💡Doublet Densities

Doublet densities are a concept in potential flow theory, where they represent the strength of a doublet, or a pair of point sources and sinks. In the script, the analysis solves for these densities on the panels, and other results are deduced from them, making their accurate calculation crucial for the analysis.

💡Kutta Condition

The Kutta condition is a principle in aerodynamics that states the circulation around a finite wing must be such that the trailing vortices leave the wing at the trailing edge. The script mentions that the wing trailing edges need to be unconnected to create a vortex and apply this condition.

💡Numerical Issues

Numerical issues refer to problems that arise during the computation of numerical solutions to mathematical problems. The script discusses numerical issues related to the connection of the wing and fuselage meshes, noting that while theoretically possible, it is not recommended due to the potential for inaccuracies.

💡d'Alembert's Paradox

d'Alembert's Paradox is a principle in fluid dynamics that states that an object moving through a fluid experiences no resistance if the fluid adheres to the surface of the object. The script mentions this paradox in the context of the fuselage, noting that it does not shed a wake and therefore does not create induced drag or lift, but does generate friction drag.

💡Panel Methods

Panel methods are numerical techniques used to solve potential flow problems around bodies. The script discusses the use of these methods in flow5, particularly when modeling wings as thick surfaces and connecting them to the fuselage, which is essential for accurate analysis.

Highlights

Flow5 has implemented all necessary tools for analyzing planes with a fuselage, unlike xflr5.

Inclusion of the fuselage in analysis shows a significant drop in performance, such as glide ratio and drag polar.

Fuselage should be included in analysis if possible due to its important influence on performance.

Including the fuselage requires careful construction of geometries and surface mesh, following specific rules and guidelines.

Critical review of results is recommended after analysis to avoid errors.

Boundary element methods in flow5 require the union of surface mesh elements to define one or more closed, non-intersecting volumes.

Mesh of wings must be connected to the fuselage mesh without leaving any panels inside the wing thickness.

Volumes in the analysis should not intersect, such as the tail end where the elevator intersects the fin.

Flow5 uses the open cascade API to build closed volumes, requiring closed solids for wings to connect to the fuselage.

Wings must have closed trailing edges, and half-wings like the fin must be managed correctly for connection to the fuselage.

Custom mesher using the advancing front method is implemented in flow5 for better mesh quality.

Geometry for meshing should be a union of faces, each defined by a closed contour, and free of free edges.

Small edges in the model can lead to high mesh sizes and should be avoided.

Fuselage nodes must fall opposite wing nodes, with adjustments needed for edges crossing the wing root section.

Triangle mesh needs to be connected automatically or manually, with free edges displayed and highlighted in blue.

Wake panels or vortices extending from wing trailing edges must not intersect with downstream parts.

Doublet densities calculated on panels are the basis for all other results, requiring careful verification of their quality.

Pressure coefficients are calculated from the surface gradient of doublet densities, which can be tricky at wing-fuselage connections.

Design options for analyzing a plane with its fuselage include modeling wings as thin surfaces or thick surfaces using quad or triangular panel methods.

Thick triangular panel method with linear doublet densities is recommended for analysis if mesh size and computer power allow.

Simplifying and cleaning the geometry before importing into flow5 can limit mesh size and improve quality.

Defining a fuselage with flat faces is the most robust method for meshing.

Transcripts

In this 5-part presentation I'd like to explain how to run the analysis

of a plane with a fuselage.

And before I go any further, unlike in xflr5,

all the necessary tools have been implemented in flow5 to run

this analysis correctly, both from the theoretical and numerical point of views.

The kind of results we get when we do include the fuselage

are illustrated in these polar graphs.

The blue curve corresponds to the case

without a fuselage and the red curve with the fuselage included.

And we immediately notice that there is a huge drop in performance for instance in the

glide ratio in the bottom right graph, or in the drag polar in the top left graph.

The bottom line here is that the fuselage does have an important influence

and should be included in the analysis if it is possible.

However, the inclusion of the fuselage is not a click-run process.

It's not very difficult, but there are some rules and guidelines to follow

and this in particular means that

we need to build and construct carefully the geometries and the surface mesh.

Also a critical review of the results after the analysis has been run

is recommended, because it's easy to go wrong.

The way I'll proceed: in this first part,

I'll summarize the main considerations to be aware of when we

run analysis of planes of a fuselage,

then we'll dive in with our first test case which will be the quad face type fuselage.

Then the NURBS type fuselage.

These are the two type of geometries which are already available in xflr5

And finally I'll show how to import fuselages from STL and STEP files

and how to process them in flow5.

The main thing when we include the fuselage is that the boundary element methods

such as the panel methods implemented in flow5 require that the union

of surface mesh elements define one or more, closed, non intersecting volumes.

And this will be the main challenge.

It requires mainly that we connect the mesh of the wings to the fuselage mesh

and that we don't leave any panels inside the wing thickness

The other consideration is that volumes should not intersect which could be the case for

instance at the tail end if the elevator intersects the fin

In this case in the picture here I've left a gap

between the elevator and the fin so that there is no intersection.

To build this closed volume, flow5 uses under the hood

the open cascade API which is a set of very powerful and interesting libraries.

One thing that these libraries require is that the individual volumes which are the wings

should form closed solids so that they can be connected to the fuselage.

In our case this will mean that we need to close the trailing edges of the wings

and have closed trailing edges for each foil.

Also in the case of half-wings such as the fin, flow5 can not detect

by itself if the fin should be connected or not to the fuselage and

therefore whether it should be left closed or open at its inner section here.

If it is not connected to a fuselage, then we should close it so that it's one closed volume

If it is connected to the fuselage, it should be left opened.

I insist on this because it's easy to overlook and this would lead

potentially to a theoretically incorrect analysis

One word about the triangle mesh.

I tried to use some open-source publicly available meshers,

but in the end there was issues of various kinds with each of

them so I decided to implement my own custom mesher which uses the advancing front method

and it performs well in the vast majority of cases.

There are just a few precautions or considerations to be aware of

so that it builds a mesh of good quality.

What is that? well the first thing is that it expects the geometry

to be defined as a union of faces, each face being defined by a closed contour.

Also the geometry should not contain any free edge so if you choose

to import the geometry from a CAD software, be aware of this.

The other issue is small edges.

These can either be in the original model, the one which was built in the CAD software,

or it can occur within flow5 when a fuselage edge comes close

to a wing node and in which case

it will generate a very small edge locally

and this will lead to a high number of local triangles

and to very high mesh sizes.

So just watch out for small edges:

they are the main difficulty when building this mesh.

Once we've built on the mesh of the fuselage,

we need to connect it to the wing mesh.

flow5 will ensure that the fuselage nodes fall opposite the wing notes

The only exception is when there is an edge in the fuselage.

which comes across the wing root section.

In which case there will be a node generated on the fuselage

at the location of the edge.

What we will need to do when when this occurs

is just move this node to either side to the adjacent nodes of the fuselage and

I'll show how this is done in the second part of the presentation.

Once the wing mesh is connected to the fuselage mesh,

the triangle still need to be connected.

This is done automatically when the analysis is tun

but it can also be done manually.

After which the free edges can be displayed and they are highlighted in blue

If the mesh was constructed according to the rules,

the only free edges should be the wing trailing edges,

because they need to be unconnected

so that they create a vortex and so that the Kutta condition can be applied

Also the side surfaces of the wings are left unconnected

because there are numerical issues here, and the connection was

shown to generate more problems than it solves, by testing.

Another thing to be aware of is that there are invisible wake panels or vortices

extending from the wing trailing edges downstream.

These panels or vortices could not be displayed in xflr5 but they can in flow5

And the main thing here will be to check that the wake panels

which extend from the upstream parts do not intersect with downstream parts.

This essentially means that the elevator must not be located

in the same horizontal plane as the main wing,

so with a different z-coordinate.

Now once the mesh has been built and the analysis

has been run the thing to be aware of is that a panel analysis solves the

problem for the doublet densities on the panels, and that all other results are

deduced from these doublet densities.

The bottom line here is that we must make sure

that the doublet densities have been calculated correctly and that they are

of good quality.

The best indicator is the color map of these doublet densities

which should not exhibit any local unusual colors, except potentially

at the location where the wing trailing edges connect to the fuselage,

because this will be an area of strong interaction, both theoretically and numerically.

But If the doublet densities look good, then the other results should be good too.

They are deduced by two independent and separate calculations.

Firstly, there is an off-body calculation in the far field plane and this is where

the forces and other quantities which are deduced from forces are calculated.

That would be lift drag, glide ratio and other things.

Independently there is the on-body calculation,

which calculates the pressure coefficients, and from these

pressure coefficients are deduced the panel forces and the moments.

And again these two calculations are independent so even if there are some issues with

the calculation of pressure coefficients this will not impact what is calculated

in the far field plane.

The calculation of the pressure coefficients is one of the trickiest parts of the analysis.

In fact these coefficients are calculated

from the surface gradient of the doublet densities.

This is why we need the elements to be connected:

it is to enable the calculation of this gradient

from one triangle to the other.

And this is especially difficult when the connected triangles do not lie in the same plane

which will be the case where the wing connects to the fuselage.

The bottom line is that the calculation of pressure

coefficients in this area will not be very precise.

Right so all in all, what are the design options

to analyse a plane with its fuselage?

Well the first is to model wings as thin surfaces

and this is for instance the way it's done in xflr5.

The main rule in this case will be that wings must not extend inside the volume

defined by the fuselage, and this is the way flow5 will build them.

So no special care should be needed at this stage.

However one thing we have in xflr5

which will still be there in flow5 is the numerical issues where

the two meshes connect because the nodes are just not connected .

So although this is possible and correct theoretically, it is not

recommended from the numerical point of view,

and it is not the preferred method.

The preferred method is to model wings as thick surfaces in which case the

analysis must use the quad or triangular panel methods but not the VLM.

And if the wing does connect to the fuselage the only option to build the

mesh is to use the triangular panel methods.

The points to watch here are the

connections of the elements as has been mentioned before and maybe the quality

of the triangular elements which must not be skinny or stretched.

I must state however that all the testing I've done so far seems to show

that the analysis is not very sensitive to the quality of the triangles which is very good news

In fact it seems to be pretty resilient and to give more or less always the same results,

at least in the far field plane, independently of the quality of the triangles

However, triangular elements of good quality and a nice mesh can do no harm

so it's worth spending a little time to build this mesh.

When it comes to the fuselage itself, due to d'Alembert's paradox,

a fuselage does not shed a wake and therefore it does not create either induced drag nor lift.

It does generate friction drag however, and there are many models to simulate this drag.

flow5 implements two of them: the KS model and the PS model.

If there are others that you would like to see implemented,

well feel free to suggest them and I'll see if they can be implemented.

So all in all, what are the recommendations?

Well it is to use the thick triangular panel method,

with the linear doublet densities, if the mesh size is reasonable and if

you have the computer power to go along with it.

The best way to limit the mesh size

is to simplify and clean the geometry before it is imported into flow5.

The main thing there will be to remove all small edges.

Also another way to reduce the mesh size and to get a mesh of good quality

is to define a fuselage with flat faces: this is the most robust method.

And this is all there is to it really.

If you follow these design rules and guidelines which are pretty straightforward,

then I'm confident that you can run safely the analysis correctly

both from the theoretical and numerical standpoints, and

to that you will get some good results.

To demonstrate this well let's dive right in

with the first test case which will be the quad face type fuselage

in part two of this presentation.