Aerodynamic fillet?

[Anonymous]

Hi all,

I'm completely new to Fusion (and CAD tools) and am trying to model a power pod for (scale) powered glider.  The pod itself will have a teardrop nacelle shape, circular in cross section, and will be mounted in top of an airfoil-shaped post.

I'm attaching a very simple design I put together using a cylinder (approximating the pod) and a sketched airfoil pod, with the standard fillet command.  It's a good concept but nowhere near what I really want!

I need some basic, basic pointers to get me going, but think I can get most of the way there except for an aerodynamic non-constant radius fillet I'd like to make where the post meets the pod.  This will look somewhat like the fillet on many aircraft where the wing meets the fuselage (see a picture of a P-51, attached, for what I'm talking about).

I'd like to do everything with splines, at least if that's the best way to go.

Here are some basic questions:

- I assume I should model the pod and the airfoil post seperately, then join them -- correct?  How do I join them (if I'm using splines) in such a way as to make a continuous spline surface?  (I may also want to join at slightly less than a 90 degree angle, and ideally it would be nice if I can play with that angle even after doing the join without doing a lot of redesign).

- I'm thinking about modeling the pod as a series of circles in their own planes, then doing a loft.  There is a whiskey bottle video on youtube that I'd use as an example.  Alternately, I could sketch a nice arc and rotate it 360.  What's best?

- Finally, how do I make a nice aerodynamic fillet that has a very small radius toward the leading edge, gets larger towards the trailing edge, and has a kind of parabolic outline in profile aft of the trailing edge?

Thank you for any pointers!

Rick

[Anonymous]

P.S. I intend to eventually make a "negative" in Fusion from this design, then 3D print a mold which will be used for a carbon fiber layup. I'll post later asking how to make printable mating halves of a mold!

[TrippyLighting]

One thing you can try is a variable radius fillet.

[Anonymous]

Hi Trippy,

I just tried the variable radius fillet and it's close but not quite what I want.  I want a longer tail fillet aft of the trailing edge, again it should look like a parabola when viewed from above (in the case of an airplane).  I want to be able to actually shape the cross section of the fillet -- I have realized that the circular arc cross section of the fillet is not quite right and that I need control over that cross section.

Is there a way to do this with splines?  I haven't tried modeling the joining of two objects with splines and then adding a fillet to that so not sure what will happen... guess I'll give it a try...

Rick

[Anonymous]

Another stupid question: How do I join these two bodies and end up with a single smooth splined body?  One is a standard box; the other is a standard cylinder.  I have moved the cylinder to overlap and intersect the top of the box. I figure I will need to do this to join the pod to the post and this will allow me to play with the intersection of the bodies with splines.

I tried exiting splines ("Finish Form") and doing a Combine command, but that did not allow me to select a Tool object and so I could not combine.  I did try using a box and a sphere and Combine did work, but then I seemed to lose the ability to work with any splines.

Ideally I need some way to join these to make a single splined surface so I can manipulate the area of intersection to make my own fillet.

If I'm going about this the totally wrong way, let me know...

[TrippyLighting]

Have you watched any of the tutorials ?

What you want to do is easy to verbalize but not trvial to do if you've had no exposure to CAD or 3D modelng tools.

Traditionally this stuff is done with surfacing tools. You can do some of this in the Patch environment in Fusion 360. This works differnt from normal solid modeling in Fusion 360. You can combine solid bodies but you cannot really combine surfaces. You can stich surfaces together and with boundary fill turn the enclosed volume into a solid body. With the result you can continue in the Model workspace in Fusion 360 as a solid model.

Modeling with T-Splines feels yet different again. You would model the plane-body and wing transition with one contiguous quad mesh.

You cannot combine T-Splines with the combine command. Combine only works on Solid Bodies and T-Splines are not solid bodies, but can be trurned into solid bodies and then you can work with them also in the Model Workspace in Fusion 360.

Also, please be aware of terminology. A T-Spline is really someting differnt than a Spline.

I'd suggest you watch a good number of the Fusion 360 tutorials. What you are trying to do is not beginner stuff, but with some help we'll get you there!

[Anonymous]

Thanks for the quick reply, Trippy.  I really appreciate the help.  I've seen a number of tutorials but will go back, re-watch, and search for more, though I agree that what I'm trying to do is not quite what is likely to be covered in most of the tutorials!  I've done many projects like this the old fashioned way with plug and mold built by hand but wanted to give CAD a try.  FYI, this project is for  large, 1:3 scale high performance radio controlled glider -- about 20' wingspan, all carbon fiber. Here's a picture of part of the airplane, and you can even see the really nice fillets around the wings that I'd like to recreate with the power pod on this project! 

Rick

[TrippyLighting]

If you not gung-ho on the totally perfect mathematically precise airfoil shape I'd sugest doing this in the T-Spline environment.

This "feels" very similar to how subdivision surface modelrs (mesh modelers) such as 3DS Max, Blender, Modo work.

Here is a Text tutorial on how to model a plane in 3DS Max.

Also search youtube for "plane 3d modeling" tutorials.

[donsmac]

Check out this video. Modelling in t-splines what it is you want to do. Contains some very good advice on t-spline body creation.

[Oceanconcepts]

I’ll pile on here and also advocate t-splines as a basic approach. It’s not too difficult to learn how to make complex shapes in the Fusion Sculpt environment, while people spend whole careers getting adept at making perfect surfaces with very high end surfacing packages. T-splines would be very capable of handling your project.  If you can define some cross section profiles and import them as canvases you can work to match them in the Sculpt environment. Tutorials on t-splines are generally good in Fusion, the tool is a lot of fun to use. Like learning to play a musical instrument, you will develop skills with time. 

In your two bodies example you could separate the forms and use a bridge command, but I would suggest not starting with two forms, but instead grabbing a couple of the sections of the cylinder and using the Alt-Edit Form command to “grow” a wing out of the cylinder. If you apply symmetry, both  wings will go together. Go through some of the tutorials and I think you will see. 

My tips for t-splines: 

Learn to shift between box mode and smooth mode- box mode is your friend. 

Use the fewest number of control points possible to achieve your shape, add complexity only when and where you need detail. 

[Anonymous]

WOW! That video is EXACTLY on point -- highly accurate airfoil modeling, great filleting and blending, and tons of tips on creating the mesh.  Thanks!

I'm confused... again... Do I need the same software shown in the video?  (I think it's Rhino with t-splines)?  Or does all the same functionality exist in Fusion 360 which is what I have now?

Thanks!!! 

Rick

[Oceanconcepts]

T-splines in Rhino and in Fusion will differ in the exact tool set and implementation, but the basic principles and capabilities are the same. 

[Anonymous]

Thanks Ron, Thanks Don!!

I see what you mean about the alt-edit stretch -- I tried a quick test and can pull out a post and make a pseudo-airfoil by adding some edges, creasing the trailing edge, etc.  It does indeed make a nice blend into the main body!

However, I think I'll need a true airfoil... low drag really is at a premium here... and will follow the technique in the video to create the wing from airfoil data, then extrude, and finally join via connecting points between the two bodies.

Still not sure how all the very fine control over T and star intersections, adding mesh lines and moving line endpoints as shown in the video would be done in Fusion 360 but will set that aside for now.

Does anyone know how to import airfoil data in Fusion 360?  Data is typically plain ascii in a ".dat" file.  The Rhino video also showed a very clever smoothing/modeling technique.

THANK YOU all for your help.  There is some light at the end of the tunnel.. but this is probably going to be more work for me than just making a plug and mold, at least for this first project!!

Rick

[TrippyLighting]

There is some valuable information in this thread about airfoil modeling with T-Splines

http://forums.autodesk.com/t5/design-validate-document/modeling-a-wing-with-tsplines/m-p/5947389/hig...

[Anonymous]

Thanks for all the suggestions.  After watching this video (https://www.youtube.com/watch?v=WV62TqZ_x2g) about a million times, and downloading Rhino with Tsplines, and spending days making lots of failed attempts, I finally was able to join a proper airfoil section to a nacelle shape and get exactly the type of fillets I wanted.  Of course, that meant painstaking effort in cutting a giant hole in the joint area of the nacelle and bridging the gap with by-hand extruding and point welding, but the results give me excellent control over the exact shape of the fillet and give a nice almost organic shape.

Still needs some fine tyning but in the ballpark!

Next step: Creating a virtual mold (negative) of the part, in left and right halves.  How do I do that?

Thanks all

Rick

[vex]

The last picture shows a crease in your transition between nacelle and wing. I would suggest, in F360, to forego attempting to fillet the transition and instead loft the transition using guide rails (unless of course you're using the meshing portion of F360--in which case my suggestion won't help you too much).

For instance create your root airfoil on Centerline and loft to your control airfoils within the wing--Connecting the trailing edge of the wings with a spline such that you have a controlled transition. Similarly, I would do the same for the front, making sure to pick the leading edge of each airfoil. Lofting, for better or worse, is an art and it takes some practice to develop which rails to use and where. I've used iCopy in Inventor to develop the ribs of wings and supplemented the skin of the aircraft (for rendering purposes) in a similar fashion. In that instance I used three rails: Foreward, Aft, and at maximum thickness on the upper surface. Similar arrangements can be done in F360.

I had hoped to develop a airfoil generator for F360, but that has fallen to the wayside as the programming is not very similar to Inventor's.

As for your last question; in F360 you can subtract bodies from eachother. For instance, you would create a box of some depth to hold as much of the model as you desire. Then subtract the correct body from the other and you will be left with the appropriate negative mold. My suggestion however, if you're planning on laying this up, is to split the model top and bottom. If you're just making the injection solid then left-right is fine. The benefit of splitting top and bottom is the ability to ensure proper adhesion of the plys to the desired geometry. You will need to pay special attention to the leading edge; maybe another ply after upper and lower surfaces have been made to bring the wing section together without a seam on the leading edge.

[vex]

Here's a few screen shots of a simple wing I threw together in F360 (took about 2 minutes). Two airfoils setup using the vector parametric method (I think I pulled this method from an AIAA paper on doing parametric airfoil design and analysis of a large number of airfoils--but I'm not sure. I'll see if I can dig it up). This wing has both twist and taper, and as a method of modification an- and dihedrel depending on where one modifies the control point. But first the basics:

So here's a basic wing. Two airfoil profiles compose the root (the larger) and tip (the smaller). A single loft rail joins the two on the leading edge of both with a control point at the center. Modifying the location of that control point, adding additional airfoil profiles, etc, will change the shape of the wing. For instance if the control point were closer to the root you would have that 'aerodynamic fillet' you desire.

Here's a shot of the wing as a solid.

The twist and dihedrel are a bit hard to see here, but I'll exagerate the location of the control point and the resultant shape of the wing is easily blended:

The airfoil shapes are fairly easy too. Referencing any NACA airfoil shape or pulling the various control points for any number of airfoils can be mimiced (as noted in the previously mentioned paper) through use a of a single spline and 5 control vertices. Here's what the root airfoil looks like (it is not based off any airfoil, but could be altered to in the future)

Note that in your case you need only adjust the values to produce a symmetric airfoil (adjust the trailing edge CV angles, modify the location of the max camber on the lower or upper surface to be the same, etc). The tip airfoil is the same with the added variability of the amount of twist (this example had 4.5 deg worth), and where the airfoil is located relative to the root (this applies to anhedreal and dihedreal as well as taper).

Hope this helps.

[michallach81]

Tsplines are great, but I would still try to use more classical approach. I've record short screencast with very basic example of how to make a transition between body and a wing. That type of complex shapes you should create with patches of surfaces or Tsplines, but rather not with solids. Except some basic shapes, fillets are just cosmetics, you apply them to round the edges, but they don't serve to create transitions.

[donsmac]

The method you show works well for a symmetrical shape like an aircraft's vertical stablelizer but as soon as the profile changes to an airfoil shape the whole thing falls apart. I could not get it to work. Even when the symmetrical profile was extruded with a small taper, the transition failed. 

If someone could get this method to work on an airfoil shape I would love to see a video of it.

[vex]

I did a little bit more fooling around and think I have come up with a solution. It is a slight modfiication of the previously mentioned/screen cast method. The difference is I created a sketch at the trailing edge; doing that allowed the loft option to be set to tangent (possible bug? I did not adjust the profiles at all, but adding the sketch, but not referencing the rail sketch in the loft dialogue allowed it to work).

Zebra analysis:

Curvature analysis shows a few issues that would need to be addressed, but as this is just a proof of concept I don't think I'll spend the time to work on fixing those... The issue is most severe on the bottom side of the transition between the cylinder and the wing.

Solid body and shell:

Top-Down:

Bottom-Up:

I don't like the way the front of the transition looks... but I think that might just be the way the cookie crumbles with the radius transition between body and wing. In any respect, this shows that it can be down with an asymmetric wing. I do believe that lofting from the Root to the Tip of the wing is still the better way to go. Fairing the transition from wing surface to body is a bit more involved and would require experimentation to find out how much of an increase of the profile on the body requires to allow a sufficiently smooth transition.