Surface for a Sailplane Rudder

Hi all,
I’m driving nuts to create the surface of a sailplane rudder… could you please guide me a little bit about the next steps to do so I can close the surfaces and blend them with the fuselage?

Here is the file…

Many thanks for your reply.

fuselage.3dm (7.3 MB)

Some help? I need to get this done and don’t know how to continue with a smooth surface…

Many thanks.

hi Gmoreno,

the model lacks a bit on basic precision and understanding on how input curves have to be prepared. so i believe nobody responded because its a bit work to dissect and explain it all.

maybe start on those curves, the issue is here that the airfoil sections do not hit each other tangential on the leading edge if i may call it so (not an expert) which splits the curvature graph, that will lead to not having a smooth transition when you loft, sweep or however else create the surfaces.

further is that the lower airfoil section does not even touch the leading edge of that fin. when you use intersect you see that only the upper one would connect. before you get into further details you should fix that. maybe somebody may help further then.




what is this for? Scale model building, just some pictures or 1:1 plug building for using CNC milling? If it´s the last: do you just want to mill the foam, laminate and fair the plug by hand, or do you also want to use CNC data for milling the fairing compound?

Hello JoergH,
A “master” of the model will be CNC’d in scale in a kind of ABS plastic material, and once the precission of the model is confirmed, the molds will be CNC’d in alluminium.

Since you are making a model for manufacturing. Your wings shouldn’t have zero thickness trailing edges. You’ll need to trim back the naca data to avoid that.

Ok, thanks for your comments. Since it is going to be produced in a Carbon/Herex/Carbon Fiber sandwich, I will leave the trailing edges of the wings in 0.3 mm thickness.



your fuselage is quite heavy on control points. Depending on the CNC process this might get lead to some trouble, as well as for blending and matching surfaces. It should be possible to model the fuselage with one degree 5 surface with 6x6 to 6x7 controlpoints per half side. The rudder should be possible with 6x2 sailplane%2022 controlpoints in deg 5. You might read some discussions here on the forum regarding class-a surfaces and minimum number of control points.

Hi JoergH,
Did you change the curves? I mean, your fuselage image looks much more straight than the one in my design… your zebra analysis looks perfect.

I´ll read about Class-A surfaces on the forum…

Many thanks

the white sections are on my new surface, the red sections are your originals. It´s not perfectly close to your design yet, that would take some more time moving control points. I just wanted to show how I would build it, if its for CNC milling. I started with a flat plane, changed degree and just pulled cp´s.

Could you please upload your file so I can see how you did it?

Many thanks

glider.3dm (3.6 MB)

not sure if this helps I know it is a few days late.

fuselage-jh1.3dm (7.8 MB)

Here is that file. Still far from perfect, tail is wrong, no fillets between rudder and fuselage, shape is not matching your design very well, so still needs some control point massage…but maybe it shows the right direction…

Thank you all… I have been reading a lot about Class A surfaces and learnt a lot… look at my surface now, how it blends the fuselage with the rudder… I’m quite happy with the results everything is G3-G4.


best to check with zebra, emap ->neon and curvature graph

Oddly, I might do it a little differently. I design the end of the tail, so the blending process will be easier, in that the whichever blend method you use won’t have to go around the shoulder end of the underside of the tail.

This is just a quick suggestion, and I don’t have your work, but I just wanted to show what I mean.
I brought a copy of the airfoil wire down, widened it, and matched where you cut, but this is no substitute for doing this upstream. The cap I made was just a quicky curve network, but it’s using a rotated extrude, which is unsuitable for blending, as are the slight concavity.

Anyway, my idea is: make the blend as much as you can on a single plane.

BTW, you know about NACA generator, correct?

Many thanks for your reply Brenda… that’s a very good point. I´ll try to see how it looks with your system, but actually I´m quite happy with the last Zebra and Env. Analysis.

looks like you are on the right track. The render mesh of your zebra analysis could be a bit finer. To set the last entry to 20000 should help. Seems like the fillet on the front of the rudder could maybe need some refinement, but that might be depending on the size of your model. Best to make some sections through that area and analyse them.

Yes… I´m almost there.

Once you have done a BlendSrf, what are the best steps to properly match the curves from the 2 surfaces?? I would like to have a kind of "step by step " manual in order to blend my surfaces in Class-A quality.

This is how my Zebra looks now after some point adjustment.

looks much better in the forward part of the rudder. but there are still some problems in the rear part.

It is difficult to help you without a file.