Freeform Modeling Help: Helicopter Tail

Hi everybody,

I am trying model a Bongo Helicopter, there is one of them with an unusual tail (see picture).

I am stuck on the tail since none of my approaches seem to be good. I tryed to use the same method showed my James Carruthers in Freeform modeling tutorial, but the fillet between wing and the main surface is not beeing successful in my case…

Have anyone some good tip to give me on how to model this tails? Many thanks in advance!

Helicopter-tail.3dm (2.7 MB)

Can you post a .3dm file with the tail boom and fin, and your fillet? Drag the file into the are where you type your post, or use the vertical arrow above where you type your post.

Hi David,
I added a 3dm with all the surfaces involved. The fillet shown in the picture resulted using a Radius 5 mm.

Dear Giacomo

check the intersection with the tail and the body
= the surfaces do not intersect on the entire length
extend the surface
_filletSrf does your job

you still need a (design) solution for the sharp edge …

easiest would be to have a small fillet there too - which would correspond to the real, physical world.
(like R0.5)


Helicopter-tail_r5.3dm (4.4 MB)

Hello Tom,
thanks for your answer.

I adjusted the position of the surfaces and the fillet does the job as you said.
Then I went through, up to completing the junction on the trailing edge of the wing.
That’s the result:

Pretty good, but I am not satisfied: as you said, there is still the sharp edge to cope with and I would like to obtain a smooth juction even on the sharp side of the wing. The two fillets are not aligned and so as the two faces of the wing…Seems to be tricky now. Any help?

Thanks in advance.


Helicopter-tail_r10.3dm (382.3 KB)


as far as I know, in aeronautics and automotive there are no sharp edges at all. So even if it looks sharp on the image, it has still a small radius. So fillet or blend your tail-wing , and it should work. If it fails, you need to manually blend the corner and match it on all 4 edges. Its rather rare that cornerfillets are working satisfying from automatic creation, no matter which cad-platform you are using.


I tried to different ways:

  1. Editing fillets until they almost match on boundaries (adding radius .5 to trailing edge also)
    Even it is a mess, I found a way to let things going, but I used any kind of expedient and it looks a patchwork of crappy surfaces to me. I Would like to know if someone better than me can make it having cleaner surfaces.
    Also, it remains to fix the fillet junction on the picture…

  1. I tried then to cut everything down and make a blend surface: it clearly isn’t the correct way of doing… but that’s the result.

    3DM file is attached.
    Helicopter-tail_blend.3dm (2.4 MB)

Other suggestions are welcome!

A big part of the problem is that the surfaces in your file don’t fit together very well. The tips of the surfaces that you want to round off with a fillet are .139 mm apart while your tolerance is .001 mm.

However a fillet can help with the inaccuracy of your model because it will cut off the inaccurate part. But you need a fillet a little larger than 0.5mm to cut off all the inaccurate part.

In the enclosed file I used a fillet of .8mm. and set the tolerance to .01mm to keep the surfaces from becoming more dense than they need to be.
Heli.3dm (362.1 KB)

Another problem with filleting is the singularity at the tip of the wing. You need two surfaces to make a fillet - you can;t fillet between parts if one surface and the singularity is likely to make filleting difficult so I used a sweep2 to round the edge near the tip.

Here is another filleting solution that makes a little more fluid connection between wing and fuselage. See if you can figure out how that was made.
Heli2.3dm (365.8 KB)

1 Like

In the real world wing trailing edges will be squared off with two sharp corners, not rounded. Edit - That is for aircraft, not toys, etc.

In the real world wing trailing edges will be squared off with two sharp corners, not rounded. Edit - That is for aircraft, not toys, etc.

I’m by far no expert regarding aircraft wings, but if we talk about “sharp”, there is a high chance that its actually filleted with a very small radius, since this is better to manufacture. So a sharp trailing edge must have a tiny radius, I can’t imagine that trailing edges are good enough to cut my onions at home. (… if I’m wrong here, I might replace my steak-knives on the next bbq with helicopter tail wings. How cool is that!)
However, from my experience in automotive, exterior parts of cars have no sharp edges at all, for various reasons (Manufacturing, Safety, Aerodynamics …).

somehow like this, you need to adjust the fillet in its radius in order to make its edges flow like this, very simple.

1 Like

Yes, all parts need a fillet, especially injected parts to aid flow. Every inside and outside corner is filleted slightly.

When a ‘sharp’ edge is desired, the fillet typically ranges from about .25-.5mm (.01-.02"). Sometimes more or less depending…

Also, looks better in renders and real parts in addition to aid to manufacturing. Complete CAD knife edges look and feel bad.

Perhaps a simple illustration is needed.
Trailing edges 01
Trailing edges on aircraft helicopters wings, tails and other airfoils are typically squared off rather than sharp or a single fillet. The edges of the squared off section may sharp or rounded with a small radius compared to the width of the squared off section.

oh okay, I thought “squared off” is American slang for something like “end up”. :sweat_smile: That sketch helped.

Thank you Jim,

I figured out the issue with the tolerance.
The surfaces on your model are definitely cleaner and the sweep2rail at the tip
looks nice.