topic Re: Complex Loft Twisting (Plane Cowl) in Fusion Support Forum

Complex Loft Twisting (Plane Cowl)

L_Stanford — Wed, 15 Jan 2025 01:56:09 GMT

Hello,

I'm completely new to Fusion 360 and 3D CAD in general. I'm trying to build a cowl for my RC plane. I've built out the fuselage, engine, and nose cone in enough detail to model the cowl.

The problem I'm running into is when I try to do the loft function with 2x rails I get a twisting function on one side of the cowl and it unnecessarily bulges out on the bottom of the cowl instead of following the rail.

- If I do the main body of the cowl and the front section separately then it seems to come out ok, but with less blending between the 2 parts.

- I am able to sort of straighten out the twist on the left side by moving the control lines, but it's not ideal and increases the bulge at the bottom.

Of note, both sides of the cowl are NOT symmetrical and this might be causing the problem.

- Attached pictures are what the old cowl looks like and the model I'm working with...yes I'm aware that the sketches are not constrained and there are likely many issues with it.

My intent at the end is to print this in ABS, PETG, PC, or PA.

Re: Complex Loft Twisting (Plane Cowl)

L_Stanford — Fri, 17 Jan 2025 13:26:13 GMT

Update: Getting closer. I redid the entire thing from scratch cutting out 2 of the intermediate planes and focusing more on aligning points between the front and back sketches. Then I used more rails to define transitions between the spinner in front to the fuselage in back.

Seems Fusion likes rails more than intermediate planes. I'll post the finished when complete.

Re: Complex Loft Twisting (Plane Cowl)

L_Stanford — Sat, 18 Jan 2025 13:37:29 GMT

Ok, this is about as close as I can get. Added an intermediate plane back in and kept adjusting side length to push the intake area flatter. I'm fairly happy with this and can't wait to print it!

Re: Complex Loft Twisting (Plane Cowl)

TheCADWhisperer — Sat, 18 Jan 2025 14:32:31 GMT

@L_Stanford

If it were my design I would attempt to eliminate most Loft geometry and instead use Revolve or Extrude for the basic parts of the geometry.

Re: Complex Loft Twisting (Plane Cowl)

L_Stanford — Sat, 18 Jan 2025 15:34:55 GMT

@TheCADWhisperer That was my next step, revolve the cylinder top and make a block for the bottom, fillet front edges, and join. I might still try it just to work on my understanding of Fusion.

Re: Complex Loft Twisting (Plane Cowl)

g-andresen — Sat, 18 Jan 2025 19:11:00 GMT

Hi,

Have a look to see if this is going in the right direction.

(view in My Videos)

günther

Re: Complex Loft Twisting (Plane Cowl)

L_Stanford — Sun, 19 Jan 2025 00:41:07 GMT

Gents, was able to work it out. I already did a test 3D print and it came out pretty good. Need to make some adjustments for the exhaust screws and air/fuel mix valve, but otherwise I'm pretty happy with it.

Re: Complex Loft Twisting (Plane Cowl)

cjjames1800 — Wed, 23 Jul 2025 16:39:39 GMT

On topic:

I use loft a lot in my models because it is the best option for what I am doing.

It would be GREAT if Fusion would warn us that the loft will fail prior to us clicking OK and losing an hour of work.

My shapes are a bit complex and organic so there are many rails created. Too often they are twisted to oblivion and require them to be adjusted.

I spend up to an hour getting everything just right and no errors or warning showing before I click OK only to then get an error message that the loft could not be created for one reason or another. I would stop adjusting things and not waste hour after hour of work if I Fusion gave me a heads-up.

It's weird that Fusion has no problem creating the loft all twisted to death but will only bomb on the loft once the rails are adjusted.

Do we need to create lines, arc, and points in the same areas and quantities in both sketches in order for Fusion to not twist the daylights out of the loft in the first place?

Thanks for reading.

Re: Complex Loft Twisting (Plane Cowl)

jon46XEC — Wed, 23 Jul 2025 18:10:41 GMT

Looks beautiful! Being a plane & 3d printer nut myself, I would caution you on the 3d print material on not being heat stable around gas engines. They will melt and distort.

Electrics really love 3d printed cowls but not mufflers and cylinder heads. I did a cowl for a DeHavilland Beaver using Fusion and lofting last year and it was just as challenging to make:

Re: Complex Loft Twisting (Plane Cowl)

TheCADWhisperer — Thu, 24 Jul 2025 11:06:12 GMT

@cjjames1800

Can you File>Export your *.f3d example file to your local drive and then Attach it here to a Reply?

Are you familiar with the Surface modeling tools?

Re: Complex Loft Twisting (Plane Cowl)

cjjames1800 — Thu, 24 Jul 2025 15:21:44 GMT

I will cut away some of the model and share what I can when I have time.
Regardless of the model, when we adjust a rail and intersect another,
Fusion tells us by showing a warning. If there are no intersections and
Fusion will not complete the loft it would, no should, tell us before we
waste the time spent making all of the adjustments. It is one operation so
it cannot be saved or undone to the last move, etc.
I am fairly familiar with many of the modeling tools but I believe loft is
the best for this particular modification and the tech person from Autodesk
who screen-shared agreed.
A reasonable facsimile of the sketches would be to make a sketch of the
borders of the state of Illinois on one plane and a similar slightly larger
shape 0n a plane about .300" below it. Then proceed to create a loft
between the two.
This is all being done in an attempt to widen the base of the model and
eliminate vertical walls along the outside of the part so it will blend
more smoothly into the next loft which is even larger.
Thank you for the reply.