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Wellsprop's Guide to CAD Modelling


wellsprop

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Hi all,

 

I've had a fair bash at CAD modelling model aircraft for 3D printing now and I've had a few people ask how I'm designing certain things, I aim to use this thread as somewhere I can put some "how to's".

 

This should work in any good parametric CAD package and it isn't meant to be for beginners! Watch this space.

 

Ben

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Hi Ben,

What a good idea and thank you for starting this.  I am plodding on, learning Fusion 360, and have achieved a fair few things; however, there's a lot more that I haven't achieved.  This thread could be just what I need.

cheers,

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Fuselages - Using Surface Modelling

 

Fuselages are complicated shapes, they change shape along the length (in both profile and cross section). The only way to accurately model a fuselage is to create section sketches, connected to each other by guides. CAD software can then generate a smooth fuselage in one operation.

 

To do this, we need the following;

  • Plans & Section Planes
  • Top and Bottom Guides 
  • Side Guides 
  • Sections
  • Surface 

The final result is something like this.

 

1

 

2

 

Plans & Section Planes

 

These are more part of the setup, I won't go into detail as it's different between CAD packages. Import the plans into your CAD software and then create section planes in the appropriate place.

 

Top and Bottom Guides

 

This is the easiest bit, on the side plane, sketch a guide for the top guide. Create a second sketch for the bottom guide. Do not join the guides at the end, guides can only be used to connect sections. You'll notice I have separate lower guides for the fuselage and cowl for the Chipmunk, as the shape is discontinuous.

 

Fuselage 1

 

Side Guides 

 

These are similar to the top and bottom guides, but with an extra step.  Sketch one side profile on the top/horizontal plane, this will give you the profile, but when you look at it side on (see image below), you’ll notice it is in the wrong place, see it doesn't remain within the top and bottom guides at the tail.

 

Fuselage 2

 

This is where it requires some judgement. Looking at the sections on the plans, it is clear the fuselage sections are elliptical at the tail and approximately oblong around the cockpit area. What this means is the maximum fuselage thickness is going to be at about 50% of the fuselage depth, i.e. the side guides shouldn't be planar.

 

To do this, the midpoint between the upper and lower guides needs to be made. CATIA has the ability to create points half way between other points. The image below shows the points. I intersected the upper and lower fuselage guides with the section planes to create all the points that are on the fuselage guides. I used these points to create midpoints in the centre of the fuselage.

 

Fuselage 3

 

I create a sketch on the side plane, then project the midpoints into the sketch. I then sketch a spline from the front to rear of the fuselage and adjust the spline to best fit the midpoints.

 

Fuselage 4

 

Note:

The cowl is a separate part in the Chipmunk, hence my sketch doesn't go to the cowl.

Don't set the spline co-incident to every point, just do a best-fit, to keep a smooth curve.

 

This line then has to be extruded horizontally to create a curved surface, see image below

 

Fuselage 5

 

The side profile sktech can the be projected (in the vertical direction), using the extruded surface as a support, see below.

 

Fuselage 6

 

The support and extruded can then be hidden before the projected surface can be mirrored in the vertical plane, this will give you two side guides.

 

Sections

 

To create each section, create a sketch on one of the planes (you need to create planes in the correct place for each section, according to the plans). In the sketch, intersect each of the guides, to give you four points (see the points highlighted in orange).

 

Fuselage 7

 

The appropriate section from the plan can then be overlaid and the section sketched as required.

 

Fuselage 8

 

Repeat this, until all sections are complete. IMPORTANT make absolutely sure that the section you sketch is coincident with all the points you have intersected in the sketch - this ensures the fuselage guides intersect the sections sketches (this is critical as the guides and sections must intersect).

 

Creating the surface/body

 

The final step is quite simple, select all the sections before selecting all the guides and a surface will be generated.

 

 

Fuselage 9

 

In CATIA, the "closing points" must be in the same relative place on each section (i.e. the highest point of each section), and each profile must "close" in the correct direction (see all the arrows point the same way).

 

And that's how I make fuselages!

 

Ben

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Here's my earlier guide to modelling propellers!

 

 

Propellers are particularly complicated as both the leading and trailing edges are complex curves, so the front and side profiles need to be sketched, extruded and intersected to create accurate curves. Blade profiles can then be sketched and a surface generated.

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54 minutes ago, ChocolateCrisps said:

This looks like it's going to be a very handy guide, so thanks for taking the time to share all your knowledge!

 

Now if you've come up with a good way to do panel lines, this will instantly become the most useful guide on the internet...

 

I'll put that up later, it's the most difficult part of CAD modelling.

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Fascinating stuff. Way above my pay grade, but very interesting. I remember seeing a demonstration by BT back in 1999 at RIAT of an early 3D printer. I still have the small item it was producing, a miniature goblet.

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Really interesting, thanks Ben. I am rereading the side guide bit as I am not quite visualising it, and the concept is a bit new. Have you effectively created a line on a curved plane, a line that bends in three axes?

 

In Onshape, I'm not sure how this can be done. I can create a horizontal plane at the appropriate angle between the 2 guide curves and draw a curve on it which can be used as a "path" for the loft (also constrained by the 2 guide curves). This works well for a fairly regular profile, but when the widest point of the section sketch isn't necessarily where the path coincides with it (it is drawn on a flat plane after all) it can cause wierd lumps and bumps! I have to say though, 75% of the time, Onshape gets the side profile near bang on without any side profile, just the section profiles and the guide curves.

 

 

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13 hours ago, Bangseat said:

Have you effectively created a line on a curved plane, a line that bends in three axes?

 

Exactly that.

 

I sketched the side profile on a plane (therefore the line curves through two axes only) and projected it onto a curved surface. The resultant projected lines curves in all three axis.

 

I have tried not using side guides in CATIA, however, upon doing curvature analysis, I realise the side profiles are all wrong as the curve in an out, whilst also curving in the wrong places.

 

Here's a screen grab of the porcupine analysis, the what this shows is the second differential of the curve (that is,the rate of change of the curve d^2y/dx^2). For a surface with a constant rate of change (i.e. constant radius), d^2y/dx^2=0, therefore the graph would show a flat line. For a smoothly curved surface, changing from on radius to another for example, the graph would show a smoothly curving line.

 

Porcupine Analysis

 

What this shows is the rate of change of curve is not smooth and it's also "hunting" (positive then negative). This might not be too noticeable in a small print, but in a larger print with higher resolution, the "pinching" around the tapering portion of the tail and the "bulging" around the fuselage, just before the taper, will become quite noticeable.

 

Ben

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