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Some experiments with "Lofting" in FreeCad


Simon Chaddock
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When I built my Hawker Sea Hawk with both its inlet and exhaust bifurcated I really struggled to make the ducts effective within the limited space available made worse by the fact its 'portly' fuselage forced the inlet and exhaust openings to be rather far apart.

 

The Sea Hawk has a triangular wing root inlet. This has to change smoothly and aerodynamically to the circular EDF inlet. In addition it has to translate sideways to the fuselage centreline. Finally the left and and right hand ducts have to merge.

 

Lofting allows Free Cad to mathematically generate a complex profile transitioning from one shape to another. Could this be a relatively easy way to create a very complex shape that could then be 3d printed?

The first attempt going from a triangle via an intermediate circle to a bigger one with a sideways offset.

 TestDuct1.jpg.c0724f1e465baa3d43ff81fc0be6a3ae.jpg

By positioning the shapes diametrically opposite lofting forms a symmetrical "Y".

TestDuct2.jpg.c9304b0cbc9207b1cb053f4b64e5fb33.jpg

Although the inlets and outlet are positioned correctly it would not be that aerodynamically efficient.

By adding an additional intermediate profile and adjusting both their shape and position as well as quite a bit of trial and error.

TestDuct3.jpg.1327d4c94b3714bd08dff0718507f52f.jpg

Now the side ways transition is gradually applied and terminated to minimise the airflow rate of direction change.

By its nature lofting generates a hollow file which is ideal for single wall "vase" printing.

DuctPrint1.JPG.82ebd0db9cf5dd3760a0b94f845eb143.JPG 

Printed in LW-PLA but at 50% scale just to see how it came out. It weighs 2.3g. Even at the required 55 mm diameter it would still only weigh 10g.

The much less efficient shape PLA printed inlet duct actually used in my Sea Hawk weighed 25g.

Lofting can produce a really complex yet easily adjusted shapes.

Fine for something like a duct but not so obvious an application for a 'structural' component.

    

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Martyn K

This is a specific Free Cad tool so any article would only benefit its users.

I would expect other CAD packages can do similar but it would of course be package specific.

Free CaD "lofting" is not the easiest thing to use and has quite a few limitations that I have discovered so far.

I am by no means an expert in the use of Free Cad, left alone in lofting with it, but I would be quite willing to help if any body has a queries.

 

What I had in mind was the possibility of using this technique to create the inlet for a replacement of my XPS EDF Skyray which has had a rather hard life with multiple crashes. This was first of 4 to date.

21Aug22.JPG.9022219cb5970ae934b4f0dc9d121039.JPG

Each side of the inlet was built using 2mm Depron planks is a specific jig and the two halves joined. The result was quite satisfactory but was slow (days!) and tedious to do.

DprnBiDuct1.JPG.0566b708546fee67f46693b3a049294d.JPG

A lofted version should be better in terms of duct efficiency.

In LW-PLA it would be a bit heavier then in Depron and it might take several hours to print but once the electronic lofting work was complete it would only need a touch of a button! 😀

We shall see/

 

 

  

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  • 1 year later...

One of the biggest gotchas in FreeCAD - which the OP has evidently copes with - is for a sweep/loft to work best there should be an equal number of elements in each sketch, and they are in the same order. Badly jumbled sketch elements can create some unique results!

 

FreeCAD is a very powerful tool, particularly for an open source package. It is however worth remembering it hasn't quite made release version 1. One of my biggest grumbles with proprietary CAD is in some ways it is a step back from the days when engineering drawings were freely exchanged. Now to get the best of CAD's strengths you really need to be using the same software. The longer you've been using one program, the more you get tied to it with your library of old CAD getting bigger and bigger and tied to a specific vendor.

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I have used the LOFT facility in FREECAD to create complicated EDF ducts for scale planes.

Probably the most complex to date is the one for my Depron Douglas X-3 Stietto. Although a twin jet in full size for practical reason I chose to use a single larger more efficient EDF.

This meant twin inlets converging for the EDF and then diverging again for the twin exhausts. To make matters still more complex the inlets were high up and angled at 45 degrees and the exhausts are low down level with the bottom of the fuselage.

This is a CAD representation of the complete duct.

CADductCmplt.thumb.jpg.b67a976ee299ca09ed2b451f86aea241.jpg  

Made up of 7 parts. 5 lofted and 2 "simple" prints. The simple prints have oversize ends to allow the lofted parts to be glued on. The big gap is where the EDF goes.

This what it looked like printed in LW-LA and glued together.

1623843196_EDFDuct.JPG.ac1944150cbcf4e3312410a0ed944526.JPG 

The bulge in the duct aft of the EDF is required to maintain the fan swept area (FSA) resulting from the slightly larger diameter than the fan hub of the overhung motor.

EDFexhaust.JPG.018fbc4d70533469e01c0d6e5280e635.JPG 

 All the parts are single wall 'vase' printed.

The end result was smooth aerodynamically about as good as good as possible, very light yet rigid enough for the main part of the "former and planking" fuselage to be built around it.

ESCmount1.JPG.656214159a05fc5842734c3a711c67aa.JPG

Pity the finished plane has so far not flown at all well. ☹️  

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