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Composite wing ribs

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David Hall 928/03/2020 16:42:13
237 forum posts
15 photos

I am considering working with 3D printed ribs and wonder if they can be used successfully or if they might be better if combined with some other material.

I have a few sheets of Depron and only a little balsa to help me through the few next weeks. I thought that maybe I might have a go at a foam sheet old-timer with electric power, to complete the wayward trip.

Has anyone tried 3D printed ribs?

Denis Watkins28/03/2020 16:46:33
4322 forum posts
104 photos

Page 1 On this site David

**LINK**

They found that printing could be 3 times heavier than Depron but very strong

A large wing would carry the extra weight no problem

 

Edited By Denis Watkins on 28/03/2020 16:54:46

David Hall 928/03/2020 17:36:26
237 forum posts
15 photos

Thanks for the link Denis, I had forgotten about that thread, although it's a more general 3D printed parts thread.

Things are looking favourable for a hybrid old timer... the Hep Cat (Hepcat?) looks to be very suitable for a lockdown project (aside from the few others).

Toni Reynaud28/03/2020 19:47:08
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423 forum posts
58 photos

Try this thread. Simon Chaddock does a fair bit of work in this area.

https://www.modelflying.co.uk/forums/postings.asp?th=130897&p=3

Simon Chaddock28/03/2020 20:48:30
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5683 forum posts
3021 photos

I have been printing wing ribs for some of my Depron planes.

I general the issue is not strength but weight. It is quite difficult to achieve a similar strength to weight as balsa and much harder to match Depron.

I have found that for real light weight at small sizes just a rib outline with a very thin web inside is quite acceptable.

RibShearweb

This rib weighs 0.5 g

Larger ribs can be reinforced with diagonal cross bracinglightrib.jpg

Even bigger ribs can have thicker elements and do away with the intervening web.

The rib set for the inner wing panel on my Depron 2700 mm span EDF Antonov AN 124.

 

ribset1-12.jpg

I would not claim 3D printed ribs create the lightest or are the quickest to create but they are very accurate and exactly repeatable which makes them ideal for a 'plank' wing. wink 2

 

Edited By Simon Chaddock on 28/03/2020 20:50:32

Nigel R28/03/2020 20:58:58
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3730 forum posts
583 photos

Hobby craft sell artists foam board. They might do mail order?

It makes reasonable ribs. About equivalent to 3/32 balsa.

David Hall 928/03/2020 23:29:09
237 forum posts
15 photos
Posted by Simon Chaddock on 28/03/2020 20:48:30:

I have been printing wing ribs for some of my Depron planes.

I general the issue is not strength but weight. It is quite difficult to achieve a similar strength to weight as balsa and much harder to match Depron.

I have found that for real light weight at small sizes just a rib outline with a very thin web inside is quite acceptable.

................................

I would not claim 3D printed ribs create the lightest or are the quickest to create but they are very accurate and exactly repeatable which makes them ideal for a 'plank' wing. wink 2

Edited By Simon Chaddock on 28/03/2020 20:50:32

Thanks for the info... my efforts to assess the usefulness of 3D printed ribs is only a couple of days old...

I tried a glider style/thickness semi symmetrical rib of around 180mm length in a range of styles as a quick and "easy" estimate. So far, I like the lamination of 3mm Depron with two outer layers (one each side) of single layer PLA at 0.2mm each. This needs a bit too much work to glue and cut/machine and holes and slots, so now I favour a fully printed rib. At 4 to 5g it's heavier, but pretty strong. I'm still working on getting a good top layer, but the way forward is clear. For my first wing, a little extra weight shouldn't be a problem.

Getting a good 3D file from a scan of a rib really is a problem.. it takes all my time just to get a rough outline. How's it done?

FlyinFlynn29/03/2020 11:56:18
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147 forum posts
95 photos

You can obtain .SVG's of actual wing profiles from http://www.airfoiltools.com/plotter/index. You can input various parameters and then plot your profile. These can be converted to .STL's in 123D Design and then uploaded to your slicer.....the biggest problem is finding the right profile to start with from the hundreds that are available....

David Hall 929/03/2020 12:33:13
237 forum posts
15 photos
Posted by FlyinFlynn on 29/03/2020 11:56:18:

You can obtain .SVG's of actual wing profiles from http://www.airfoiltools.com/plotter/index. You can input various parameters and then plot your profile. These can be converted to .STL's in 123D Design and then uploaded to your slicer.....the biggest problem is finding the right profile to start with from the hundreds that are available....

Thanks for the link, that's a very useful resource...

For my first attempt, the Hepcat looks to have an old and very simple section. Clark-Y is very close and will be perfect for my needs.

I'll try one out now.

Barrie Lever29/03/2020 12:49:22
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162 forum posts
40 photos

David

'Getting a good 3D file from a scan of a rib really is a problem.. it takes all my time just to get a rough outline. How's it done?'

So lets just consider the scanned airfoil to start with, drop this image into a vector CAD package, such as Autocad or any clone. Now trace a spline around the airfoil image, I find it is best to a top half and bottom half spline with a straight line drawn from the extreme leading edge to the extreme trailing edge.

Play around with the spline fit to get a really nice following of the airfoil. now check the chord of the original airfoil and scale the CAD file to exactly that size.

At this point you have data that could go to a CNC router or laser cutter but 3D printers require an STL file normally.

So now extrude the CAD data to the thickness of the rib that you want and add any other features and save as an STL file, this can then be printed.

The above process is marksman accurate if done with care and precision.

The above pylon race models had their wing sections produced in this way.

If more detail is required then just shout.

Regards

Barrie

David Hall 929/03/2020 13:16:44
237 forum posts
15 photos

Thanks for the detail Barrie.. I hadn't asked before, but I use PLA, ABS is apparently stronger, does anyone find ABS better (I understand that ABS is more difficult to work with)..

Barrie, those plyon racers look the part...

Simon Chaddock29/03/2020 14:14:47
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5683 forum posts
3021 photos

David

You only have to looks at a full size rib to get an idea of how 'spindly' they are when compared to the massive proportions of a spar.

This is a Tiger Moth wing.

Tigermothwing

It would be virtually impossible to reproduce this at a small size in wood and the geometric scale effects would actually make it impossibly weak to resist any compression forces.

The accuracy of 3D printing means you can technically create a small size scale wing rib and although the structure would be plenty strong enough in tension the members would need to be supported in some way to prevent them buckling under compression. The trick is to find ways to do this but only adding the minimum of extra material.

It took some experimentation to produced 3D printed ribs that were even close to the weight of a comparable wing rib cut from the same thickness balsa.

How light you want the rib to be really comes down to how much time and effort you want to spend designing it and how ingenious you become in using the 'features' of the slicer programme to give the result you want. wink 2

Barrie Lever29/03/2020 15:06:22
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162 forum posts
40 photos

David

Just to be clear those models don't have 3D printed wing ribs, rather wing sections reverse engineered as described.

Try the Carbon filled HTPLA from protopast, it gives superb results, has a higher temp. resistance and is stringer than normal PLA.

Regards

Barrie

David Hall 929/03/2020 19:50:37
237 forum posts
15 photos

Thanks for your help and suggestions guys...

I have a small collection of trial rib prints. It was a good exercise, several new things learned and gathering more experience on the 3D printing side is a plus.

rib2.jpg

Now I have a workable (simple) design for the rib for this project. 2mm thick, 3g and in my wife's favourite filament colour... rib1.jpg

FlyinFlynn30/03/2020 09:58:11
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147 forum posts
95 photos

Very nice David, I would suggest you add a chamfer to the underside of the lightening holes to combat the elephants foot and add just a touch to your extrusion rate as that last rib looks a mite under extruded.

Barrie - with respect, you don't need to create a spline with 123dDesign. You import the .SVG as a solid, remove any lines you don't want, merge the sections together and reduce it's height to 1mm and export it as an STL - no hand made artwork needed and the size is as set in the aerofoil plotter.

123DDesign has been withdrawn by autodesk but is still available for free to download.

as imported as a solid.jpg

As imported as a solid from Aerofoil plotter

removing the unwanted bits.jpg

Removing the unwanted bits from the aerofoil solid and the 2D sketch parts ( and the block artifacts)

merged the sections together.jpg

Merged the sections together

ready to export.jpg

Reduced to 1mm thickness

cutouts added.jpg

Cutouts added and ready to export as a 3D .STL (this is actually a different profile I had made earlier to save me the work putting in the cutouts to the clark-y profile...but it shows what is possible.)

This only really works for a parallel chord wing as scaling the profile will alter the spar cutout sizes... if you want to build a tapered wing then you will need to buy profili or somesuch application....

Barrie Lever30/03/2020 13:58:00
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162 forum posts
40 photos

FF

Can you describe the process of getting to the SVG files? I have not worked with those before.

My experience has been with working from scans of templates or plans and then creating the vector data from that.

Coraldraw does have a feature called Coraltrace which vectorises scans if required, but there is nowhere near the finesse in that as compared to manual tracing.

I never machine from STL files, I just use those for 3D printing.

Barrie

David Hall 930/03/2020 15:20:02
237 forum posts
15 photos
Posted by FlyinFlynn on 30/03/2020 09:58:11:

Very nice David, I would suggest you add a chamfer to the underside of the lightening holes to combat the elephants foot and add just a touch to your extrusion rate as that last rib looks a mite under extruded.

.........................................................

Thanks, 3D printing presents a challenge to get to grips with all that's happening. Interpreting the myriad of options within the slicer settings is a slow process, especially when determining what features and faults can be overcome with what option. It can be a complete hobby in itself.

FlyinFlynn31/03/2020 11:18:36
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147 forum posts
95 photos

"Can you describe the process of getting to the SVG files?"

Sure.... go to **LINK** , the tricky part is knowing which aerofoil you want as there are hundreds of them listed, it is just a case of trolling through the 'Aerofoils A to Z' on the left hand side of the web page. A good place to start would be either the ubiquitous CLARK-Y for a flat bottomed profile or maybe a NACA 0018 for a Zlin 50 / Wot4 symmetrical profile. There is a search box top right if you know the name of your desired profile. Once you have selected one, select 'Send to airfoil plotter' [sic..american site!] from the list of four options on the right side of the page. From there you can change the parameters to suit your airframe - chord and, if required, the thickness. In order to tidy up the finished image, uncheck the data box and change the X grid and Y grid values to be bigger than your profile dimensions or else those grid lines will dissect your profile - it is not a fatal flaw if you forgo this step as you can stitch the resulting sections back together in your CAD software. Once finished you can select 'plot' at the bottom of the parameters section and the grid lines should vanish from the plot picture and then select 'SVG image as text file' between the plot picture and parameters section and you will receive the download......happy hunting.

David...indeed...and as soon as you think you have got it something changes!

You seem to have a large 'elephants foot there on those ribs, maybe your first layer height is too low or it could be your first layer extrusion level is too high!..either way, changing them could impact the part cohesion to your print bed so perhaps the best 'cure' would be to add a very small radius to the bottom of the holes to just reduce the 'squish' right on the edge...that way the adhesion will not noticeably be impacted. Conversely...you can see a couple of holes in the plastic where the infill is supposed to touch the hole walls..this is a touch of under extrusion - I would try increasing the extrusion rate 1% at a time until it stops. This may well change when you change spools.

Edited By FlyinFlynn on 31/03/2020 11:21:40

David Hall 931/03/2020 11:47:36
237 forum posts
15 photos

Posted by FlyinFlynn on 31/03/2020 11:18:36:

........................................................

David...indeed...and as soon as you think you have got it something changes!

You seem to have a large 'elephants foot there on those ribs, maybe your first layer height is too low or it could be your first layer extrusion level is too high!..either way, changing them could impact the part cohesion to your print bed so perhaps the best 'cure' would be to add a very small radius to the bottom of the holes to just reduce the 'squish' right on the edge...that way the adhesion will not noticeably be impacted. Conversely...you can see a couple of holes in the plastic where the infill is supposed to touch the hole walls..this is a touch of under extrusion - I would try increasing the extrusion rate 1% at a time until it stops. This may well change when you change spools.

Edited By FlyinFlynn on 31/03/2020 11:21:40

Yes, i know what you mean, the "flange" around the entire piece.. This was my first part printed with an offset for the "elephant's foot"... I added 0.5mm when it now appears that I should have used a negative value.... every print is different!

So far, so good. I am really pleased with the last couple of day's efforts. They will make very practical ribs, they glue well with cyano and gorilla glue...

The holes in the print appear where the infill meets a wall... there's an adjustment for that.. overflow or some such option. More to try.....however, the ribs are good as they are, so it's a minor issue for me.

Barrie Lever31/03/2020 12:53:41
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162 forum posts
40 photos

FF

Thanks for the explanation of the SVG work flow.

The composite wing in the pylonracer started with an unknown but good section, we splashed a template off of a good wing and then reverse engineered it as I briefly described. from that 2D data I then modelled the wing in 3D and machined the patterns.

The SVG technique relies on using a known section which is no bad thing of course.

I might run off a composite printed rib later for the fun of it using the carbon filled HTPLA

Barrie

primed wing pattern.jpgwing section.jpg

Edited By Barrie Lever on 31/03/2020 12:54:28

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