Printed wing ribs

[Simon Chaddock]

Printing ribs has always been an attractive proposition as both the profile can in theory be created from true plot coordinates, is repeatable and if required accurately scalable.

One of my early applications for printed ribs was for the Big Dragon. A simple plank wing pylon pusher.

 

The fuselage is also printed. The wing is skinned in 2mm Depron over printed Clark Y section ribs.

 

Each rib is 3 mm 'thick' and is printed with a single bottom layer with a carefully positioned 'triangle' infill. It looks like this on the printer.

At just over 1.5g it is light and remarkably strong. They have served well flying regularly for 3 years but it has had to survive a could of crashes so I was considering building a replacement wing as it is quite a high stressed structure.

As a start I printed out 1 rib and weighed it using my jewellery scale. It came to 1.569g. There are 28 ribs in the wing so the total weight of the ribs is 44g. This was a bit of a surprise as the total weight of the wing is only 144g. Was this rib design as light as it could be and still be 'fit for purpose'?

After some experiment this was result.

Exactly the same profile as before and it certainly feels just as stiff.

The diameter of each hole is set so its flange just touches the inner surface of the rib flange so giving a double thickness at that point.

The rib weighs 1.239g, a saving of 21%. That will result in a saving of 10g over all 28 ribs.

I also discovered that adding more smaller holes did not reduce the weight as the material saved by the 'hole' matches that used by the 'flange' that surrounds it.

 

I appreciate not everyone is so weight paranoid but "every little helps".

Now I have just got to print out 27 more ribs (6 hours) and build the new wing (many more hours). ?   

 

 

    

Edited March 7, 2022 by Simon Chaddock

[Simon Chaddock]

Well I did and this is what it looks like so far.

Exactly the same as the original wing. No problem running the aileron servo wire through the wing. ?

There is a 2 mm thick balsa spar flange in the lower skin under the variable thickness Deron shear web. The same is on the rear edge of the front portion of the top skin. Note the skin preform.

The rear portion of the top skin only goes on once the wings are joined and the aileron servos installed and tested.

 

It follows my preferred method of only using printed components where their properties result in a net benefit to the finished structure.

[Andy Gates]

That is smart.

 

What sort of weight is each rib in comparison to standard balsa rib?

[Simon Chaddock]

Each rib is 200 mm long, 25.4 mm tall, 3 mm thick and weighs 1.24 g.

I have not made a direct comparison with a balsa rib but it would have to use pretty soft stock to match the weight and as a result would not be as stiff.

 

This printed rib was intended to be match one cut from 3 mm thick Depron sheet. The printed rib ended up about the same weight is considerably stiffer but is of course far more accurate in profile and is readily repeatable, just by pressing a button. ?