Printed spars for foam.

[Simon Chaddock]

I recently managed to severely damage my modified Cheetah RC conversion in that the centre section extension had to be scrapped. Rather than be rebuilt it was to be returned to use its original foam wings.

This resulted in a significant reduction in wing area from this:-

  

To this:-

The wing area is reduced by just over 60%, although there is a small weight reduction from 198g to 189g.

As a chuck glider the Cheetah weighed only 90g but now twice that weight has to be carried by just the original foam wings. 😲

The usual remedy is to reinforce them with a carbon rod let into the wing under surface.

This is a poor solution from a structural point of view. The foam wing is quite thick so much less reinforcement is required if it is placed at the top and bottom of the foam. Using balsa it is a process I have used on many of my Depron skinned planes.

Rather than balsa I chose to use a very thin (0.35 mm) printed flange. With such a thickness it could be simply glued directly onto the top and bottom surface.

By ensuring the filament bead runs only along the line of the flange for both layers even in PLA it has considerable tensile strength.

A 200 mm long flange top and bottom across the wing root greatly increased the wing stiffness.

At the same time I added sub spar flanges to counter the weakness created by the cut outs for the aileron servos.

 

All four spar flanges weigh just over 1 g.

The final bit of printing was to create a one piece cowl to cover the motor.

 

Quite a complex 'loft' in FreeCaAD from the shape of the fuselage at the motor bulkhead to a circle, with a small cooling air intake, to match the prop spinner.

The finished nose showing the prop folded.

It all took far more time and unsuccessful attempts than the plane is worth but on the plus side such tiny light parts use very little filament.🙂