Another Biplane Ballerina

[Simon Chaddock]

At the moment this is just an idea,

I posted in another Ballerina Biplane blog that my small 3/4 size Depron Ballerina(mod) already has a 9 cylinder printed radial'

and if made into a biplane it would be no bigger - just taller!

However the more I cogitate on this the more issues I see.

1. Being made of Depron it is already very light at 13 oz (369 g) even with the printed radial so it needs a second wing like a hole in head!

2. My supply of 2 mm Depron is limited and irreplaceable so I don;t want to use it needlessly.

3. To keep the current wing loading but as a biplane the wings would have to be smaller but that would mean making both sets which would use the same amount of much Depron.

Out of curiosity I doodled what size wing as a biplane would give the same wing area as the little Ballerina but retaining the same aspect ratio. the answer was 24" (600 mm) span with a 6" (150 mm) chord.

It would look more like a Pitts Biplane than a Ballerina - a Pittserina?

My next thought was as the wing is so modest could it be 3D printed?

Indeed could the whole thing be printed? My apologies to Peter for even thinking of such a thing! 

I've done quite a bit of printing 'parts' but never a whole air frame and keeping to my stressed skin type construction do it with no reinforcing without ending up stupid heavy. However being realistic it may be wise to use Depron for the tail feathers.

So the proposal is a 3/4 size Ballerina fuselage with a 'electric' radial engine coupled to 24" biplane wings and a target weight of 20 oz.

My heart says 'yes' but my brain says 'prepare for tens of hours of printing and lots of disappointment'.

Edited By Simon Chaddock on 15/05/2020 22:49:11

[Simon Chaddock]

Just a small advance.

I put together a CAD design for the LH wing. 12" long with a Clarke Y section and a simple tape top hinged aileron.

A position for a micro servo is included as is a channel on the underside for the servo wire.

The way it is to be printed and bed limitations means it has to be printed in 4 pieces plus the aileron.

Some initial test prints on two parts showed it was practical and would have more than sufficient strength but it was indeed going to be 'crazy' heavy. The biplane pair would be about 4 times weight of the single 3/4 Ballerina Depron wing of the same area.

Unless there is room for a considerable improvement the printed route will seriously miss my target weight.

[McG 6969]

Hi Simon,

I really don't want you to get furious on me...

... but couldn't you consider to have the second wing made of 3mm Depron? ... possibly 'scraping' the inside of the foam a bit to reduce weight? ...

Apologies in advance.

Cheers

Chris

[Peter Miller]

Do what you like SImon. THat is the way we learn what works and what doesn't.

My own opinions?

Biplanes have lots more drag.

There is no substitute for balsa.

[Simon Chaddock]

Chris

I could indeed make the second wing out of Depron or both for that matter but my challenge was to see if I could design a printed air frame that was not too heavy by my lightweight standards.

the best solution so far is to print the front and rear parts separately face down along the line of maximum thickness like this.

I have printed out a 6" (250 mm) section of wing that includes the aileron.

With a suitable area for a flush mounted micro servo.

It certainly produces a smooth section and has plenty of strength and stiffness but so far it rather confirms that a printed wing is going to be heavy. At the moment just the pair of 24" wings would weigh nearly as much as the complete Depron 40" Ballerina ready to fly.

I still have some more angles to try but at some point I may have to admit that printing as a direct substitute for Depron is just not feasible.

[Simon Chaddock]

In my normal Depron builds I had used 'printed' wing ribs that were a similar weight and strength to balsa so the next step was to use just the front part of the printed wing as a D box spar. simply glue ribs to it and cover with tissue.

And with a printed trailing edge

This would dramatically reduce the printed surface area and substitute it with a much lighter tissue covering.

Of course this meant not only hours of printing but quite some assembly time as well.

The result is a 24" 'light weight' printed wing.

The term 'light weight' is of course relative. Although plenty strong enough but when covered and doped each wing was going to be close to 80 g so160 g for a biplane which would still be nearly as much as the total weight of the small Ballerina Depron complete fuselage including the motor & flight battery.

Given the very poor crash resistance of a printed structure I decided to call a halt to a 'printed' Ballerina biplane.

On the other hand the concept of a conventional 'stick and tissue' wing but made entirely from printed parts did look interesting.

I wonder