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Simon C's Depron Ballerina

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My 3/4 size "Petiterina" went together so well I have just got to try a Ballerina using the same sort of techniques.

I am not sure I will get it finished in 2016 though!

None of my Depron planes have retracts, so this Ballerina will - just because!

Keeping it simple (& light) it will look something like this from the underside.

Underside 1


The big challenge is how to mount the retract units in the Depron wing in such a way that they will not rip out on the first landing without adding a huge amount of wood to fix them to.

I am sure the wing with a balsa/Depron/balsa spar and stressed Depron skin will be plenty strong but the retract unit represents a very concentrated point load. Not good news for a stressed skin structure. wink 2

The Hobby King retract units are commendably compact, light (25g) and cheap!.

HK retract Whats more lifting a lightweight wheel it only takes 0.5A.

On the Petiterina acetate sheet gave sufficient local reinforcing to the Depron fuselage to carry its wire undercarriage. Would it be possible to use the same technique to hold a retract unit in a Depron wing?

I feel a test piece coming on. wink 2

Edited By Simon Chaddock on 04/11/2016 22:19:33

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Nice one Simon.

I've considered retracts for some of my foamboard builds but have also wondered about mounting too. I have not done it but I have an idea of how I would.......

My intention had always been to run a Birchwood wing spar and to mount the retracts on that. The other side could have a partial spar to mount too.

This way, the load is spread, the mounting is fairly solid and the mount also doubles as a wing spar so it is not wasted weight.
The spar by the way is glued upright so it also acts as webbing.

I would add a pic but I can't from my phone. I hope my explanation is understandable?


Edited By Rosco on 05/11/2016 03:19:06

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Hello Simon,

I can't really believe that you wrote that your Balleretracta would maybe NOT be finished in 2016... surprise

I suppose that your fuselage and tailplane parts are all cut by now and that they will get together before the end of the weekend.

@ Rosco > I'am not so sure that Simon likes the word "birch" - in regard with spars - very much. wink

At the other end, I don't think I'm in a very comfortable position to give some advice, but I'll give it a try anyway.

What about a flat centre section of the wing, with a flatted 'U' shape dihedral brace instead of a 'V' one. This gives a little more room for the wheels as well. The brace could be prolonged up to the outer edge of the retracts with a 'mounting box' attached at both ends having the retracts consolidated from each other. Some laminated Deply might work for that brace.

I can leave the classroom right away of course, but please don't get angry at me now... angel

Hakuna matata


BRU - BE / CTR Retracts Control


Edited By McG 6969 on 05/11/2016 08:11:50

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Rather than use wood my aim is to use acetate sheet as the method of spreading the load directly from the retract units into a large area of Depron as I did with the Petiterina..

I just so happens that the retract unit spans two ribs. What if the rib faces were covered in acetate sheet?

So the test piece will be a small element of wing 2 ribs wide and just from the wing spar forward.

The wing spar itself is a balsa/Depron/balsa box all 3 mm) with the ribs glued directly to its front an drear faces..

Boxspar T

The retract unit across the ribs. The face of the ribs are covered with acetate and incorporates 'lugs' to pick up the mounting screws.

Retract T3.jpg

The underside.view.Retract U1

The hope is that all the loads created by the retract unit will be transferred through the acetate sheet to the rib and then to the wing skin.

Once the test leg is added and functionally checked the test piece will be covered with Depron to create a realistic part of the wing..

Of course it may look 'fragile', and it probably is, but as long as it is as strong as wing then it is good enough.

I can't tell at this stage what the final weight of the Ballerina might be but most likely "not a lot"! wink 2

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Hi Simon,

It seems that there is some acetate at both sides of your rib, but looking at the picture one can't really be sure?

If so, are your 'lugs' passing through the rib and incorporates the other side's acetate piece?

I would make the acetate doubler the full size of the front rib & would make the glued 'base' of the lug wider - ie more like a 'trapezium' instead of a 'rectangle' - spreading the twisting load to a larger base.

But then, who am I? angel





Edited By McG 6969 on 05/11/2016 16:25:52

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I think the key to success here Simon will again be your ability to create incredibly light structures, you might get away with it. In the Super Marauder I confess that I've put the retracts on short 6mm ply bearers and epoxied those into 3mm Liteply ribs. However, I'm just looking for a "lighter" build, not a "super light" one.

I like the way you've solved the problem of having somewhere to tuck the wheels away when retracted, a bit like the P51! I hope,you won't get a pitch-up problem like they did when they extended the inner leading edges forward on the Supermarine Swift to squeeze in two more guns!

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You may all be right about it ripping out but actually the top and bottom lugs are there just to stop the retract moving sideways.

The acetate sheet at each end is an 'interference' fit around the retract unit so the torque created by the leg on landing is transferred directly to the rib Depron. The rib itself is then supported all round by the wing skin and also by the front face of the spar. Having built it I can see some slight improvements for the final version but then that is what the test piece is supposed to do! wink 2

The completed test piece.

Retract T4

Retract T5

The leg is also only a 'test' bent up from a bit of wire coat hanger! It is actually 1/2" longer than it need be.

A video of the retract in action with a light weight 2.5" dia Depron wheel.

It takes just 650 mA peak.

Yes a lot will depend on its very light weight (i.e. landing speed) enabling such an undercarriage to work.

We shall see.

Edited By Simon Chaddock on 05/11/2016 22:29:39

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I needed to find out just how much load the retract mounting could take.

Holding just the Depron wing section it was possible to lift the 2lb weight, likely to be significantly more than the total weight of the Depron Ballerina.

UC load test

In fact the load actually put a bit of a bend in the coat hanger wire!

Because of the retracts the Depron Ballerina's wing will built slightly differently. On the Petiterina where the spar flanges were incorporated intot he wing skin itself but in the DepBal? there will be a 'stand alone' spar and ribs which will then be conventionally covered in Depron.

Wing spar 1

The spar itself is a 'built up' structure. A box spar at the root tapering to an "I" beam towards the tip. The actual tip will have no spar, just a Depron shear web.

The spar internal webs correspond to each rib position.

Next is to start cutting the ribs!

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To give the acetate sheet the maximum glue area it is now sandwiched between two Depron layers.

UC mount 1

The actual mounting has been simplified a bit there are now no screws at all.

UC mount 3

The retract is restrained from sideways movement by Depron end caps.

UC mount 4

Unusually the wing is built up directly off the face of the spar.

Wing ribs 1

The next big job is to build up the wheel well but at least I will have the benefit of a fully operational leg and wheel to allow a close fit. .

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The 'standard' Ballerina's u/c was indeed foreseen into the wing, Simon...

... but being a bit of an 'apprentice' rebel as well, I did foresee these parts >


... to have my u/c mounted like this >


devil .. hihi

... and now, while starting to build my wing, I'm really glad I did. wink



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A 'fuselage' u/c certainly makes the wing easier (and a bit lighter) to build and it should have quite a bit more 'suspension' travel.


To avoid damage to the wing structure the u/c legs should bend quite easily which is likely to be a problem with the tight fitting wells on anything put the gentlest landing on a smooth surface, but as the Depron Ballerina is going to be seriously light its landing speed should be suitably low.

However for its maiden on my rough grass field it might well have to be hand launched with the wheels down, to allow the wheel wells to be use as hand holds and the u/c then retracted for a belly landing.

Bizarre or what!

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Posted by Simon Chaddock on 12/11/2016 14:45:26:

However for its maiden on my rough grass field it might well have to be hand launched with the wheels down, to allow the wheel wells to be use as hand holds and the u/c then retracted for a belly landing.

Bizarre or what!

... there is a French saying, Simon.

Translated it goes like this: "Probably even more strange than bizarre"... wink

And yes, I'm really chuffed with my fuselage mounted u/c. angel



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U/c redundant?

Well I suppose it might be, but I did say in the first post I would be adding retracts simply because none of my Depron planes so far have them! wink 2

It is really an experimental exercise as with the Ballerina's thick wing high speed is not really an issue and even less so on a super lightweight version.

In fact I have a suspicion that necessary cut outs in the lower skin even when the wheels are retracted create as much drag as an unbroken skin and the wheel and leg left in the airflow. Then there is the significant additional weight of the retract units themselves.

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