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How to triple strength of Depron without adding weight.

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kc11/04/2018 11:32:10
5511 forum posts
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I notice that some of the commercial models made from EPP use a horizontal layer of foam to form a cruciform fuselage all the way back to the tail.. Would such a shape be easier/.lighter/quicker than putting 1/64 ply framework onto the fusealge to stiffen it? It makes a neat way of hiding the radio etc and might make the model easier to see.

David Mellor11/04/2018 14:39:41
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Hi kc

I've not been very systematic with coverings, so there are lots of things I haven't tried.

I did get a very lightweight brown paper with good wet strength. It is 40 gram per square metre (normal parcel paper is about 80 gsm). Also, it has good wet strength. It is available on 100 metre rolls as wet-strength lightweight masking paper for car body spray booths. And it is very cheap.

Do you have any pictures of that cruciform arrangement - a sketch might do?

The 1/64th ply construction is pretty tedious to do and anything that was quicker/lighter/stiffer would be a great idea.

I've noticed that the most "twisted" area (highest torsional load) of the fuselage is just behind the wing TE...... thats the toughest area to deal with in a simple profile-type of flat fuselage,

kc11/04/2018 15:08:42
5511 forum posts
161 photos

If you look at the photos of the Hobby King Hummer it shows the type of cruciform fuselage I meant   The red parts are the horizontal layer.   It is just a profile of the plan view as well a profile from side view - nothing special. However it was more pronounced an a colleagues 3D model ( don't remember the name) as the fuselage horizontal sheet seemed almost to add to the wing area as it went right back to the tailplane.

Presumably just widening the fuselage of profile models would make them stiffer as the side pieces are further apart.

Edited By kc on 11/04/2018 15:10:38

David Mellor11/04/2018 17:10:40
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Thanks kc.... I see what you mean. Yes, looks like a good idea. I've seen some super-light indoor 3-D models with a similar arrangement plus some thin diameter carbon rod strutting at 45 degrees diagonally connecting the vertical profile to the horizontal profile.

Its not so much the stiffness that matters (though that is important), but the resistance to twist. That seems to be the hardest bit to sort out.

eflightray11/04/2018 20:08:12
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Posted by David Mellor on 11/04/2018 14:39:41:

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

Do you have any pictures of that cruciform arrangement - a sketch might do?

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

My Mosquito picture back on page 2 shows a 'cruciform' build, (if that was what was meant), both the fuselage and motor nacelles have the side cheeks, to form a cruciform.

It allows for a thin fuselage, (the Mossy is 6mm), with the side cheeks giving additional strength.

Ray.

David Mellor12/04/2018 08:57:33
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Thanks Ray.

I'm sure you are right - that must be what is generally meant as a cruciform structure.

It is something I haven't worked with up to now. A 6mm thin fuselage on a 50" span is very thin - and presumably very light too ?

By the way, what size motors are they on that Mossy?

Have you built other planes that way?

Dave

eflightray12/04/2018 14:08:14
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The motors on my Mossy are quite old Tower Pro 2408/21, often referred to as 'bell' motors, (they look a bit like bicycle bells I believe). Turning 8" x 4" HD props

I have built a couple of profile models in that method in the past, but prefer the more rounded built up fuselage, easier to hide things.

I wont show a picture of the Mossy underside, as that's were all the gear etc is stuck on, (look quite embarrassing, as well as ugly). But in the air the cruciform shape fools the eye into thinking it's a full fuselage.

Flying weight was 20 1/4oz (570g), 3s 1800 Lipo.

Early build shot --

may06#09.jpg

Ray.

 

Edited By eflightray on 12/04/2018 14:08:57

Simon Chaddock12/04/2018 16:05:12
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Cruciform is an easy way of building an aerodynamic test model. My half size chuck glider (6 and 3 mm Depron) to find the appropriate CofG position before I risked the delicate double size EDF version.Glider top

Glider under

I felt it should be sufficiently aerodynamically similar for the RC's maiden (a hand launch). It was an no ballast has been added since!

David Mellor13/04/2018 09:48:15
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757 forum posts
230 photos

They are very nice examples, thanks Ray and Simon.  I especially like the idea of a cruciform "chuck glider" to get the CG right.

 

I did find a couple of cruciform structures that I'd built a few years ago, but forgotten about. In fact I didn't really think of them in those terms.

 

The biplane is a sacred-up BYOB (Back yard Outdoor Bipe). It is made from thin sheets of pink foam that I hot-wired from a block. The other is a disc-wing called a PiYak (a cross between a Pi-plane and a Yak).

rimg0615.jpgp1000322.jpg

Edited By David Mellor on 13/04/2018 09:50:42

kc13/04/2018 17:38:29
5511 forum posts
161 photos

David,

What exactly should one order from Screwfix etc for the material used in these models - Vitrex seems the name for several products. And how many 850mm by 600mm sheets do you get in a pack? It seems to say 1sheet for 17.99

David Mellor13/04/2018 19:32:21
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757 forum posts
230 photos

Hi kc

The full product name is "Vitrex Wood & Laminate Underlay Boards".

There are 19 sheets per pack and 1 pack costs £17.99, so the price works out at 95 pence per sheet.

The individual sheet size is 23.5" x 33.625" with a thickness of 5mm.

The Screwfix product code is 68038.

p1050858.jpgp1050859.jpg

kc13/04/2018 20:00:17
5511 forum posts
161 photos

Thanks David.

19 sheets seems an odd number! Enough for about 2 models I suppose.

It's like buying balsa at about 20 pence for a 4 inch sheet!

David Mellor14/04/2018 08:35:16
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757 forum posts
230 photos
Posted by kc on 13/04/2018 20:00:17:

Thanks David.

19 sheets seems an odd number! Enough for about 2 models I suppose.

It's like buying balsa at about 20 pence for a 4 inch sheet!

 

Yes, 19 sheets is a weird number.

And it is very cheap stuff..... one pack contains just over 104 Square Feet, for just under £18.

That is 17 pence per square foot. Makes for lots of very cheap experiments.

 

You should get more than two models from a pack, depending how big they are, of course.  I got the 6 foot Vulcan (photo on page 2 of this thread) from one pack and still had some left over.  And that is 11.47 Square Feet of wing area laminated up from 5 layers!

Edited By David Mellor on 14/04/2018 08:38:53

Mark Kettle 120/04/2018 16:28:58
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2117 forum posts
1289 photos

Sorry I didn't reply to your invite for ideas David when you posted a few days ago, I was on holiday and have been work busy since getting back. David, I've just ordered a pack of Vitrex Wood & Laminate Underlay Boards to have a play with, I'm picking them up tomorrow from the Corby branch when passing though.

David Mellor20/04/2018 17:39:52
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757 forum posts
230 photos

Great stuff Mark.... Hope you like it!

Let us know how you get on.

Dave

Simon Chaddock20/04/2018 19:45:10
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5169 forum posts
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That 5 mm underlay board is not the same as Depron. It is not nearly as rigid.

Used in conjunction with a suitable 'skin' material it works well and is admirably cheap.

The lower wing uses a 'skin' of cereal packet cardboard over underlay board ribs.Cereal wing 1

The all Depron version above is about 25% lighter for the same strength. Neither wing has any form of spar!

David Mellor20/04/2018 20:26:34
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757 forum posts
230 photos

Yep... it is cheap and endlessly versatile!

The main factor determining XPS sheet strength is orientation and lamination.

Having said that, the next factor is density. Tensile strength is pretty much a linear function of the foam density and flooring underlays aren't quite as strong (they are slightly less dense) than the original Depron. The differences are less important when the sheets are laminated and more important when using single-sheet rib & skin techniques.

Interestingly, colour is a rough guide to strength. The original Depron was un-dyed white and is still the densest and stiffest of all the XPS variants and is at the "higher end" of the flooring underlay market. The cheaper coloured flooring underlays are slightly less stiff and slightly less brittle - they are normally shades of green or blue. To alter the thermal properties, graphite powder is added to make grey XPS. The graphite addition makes for the least stiff of all the XPS foams and it has a soft, "squishy" feel to it. But it still has good tensile properties and there have been many excellent models made with it.

Mark Kettle 128/04/2018 18:09:40
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2117 forum posts
1289 photos
Just purchased some carbon arrow shafts 5mm diameter 27" long for spars ideal for placement, in a gap I'll produce around / in the 5mm thick foam.

Edited By Mark Kettle 1 on 28/04/2018 18:10:28

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