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-   -   Stiffening Depron without rods ... (http://www.Wattflyer.com/forums/showthread.php?t=73809)

solentlife 05-29-2014 06:04 AM

Stiffening Depron without rods ...
 
Depron for me is available in two thicknesses only ... 3mm and 6mm ....

My thoughts are that 3mm laminated with epoxy with another 3mm should produce a 6mm wing or structure that has far more stiffness / strength than a 6mm sheet version.

It may even be enough to not need insert rods etc.

My Concorde and other models have used this idea but I still added BBQ skewers etc. as I was using non-resin glue. Flex was still there without rods.
Change to laminating epoxy could the answer ............

Anyone use this idea already ?

Nigel

AirmanAirhead 05-29-2014 06:43 AM

Nigel,

Sounds like a great idea. I have not tried that, but it reminds me of something Steve Schumate (Polaris Sea Plane, various foamy jets, etc. and real live Boeing Engineer) wrote once. Something like creating even a thin but rigid surface layer as a skin over foam creates great strength. Your lamination idea sounds worth a try, and it seems a simple experiment is in order!!

Jeff

solentlife 05-29-2014 08:34 AM

I built a 70% version of Steve's F15 ... fabulous flyer ...

Only drawback with epoxy laminating resin is the cure time is much longer than usual bench type.
You cannot use Polyester as it will eat the foam.

Nigel

CHELLIE 05-29-2014 11:14 AM

Quote:

Originally Posted by solentlife (Post 949271)
Depron for me is available in two thicknesses only ... 3mm and 6mm ....

My thoughts are that 3mm laminated with epoxy with another 3mm should produce a 6mm wing or structure that has far more stiffness / strength than a 6mm sheet version.

It may even be enough to not need insert rods etc.

My Concorde and other models have used this idea but I still added BBQ skewers etc. as I was using non-resin glue. Flex was still there without rods.
Change to laminating epoxy could the answer ............

Anyone use this idea already ?

Nigel

Hi Nigel :) Know What ???? I laminated 2 pieces of 6mm depron together with epoxy and know what, it was the same thickness as my 9mm depron :D Dont ask me why, i measured it, I wanted a piece of 12mm depron and it came out to 9mm :silly: the epoxy did not make it any stiffer, it still flexed, I have always wondered if i could take a thin piece of card board, cut a zig zag pattern in a flat foam wing,and insert the cardboard in the wing, the same thickness as the foam wing, then hit the cardboard with foam safe CA to stiffen the card board with and to glue to the foam. What do you think about this idea ????

fhhuber 05-29-2014 11:18 AM

Laminating 2 layers of 3mm depron... It will be stiffer but not stronger... You are putting the stiffening material in the center where it does the least good. The stress that would have just bent the 6mm will be breaking that epoxy layer then creasing your wing. You'll come down looking like your airplane broke a spar instead of having the wing straighten when you relax G forces.

You'd do better for the same weight by very thin film epoxy coating the outside of 6mm foam... or even better just apply clear packing tape on the spar line.

solentlife 05-29-2014 02:38 PM

I realise that having the epoxy inside the lamination is a compromise ... but given that plywood uses that principle to basically use up crap ood stock to produce a 'building material' ... think it has possibilities.

At present I use the durability of Depron laminated with the stiffness of green floor insulation laminated to stiffen wings (6mm Depron + 5.5mm FI).... it works but of course results in a thick wing.

Here's top of model during build in 6mm Depron ..

http://i6.photobucket.com/albums/y24...pse58b517b.jpg

and the bottom Green FI laminated on bottom ...

http://i6.photobucket.com/albums/y24...ps8892b26c.jpg

Because I used PVA glue to laminate .. it stayed flexible rather than stiffening up ...

There is another factor that is convenient .... if you laminate with epoxy - you get a hard joint line. So when sanding LE / TE - you have a natural centre line that resists sanding.

When I get home .... more like IF I get home !! I want to experiment with a foamboard plain delta and a conventional wing form .... see what can be developed from the idea ... it may be that finishing cloth can be used with the resin to increase it's strength ... that is literally similar to lightest tissue in grade ...

I'm always trying to find ways to avoid packing tape !! ;)

Nigel

fhhuber 05-29-2014 03:29 PM

Can't compare with plywood really... part of the idea of plywood is cross-grain lamination which helps prevent splitting.

Depron you are not worried about splitting along the grain.

solentlife 05-29-2014 03:35 PM

Quote:

Originally Posted by fhhuber (Post 949299)
Can't compare with plywood really... part of the idea of plywood is cross-grain lamination which helps prevent splitting.

Depron you are not worried about splitting along the grain.

Partly ...

My laminating of Depron and the FI foam - does have a grain factor ... the FI has grooves that run parallel along it's length and is part of the reason it's not good enough on it's own. But it allows glue to have a corrugated form ... making ridges of glue run the length or breadth of the lamination.

My point about plywood - is the use of what is basically scrap wood to create the sheets ... imparting strength to the otherwise useless material.

Nigel

carpetbagger 05-29-2014 04:48 PM

The "plywood" you describe sounds like what we call chipboard - hand size wood chips bonded with glue and used as a cheap substitute for plywood such as CDX. It is junk. The interior lams of standard plywood has all sorts of splits and knotholes, but marine grade plywood such as BS 1088 has interior lams as good as the AB exterior, and aircraft plywood has A lams through and through.

Back to depron lams - I don't think just epoxy glue on the centerline will so much for stiffness. Real foam planes (homebuilts) use epoxy attached fiberglass cloth on the exterior and that is very stiff. Glass doesn't have to be heavy as it works under tension.

solentlife 05-29-2014 04:59 PM

Quote:

Originally Posted by carpetbagger (Post 949309)
The "plywood" you describe sounds like what we call chipboard - hand size wood chips bonded with glue and used as a cheap substitute for plywood such as CDX. It is junk. The interior lams of standard plywood has all sorts of splits and knotholes, but marine grade plywood such as BS 1088 has interior lams as good as the AB exterior, and aircraft plywood has A lams through and through.

Back to depron lams - I don't think just epoxy glue on the centerline will so much for stiffness. Real foam planes (homebuilts) use epoxy attached fiberglass cloth on the exterior and that is very stiff. Glass doesn't have to be heavy as it works under tension.

I am well aware of Chipboard ... OSB ... Interior grades, exterior grades, WBP, Marine and various other ply / laminates / sheet wood types.

I've built boats, worked in boatyards, rebuilding my house and other stuff that introduces you to so many different materials the mind starts to go nuts !!

Interior and cheap exterior grade plywood is often made up of reasonable sheet outer veneers with scrap interior veneers - often with a lot of voids. But it's strength as against it's components is far greater.

I'm not comparing our model needs to full size or serious stress carrying beams etc. Just putting forward a possibility .. having already near proved it works. I just need to test without the BBQ skewers and with fully cured epoxy instead of my usual PVA.

One material I've found that is it magic for E power firewalls ... pressure pressed paper board ... commonly found as floor laminate ... or factory Parquee.

Nigel

carpetbagger 05-29-2014 05:28 PM

Yeah, that floor laminate stuff is hard. I ripped strip #1 on my first laminate job and wound up choking on smoke = the standard saw blade was cooked beyond all recognition. Switched to a 40 tooth carbide to continue.

I use leftover scraps of marine ply in 3mm, 4mm, and 6mm for various ply spec airplane parts. Plenty good since my largest motors are 300 series.

quorneng 05-29-2014 07:01 PM

Is flexing such a bad thing?
By trying to eliminate it you are loosing much of the potential strength in Depron.
The trick is to keep the aerodynamics uner control whilst allowing the flex.
This was critical issue with the advent of glass and later carbon full size gliders where wing flexing reached proportions that would be unimaginable in wood.
Like this:
http://www.mandhsoaring.com/PhotoGal...hweuphrata.jpg
So don't stiffen it - let it bend! :D

Larry3215 05-29-2014 07:28 PM

Quote:

Originally Posted by fhhuber (Post 949282)
Laminating 2 layers of 3mm depron... It will be stiffer but not stronger... You are putting the stiffening material in the center where it does the least good. The stress that would have just bent the 6mm will be breaking that epoxy layer then creasing your wing. You'll come down looking like your airplane broke a spar instead of having the wing straighten when you relax G forces.

You'd do better for the same weight by very thin film epoxy coating the outside of 6mm foam... or even better just apply clear packing tape on the spar line.


+1

As far as plywood, it has multiple layers and the outside layers are the ones that carry the bending loads. Thats why they are normally the highest quality layers in the sandwich and have the grain orientated to carry the load in the "long" direction.

In any kind of bending situation, the outer portion of the "spar" (the caps) carries virtually all the load. The center is only carrying compression and shear loads.

Just look at any steel 'I' beam or one of our model spars - thin balsa sheer webs with wide but thin spruce spar "caps". The caps do all the heavy work while the webs are just there to keep the caps apart when bending loads are applied.

Beefing up the center of a sandwich is mostly just adding weight to no purpose.

solentlife 05-29-2014 08:05 PM

Seems people are getting hooked up on strength .. rather than practicality.

Many models are made in straight Depron ... Schumate's examples are great for this.

I've put together various Depron jobs and not flown them because I found them too flexy or flimsy. I have a profile ME109 that I blow on the tail and it moves ! It's built exact as plan.

I've built many others that the BBQ skewers have given me excellent solution ... but burying them always ends up with tape or other to try hide them. Having experimented with laminating already - I know that in many cases it will work ... and I am now putting it to you guys to see how many may have used it ...

I am fully aware of Beam theory etc. having had 4 years of Marine Academy teaching me about how ships bend etc. - it's all beam theory ... whether it's an I beam or a structure.

Plywood actually has more factors in it than just plain outer veneers for it's strength. Having spent many an hour waxing lyrical with yacht designers etc. on virtues of Plywood against wood strakes ...
Plywood in fact gains strength and durability from the layers and internal friction allied to the bonding. Quite often the outer veneer is THINNER than inner for good reason - it's the cosmetic finish. Yes it contributes to the resistance to bending stresses but sheer is provide by the whole structure through. The inner laminates are not just fillers. Fillers are the case with 'packing case' ply's - not serious building quality.

We can go on for hours on this ... so please better left alone !! ;)

Nigel

Larry3215 05-30-2014 02:31 AM

Well, you did say you were looking for more stiffness/strength.

Beefing up or adding to the center of a laminate structure just isnt the best way to do it. In fact, its just about the least efficient way to get where you said you wanted to go.

I suppose it has the advantage of being easy, but as fhhuber said above it makes the structure more subject to breaking rather than bending and recovering under load.

When your center epoxy layer breaks under a bending load, it will create a stress point and the depron will tend to crack along with it at that same point - long before the depron would normally fail.

To my mind that's kind of going the wrong way from where you said you wanted to go.

fhhuber's suggestions in post 5 - adding a thin skin or tape to both sides - are the best ways to beef up a foam plate structure if your not going to use tubes or rods.

mclarkson 05-30-2014 03:37 AM

Not to incite further argument, but I am reminded of an episode of Nova talking about the strength of a Toucan's beak and the materials it derived from. It reminded me at the time of foamie construction. The bulk of the beak is made up of a soft, spongy bone, which is wrapped in a thin sleeve of flexible plastic-like material. Combined, they are amazingly strong.

The show demonstrates the principle using styrofoam and a thin plastic sleeve:
https://www.youtube.com/watch?v=4VP9Rkhu96s#t=115

solentlife 05-30-2014 06:42 AM

I admit that my original post was badly worded on this and does incite more on strength ...

My idea is to get away from packing tape, rods and similar - to arrive at a system that in small to medium size can survive flight loads and have cosmetic appeal. Packing tape and rods visible are my items to avoid !
I'm also trying to avoid filllers and surface finishing as explained further below ..

Laminating epoxy in fact has much more tolerance of bending than 5 min variety which is quite solid. Having built veneered foam wings in past .. joining with resin and then bandaging joint (whatever happened to that ?) ... The bandaging was usually done with Polyester, being a 15min set-up time and quick. But occasionally I used Epoxy resin .... A few models I used 5 min epoxy thinned to be able to wet out the bandage. WOW the difference between the 5 min and laminating variety was immediately apparent. And later flight loads did create stress lines / cracks in the 5 min variety, but the laminating variety (Gudgeon Bros. yacht Epoxy) was able to absorb the stresses and remained glass clear with no signs of stress. In fact it outperformed Polyester. It's downside was the cure time.

I've seen epoxy plates (glass cloth and resin) subjected to bending loads and amazed at just how far it will bend. Try it with Polyester or quick set and you will fail.

So lets be sure we are talking same materials ... forget about the quick set epoxy ... and that includes varieties of 1 .. 2 .. 3hr cure time ... the laminating epoxy can be up to 24hrs ...

On to the whether glass up outside or inside ... I agree that better strength result is achieved by outer skin ... but then with epoxy .. in fact with any resin based system - you come up against quality of finish. I certainly do not want to start filling surface to have a sandable medium to smooth out glass work. The intent is to have a cosmetically good product and light weight. The only way to have a good finish to resin based finish is to either wrap in film to 'mould' the surface or to actually create in a mould. (Avoiding filler).
Put the resin between the foam sheets and although we may incur a reduced increase of strength - we gain in cosmetics ability ....

I'm a lazy SOB !! and I'm all for a quick 'finish' !! I think we all have tried sorting out glasswork finish and what a pain it is ... resorting to fillers to sort.

Goood laminating resin such as Gudgeon Bro's is not expensive ... available in small and large quantities. It's thin and wets out beautifully ... has excellent working time.
It also has a characteristic that the surface does not stay tacky as with normal epoxies. It's a factor of oxygen and resin that causes the tacky surface of cured epoxy.

If anyone wanted to really build in strength - the laminating resin with CF tissue is possible ...

Yachts often have Kevlar (CF) matt inserted into the laminate to increase strength in required areas .. surface is still the Gelcoat or finishing resin ... if it works for them - it can work for us ;)

Because of lack of available CF over counter for me - I shall have to explore this concept without ... but I most certainly will post back what happens ... In my mind any system that gives alternatives is always welcome ... we don't always need factor x or y in our builds and w may be fine.

Packing tape ? Lets be honest - 30 yrs ago - you'd be laughed of the flight line if you turned up with a packing tape covered model. I remember arriving at Butser Hill in UK with a Slope Soarer I'd covered wing in thin Fablon (a self-adhesive film covering used to cover paper sheets ... the thick variety was for benches etc). Boy did I get some 'stick' for that ... I thought it was a quick easy way to cover the wing ...
Packing tape - what does it really do ? Strength ? Not really. IMHO it only provides a support for flexed structure, as the structure bends - the tape resists stretching ... It has no real strength in it.

I'm going to try out laminated sheet on the Tu144 build - IF I EVER GET BACK HOME !!

Cheers everyone !!

Nigel

carpetbagger 05-30-2014 11:13 AM

I believe laminated depron will gain some stiffness since laminating strips of wood does, such as laminating a curved piece out of wood strips. The lam will want to return straight to a small degree, known as :spring back", and more layers equals less spring back.

As for surface finishing - peel ply is your friend. Peel ply does not like compound curves. Wet fiberglass with resin, or just slobber on resin, then squeegee peel ply over it. Press squeegee as hard as the depron will allow without deforming. When cured peel of the peel ply = smooth surface, no nasty sanding required. Peel ply is a fine woven polyester cloth, so any polyester cloth should work as well.

Ohm 06-18-2014 07:31 AM

2 Attachment(s)
Colored packing tape (they very thin stuff) on Dollar Tree foam (Blu-Baby)and applied to EPP with extra contact glue (FF Edge 540). Not necessarily a rod replacement but it defiantly stiffens up the frame and seems to add a level of durability to the plane. The Blu-Baby has been tumbled and bumbled all over the place and practically no damage and has not needed any airframe repair to date, although a slow flyer it is being flown by folks who never touched a stick on an RC often in breezy conditions. Although I am modifying the motor mount so I can run a prop shaft saver. It's costing a small fortune in prop shafts and props. The Edge is designed for CF struts at the tail and rear fuse with the application of tape that was CF was eliminated.

flypaper 2 06-20-2014 04:48 PM

2 Attachment(s)
A buddy and I built this Sea Fury Reno racer quite a few yrs ago. There are 3 layers of carbon fibre of different lengths. First layer goes out to 1/3 of the wingspan. second, 2/3s out and the third full span. The plane weighs about 60 lbs with a 24 horse Quadra 200R engine. The couple standing on it are not lightweights either, and the wheels are actually off the ground.:D


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