Would you use expanded foam sheets for wing ribs?

[old man emu]

I've seen that a few homebuilts emply blue styrofoam for wing components, in fact the Long-Eze wing is solid foam. I recently came across a produce called CELUKA sheet. This stuff is expanded PVC foam, sandwiched between plastic veneer sheets.

 

I've done some experiments and found that it is not affected by dope, MEK or general thiiners. If a flame is applied to it, the material will smoulder, but burning stops as soon as the external fire source is removed. It is pretty well waterproof.

 

Some specs are:

 

Density: 0.02 lb/in^3 (0.55 gm/cm^3)

 

Tensile Strength: 2266 psi (160 Kg/cm^2)

 

Tensile Modulus: 144,000 psi (10124 Kg/cm^2)

 

Flexural Strength: 3329 psi (234 Kg/cm^2)

 

Fexural Modulus: 144,219 psi (10139 Kg/cm^2)

 

How do you think this struff would go if used for wing ribs? The wing loading of the aircraft is about 7 lb/sq ft.

 

OME

 

 

[fly_tornado]

The tornado wing has a "D" cell construction, aluminium over foam leading edge to the spar.

 

 

[Gnarly Gnu]

Perhaps this question could be asked on HomeBuiltAirplanes.com or similar, that sort of thing comes up there frequently.

 

 

[Deskpilot]

Perhaps this question could be asked on HomeBuiltAirplanes.com or similar, that sort of thing comes up there frequently.

Make sure you use their search function first. They tend to get a little annoyed at repeat questions. There is good coverage there on this subject.

 

 

[turboplanner]

The technique you are talking about is honeycomb, where the core is simply used two skins apart. As the component tries to bend, the tensile strength of the outer skin holds it straight.

 

You'll probably find a lot od engineering data on honeycomb structures and calculations around the web. The key issue is a strong bond between the skin and the core.

 

One of the biggest volume examples are Australia's refrigerated vans where polyurethane is used for the core and fibreglass reinforced plastic is used for the skins. They'll roll upside down and hold the truck in the air.

 

 

[RickH]

there are many and varied types of foam available I'm not familiar with the type you mention but one which is certainly worth a look is a product called multi panel not cheap tho.

 

 

[Deskpilot]

TP, honeycomb sheets are entirely different from blue foam. The core of Honeycomb looks just like that, honeycomb. It's of fibreglass construction I believe and is skinned on both sides with f/glass. It's very light and can be used for flat items such as bulkheads but the edges are hard to seal, even if the are straight. A curved surface like the profile of a rib would be very labour intensive. You'd probably have to fill all the open holes,(top and bottom, not sides) then glass over the filling and down the sides in order to fit caps and bottoms to which your skin will eventually fix. Do some research.

 

Blue foam with ply-wood sides would be the way to go if any. I've looked at doing it this way myself (eventually) but even then, it has to be designed right.

 

 

[turboplanner]

Sorry Deskpilot, you're stepping into my design profession here. The honeycomb name comes from the time when sheet material was used, and a practical way to produce the maximum crush resistance from the minimum thickness and the lightest weight was to start with a flat sheet and for the material to form hexagons, which looked like honeycomb. I've used paper, and aluminium as the core to produce truck and race car components.

 

When polystyrene and polyurethane foam came on to the market around the sixties we were able to take a shortcut and reduce the labour time by just sawing up foam then bonding on the skins, the holes in the core were no longer hexagons, but we still call it honeycomb.

 

Where there is no curve, and weight is not critical, plywood is a popular skin material.

 

Where there is a curve in one plane, aluminium is a popular choice, and is immensely stronger and more weather resistant that a rivetted construction.

 

In the US most van bodies are constructed by drawing off two rolls of aluminium and feeding the core between them, bonding each side in the process, virtually a continually moving process.

 

Where there is a compound curve, the high tensile strength of fibreglass reinforced plastic, or carbon fibre allows more complex component to be made very strong at very light weight.

 

I've used it in truck components to stop drumming and race car bodywork to stop flapping at high speed.

 

The bond between the skins and the core is critical, and if done well also seems to stiffen the skin against localised impacts.

 

When I was working on refrigerated van design, and was giving talks to convert operators from aluminium construction, I would take a small piece of wall section around and invite people up, give them a hammer and ask them to try and break it.

 

Mostly they just hit it tentatively and the hammer bounced off sharply, but one milk truck operator who lifted hundreds of crates per day and had arms the size of my thighs had seen me do it before, and he raised the hammer above his his head, then smashed through both skins and took a lump out of the concrete floor, so I had to give that idea awsay.

 

For critical design areas I used to test to destruction first but I wouldn't be surprised if there are calculations around today for given materials and dimensions.

 

Apart from the core and skin material chosen, the key part for long term life and strength is the bonding material and the method of bonding - without that you very quickly have two floppy skins blowing in the breeze

 

 

[Deskpilot]

I bow to your superior knowledge TP, and thank you for your insight of this material. I only looked at it once and that was for bulkheads only. I decided against it.

 

So, would you recommend ply/foam sandwich for OME application, or not?

 

 

[turboplanner]

Put me on the spot DP, I haven't done any wing engineering apart from race cars where the wing has to be heavy enough to resist damage from flying clay, rocks etc, so I'm not sure whether the final weight would be satisfactory, but I would see the application as better suited to a full wing sandwich (like the Vari Eze example OMG gives which could potentially save the trouble of making ribs.

 

Simplifying things down to just one load, if we stand directly in front to the aircraft and pretend it is moving towards us, at the take off transition the wing tips want to come up as the wings take the load.

 

With a full honeycomb wing structure this is resisted by tension on the bottom skin - if it can't stretch it can't bend.

 

If you look at the same force, using a series of ribs they have to be strong enough not to crush as the tension on the bottom skin tries to take a short cut by flattening in the centre - so they need maximum strength to hold the wing skins apart....but a vertical sandwich panel is not strongest in that plane, and if you start adding complications ,like control rods/wires, electrical lines, fuel lines, tanks etc. manufacturing is also more complicated than a single skin rib.

 

Maybe you could mould box structures with all the angles shapes and flanges of allow rib apertures, then foam fill the voids, but this is more suited to mass production that a home build.

 

 

[Deskpilot]

OME, try this link and sub-links. Might be of use to you.

 

http://www.homebuiltairplanes.com/forums/composites/9011-foam-core.html

 

 

[old man emu]

Thanks for the link to homebuilts. A handy reference.

 

This discussion has gone off on a tangent when it discusses solid core wings etc.

 

What I wanted to do was to cut ribs from 10-12mm thick sheets of this material. The original plans call for the ribs to be constructed from 1/8 x 1/2 sitka spruce. I was going to cut them from 1/4 hoop pine ply, then I found this stuff and was wondering if it would be lighter, but just as strong as ply.

 

Here's a picture of a plywood rib that I had cut as a proof of concept piece.

 

OME

 

 

 

[Gnarly Gnu]

I reckon ply would be stronger and cheaper but then I'm not familiar with the Celuka product (which has the obvious advantage of moisture tollerance). The obvious answer is, rather than relying just on product data, knock up a handful of ribs in each material and load them, stretch them and generally tear them to bits whilst measuring the force used.

 

 

[old man emu]

Yeah, making up a test section of wing is one of the things I was planning to do.

 

If the wing loading of the plane is 7lb/sqft, what weight do you think the rib should support during testing. I was thinking of making the section; laying a plank across a couple of ribs and standing on it.

 

OME

 

 

[cherk]

Wing Loading X area X ?G's / No of ribs = ?........or something like that.....maybe!

 

 

[turboplanner]

Guys, you're scaring the living daylights out of me - when you read about an aircraft which broke up in flight, do they ever talk about a crushed wing profile?

 

I'm sure there'll be some reasonably straightforward wing design data around which would show you the design calcs and methods of testing.

 

 

[old man emu]

I suppose that I should have added that the main spar is 3-7/8"x 1" sitka; the rear spar is 3" x 1" sitka and that every fifth rib is a compression rib which has a solid pieces of 3/32 ply on each side. The chord length is 36". Ribs are on 12" centres.

 

OME

 

 

[Deskpilot]

I suppose that I should have added that the main spar is 3-7/8"x 1" sitka; the rear spar is 3" x 1" sitka and that every fifth rib is a compression rib which has a solid pieces of 3/32 ply on each side. The chord length is 36". Ribs are on 12" centres.

My humble opinion, I would only lighten the foam ribs by cutting circles and not go too close to the edge. A variety of sizes will take out a lot of weight and leave a good web with no 'in line' weak points. Personally I would use more ply strengthen ones as well. What you've described is one every 5 feet. That's too far apart for my liking. I would aim for 4 feet or less but as others have said, testing is the only way to prove your concept.

 

 

[old man emu]

Actually, the distance between compression ribs is 4ft. The first rib is at 0 ft. This is a compression rib, then there are ribs 2,3,& 4 each 1ft apart, then rib 5 is a compression rib. Rib 5 becomes Rib 1 for the next bay.

 

My enquiry really relates to the suitability of this material to withstand the Lift forces generated by the aerofoil shape. In other words, would this stuff collapse upwards due to the higher pressure on the lower surface, or separate internally due to the lower pressure at teh upper surface?

 

OME

 

 

[Deskpilot]

I would say no, otherwise it would be banned.

 

 

[Old Koreelah]

Very interesting topic. I have mobs of klegecell (PVC foam with 75kg/m3 density) left over from another project and used some of it in a sandwich with 1.5mm hoop pine ply. Mobs of stiffness, and pretty light. but I have no idea of the numbers.

 

 

[Deskpilot]

Very interesting topic. I have mobs of klegecell (PVC foam with 75kg/m3 density) left over from another project and used some of it in a sandwich with 1.5mm hoop pine ply. Mobs of stiffness, and pretty light. but I have no idea of the numbers.

OK, I've heard of 'Mobs of Kangaroos' but never mobs of anything else. Must be a local area measurement. Care to enlighten us OK. How much/many constitutes a mob ;-))

 

Serious now, what project did you use it on and was it successful?

 

 

[Old Koreelah]

OK, I've heard of 'Mobs of Kangaroos' but never mobs of anything else. Must be a local area measurement. Care to enlighten us OK. How much/many constitutes a mob ;-))Serious now, what project did you use it on and was it successful?

The project is the Jodel D9 you see on my avatar, and yes it's been successful. Unfortunately I can't advise in more specific terms about it's efficacy.

 

(A "mob" is 100 times larger than "heaps big", 1000 larger than a "smidgeon" and a million times the size of a "poofteenth")

 

 

[Deskpilot]

 

 

[old man emu]

Oh! Does that mean that a "tad", a "touch" and a "smidge" are Imperial measurements?

 

OME