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Jul 11, 2019, 04:09 AM
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Screen window aluminium for pylon style wing ?


Is there another name for screen window aluminium, i combed through spadworld for quite some time to find out how they did the club pylon racer as key information was not easy to come by.

In one discussion i picked up that they used screen window aluminium for the spar and that it was 4 inches short of each tip.

Immediately i thought this thing will flutter like crazy because it has no spar reinforcing the tips the last 4 inches of each tip ! !

But one guy said it works and will be fine for anything below 130mph.

So as a cheaper alternative i am looking at getting this spar (and at least put in 4mm correx at each tip instead of nothing) but i have no clue what it's called locally and what it actually looks like ? Are there different sizes ? Are they a standard size ? How hard is it to get hold of and what do they cost ?
Last edited by Axefly; Jul 12, 2019 at 05:57 AM.
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Jul 11, 2019, 08:54 AM
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It's the framework around a window screen like the ones this guy is replacing in this video. Not sure if there is a standard size, but most of all the screen frames I've seen seem to be about the same size.

Replacing a Window Screen 5-Steps (4 min 10 sec)
Jul 11, 2019, 10:18 AM
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Good luck. Bug screens are not as common in SA, but they do exist. I've just never seen the frames sold anywhere. I've spent countless hours scouring through Builder's Whorehouse for suitable alternative spar materials and ended up deciding that the time would have been better spent making composite spars from scratch. Aluminium is both quite heavy (density 2700kg/m^3) and the 6063 grade commonly sold in SA is flexible (unlike the 7000 series used in quality tent poles - which would work well if you could find some). So you ideally need a section that has a high second moment of area and made from thin sheet. But most suitable sections I've seen have a minimum thickness of around 1mm....which is too heavy for my builds. A pressed section made from ~0.5mm alu alloy might work, but I've never found such a thing.

If you don't find a suitable Alu section, ask the BW staff to rip a carefully selected strip of 8mm meranti or pine to the thickness you want for the spar. Then buy some CF strips and bond them to the top and bottom of this spar (use good glue - the joint quality is crucial - even consider wrapping with dynema fishing line). This will be lighter and stiffer than Alu, and easy to build so long as you don't need a taper.

Other options are old CF fishing rods or golf clubs...if you are lucky enough to find just the right size when you need it. Maybe try a local tent manufacturer....there are quite a few and they might have alu tubes with the right OD and a thinner wall thickness than anything you'll find at the local hardware store.
Jul 12, 2019, 06:10 AM
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It seems like an uphill battle to get the stuff locally. Someone mentioned Chamberlin's might have, so will go look there first.

Did not know builders would plane it down for you, thats nice to know. So might get some meranti anyway later on.

The problem is getting carbon tape and the cost, as well as the glue. So while it would be very nice to build one and have some tips on how to do it nicely... it's just going to be expensive at the end which is what i'm trying to avoid.

But perhaps i should just stick to getting meranti ripped and using doublers in the middle if i can't find screen alu at chamberlins.

The Tent aluminium, golf and fish rods seems interesting too.

Both of these options are tapered so i guess would have to get 2 of the same type to work.

Anyway... guess it might be harder to get easily after all. Maybe i get lucky at Chamberlin's.

This is for future build not right away, but it's nice to know possibly where one could get something useful off the shelf.
Last edited by Axefly; Jul 13, 2019 at 01:48 AM.
Jul 12, 2019, 09:29 AM
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I'll wager that:

[Cost of your time spent trying to find marginally cheaper alternatives + cost of eventual solution] >> [cost of CF strips from Aerial Concepts + strip of meranti + time to go straight to the store and buy them]

Holy Grails generally don't exist.

Tapered spars at ~2m span are always going to require more work. Builders will rip a strip of meranti to a thinner strip, but they won't plane it to a taper for you. So either you must cut it yourself, or build a built-up balsa and CF spar (or similar technique), or get lucky with a golf shaft/ fishing rod. Or buy a simple wood planer and learn to use it...

If you want a light, strong, rigid tapered spar for a 2m span thin profile wing you are at some point going to realise there are no SPAD shortcuts. The best I've come up with is a CF tube for strength and a tapered strip of depron for shape.
Jul 13, 2019, 01:49 AM
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spelling and 1 or 2 words changed in my post above shees...
Jul 13, 2019, 01:54 AM
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Makes sense... yes weighing it up it does make sense.. i will let it stirr inside my head for a while...

so far i'm happy with my current 2 meter wing but something with a taper or thinner outer section in future might me nice for sure.

Could possibly a piece of meranti with fiber tape on it wrapped tightly in 600kg non stretch fish tow perhaps work ?

crude illustration per pic.
Jul 13, 2019, 04:27 AM
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Quote:
Originally Posted by Axefly
Could possibly a piece of meranti with fiber tape on it wrapped tightly in 600kg non stretch fish tow perhaps work ?
Sounds like a genuine 'SPAD' version of a proven technique....might be worth a try...

I can't see from your picture if you've put the tape on the top or the side of the spar - it must go on the top and bottom. Also, not sure what the fibers in the tape are made from - if glass fibers, they will still stretch too much. If CF or dyneema or similar, it could work.

The one caveat I have is that the main force between the wood spar and the tape is shear. I'm not sure how the tape's glue will handle this force - it is huge. Usually the CF tow is bonded on with epoxy. But the combination of the tape's glue plus tightly wrapped fish tow glued in place might be enough for moderate flight loads - you are not designing for winch launches. Another idea is to wrap at least two layers of the fiber tape around the spar lengthwise - likely to give a much better 'tape to tape' bond.

I've been finding many uses for Dyneema fishing line bought from MIAS angling stores. I wonder if several tensioned wraps of that lengthwise along the spar (instead of the tape), followed by close, tight wraps (~every 2-3mm) around the spar would work? Guess the trick would be to keep the ends of the dyneema from slipping.
Last edited by Extreme Sports; Jul 13, 2019 at 04:33 AM.
Jul 13, 2019, 12:03 PM
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How about that plastic /glass/fiber strap they use on boxes (as per pic)

Try as i might i cannot break those strips... i would not be surprised if you could lift the rear of a car with it... and that's just the white strip (the green one is almost 3 times thicker ! )

But i think your spanwise wrapping of the angling tow thus far might be the best.... interesting idea indeed... im definitely going to do a test on that.

Perhaps making a notch on each end of the spar and superglue it to harden the meranti where the tow could "bite" into so it does not slip, then as you say wrap the tow also around the spar every 3mm, to keep the spanwise tow on the top and bottom on its track.... Indeed i too like this tow... i bought mine at that angling shop next to builders in randburg, not sure if it's the same as where you got yours (those jewish) or is it muslim guys ?... but this stuff is nice.
Jul 16, 2019, 03:46 AM
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Quote:
Originally Posted by Axefly
How about that plastic /glass/fiber strap they use on boxes (as per pic)

Try as i might i cannot break those strips... i would not be surprised if you could lift the rear of a car with it... and that's just the white strip (the green one is almost 3 times thicker ! )
Interesting idea. My first reaction is that these strips are plastic based and therefore likely to have some stretch (no stretch is more critical than absolute strength in this application), but if you think about it, when they bind up pallets of bricks there is no discernible stretch and those straps stay tight over time as well. I've no idea what fibers they use to give the low stretch, but the idea has a definite 'genuine SPAD' feel to it.

So I'd say it's well worth a try. Just pay attention to getting a good bond to the wood. I'm thinking contact adhesive plus a decent overlap before winding on the dyneema.
Jul 18, 2019, 07:15 AM
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I have no idea where to get the green strip, would love to get a roll somewhere i picked it up in the street. But i guess the white strip should work like you say they strap bricks with it. And i can't see it being easy to stretch it...

Still trying to decide if i should try your fishing tow strap x 2 spanwise and then wrap around the spar...without using the plastic strip. Just the fish line.

If i use the plastic strip, do you mean wrap it around spanwise with overlap instead of one separate strip on top and one separate strip on the bottom ? (and then obviously tow around it.)
Jul 18, 2019, 10:08 AM
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Yes.

For a spar, the predominant force is a shear force between the stiff cap (the plastic in this case) and the web (the wood). Problem is most glues are not all that good at sticking to plastic, especially in shear. It only takes a minuscule amount of slippage before the cap is no longer effective. By doing the wrap you get the strength effect of the 90 degree bends at the ends, plus the overlap being held in place by the wrapped tow. Might well be an overkill, but since you are going to the effort of making a more complex spar, a little more effort for the overkill seems worthwhile. Put the overlap on the top side of the spar, since the bottom will usually have the higher tensile load.

I'd try the strap - it seems like an interesting SPAD idea! I suspect contact adhesive (e.g. Genkem) will be the best glue for wood to plastic.

Suggest you do a test. Take the wood spar and measure the deflection when a weight is hung off the end. Then do the "strap and wrap" and remeasure the deflection (for the exact same geometry). If you notice a significant reduction, then you know the strap is doing what it needs to do.
Last edited by Extreme Sports; Jul 18, 2019 at 10:14 AM.
Jul 20, 2019, 05:24 AM
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Ok not exactly sure how well it is going to pan out, since if i "strap" it i can't add "tension" i can just fold it so goes from top to bottom but not not add tension. (The tool they use specifically with these straps would've been nice, the bond is extremely strong, not sure how they do the bond) But it seems (i moght be wrong) that with the tool you can "tension" the strap then bond it at the same time... maybe not ? but if you look at packaging those strips seemed "tensioned" over the packaging)

But anyway, since i don't have the tool and whichever means they use to create such a uniqly strong bond with added tension, the Tow will have to be the alternative for bonding like we discussed. Even though there won't be tension added.

If i use the tow to "bind" it, should just adding two separate strips not be suffice ?

If not/or maybe i should not be asking this "question" as you already gave the answer that said:

"Might well be an overkill, but since you are going to the effort of making a more complex spar, a little more effort for the overkill seems worthwhile"

And I agree rather with that.

i have just done a contact adhesive only test, it seems like the bond is not so good... well... it can be torn off easily...... side to side movement of the strip seems very solid though

But i guess the glue will be basically just to keep the strip on track and the tow will do the magic of "bonding" it to the spar.

Will do a superglue test next and see, whichever will give the best bond i will use even though the bond might not be that good, like mentioned, if i do a full wrap top to bottom and use the tow, surely that should be good enough ? (with overlap)
Again wish i could add tension, but also this strip is non stretch... or heck i don't know what kind of tension would "stretch" it but it would have to be a freekin concrete slab hanging from it or something.

Now i guess all that is left to do is go bug a few storage facility workers at builders or game for the stuff..
Last edited by Axefly; Jul 20, 2019 at 05:30 AM.


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