Jart Build - On Hold Indefinately

[jase]

Ok. so i've got full scale printed plans for the jart. I got 2 copies made so i can cut one and have a backup for a good flat template...

Today i picked up some Styrofoam.. ( i would have got blue or pink foam, but it was a bit more expensive and the fact that i'm fibreglassing the whole plane it shouldn't make any difference with the end result.

Next i have to build a wire cutter.. i'm going to go off the worlds cheapest and easiest wire cutter thread on rcgroups so i should be able to make some nice foam cores... the few places i have sent e-mails through for a quote haven't gotten back to me, but i think i'll save a lot more money doing it myself.

So at this stage, until everything is cut out and ready to fibreglass, i'll be shaping and getting this plane ready.

A few things i have in mind for this plane is to make it travel safe. I want both wings removable but also make them solid to mount.

[Pij]

This sounds like an interesting project, Jase. I'll be watching this thread with interest, as I've never tackled anything like it myself. Don't throw out the templates for those wings, will you?

[jase]

lol. nope... oh and some more good news in regards to the hot wire cutter.. i was doing more research online... through google and rcgroups and a lot of people just use a standard car battery charger 12v with 2 - 3amp output.... just so happens my charger is 12v and 2.5A output. so i won't have to do any modifications what so ever and as i already had the car charger it's only cost me $4.95 for the wire (assuming the wire works)

[thevon]

Jase, the type of foam does have a big effect. (sorry to be a pain!). If the foam is harder, it makes the wing much stronger in resisting bending, particularly upon impact. When the wing is bent hard the inside (concave) side pushes into the foam and if it's soft, it will compress the foam enough to allow the fibreglass to crack. This is a well known occurrence called a compression fracture.

The 160PSI blue foam that the guys have been using for vac bagging is heavier than standard blue foams and more fussy to cut, but has enormous advantages for strength, stiffness and durability. Quite seriously if you can get some pink or blue foam for a few more bucks I'd recommend going that way. It's also much easier to cut and sand etc. compared to PS.

[sean]

Yeah as Andrew says the foam does matter for strength but lots of bagged wings have been built successfully with white foam. The heavier/denser foams allow a lighter layup to be used for the same strength. But when it comes to the fuse, blue foam is just so much better to carve/sand.

[jase]

fair enough. the problems i encountered were mostly price and the fact that i am on a rather strict budget.. the 50mm PS foam was only $50 for 2.4m x 50mm and they only sell it in sheets.... the blue foam was $115 for the same quantity, so it was considerably more expensive. i would have gone that way if they sold 1/2 sheets.

however, seeing that i am fibreglassing the bird i figured i was really only using the foam as a "template" anyway, so the real strength will be in the glassing, not the foam. if i was concerned about the foam i would have bought EPP from wowings.

appreciate your thoughts though... it's also my first build. if it works, then i can go better next time.

[stevew]

Jase,
Just to interject here for a second ,by fibreglassing you mean using epoxy laminating resin dont you ? polyester resin and EPS = big melted lump of white goo . BTW I used and still do HD EPS in all my Jarts using 8.0mm carbon tube spar and 1 layer 8oz carbon cloth with a layer of glass tissue which is solid as a rock .
but if your bagging the wing you only need a max of 4 inches of mercury(bout 13.5 kilopascals) for EPS otherwise you'll end up with a pizza instead of a wing.
SteveW

[stevew]

P.S. a 4amp charger is better for supplying the battery with the power to cut.
I have a few bows but the most used is the 30 inch bow (which is actually 36 inches long in the cut),but perfect to cut 30 inch cores. I use 2 x12 volt batteries in parallel and a 4 amp charger , the wire I use is .020" stainless steel trace single strand not that horrible nichrome stuff. It will cut EPS ,blue ,pink and EPP foams with ease .Ive use this method since the early 80's and has always delivered a good set of cores when ever I need them. On the other hand the finish on your templates will directly relate to the finish of you cores surface ,so make them as smooth as possible I use .030" ally plate for my templates you can get a polished edge on them with a bit of scotchbrite . Well I guess this is the longest P.S. ever written lol
Cheers
SteveW

[jase]

yeah mate, i'll be using epoxy.

the wire trace i bought yesterday was 20lb 7strand wire. it's quite soft, but similarly to ni-chrome you can't kink it and it stays straight. unfortunately all the wire they had at the tackle shop didn't say how thick it was. but now atleast i've got somewhere to start in terms of thickness.

as for wing core templates... the local sign store will be getting me some off cuts of this new product which is a polycarbonate/aluminium sandwitch.

[jase]

well, this afternoon i got around to cutting out the main wing core templates. the wing tip templates came out really well, but i think i'm going to have to redo the large templates.

[jase]

well, they say we learn from our mistakes, so i guess the following was a successful failure..

i constructed a wire cutter which now that i've done it is incredably simple to make..
motorcycle battery, electrical wire, 7strand wire trace..

i have the battery on a tackle box under the table when i'm cutting. but this is the simple setup i used. unfortunately my battery charger was unable to supply a charge through the wire.


wing template cut out. i really need to have guides set when cutting straight lines, but came out ok.


first 1/2 of the wing core cut out... didn't come out too badly, however the trailing wire at the wing tip lagged and looked a little funny. the wire also stopped a couple of times on the template (3ply template) which caused some buldging, but apart from that, i was happy with my first real cut.


ok. this bit was a bit of a disaster, for what ever reason (unfortunately i didn't pick up on) the wire caught on the wing tip template through the whole cut. by the time i reached the end of the main template it was too late. from the previous cut i pushed the wing closer to the apex of the wire so both ends should come out evening and on time this time, but due to the wire catching, this was the result.

overall i was preety happy...... this is what i learnt.
need more heat in wire... i'll drop down from 20lb wire to 15lb.
the battery was only 75% charged. i'm sure this had a big affect in terms of heat put into the wire. if this fails. i'll use my car battery
templates need to be seriously looked at. smoothed out even finer with no stray edges or flaking wood (i'm waiting on a poly/alum board from the signwriter). apart from that, i'm sure they will cut a nice wing.
and though i can only do this outside because of room restrictions, the cold breeze probably didn't do much to help keeping temp in the wire... still need more heat

[Garth]

make your ends out of alloy. its all good mate you will get there. maybe go a little slower, take your time

[jase]

yeah. i was going at a nice pace on the first cut. i could also have had too much angle in the wire leading up onto the wing tip template causing the grab. as you can see i had that lattus there to get some more height into the bend in the wire. but i think i'll take that out.

[sean]

Getting clean foam cuts is a balance of so many different factors! The first thing you need though is perfectly smooth templates. Sand them with progressively finer grades of wet/dry (I go down to 1000) then finally rub the surface with a lead pencil. The wire travels pretty slowly so any tiny knick on the surface of the template will cause the wire to stop momentarily, burning a little valley in the core. I've used circuit board material bought from Dick Smith for my templates to date, with pretty good results, but my next ones will be made from formica (laminex). The circuit board material can be sanded really smooth, but it has tiny fibres that I think can cause the wire to skip rather than slide along the template.

[jase]

Ok. well some more progress.

I went back down to the tackle store today and the owner was there and he gave me some single strand wire to use instead of the 7strand. connected it up and tested it out on a scrap piece and it was 100 times better. Used the motorbike battery as before and it had plenty of juice this time.. just enough.. perfect.

I also found some 800grit wet n dry sandpaper so i got stuck into the wing templates smoothing them off and taking away the rough edges. It's 100x better now, but i'm still waiting on this stuff from the sign writers to make some really good templates.

i didn't raise the wing cores up like i did in my previous cut so there was less angle and less friction over the wing core template.

So this afternoon, i managed to cut 2 good wings out. though not perfect, i was very happy with the results.

enjoy the pics.

few little streaks, but teh photo make it look worse than it is. i'll sand and spakfil the cores later to a smooth finish.

these are my wing cores. i used some spray glue (3M77) to glue these to the 3ply. and extremely slowly and cautiously used a jigsaw to cut out the templates as shown.

wing core and bed looking side on.

frontside of wings put together. the matched up perfectly.

and the back.

with the equiptment i used basically maxed out the cutting wire length. With better wing core templates would definately cut better wings. However, i'm glad i've got 2 usable wing cores on only my 2nd attempt cutting.