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#1 Joe Yap Nov 27, 2006 10:38 PM

Jarts ( AGAIN?!) from Singapore
 
I've never heard about the Jart until about few months ago, when one of my close friend brought it up, while we were discussion what new planes shall we get for the upcoming season. I was planning for another scale design but I took a glance at the Jartworld website and found this interesting and unique design. A chat over the email was done and Mr Reed kindly and promptly sent me a copy of the plan, which even mention "This design sheet is created exclusively for Joe Yap"! How nice with this personal touch! Thanks again, Reed.

So, after clearing my work bench, my work on the Jart has started. I'll be building 3 models, which are meant for myself and 2 other very close friends. The plan does not give full details on the construction, which is good enough for modellors who are experienced enough to build and fly one of these very unique birds.

I deviated from the original construction design to customise to my limitations and equirements. First of all, I don't want to spend too much on carbon materials and I do not have vacuum bagging facilities. One of my friend do not have personal transport which means an one-piece design is out of the question. The models will be flown at slope with somewhat unforgiving terrains and survivability, repairability and performance must be considered. I do not want to go for EPP as the finishing will be greatly compromised.

I decided to use the same techniques I have tried on my Mini Duo Discus. The wings will be pink foam core sheeted with 1/64" plywood. I usually use balsa skin but I want to experiment with very thin plywood sheets, which are large enough to cover the wings without the need to join. This cost more but saves time. This kind of wings are proven to be very robust, and even if you managed to break it, it can be easily repaired, or even rebuilt. The wings will be 2 pieces with carbon or steel joiners going through the fuselage, which most scale gliders use.

The fuselage will be shape from pink foam, hollowed and covered with a few layers of glasscloth. Instead of dissolving the foam away, I'll leave it inside as glassed foam structure can achieve very good stiffness, with just a thin glass skin. Somebody has even suggested than I'll glass the inner foam walls to achieve a true sandwich structure, which is what I'm intending to try out.

#2 Joe Yap Nov 27, 2006 10:45 PM

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I first got the fuselage plan and side profile templates cut from cardboard. I used 2 sheets of 2" pink foam and tack them side by side together with a coat of 3M77. This is the only adhesive I've tried that allows hotwire cutting through the joints. I hotwire cut the foam block edges square before pinning the plan profile templates on both top and bottom. After cutting the 2 side surfaces, I tack the foam beds back with 3M77 to form back the foam block. This will help to align the side profile templates for hotwire cutting the top and lower surfaces.

Another template was fabricated and pinned on the foam to cut the canopy out, before a hole was bored through the centre on the fuselage for the wing joiner tube. After that, all the waste foam beds were removed and the fuselages were carved and sanded to shape.

The fins were also hotwire cut using conventional hotwire cutting method.

Sorry, I didn't take much pictures up to this point.

#3 Joe Yap Nov 27, 2006 11:11 PM

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At this point, I will need to get the fins ready as I need them to carve the fuselage to fin joints to the correct contour. I do not have vacuum bagging facility but I cheated on this one. I laid glasscloths with epoxy resin over a sheet of waxed PVC, peeled them off when the resin has cured and epoxy them over the fin surfaces. The result is very similar to the vacuum bagged ones but I have to touch up on the LE and the TE.

#4 Joe Yap Nov 27, 2006 11:16 PM

6 Attachment(s)
After using the templates to mark out the sectional parting lines, I dissect each fuselage half into small sections with a sharp penknife. Templates were pinned and the inner cavity was hotwire cut out accordingly. Once done, the sections were epoxied back together one by one.

#5 Adrian B. Nov 28, 2006 01:28 AM

Go dude go. I love the approach.

Have fun, its deffinetly worth the ride..

#6 samotage Nov 28, 2006 02:01 AM

Sweet, I really enjoy Jart threads :D

I wonder if it is true, some say 4 lost foam fuzzes can be made for the time of one mold. :D

Go Joe!

Sam,
who has subscribed.

#7 Hangtight Nov 28, 2006 04:17 AM

I like the idea of using the pre cured laminate as a covering. Was flicking through a model magazine the other day and saw a large scale turbine powered Hawker Hunter that used a built up ply wing with something called Proskin as a stressed skin bonded on with PU (Gorilla) glue. I recognised the Proskin from my days building high powered medical imaging equipement. It's an epoxy/ glass laminate known as FR4 used as an electrical insulation and is available in big sheets from 0.1mm thicknesses up. Mechanical properties are excelllent as there is a minimum of resin and the manufacturing gives a very consistant thickness and smooth gloosy finish. Something else to add to the 'must try that' list!
Joe, can you explain how you did the LE and TE of your fins using the pre cured epoxy glass?

#8 Joe Yap Nov 28, 2006 05:21 AM

Hi Samotage,

I'm not too sure, but seems to be true. Lost foam and my sandwich construction technique should be faster to produce if you don't mind somewhat poorer finishing. The reason that put me off for making a female mould is that I do not have the expertise and material to do so. I tried but with poor results. I only took me 2 days to produce a SkaT pylon racer fuselage using the lost foam method anyway.

My mini Duo Discus has only 2 layers of medium and 1 layer of lightweight cloth but proven sufficiently strong for most abuse. I had smashed it into concrete walls and boulders, and the damage is isolated to the area which can be easily repair on the field. The foam seems to absorb quite a bit of impact and the fibres will tend to hold up the structure to a certain extend.

Hi Hangtight,

Actually, the pre-formed epoxy glass sheets do not meet well at the LE and the TE which produce a small gap between them and the foam core. What I did is to mix and brush some epoxy resin into the cavities to fill them up and sand them down once cured. Having said that, the total gap at the LE is only 1/8" wide and the TE is less than 1/16". Very little resin is used.

#9 samotage Nov 28, 2006 05:22 AM

Many of the full sized "proper" gliders with foam cored wings use this precured skin approach. It's then filled and sanded, and can avoid vacum based depressions forming.

#10 Joe Yap Nov 28, 2006 05:25 AM

And best of all, I do not need to turn my study room to look like a full-blown factory. My wife will nag at me for that. That is also one of my constraint on choosing suitable building techniques. We stay in flatted apartments with limited space.

#11 Joe Yap Nov 28, 2006 05:33 AM

3 Attachment(s)
By now, I have epoxied the sections back in place and I have 2 halves. This surely resembles the 'Airfix' plastic models at this point. I filled the imperfections on the inner walls with model magic as I'll be laying glasscloth on them later on. Right now, I'll put this fuselage on hold and move on to the other 2.

I'll be on a short 5 days vacation tomorrow and will take a well deserved break. I've been quite stressed lately due to the electric pylon race held on last Sunday. I have scratchbuilt a brand new SkaT to accomodate a brushless outfit for the past few weeks, and I'm very happy with the results. I came in first by the way, with this missile.

#12 Reed Nov 28, 2006 08:54 AM

Two beautiful models, Joe (wife/pylon racer). Soon you'll have three!

I like the idea of hollowing the foam before you glass it. How will you hollow back to the empanage for the elevator linkage?

#13 Alfred Nov 28, 2006 09:13 AM

Hey Joe - Long time no see.

Great to see that there will finally be a Jart in Singapore!

Alfred

#14 steve wenban Nov 28, 2006 12:30 PM

Oh god not another Jart thread :rolleyes:
Only joking welcome to the club Joe nice approach :cool: be warned however the Jart is no ordinary sloper its more like an addiction :eek:
my first born Jart is currently under major over haul due to a mid air ,but I'll have a second one ready to fly as well around the time I finished the repair job ,your going to love it :D
SteveW who thinks back to Hendrix singing Hey Joe where you going with that jart in you hand ;)

#15 ajroahkni Nov 28, 2006 12:49 PM

Joe Yap

Your Little Duo Discus thread is one of my all time faves. The hollowed out fuse is a great idea and your craftsmanship is amazing.

I look forward to seeing the process on this jart and the finished product!

John


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