[smuzz]

Be careful ironing film directly on to foam, if that's what you plan on doing - I experimented with covering a PZ Trojan wing with solarfilm once and it didn't go well because the foam cells ended up swollen under the film giving the characteristic "orange peel" surface.

[secant0give]

Thanks for the Warning smuzz,

The Trojan is made of EPO and I'm using EPS, I've never tried to Iron on film to an EPO model. I have had some trouble with EPP in the past.
EPS is no trouble to cover with Iron on film. You do have to be carefull that you don't get it too hot. So you cant bore the heat into it like you can with balsa but it can certainly be covered quite nicely with Iron-on film. I've done it quite a few times in the past. My Skyfunjet is covered with Red Iron on film.

[smuzz]

All good mate - I didn't think you'd be going into this with your eyes shut, but I thought I'd mention my past escapade anyway.

In spite of having had loads of foamys, I've got to admit that I still can't identify the type by inspection.

[secant0give]

Here's a Pic of the Templates for cutting the wing cores, just about finished.

   

[secant0give]

Finishing off the wing core templates:
At this stage they have been cut and then filed as closely as possible to the printed profile, which is applied using double sided tape. the templates are made from some peices of 3mm alluminium sheet.

It's important to file it nice and smooth as any grooved or deep scratches will snag on the hot wire and impact on the quality of the finished core. Since these won't be sheeted in balsa I'm wanting the best surface I can get to Iron film on to.

   

After the templates are filed flat right to the line, I then darken the working edge with black artline pen. This makes the next step much easier

   

Then I champher the edges to leave about 1mm or a little less of the pen mark on the working edge.

   

The idea of this last step is to reduce the contact area so the wire doesn't cool down near the edges and also with a narrow contact area the wire will follow the shape much better. The very last step is to put a fine finish on the working edge with 600 grit emery followed by a quick polish with a scotch brite pad. and drill some mounting holes to fasten it onto the jig

Here is a pic of the jig that will hold the templates it will be reconfigured four different ways, to cut the profile of the left, right, top and bottom of the two cores that make up the wing.

On it you can see the lines I've drawn to align the peices of foam that the cores will be cut from. Both the left wing core and the right wing core are drawn on top of each other. Don't worry if it looks confusing All will become clear with the fullness of time and some more pics of the next steps
   

[wingtipper]

love ya work mate . i have witnessed some of the technics you use making all types of planes , and its superb engineering work beyond the average persons skills . great to see though .

[gbanger]

Very nicely done Jason. Its good to see the fine detail and effort you've put into it. Looking forward to seeing the finished product.

[secant0give]

Thanks for compliments guys, but to be honest this is not anything more than very simple wood work and metal work. It's not beyond the ability of anyone who is willing to pick up some basic hand tools and have a go at it.

I'm hoping to show just one way of makeing a set of hot wire cut foam cores. I found this method the easiest way to make a nice wing section. If their done carefully wings can made be very light and strong as well quite accurate. Truthfully it's not a difficult technique at all if you follow the steps carefully. My philosophy in building anything is "do it by the simplest, fastest, most efficient method that will give the required quality". If I do this I often find there will be time and funding left over to make more !!

I'm taking photo's and posting this part of the build in the hope that someone will see it and say, "Oh that's how it's done, that doesn't look too hard, I'll give that a go." Or something "like That looks a little better than the way I've done it in the past." Or the ultimate would be "Jase I can show you an easier, even better, lighter, way to do this"

[secant0give]

I ordered some foam yesterday to make the cores, EPS foam comes in different grades. In the past I've mainly used M grade foam which has a density of 20 kg per cubic meter and it's reasonably tough stuff and feels harder than most EPS used for packaging. I thought that since this is an extra thin wing, It's thickness is only 7.3% of the chord length, I would like to try a denser, harder grade of EPS. After all with the wing being so thin it will be made of less foam and hence reasonably light. Because it's not going to be sheeted with balsa I want it to be a bit tougher and hopefully dent resistant.

So next week, enough foam to make several cores, with a density of 24, 28 and 32 Kg per cubic meter should arrive. EPS is not dear to buy. It cost $100 for enough foam to make 15 sets of these wings plus a foam box to transport my Big Yak (disassembled) and another foam box to transport my shoestring. All the pieces for the two boxes should arrive, already cut to size so all I have to do is glue them together. The other 3 pieces, enough to make 5 wings from each, of the 3 different grades are cut to length in one direction and will need to cut to the plan-form shape to make the individual cores.

The company is called Polyfoam their number is 02 9734 9555 and the man to speak with is Trevor.

But not wanting to wait 'till next week to start on the cores I managed to find a reasonably dense piece of EPS lying about the place and decided to make a set of cores for a bit of practice and to test out the jig. It's a bit of a daggy looking old piece of foam but I should be able to make something good out of it. This piece is quite hard tho’ about as hard as soft Balsa, quite a bit harder than the EPS a SkyFunjet is made from.

Here's a pic of the planform of the cores marked out on the piece of foam I found.

   

For cutting EPS foam there is nothing easier and cleaner than one of these. It’s simply a bow made from a piece of PVC pipe, strung with a piece of .7 mm mig welding wire. To heat the wire, simply connect it to your 12V power supply with an alligator clip and some cable. The wire is just normal mig wire used for welding steel. It has a very thin flashed copper coating on it, but this makes little or no difference to how it cuts foam.
And you want enough length between the clips so it draws about 1-3A at 12V and it should cut like a ripper.

   

[secant0give]

The core is aligned with the lines marked on the jig, and weighted down with something heavy, in this case a peice of wood and 3 peices of steel, could easily be a couple of bricks or lead weights or anything heavy

   

The jig is first configured to cut the bottom profile of the right wing. The hot wire is passed over the templates, which are marked with Artline pen at evenly spaced intervals this helps me to guide the wire evenly across the templates. Remember this is a tapered core so the wire has to travel further along the root cord than the tip cord. the marks help a lot with this

This is the resulting cut

   

The jig is then reconfigured to cut the top profile of the right wing and the core replaced and realigned.

   

The hot wire is then passed over the templates again and here is the final result

   

   

[smuzz]

Very interesting!

[secant0give]

Finishing the cores:
Freshly wire cut EPS (Expanded PolyStyrene) cores have this strange stringy finish that has to be removed, (see the photo below) If you try to sand it off then it seems to roll around under the sanding paper and slightly mar the surface underneath. I found the nicest way to remove this stringy stuff is to lightly rub it off with a “scotch bright” pad. If there are any ridges sticking up after hot wire cutting I wouldn’t throw away the core. It can be easily saved by carefully sanding back to match the correct profile using 60 or 120 grit emery glued to a block.

   

After the stringy stuff has been removed, the cores get a light sand with 600 or 800 grit emery, just enough to make it smooth to the touch, we don’t want to modify the profile.

   

Then it’s on to making out where the spars will go. I guess that these are not really spars, but more correctly sheer webs. Their main job is to be strong in compression and stop the foam from crushing and causing the loaded side of the glass skin to kink in. All the tensional stress on the unloaded side of the wing will be taken locally by the glass skin. The plywood spar in this wing is used quite differently to how a carbon rod or tube would be used. It will add little to the span wise stiffness of the wing it’s there mainly to hold the skins apart when the wing comes under load.

Another thing I’ve done is to end these plywood pieces at different points along the wing. In any structure we never want the strengthening to end abruptly all at one point. Because it does, then a localised weakness is created, a place where it will always fail. By gradually tapering off any reinforcement the load is spread across a larger area and the whole structure becomes inherently stronger. When the skin is applied it will extend out span-wise past the ply continuing this principle.

   

I’ve used the off cut to support it while I cut the slots and the slots go through the bottom off cut as well. The best thing to do is find a saw with the same kerf as the thickness as the ply. Two hacksaw blades is about the right thickness. It’s better to be slightly looser than too tight.

I should have mentioned earlier that the off cuts from the wire cutting are used to support the cores for all of the sanding, cutting and glueing work that will be done to them.

[smuzz]

Jason - thanks for the kind offer to cut me some cores, which you mentioned today when I was considering what to do with the power-train from Junior's "folding Spitfire" ... but before you do it, I've decided to take another tac and scratch build a racing twin-engined job!
With the innards from the Spitfire and the spongey Bf-109 I'm going to build a twin FunFighter ... would appreciate your input as I firm up some ideas though.
Cheers
Steve

[secant0give]

That sounds like an interesting project, I'd be half inclined to scratch build a dornier arrow if I was going to do a twin but it would have to be taking off on wheels, don't fancy hand launching with that second prop spinning on the rear of the fuse!!

The main advantage of having the engines in line is the reduction in drag more power with less projected area.

[smuzz]

Yes, you're right mate - and of course with a conventional twin you've got a greater roll inertia too, so the roll-rate is reduced. These were things that didn't hamper the Do-335 which was not a very handsome machine, but a potentially devastating bomber destroyer.

I don't want to detract from your build thread, so I'll do a little more digging about - I have some plans for a deHavilland Hornet and an Me-410 and to keep things simple it might be one of these two. I haven't built from plans since 2009, so I'm overdue to build a model properly for a change

[samste]

Very Cool Jason, You Got me digging out my very old roll of cutting wire that i had in the garage Ready to make my own hot wire cutter to have a play with.
My brother and I use to cut foam when we were kids with a home made wire cutter and expanda foam in the tins. We made moulds to pour fiberglass and matting cut up in and made a kayak.

Steve

[secant0give]

Excellent, Steve. If your after some EPS foam, Give me a yell, I've got bit spare.

Here’s a pick of the strips of ply that will be inserted into the wing cores.
   

Here they are trimmed to length and sanded to thicknes and fited flush with the core.

   

Next pieces of thin plastic sheet are cut to the same plan-form dimensions as the wing, but with a little extra extending beyond the trailing edge. This last bit will form a nice sharp T.E. and the excess will be trimmed off later.

   


The pieces of plastic sheet are held with tape to the upper and lower offcuts of the foam core, this will form the mould to which I will lay up the partial skin of Uni-glass and brown paper.

   

   

Here's a Pic of the whole lot assembled for a trial fit

   

The thin plastic sheets will not be part of the wing they are just to give me something to layup the glass and resin on. After the layup has cured the thin plastic sheet will peel off to reveel the almost finished wing.

[secant0give]

This is unidirectional glass mat. It’s very handy stuff if you want to build great tensional strength into a layup in one direction. It isn’t woven together just joined every inch or so with a thread glued to it. As far as glass goes it’s very easy to wet out and works pretty well for this kind of application.

   

Here’s the pieces I of uni glass allready cut to go span-wise along the top and the bottom of the wing. Note that they are tapered to spread the load and cut so the tapered edges are not directly above one another when layed up.

   

Here are the strips of light glass that will reinforce the trailing edge of the wing and toughen up the ailerons as well.
I find the easiest way to to cut glass cloth is with a straight edge and a sharp exacto blade. Scissors can easily just make a mess.
The light glass cloth is crows foot weave which is quite nice to work with as well.

   

Brown paper strips are cut and tapped with very small pieces of masking tape to the edges of the plastic to hold them in place. These pieces of masking tape will become part of the layup so I keep them as small as possible.

   


Now the glass is layed on top of the paper, epoxy resin is mixed and applied to the glass. It will completely impregnate the brown paper and the paper will give the wing a pin hole free finish and cover the texture of the glass cloth. I should mention that the paper is put down shiny side out. Also a small amount of resin is brushed onto the edges of the plywood inserts in the core and a little resin is lightly brushed on to the area of the core where the layup will be bonded, a little extra needs to go on the core along the trailing edge as well.

   


And here it is all layed up and weighted down. Notice the extra material on the trailing edge and the decent amount of weight needed to press the whole sandwich together

   

It will be interesting to see just how it turns out tomorrow !!!

   

[smuzz]

You've got more patience than I have mate!

By the way, I trust you're familiar with Martin Hepperle's work on model aircraft aerodynamics?
http://www.mh-aerotools.de/airfoils/index.htm
There's hours of distraction there when we should be doing work
I'm thinking of using the MH 33 aerofoil for my twin ... although I think if I can get within a bull's roar of the actual shape, I'll be happy!

[gbanger]

There is some vey very interesting stuff in that there link matey, thanks Smuzz!

[secant0give]

You've got more patience than I have mate!

By the way, I trust you're familiar with Martin Hepperle's work on model aircraft aerodynamics?
http://www.mh-aerotools.de/airfoils/index.htm
There's hours of distraction there when we should be doing work
I'm thinking of using the MH 33 aerofoil for my twin ... although I think if I can get within a bull's roar of the actual shape, I'll be happy!

Yes I've read most of the info on Martins site. There is heaps of interesting stuff there. He would have published a lot more, I think in the form of Papers, It would be interesting to have a search through the journals and see.

The MH33 section looks like a beautie that's why I chose it for the Dynamite's wing. I've allready got the templates on cad for it so If you want an accurate printout of the shape just let me know, it's the work of moments to scale it to any size you like.

One thing you will have to be carefull with the MH33 is how you do the construction of the wing, it needs to be very strong because it's so thin at only 7.3%. This will be even more important if the model is a heavy one.

[smuzz]

One thing you will have to be carefull with the MH33 is how you do the construction of the wing it needs to be very strong because it's so thin at only 7.3%. This will be even more important if the model is a heavy one.

Yes - the strength vs weight problem. Well, one thing I bloody-well won't be doing is butt-joining two pieces of 3mm carbon-fibre tube at the fuselage I've yet to have a whinge to HK about that and I'm going to have a rant on RCGroups as well!

[secant0give]

Well Here it is,

   

One strong, light, accurate and thin wing section, ready to have the ailerons cut out and hinged, the tips shaped and some Iron film applied.

At this stage the wing's all up weight is 120 gramms which is about 4oz. I used about 60g of resin, a little of which stayed on the brush but most of it went on the wing. So about 1/2 the weight is epoxy. The rest is foam ply and glass.
I'm sure I could have reduced the amount of resin quite a bit if I vacuum bagged it, but I wanted to do it without Vac bagging just to show that a decent result could be obtained.

It certainly feels very stiff !!

   

[secant0give]

The wing's trimmed up a bit with the tips going on, just made form balsa, cut and sanded to shape.


   

[wingtipper]

nice work . looking forward to seeing how this thing will fly .

[secant0give]

I'm predicting it will fly fast !! and hoping it will be reasonable to control...........time will tell old mate, time will tell

[secant0give]

I havn't done much work on this for a while, mainly because I've had a couple of other distractions but I did manage to a few sets of cores from the foam I ordered a while back.

That's the good thing about having a decent jig and templates set up to cut wing cores, once the jig has been made it's a quick and easy matter to produce nice fair foam wings.

I have a couple of sets spare and a set to fit up to, a very nice light glass, lowing "Bandit" fuse I was given. It should go well with this wing!!

The 28Kg/cu meter foam is quite hard and cuts very well with the hot wire, all be it quite slowly.

   

[secant0give]

Mounting the wing on the fuse:
The original design called for a single 4mm dowel at the leading edge which went through the Leading edge timbers of the built up wing. I decided to change things; I had to because I’m using a foam core wing with no timber leading edges. The idea of a single 4mm dowel glued into foam didn’t appeal to me very much, this Dynamite will be going a lot faster than the original design. so I decided use a piece of ply with two 3/16 dowels. This should be strong enough.

   

The wing needs to be a pretty nice fit because it must be removed to change batteries. It needs to go back in precisely the same place to avoid altering the trim of the model between flights and I expect it will be removed and replaced many times.

   

This is what I came up with, Once the plate with the dowels is nicely trial fitted to the fuse It is then put in place with cling film underneath. The beauty of cling film is that nothing sticks to it. You’ve gotta love that !! I use it all the time.

A nice light adhesive bog is prepared using west systems epoxy and microlite filler, a little micro fiber blend filler is also added for extra strength. The resulting bog is very light, easy to sand or carve and also quite strong. When preparing bog like this I keep a little mixed resin with no filler over to prime the surfaces with and ensure the best possible bond.

   

   

Then it's just a matter of priming the area to be bonded by painting some reisn that I kept aside from making the bog and putting the wing on nice and straight. a small fillet of bog fairs in the top of the wing as well.
   

[wingtipper]

getting there mate. what an awesome build .

[secant0give]

here's a pic of the wing retaining dowels bonded in place


The ailerons have been cut out of the wing despite being quite thin they are remarkably rigid. They will be attached with tape hinges after being covered.

   

The wings are so thin, that the 9 gram servos that drive the ailerons, stick out of the underside of the wing by about 5mm. So I’ve moulded some fairings for the servos as well as the linkages