DeHavilland DH90 Dragonfly

[Steve Rickett 2333]

Hi John,

Thats very interesting, there can be so many ways to do things. Everyone has there own methods for sure! I didn't have any trouble with the fabric cement, in fact I really like it.

I have always had trouble with Solartex bubbling over sheeted surfaces...the fabric cement eliminates that completely. How does the Balsaloc compare?

Steve

[Tony morris]

No problem Steve i'm usually up the Hangar once a week

[John Greenfield]

Steve

I only put the balsaloc around the edges so the dope adheres the diatex to the structure so no bubbling issues.
On the odd occasion that I have used solartex I coat all of the sheeted surfaces with balsaloc and than apply the solartex with an iron on LOW heat working out from the middle of the panel exactly as per the instructions that come with Solartex. No problem with bubbling.

John

[Steve Rickett 2333]

Ok, thanks John

[Steve Rickett 2333]

With the (slightly!) warmer weather we've been having recently I decided to get the moulds laid up for the wheel spats, tail cone and nose cone.

The plugs have been finished a few months now to give them time to properly harden. I'm not sure what I did wrong last time I made moulds but they stuck like glue!!!

I gave the plugs 5 coats of Mirror Glaze wax before I made some parting boards from conti-board, not sure if this is a universal name but it's basically chip board with a plastic surface each side....for making cheap shelves and stuff. I bonded this to the plug with some hot glue which did a good job.

The gap between the parting board was simply filled with some plasticine to get a perfect fit before I have the whole lot another 2 coats of wax.

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You're probably wondering why I made the tail cone integral with the wheel spats! Basically, I didn't have enough conti-board and being the cheap skate that I am I made do!!

An area I had never had much luck with was PVA release agent! Usually it just beads up and looks awful....I had tried wiping it on with a cloth, better but you could still see the wipe marks in the finished moulding. I have it a lot of research and bought some fresh PVA release agent and tried again. Using an iwata airbrush turned upto 80psi, thinned PVA with 30% water and spraying ever so lightly I hit the eureka moment! Too little spray and the PVA simply dusted onto the surface and left a mottled effect, too much and it beaded up again, but at a fraction more than a light dusting it settled down a treat and left no discernible layer!

The gel coat was mixed up using West Systems Epoxy, 10% colloidal Silica to thicken it and also 10% graphite powder to provide the dark colour and also provide a low stick surface for the final mouldings.
This was left to go 'green' a state where the epoxy has lost its tackiness but you could still indent it with your thumbnail. I mixed up some West epoxy and milled fibers to a thick paste and went around the sharp concave corners that you know glass cloth won't conform to!

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Then 6 layers of 165 gram cloth was laid up with west epoxy and left to harden over night. I tried using a powder bound chopped strand Matt for building up some thickness to the moulds but it needed a huge amount of working to get the resin soaked in.....not to mention a huge amount of resin. I gave up a just used the cloth I had to hand!

After removing the parting board and laying up the second half.....the moment of truth arrived. Will I get this great mass of epoxy apart?

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I'm not sure what did the trick, leaving the plug to properly harden, the PVA or the graphite in the gel coat, maybe a combination of them all, but the two halves released easier than I have ever known moulds release!!! I don't know what I did, but something went right!

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[Stuart Solomon]

Steve, if you wet the cloth out with the resin before it goes on, you will find it easier to go into corners. The f/glass lads next to me do exactly that and they do some wierd shapes. :-)Solly

[Stuart Solomon]

P.S. where's my logo going ;-)

[Peter Siggins]

What Solly - SS !!

[Stuart Solomon]

Careful Siggy, might unglue that fat wallet of yours from your pocket ;-)

[Steve Rickett 2333]

I'll give that a go next time....though I bet they are using better quality fiberglass cloth than me!

Err...your Logo was going to go on the bottom....somewhere!

[Steve Rickett 2333]

The tail cone popped out the mould earlier! 2 layers of 160g cloth will be plenty and keep it nice and light.

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I also made the first undercarriage spat. This one will not be used ultimately but serves to test the strength of my layup (2 layers of 160g gram cloth and 2 layers of 200g cloth) and also as it is moulded in one piece, it makes it easier to check the fit and make the mounting brackets before I make the final parts from multiple lay ups.

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[Steve Rickett 2333]

The nose mould has been finished too and a final moulds taken from it (again 2 layers of 160g and 2 of 200g cloth).

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My little helper popped in too....and found a new use for the mould!!!

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[Steve Rickett 2333]

I have also begun the layup of the actual spats/cowls......

Starting with the opening cowling hatch.....before anyone points it out, I've cheated slightly with the size of this and made it a bit smaller. The Dragonfly actually has an unusual arrangement whereby the split of the cowl is right down the front just behind the propellor. I think that I'll lose too much strength this way and also have a job holding t closed...so I'm going to copy the Tiger Moth style and not tell anyone!!!

[Steve Rickett 2333]

The cowl hatch is again 2 layers of 160g and 2 of 200g but it also has a layer of thin acetate sheet to create a smooth internal finish. This gives me something that I can then mould a lip onto......

Once dry I trimmed to shape, covered the edges in thin sellotape and then gave the sellotape a couple of coats of wax before placing the hatch back in the mould. High power magnets and clamps help to keep it seated firmly.

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The next stage was then laid up over this hatch....hopefully giving me a perfect fitting lip for the hatch to seat against.

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The other side of the spats has had the same treatment but no opening hatch this time. You'll notice that the spat will be able to be split in half vertically to fit around the undercarriage.

[Peter Siggins]

Stunning stuff Steve - an education in moulding

Pete

P.s. Should market the hat !

[david clarke]

Yes, I had the same problem on the Libellula which was covered the same as I described above, but I used Sig Koverall instead of the Diatex. It must have taken 6 plus coats of Nitrate dope to fill the weave (and even then there was still to odd patch of pin holes!)

I'm hoping that the Diatex will only take 3-4 coats.

I thin the Fabirc cement with standard cellulose thinners, seems to be ok. The Libellula is now 6 years old and the covering is still in great condition and no sign of bubbling in the heat or cracked/crazed dope. That tin is the smallest that Skycraft do, and it is certainly way too big for our uses. I think it is 1 litre, it does seem to store fairly well though.

The final coats of Dope will be Nitrate Dope (as most paints will stick to it!) thinned about 50% with the same cellulose thinners. The first coat brushed to get it into the weave, but the subsequent coats can be sprayed...I'll see how I get on with that, I've always just brushed it in the past.

Hi Steve,
Do you use a shrinking dope or none shrinking dope ? got a Half scale GeeBee to do so want to get it right.

Many thanks,Dave.

[Stuart Solomon]

Dave, use a non shrinking dope on Diatex as the cloth will shrink with the heat gun. Solly

[david clarke]

Thanks for that Solly,why i have your a attension what sizes and lengths of Cyparis do you have ?.

Dave.

[Stuart Solomon]

From 6x3 up to 25x12 all 6fts :-) Will be at Cosford

[Steve Rickett 2333]

Thanks Stuart. Yes, I always use no shrinking dope, despite its name, it will still shrink a bit....just hopefully not enough to warp the structure....the covering should still go nice and tight though.

I would suggest not using a heat gun though. All heat shrink fabrics have a maximum temperature (125 degrees for Diatex) if you exceed this, youll damage the fabric and run the risk of it slackening off in the sunshine.