A4 Skyhawk - Building tips and Plan queries

[Phil Cooke]

Hi Shane,

The canopies can be bought separately from Traplet both in the UK and 'in region' from Traplet Australia.

**LINK**

The canopy (bought separate) has a product code SKU: CA3775CY and retails at £6.99

Ordering direct from Traplet (Aus) would get you the canopy much sooner than getting one to your friend whilst in the UK, and the logistics are simpler too.

[Shane Talbot]

Thanks for that info Phil, my mate Craig recons he can get one and bring it back quicker than the Aussi store can get one. I'll let you know when I get it, meanwhile I will make a balsa one.

[Mark Kettle 1]

Shane - via a repeated dispatch error on a Jet Provost plan order - it happens that I'm the owner 3 extra canopies.

So get in touch -via the message system- if it helps.

[Steve Houghton]

In fear of revealing my poor workmanship, I would like to ask if anyone has had problems with the laser cut formers being approx 1 to 1.5 mm undersize?

Having glued all the triangular sections and doublers to the fuselage sides, I was about to cut F1 down by 3mm in height to suit my modified nose profile for the A-4C, and found that it fitted perfectly without any trimming. Investigating further, I found that all of my formers have significant gaps at the edges against the triangular sections, and I think they are also too short.

My triangular sections from SLEC are, admittedly, slightly undersize (perhaps metric rather than imperial?), but I don't think that's the full story. I also don't think I've cut the fuselage sides too large, but I shall make some more measurements tonight to confirm.

Plan Notes:

I have noticed that the bottom of F7 on the plan is not drawn to the surface of the fuselage bottom sheeting, which can be seen in Phil's posting of 8/6/16 above.

I would also like to see an extra arrow for the extent of the fuselage side at the small vertical section in front of the wing, showing that it overlaps the 1/16" ply facing on F3.

Apart from that, I'm having fun with the build - just need more spare time!

 

 

Edited By Steve Houghton on 30/06/2016 12:51:49

Edited By Steve Houghton on 30/06/2016 12:52:29

Edited By Steve Houghton on 30/06/2016 12:53:21

[Phil Cooke]

Hi Steve,

I think we have all experienced this, the parts aren't wrong, but the gaps are a function I think of a stack of tolerance and angle effects working against us...

In my build, I think I've cut the fus sides to the outside of the line, my triangular section (Balsa Cabin stock) is a little small and the angle at which the triangular section interfaces with the square (laser cut) formers means you get a 'tight' side and a 'sloppy' side as the former (typically vertical) is not perpendicular to the triangular section (which is either angled vertically or tapering in at the nose or tail)

Here's my F7. Aligning it tight to the top triangular section (which IS almost perpendicular to the former) I had to add almost 1/16" in balsa at the base so that there was no gap at the front. This material had to be chamfered down towards the rear face so it fitted well on the rear side of the former, as the fus tapers and the triangular stock is not perpendicular.

You might see this better here?

So the Traplet parts are cut exactly to my drawing, I have no concerns there. But neither my drawing or the 2D laser cutting process allow for the tapering of the triangular section in a realworld 3D build. In my plan sectional views (F7 example below) the triangular section should really be slightly distorted (vertically) to accommodate the angle at which it would be really sectioned. But being a hand drawn plan and not a 2D sectioned CAD drawing, its not that clever.

The fix is simple enough and the phenomena understood, but apologies Steve if this has caused frustration - no one likes a gap in a joint I appreciate that.

[Steve Houghton]

Hi Phil,

Many thanks for your lightning response. Since I posted, I found that you had already posted the photos above in your own blog, so my apologies for not noticing, but thanks for the repeat.

I was going to use the same solution as you, so your confirmation has helped. I'll get my extensions on tonight!

[Andy Meade]

Here is a quick view from 3D CAD that I hope shows the problem as a I understand it.

The yellow plate is the fus side, the blue part is a cutaway of a former. The triangular section is set as transparent.

The former is the right dimension at the front, but tapers away from the triangle section at the back - thus Phil's additive balsa which was shaped to fill this gap.

[Phil Cooke]

nice. Must learn to do my plans in CAD. I MUST!

[Steve Houghton]

Thanks for the drawing, Andy.

As Phil said, I think it's a combination of factors, as the slope on the triangle section would only account for a fraction of the gap.

I have just finished extending my formers. Most needed 1/16" to 3/32" adding, but F7 needed 1/8". These figures are before sanding the angles to butt against the triangular sections.

Must get to bed soon!

[Flyer]

Hi Guys,

Just about to try 'ammonia' to make the curved intakes. Cam I obtain ammonia from Homebase or wickes or Wilkos?

Have done a search on their website but have drawn a blank. Only ammonia I can find is to dye my hair !!! ........

Cheers

Ade

[Phil Cooke]

I found mine in Homebase... in the household cleaning section. I struggled to find it in normal supermarkets but Homebase came up trumps!

[Flyer]

Aha,

cheers Phil, I thought I'd seen that somewhere on here. Now getting excited mine's actually starting to look like an aircraft !!

[Phil Cooke]

A session with the sanding block once the intakes are on and she soon starts looking like a proper little A-4. Glad you are enjoying your build. DO make sure you use the Ammonia in a well ventilated area, its pretty potent stuff when you are applying it... have fun!

[Flyer]

Quick question regarding the use of ammonia.... is it applied ( by brush?) on the outside, or inside of the curve?

Cheers

Ade

[Phil Cooke]

Hiya Ade,

I've used both brush and rag methods of application but my recommendation is to use a swab made up of kitchen roll as it runs off the brush very quickly and you'll have drips all over the place. The swab method also allows you to 'push' the ammonia into the grain a little too - once the balsa surface is impregnated it will then soak in and take more ammonia if required... at first the surface of the balsa can be a little 'greasy' and the ammonia doesn't penetrate without a little rubbing.

Apply it to the outside only, that should be fine on 1/8th balsa, rub it in, rub in some more, and let it permeate for 10 to 15 mins, with the intake skins flat on the board - then you'll find the balsa is ready to work. You can't glue it when its wet but you can form it and you will find it will stay in this exact form making gluing later with PVA a doddle.

[Peter Garsden]

I know I have said this before Ade but you can use white vinegar as an alternative which doesn't stink and works, I think, as well. I used Malt Vinegar which does smell but only because that was all we had. I read that all one has to do is upset the ph balance of the wood to soften the grain, so acid or alkaline will do the same job. Up to you. Also white vinegar should be easier to source from supermarkets.

I am alone, however, as most people seem to use ammonia, which I think is a bit more toxic.

[Shane Talbot]

I used vinegar and hot water 50/50 and it worked very well. I brushed it liberally on the outside, rapped the balsa around a suitable can and left it over night.

[Andy Meade]

Hello builders!

I have created this for Phil's model :

Which builds into this :

Which you add McDonald's straws to from this kit of parts :

Yes, it's a fuel probe for a Skyhawk. I'll be handing over the prototype parts to Phil tomorrow, and hopefully he can check fit and form ASAP. If anyone else wants one printed after a successful fit, let me know

[Flyer]

That looks good Andy. I may well be interested.

Ade

[John H. Rood]

Hey -- isn't this a FAMILY website?!!! 

Just shocking, what these naughty Skyhawk people are about these days. cc: Dr. Sigmund Freud & His Disciples.

BELOW:  Oh well, if ya cain't beat 'em, might as well join 'em...

Seriously, how great are these underwing flying target drones?!!!   Me wants!!!  

A-4B (A4D-2), Late 1950s or early 60s, NAS Point Mugu, California.

Edited By John_Rood on 14/07/2016 17:25:17

[Peter Garsden]

Definitely one for me Andy please. Looks brilliant. Will fill her up from your Buff?

[Phil Cooke]

hehe looks great Andy many thanks for your skills in CAD. Does the drinking straw 'fuse' work if you twang the end as per the design intent??

I knew these would be popular, so you'd better agree a unit price to cover your costs quick!

Ill do a trial fit on Saturday once we are back home and will show it off against the fuselage for good effect. Everyone run down your local McDonalds to get some spare straws in!! - we chose this as we thought it would be a universal fit for those being built in the US, in Europe, Asia and Australasia.

[Flyer]

Is the preferred glass-cloth finish now to use Poly C? Or would I get away with finishing epoxy, as long as I seal the surface first?

I'm just beginning to get concerned that mine will not be a 'floater', but I suppose it will save on ballast!

Hoping to try the pop bottle canopy moulding this weekend, before SWMBO gets back from holiday and notices the oven smelling......

Cheers

Ade

[Andy Meade]

I've just used a covering heat-gun to form a pop bottle around a plug recently, didn't need an oven. In fact I'd argue that the heat gun is a better method, as it allows you to heat the bottom up separately to create the required tension that will pull the top in tight.

Each to their own on the poly C / epoxy I think Ade, but either way, I'd recommend that the wood is sealed first or your resin of choice will end up soaking into wood instead of staying on the surface.

[Flyer]

Thanks Andy, I'll try the heat gun first.

And I do hope to try the epoxy finish, even if it's starting to get heavy; just will need a little more wind lol. (I've a lighter model on the go now for those light wind days).

Cheers