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A4 Skyhawk - Building tips and Plan queries


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  • 2 weeks later...

Another small issue has been pointed out on the A-4 plan by fellow builder Nick Rees which is easily corrected and shown here for clarity.

It relates to the extent of the 1/2" triangular section along the top of the fuselage sides. The plan as currently presented suggests this stops behind F4. But a look at the former as cut with the 1/2" chamfered corners shows the design intent is for the 1/2" triangular section to run through the former and terminate at the front face.

So the plan, currently drawn like this...

f4 triangular short.jpg

should actually be drawn like this...

f4 triangular fixed.jpg

and your build should look like this, with the triangular section just protruding beyond the front face of F4.

fus doublers 3.jpg

Hope that is clear - I will contact Traplet to ensure all future copies of the plan are amended in this way.

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  • 2 weeks later...

Hiya Steve,

Yes, my hand drawn plan was submitted to Traplet very soon after the test flying and I forgot to add the control throw recommendations to the drawing so they never got added...embarrassed

I have no idea where Traplet got the figures they quoted - you are right they are way too much, we spotted this in the Feb issue of RCMW and I asked the editor for it to be corrected in the March issue but this was not achieved.

The A-4 is a responsive model as you might expect, and the throws needed are only small, I would set up something like;

Elevator +/- 12mm

Aileron +15mm -10mm

with a healthy dollop of expo to keep things smooth around the centres.

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Something I didn't own before- clamps!!!

I noticed Phil was using the Irwin Quick-Grip ones in his build so a quick look on the ScrewFix website found the 12" versions readily available and currently on offer - 2 for £10.

Needless to say I ordered 4 and have just been to collect them!!

I think these are a must have for any workshop really!

image.jpeg

Dave

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  • 1 month later...

POST COPIED FROM PETER GARSDENS BUILD THREAD WHERE SOME ADDITIONAL CLARITY ON THE WING JOINING METHOD WAS DISCUSSED

Thanks Pete, I did see your previous posts yes, I'd struggled to add a response with 100% confidence as on the prototype (to reduce build time) I built the fus and Matt actually assembled the wing in parallel. My original drawings left the wing joining method a little 'open to interpretation' and the resultant method (as explained in the magazine article) was devised by Matt during that first prototype assembly.

The additional 1/8" webbing aids to bridge the stagger between the top and bottom spars - helping us then to add 2 identical 1/4" triangular gussets top and bottom to enable a vertically orientated wing joiner. Its not there for strength - just joiner alignment. If you made the top and bottom 1/4" gussets slightly different sizes you could still achieve the same vertically aligned joiner. That's all the additional 1/8" webbing does. From memory it sat ON top of the bottom spar which was rearward of the upper spar.

Here's a couple of Matts photos during the first build, as used in the mag. They show the 1/8" balsa web on the bottom spar but mating with the rear face of original 1/16" webbing in between the 2 spars as per the rest of the span.

wing join 8th balsa 1.jpg

wing join 8th balsa 2.jpg

Its not easy to show this on the plan as the 1/8th balsa overlays the bottom spar - perhaps an isometric sketch of the assembly would aid the builder in future - all I can offer now is these photos and wording to outline the design intent - as per the write up and photos in RCMW.

As for the joiner breaking THROUGH the top and bottom skins - this is just the easiest way to ensure the join is good and the top and bottom skins are acting structurally regarding the wing join - the strength on these little skinned wings comes from the skins and its good practice to ensure the joiner is integrally joined to both to ensure max strength. Leaving the wing joiner proud through the skins and sanding back flush after the wing is joined is the easiest way of building absolute confidence in the joint.

Hope that helps others - Ill copy this post and add it to the general building guide thread too I think...

Edited By Phil Cooke on 30/04/2016 07:52:05

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Tiny wee suggestion having considered the above and been suitably reprimanded for not looking at plan sheet 1 which was folded away ready for fuselage construction - add an arrow and the words "1/8" vertical webbing between ribs 1 and 2 to aid gusset alignment" on the wing plan. I appreciate it is difficult to show pictorially but an extra note might stop me making the mistake I did.

I think, though I haven't checked the plan as I am in Anglesey, that the diagram above is on plan sheet 1? So I wasn't looking at it when I was making the wing. If it is on plan sheet 2, it is folded away underneath the visible wing area. I appreciate no one is making a wing yet, so it will become relevant as and when.

Raining badly today so no Orme flying, but might be there tomorrow on way home?

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You're right Pete this composite section WAS added to Sheet 1 by Traplet, not sure why as the wing plan Sheet 2 is hardly challenged for space or over cluttered. I can recommend the changes for future copies of course. Builders with plans as currently issued will have to self amend.

In a similar way, the fuselage cross sections are all on the wing plan Sheet 2 when they would be better placed on Sheet 1.

This plan layout and annotation is definitely a skill we are still learning. It's iterative of course too, as the drawing gets utilised then issues become obvious. So not easy to get right first time, natural I guess that tweaks are needed then AFTER initial publication.

As the designer you have a better image of the structures in your head of course, and sometimes notes are missed that should be added for another builder.

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  • 2 weeks later...

Am I being stupid or ......

The plans show the vertical height of the nose block adjacent to F1 as being 2 9/16" high. My nose profile former measures only 2 3/8" high. Now I have added the 1/2" top planking from F2 to F1 I have a 3/16" step in the profile which is not as per the plan. Is there a problem with the laser cut profile former or have a done some thing wrong?

Apart from that I am really enjoying my first PSS build

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Hi Stephen,

Glad to read you are enjoying your A-4 build. Are you doing a blog for us??

I'm not sure there is a problem with the 1/8" light ply nose profile. I've double checked your plan measurement and agree that the profile as drawn is 2 9/16" tall. I've overlayed the woodpack part and it's identical, within tolerances, maybe just slightly under all round the profile due to the laser cutting process but certainly within spec.

That all said, I'm happy there is nothing wrong with the laser kit component. Equally, I'm not sure you've really done anything wrong, certainly nothing that can't be fixed. First, take a look at a photo from my recent build - you can see when I added the front top 1/2" sheet mine too has a step ~1/8" - the sheeting is proud of the nose profile. Is this what you have? This can be feathered in with a little sanding between F1 and F2.

front top sheet 5.jpg

If you think your step is bigger than this, then I think one of two things might have occurred. Are you sure your 1/8" light ply nose profile was fully 'seated' down through the 1/4" fuselage floor - and was it glued at the right angle, such that the back edge was vertical? Or is your 1/2" sheet perhaps slightly thicker than 1/2" giving a bigger step?

Either way, sand it all up to the correct 2D profile by eye relative to the plan and you will be back on track. Hope this helps. Do send us some pictures of your build we'd love to see how you are getting on!!

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ju882.jpgju88nose 3.jpgConcerning canopy making check out my posting of 2014. I followed internet concepts to make both nose and canopy glazing for a JU88 model. My plugs were made of plaster of paris built up on a one sixteenth frame substrate. A toaster oven warmed up the canopy material. (Same stuff pop bottles are made from.) After some trial and error it worked satisfactoriily.

John Dennier

mold on the vacuum box.jpg

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Thanks for the help Phil.

The problem was two fold. Firstly my 1/2" sheet is slightly over thickness so the step was bigger than you had. Secondly I had interpreted the plan too literally. It shows the 1/2" sheet at full thickness where it meets the nose block so I assumed that no sanding was required but to get the correct profile I had to sand it back a fair bit. All is now hunky dory.

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I got some sanding time in the garden today and I've highlighted a slight error in the build, well, in my build anyway - this didn't occur on the prototype!? In shaping the fuselage around the deepest part of the intakes just ahead of F4 I've ended up with 2 small triangular holes where a little more material is needed on the inside to avoid a thin section. In this view one of the holes is clearly visible towards the top right hand corner.

hole1.jpg

And here, in plan view from the top, two symmetrical triangular holes. Hmmm - I've spent some time trying to ascertain what's different between my 'production kit' build and the original prototype build, where as stated this did not occur. Either I've cut the holes for intake formers A1-A3 slightly too low in my fuselage sides, meaning the intakes are sat 1/8" low - or the size of the intake formers are very slightly small resulting in a similar problem.

hole2.jpg

This is where the problem is - just ahead of F4 where the 1/2" triangular section runs out.

f4 triangular fixed.jpg

Here's the holes again after sanding, viewed from the inside...

hole3.jpg

Simply extending the 1/2" triangular section another inch forward along the top of the fuselage sides would fix this issue (if indeed it IS a problem with the kit and everyone suffers the same?) but the bend in the fuselage sides at this point would mean its better to glue a separate smaller piece of triangular section in after the fuselage sides are pulled in and glued upto F3. I've retrospectively corrected my build in just this way, by gluing 2 short lengths of 3/8" triangular section to bridge the thin region and the hole.

hole4.jpg

My recommendation would be to all - before sanding your fuselage to the final profile - please add these short sections of additional material into the top corners just ahead of F4 to mitigate any risk of thin walls or holing.

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Hi Stephen, I would recommend you create your own thread from the main A-4 section linked below;

**LINK**

You don't have to write about it as a blog, but it will give you your own place to post pictures, allowing others to see your build. You'll always have a place to post questions or comments then too, should you wish.

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  • 3 weeks later...

Hi Phil, I have a problem with my wing T/E. The plan shows it as 3/16, your balsa list says 3/16 x 1/4, so I made the wing up with the 3/16 x 1/4 T/E and being 1/4 high it way above the end of the ribs. What have I done wrong? It would be better now if the top cover butted up against the T/E not over it.

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Hi Shane,

There's no problem here, we will all face this in part of our build. The T/E of the wing ribs is approx. 4mm high but no standard T/E section would ever naturally suit due to the sloping taper of the ribs upper surface - so every builder has to sand down the T/E section to suit that angle, enabling a smooth top skin. Similar on the sub L/E, which is tapered too and also varies in height from root to tip.

torque rod clearance.jpg

Using a suitable length of sanding block - gently sand the T/E back down to the ribs upper surface creating the angle as you go. Its important for strength and build accuracy that the top sheeting overlays the sub T/E material in my opinion.

Edited By Phil Cooke on 08/06/2016 06:53:36

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Yes that's right Phil.

What I did Shane was attach the T/E with the edge proud of the ribs by quite a margin, then put some masking tape along the edge of the sheeting, and plane it down until it was nearly there then sand the rest. The masking tape protects the sheeting. I have shown this on my blog here **LINK** but in relation to the leading edge which is actually the same idea.

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  • 2 weeks later...
  • 2 weeks later...

Hi Phil, my A4 is nearly finished however I have lost or misplaced my canopy, I have a friend currently in the UK, is there some where that he could get me another one. He will be there for several weeks visiting his parents so it could even be posted to their address. thanks Shane.

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