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Black Knights Skyhawk

A-4SU display team of RSAF

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Steve Houghton12/06/2016 10:09:33
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Republic of Singapore Air Force 'Black Knights'

My subject for the 2016 PSS mass build of the A-4 Skyhawk is the A-4SU variant used by the RSAF.

As well as a mainstream RSAF service aircraft, it was used during the 1990s in a team of six A-4SUs by the RSAF Black Knights aerobatic display team, in the national Singapore colours of white and red.

skyhawk-1024x686.jpg

The A-4SU was based on the A-4C, and will require a slightly shorter nose and more upright air intakes than the A-4E shown on the plan. Other differences may come to light as I progress.

I don't intend to present a full blog, as time is rather short to get it finished. I shall concentrate on the building and post anything interesting that I haven't seen in other blogs.

On your marks....

To the building board...................

J.H. Rood13/06/2016 02:42:33
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170 forum posts
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Best wishes, Steve! What a gorgeous colour scheme, too.

Phil Cooke13/06/2016 07:01:48
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2203 forum posts
1513 photos

Good to see you underway with the A-4 Steve, and a great choice of scheme! Enjoy the build!!

Steve Houghton13/06/2016 14:08:52
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512 forum posts
342 photos

Hi John,

Since I started this thread I noticed that you have also posted pics of the Black Knights on Graham Gilliver's thread. Great minds think alike!

I hope one or two others may follow the same Black Knights scheme, so that we can give those Blue Angel builders an aerobatic team challenge!

Phil, I'm glad to be on the job at last. I hoped the Zlin 526 ASM would be finished for this weekend on the Orme, but I didn't quite make it. I will now leave the Zlin until I have made significant progress on the A-4.

Wing

I have started on the wing, and haven't seen much information about this on the other threads yet. The ribs, spars, webs are glued down to the lower sheet.

Sub-LE

I'm puzzling how to shape and attach the sub-LE 1/8" sheet without having access to plane it after glueing in place, as it will be tucked under the lower sheet. I think I shall pre-chamfer the bottom edge, and hope it is somewhere near to the right angle, and stuck in the correct alignment with the bottom of the ribs, when I roll the lower sheet onto it afterwards.

I did wonder about wrapping and gluing the lower sheet onto the ribs first, then add the sub-LE, but the structure would be a little flexible and I might add twists.


Phil Cooke13/06/2016 16:01:31
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2203 forum posts
1513 photos

Good question Steve, you can do it either way I think... I will have to re-read my build article but I would suggest its easier to roll the bottom sheeting up first THEN add the sub L/E into the resultant 'corner' groove.

Ribs are aligned fine and glued to the bottom sheeting with the bottom spar and the rear sub T/E and once that's dry you would be able to add the top spar and cross spar webbing - adding further stiffness. Then you can roll the L/E sheeting up onto the underside of the ribs prior to adding the sub LE. I would try to keep the wing pinned down and chock up the skins where needed at the front using a section of thin elevator stock positioned at the right angle to give the varying degrees of elevation needed along the span root to tip.

You can chamfer the bottom edge of the sub L/E to achieve an almost perfect fit if you wish but the amount of curvature on the underside of he ribs is fairly limited and the resultant gap if it was left square would likely be much smaller than the bead of PVA used to retain in in the groove.

Steve Houghton13/06/2016 21:17:00
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512 forum posts
342 photos

Thanks for the ideas, Phil. The magazine article (presumably from Matt's notes) suggests glueing the sub-LE before lifting the TE of the wing to roll the sheet onto the bottom, but I couldn't see how to attach the sub-LE correctly using that method. I may try the sheet first technique.

I've just added the 1/8" webs to the two centre rib bays, and when dry I shall study form to decide on the best approach.

Steve Houghton13/06/2016 22:35:45
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512 forum posts
342 photos

Pack It Up

I adopted Phil's suggestion for the lower sheet and sub-LE, and left the wing flat on the board, with the lower LE sheet packed up with a piece of TE stock. The tip needs very little packing, and the root needs much more, so the packing is angled.

1606_1301_lowersheetpacking.jpg

Wick-ed

I've finally found a use for a bottle of Super Phatic that I bought last year. Instead of squeezing aliphatic into the gap between the ribs and the sheet, just apply the super-penetrating glue once everything is held in place and let capillary action do the rest. I suppose some techies would use super glue, but I never get on with it, preferring the sticky white stuff.

1606_1302_applysuperaliphatic.jpg

R1 will have to be glued later, as it needs an altogether different packing angle. I haven't fitted R1A yet, preferring to wait until the sheet has been stuck to the ribs.

1606_1303_r1gapdonelater.jpg

While the ribs dry, the sub-LE can finally be added. I did chamfer the lower edge slightly, but it needs less than 1mm, so a gentle application of the sanding block was sufficient. Real aliphatic used for this.

1606_1304_subleglued.jpg

Phil Cooke13/06/2016 23:34:16
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2203 forum posts
1513 photos

Great job Steve - your photos help describe the method perfectly and it looks like that's worked well - the use of T/E stock as packing with variable amounts via the angle change from root to tip is well shown in your photos and will prove valuable to others I'm convinced! Glad its gone together quick and in a straightforward manner.

Piers Bowlan14/06/2016 08:34:16
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1705 forum posts
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Thanks Steve, I will use that. The simple ideas are often the best.

Steve Houghton21/06/2016 22:01:06
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512 forum posts
342 photos

Bracing Stuff

Here are a few photos of my centre section bracing. I went for the full depth 'skin piercing' 1/16" ply dihedral brace, rather than using the narrow one supplied with the laser cut parts.

My servo box is sized to fit a single HS-225MG, but I made it slightly wider so that it will also accommodate two 13mm micro servos if I choose to change to a two-servo setup in the future.

I added some additional gussets at the centre TE, cut from TE section, to spread the load from the wing bolt into the structure. I might then reduce the thickness of the Lite-ply load spreader fitted on the lower surface.

1606_1901_wingbraces.jpg

Here's the dihedral brace dry-fitted, protruding above the spars and gussets.

1606_1902_servobox.jpg

The slot with the brace removed.

1606_1903_braceslot.jpg

The final bare structure photo before adding the top wing skin. Two small blocks fitted to take the pin and leaf hinges for the ailerons.

1606_1904_wholewing.jpg

Finally, the slot in the lower wing skin for the dihedral brace to penetrate and bind to the wing skin when glued later.

1606_1905_lowersheetslot.jpg

Steve Houghton21/06/2016 22:08:04
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512 forum posts
342 photos

Leading the Way

The top skins have been added uneventfully. I used Evo-Stik 'green' PVA, which has a slightly slower setting time than my Aliphatic, giving more time to spread the glue well across the ribs and spars before plonking the skin on top.

The leading edge has now been added.

NOTE: I ended up with a slight forward curvature in the sub-LE near the root end. I spent some time dressing it back to remove the curve, and as a result the LE strip needed to be 9/16" deep, rather than the 1/2" deep quoted on Phil's wood list. If you build straighter than me, you may not have this problem. Hence, I cut the LE from sheet rather than using the strip I bought.

1606_2101_le_glued.jpg

The LE is pinned for locating in place, then taped to pull it back against the sub-LE.

1606_2102_le_taped.jpg

 

Edited By Steve Houghton on 21/06/2016 22:09:54

Steve Houghton27/06/2016 12:39:03
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512 forum posts
342 photos

Black Knight to 4C

The A-4SU is based on an A-4C, so I have a small redesign to do around the A-4E nose on the plan, to shorten it by 13 inches (full-size). This is how I propose to do it:

The nose former will be shortened and re-cut to the new profile shown below. The top of the tab that supports the battery box is unchanged, but the lower edge is more curved. I may add a building tab to support the front at the right height during assembly, to be cut off later.

The lower nose sheeting needs more curvature, but this will leave the sheet a little too thin for comfort. Hence, I shall raise the front of the 1/2" triangular sections by 1/8", with a sheet wedge that tapers down to nothing at 2.5" from F1. This needs a cut in the triangluar section at the end of the wedge, to change direction.

The bottom 1/4" sheet can still lie flat on the board during construction, and will be planed down later.

1606_2603_noseprofilechanges.jpg

Nose Job

This revised F1 section below shows the changes more clearly. F1 will be reduced in height by 1/8" (chamfered) at the bottom to allow the extra 1/8" sheet wedge to be inserted.

The raised trianglular sections will now clash with the lower corners of the front part of battery box, so I'll cut some small recesses into the trianglular to match.

1606_2602_formerf1changes.jpg

I deliberated whether to just risk keeping the nose design unchanged and just plane away, but I'll feel happier to have more wood to plane to shape at the front.

I shall post a few pics when the glueing starts. (does gluing have an 'e' in it? I think one is British and one is European, but we won't go there!).

 

 

 

 

Edited By Steve Houghton on 27/06/2016 12:59:02

Steve Houghton03/07/2016 15:18:00
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512 forum posts
342 photos

Nose Surgery

I cut the revised nose profile from 1/8" Lite Ply, and added a couple of jigging tabs fore and aft to hold it at the right height during assembly:

1607_0104_a4c_noseprofile.jpg

Here's a direct comparison between the A-4E original and my A-4C revision:

1607_0105_a4c_noseprofileoverlay.jpg

Steve Houghton03/07/2016 15:43:28
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512 forum posts
342 photos

Nose Assembly

Assembly of the nose section followed the magazine instructions with no probelms. I was glad to have the jigging tabs on the nose profile, which made easy work of aligning it.

These pics highlight the extra 1/8" wedge I added on top of the 1/4" lower nose sheeting, below F1:

1607_0201_nosewedgeside.jpg

I had to extend the sides of F1 slightly, as a dry fit showed that the 1/4" nose doublers clashed with the top sides of the battery box, stopping the sides from touching F1. I think I was too ambitious with chamfering the sides of F1 and took too much off:

1607_0202_nosewedgefront.jpg

Steve Houghton03/07/2016 15:49:10
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512 forum posts
342 photos

Padding Out

As mentioned on the main A4 Building Tips thread, a combination of circumstances led to most of my formers being undersize to mate accurately with the fuselage sides.

I used the same solution as Phil, by adding some thin strips of 1/8" sheet to the top edges. These pics show what remained of the padding on F4, F5, F7 and F8 after I had sanded them (with chamfer) to fit against the triangular sections:

1607_0101_f4f5extensions.jpg

 

1607_0102_f7f8extensions.jpg

 

 

Edited By Steve Houghton on 03/07/2016 15:50:12

Steve Houghton03/07/2016 16:03:50
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512 forum posts
342 photos

Add Nose to Body

After completing the nose sub-assembly and joining the fuselage sides with F4-F7 as per instructions, I deviated from the instructions. I used my fuselage jig to clamp the nose onto the fuselage sides

This method only works if the 1/4" lower nose sheeting is first cut to size, slightly smaller than the width of the fuselage. Without this, the jig uprights would not touch the sides.

1607_0301_fusjigfront.jpg

To pull the sides in a litltle at the top, I used these dinky little 6" bar clamps (£7.49 a pair from Homebase). Only clamped enough to provide a slight inward pressure, so that they don't pull the lower edges apart.

1607_0302_fusjignose.jpg

The obligatory balsa blocks prevent the clamps from damaging the sides, and the rubber bands provide the usual assistance that the SLEC jig needs to keep the tops of the sliding clamps pulled in:

1607_0303_fusjigclamps.jpg

The fuselage was still built flat on the board, with the 1/8" packing under the TE of the wing seat:

1607_0304_fusjigtepacking.jpg

This shows the slight inward bow from the additional clamps, which also ensures there is no gap at the top of the part-glued former edges, where glue could pool up and make a lump:

1607_0305_fusjigtop.jpg

Centre lines on everything, to ensure alignment with the guidelines on the building board:

1607_0306_fusjigdrawnin.jpg

The jigging tab on the nose profile is still providing assistance in keeping the nose level with the board:

1607_0307_fusjigleft.jpg

 

Now, I'm off to read up the notes on Ammonia treatment for the next stages!

 

 

 

 

Edited By Steve Houghton on 03/07/2016 16:06:11

J.H. Rood04/07/2016 00:07:28
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170 forum posts
204 photos
Posted by Steve Houghton on 13/06/2016:

"...I'm puzzling how to shape and attach the sub-LE 1/8" sheet..."

Steve, I'm late to the party on this one, as you're far past it now, but... you COULD'VE tried my patented NUMPTY method:

After successfully attaching the underskin to the forward section of the ribs and spar, next, with Super 'Phatic well in hand, simply space-out altogether, send your mind to la-la land, and just glue the wrong part there altogether: attach the LE, NOT the sub-LE.

((( brilliant! )))

After it all dried quite nicely, and looked so fit and pretty and nice, I then and only then awoke to my mental-prang. So I next had the delightful opportunity to either cut off / saw off / hack off / dynamite off (etc.) the damn thing, or leave it there and plane it, sand it, sandblast it down to spec for a GLORIOUS Sub-LE !!! I chose the latter; Lord only knows what next savage error I shall inflict, but trust, it WILL happen! HAHA!

Here it is, after the fix...

img_3796.jpg

Whilst sanding it I shall we say "lightly caressed" the tops of the ribs, because naturally I was not going to take TOO much care with this delicate process... ... actually, the damage to those ribs ain't nearly as bad as it looks, it's actually just a bit of the "char" coming off.

I'll need to sand the thing down a bit more when the time comes to attach the LE, but otherwise things are looking SYSTEMS GO! for more misadventurings in SkyhawkVille. And meanwhile yours is looking GREAT!

p.s. My wife says your Singapore Black Lions scheme is the nicest. The rest of us are chopped liver.

 

Edited By John_Rood on 04/07/2016 00:10:24

Steve Houghton04/07/2016 10:08:45
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512 forum posts
342 photos

HI John,

I had a chuckle at your post - not your misfortune, but your comedic way of presenting it. I have to say that if you hadn't let on, no-one would have noticed. The end result is perfectly acceptable.

I'm sure you know, but a good way to reduce the risk of rib damage when sanding the sub-LE is to lay a length of masking tape along the ribs from root to tip, just behind the LE, then plane/sand away until the top of the tape starts to get scuffed. Then peel the tape away and finish off very slowly to achieve the perfect finish - at least that's what I'm told!

I hope my Black Knights scheme will look as good as the full-size. I do like camouflage schemes as well, but I thought this one might be quicker, since I'm always short of time.

I'm following your thread to see how you progress with your nose modifications, which are somewhat more extensive than mine. Keep up the good work.

Steve

Steve Houghton04/07/2016 10:29:51
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512 forum posts
342 photos

Ammonia a poor little learner!

Donning full PPE and looking like a post-apocalypse survivor, I ventured into the garden to find an isolated corner to implement the ammonia moment.

I liberally dosed the fuselage ahead of F4, and the nose doublers, with several coats of the ponging stuff, waited for 5 minutes and then tried to coax the sides into place. Whilst they did bend a little more easily than when dry, I managed to crack the top part of one side behind F3.

Reading the magazine article again, it clearly says to wait until the ammonia is dry before bending the wood. Is that what I should have done?? I thought it was more likely to bend whilst wet. Maybe I was in too much of a rush.

However, here's the crack (above the slot) after some remedial glueing, and a slightly too tight clamp that left its mark:

1607_0402_fussmallcrack.jpg

The end pull

With the sides clamped while the pong dried out, I fitted the top 1/4" sheet from F4 to F7, then pulled the rear fuselage ends together. I had to apply saw cuts to the triangular sections, top and bottom, because there's no way they would have bent without:

1607_0401_fustailjoined.jpg

Waist reduction

With the side now dry, its was easy to dribble some glue down between the sides and the formers to lock the sides into place. Waist reduction with a vengeance!

1607_0403_fusnosepulledin.jpg

I notice that the nose now increases in width again between F2 and F1. I'm not sure if this is correct, or whether further ammonia surgery is needed.

Phil Cooke04/07/2016 14:32:21
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2203 forum posts
1513 photos

That's correct Steve - its a function of the fuselage sides getting taller as you move from F1 to F2, that's all. No more ammonia needed (until you get to the air intake skins of course)

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