PSS A-4E 'Top Gun' Skyhawk

[Phil Cooke]

My A-4 build progress has suffered an inevitable 'stall' with the recent opening event of the PSSA's 30th Anniversary season - but with that event successfully run and with the write up and photos now all online HERE we can recommence getting the Skyhawk fuselage completed! Afterall - there's only 19 weeks left to go until the Mass Build Fly-In in September (for anyone not yet started!)

The intake skins were cut to size as per the plan but when I rolled them around the intake formers I found them just a little too big, so I altered the dimensions slightly to suit my build. The skins should sit flush around the full circumference of the intake formers A1 to A3 - too long and they will 'sit off'. I've added the dimensions I needed to suit my build in the photo below in case it helps others (dimensions in mm) - my skins are a little longer at the front to enable them to overhang and be trimmed back to A1.

With a pair of skins cut from medium 1/8" balsa I gave to outer surface a good rub of ammonia and let the pair dry around a suitable pot of filler whos diameter was almost identical to that of former A1. Allowing the ammonia to soak in and dry with the skins held like this will enable a stress free fitment of the skins to the fuselage very soon.

[Phil Cooke]

A bit more fin and rudder work whilst the ammonia is drying on the intake skins...

FIN3 from the kit has been discarded and in the quest for a scale rudder with external rib stiffeners I've cut 3 pieces from 1/8" balsa to make a 3/8" sandwich. The central core is marked up with parallel lines at a pitch of 14.5mm - these are the centrelines for the ribs.

The ribs were also cut from 1/8" balsa with the grain running lengthwise - 2mm thick at the T/E with a constant taper - the length has to be cut to suit as they are glued into position.

10 minutes with thin cyano later the ribs are all in place - comparing here the new 'scale' rudder with the 3/8" part supplied in the kit.

Finally tonight the fin assembly was glued up and pinned down to the board to cure.

[Tim Mackey]

Great work Phil

[Peter Garsden]

Excellent suggestion and explanation Phil. I was thinking of using the original rudder and applying some dummy ribs with strips of masking tape which would show up under the 25 gram fibreglass cloth we are both going to use. Might still do that, as I don't think rudder flutter will be a problem for us without a moving rudder! Will give it some thought - as you say the Gods of scale might be offended and send us South Easterlies of a light variety in September.

[Peter Garsden]

Excellent suggestion and explanation Phil. I was thinking of using the original rudder and applying some dummy ribs with strips of masking tape which would show up under the 25 gram fibreglass cloth we are both going to use. Might still do that, as I don't think rudder flutter will be a problem for us without a moving rudder! Will give it some thought - as you say the Gods of scale might be offended and send us South Easterlies of a light variety in September.

[Lorenz Mueller]

Same as you, Phil - only my sandwich is 4mm/2mm/4mm balsa. Not having the build article I am interested how the intakes get onto the curvy fuselage sides, and how the intake splitter lips get treated. I am also thinking about adding the scale horn balance to the elevators.

Lorenz

[Tim Mackey]

Tut, typical thespian that Garsden, has to say things twice to make sure everyone hears him

Edited By Tim Mackey on 01/05/2016 18:03:31

[Phil Cooke]

Hi Lorenz - hey your A-4 is really coming on - great to see another so close to structural completion!!

PM me your email address if you would like copies of the 2 RCMW articles I have them here scanned and ready.

As for the front intakes, I'm not quite at that stage with my 'production' build but I can take you back to a photo I took of the prototype built last summer. You are right, the fus sides curve in around F3/F2 and these basic blocks cannot fit true against all mating surfaces.

They are best fitted such that the mate neatly against the intake former A1. This way the bottom edge of the intake block will also seat against the fuselage side, and you will have a 1/4" gap at the top, like in the photo. I dabbed the balsa blocks on against A1 and the bottom edge with 5 minute epoxy - and once dry I just filled this top gap with sheet and sanded it all up smooth - same for the front face - a 1/4" balsa wedge was driven in and sanded up flush.

As for the splitter fairing this was not modelled on the prototype. It would be nice to see that implemented as another scale feature, but its not shown on the plan as this is only stand off scale.

[Peter Garsden]

I put it to you Time Mackey that forsooth I am not a veritable thespian, and even if I wasn't, I wouldn't stoop to eating other people's waffles, or even waffle inappropriately in other people's blogs!

I have to say that when I look at the complications of the jet intakes and the compound curves I start thinking once again of lost foam! Have had enough of it for 1 year.

I have now decided to go for the Cooke method of Fin even if it is a bit more effort as thankfully, at this stage, we are not in a rush.

This software does annoy me. Just can't work out why sometimes you get duplicate posts. Also if you are not logged in, it lets you do a post then loses it when you realise you are not logged in. I can see it is old because it is based on .asp technology.

[Phil Cooke]

So like others today I assembled my tailplane flat onto the plan. Once cured, I tried it for size on the assembled fin and rudder. Note how the tailplane incidence is set by the base of the rudder.

This is all complete now, just in need of a good sanding to section. Important to round all the Leading Edges and feather all Trailing Edges down to 1/8".

[Phil Cooke]

Back to the intakes.

With the ammonia well and truly dry the sheeting was unclamped from the form I'd used and we were left with a pair of well curved skins.

I used PVA on the 3 intake formers and thin cyano to help 'grab' the tops and bottoms of the skins as they interfaced with the fuselage sides. Just ensure you've got them aligned before you add the cyano - I drew a guide line along the bottom edge of A1, A2 and A3 to help.

Just a couple of pins here and there to help ensure we achieve the full curvature around the 3 formers...I've been left with a small gap top and bottom around A1 which I will fill afterwards with 1/8" balsa, but the general form is there.

She looks a lot more A-4like with the intakes on!

[graham Gilliver]

good stuff Phil, I like the blow by blow you are presenting. for quite an easy model there are a few traps/hard bits , that you are demonstrating very well. any deviations from your techniques would appear to be at our risk .G

[Phil Cooke]

A 'fiddly' couple of hours spent refining the edges of the intake skins tonight, a lot of fettling with little progress to show

First job - cut back the intake skins which were overhanging beyond intake former A1.

This shows clearly the gaps which need filling towards the front where the skins I'd cut didn't quite meet the requirement - easy to rectify with some additional 1/8" sheet planking. With the skins cut back like this you can dry fit the front intake blocks, position them centrally up against A1 and mark them up to aid pre-shaping with the razor plane. Best to get as much material removed as possible before they are glued on for final sanding.

The rest of the session was spent fettling the underside join between the intake skin and the fuselage side. The bottom joint is more critical than the top as there is no profile sanding along the bottom edge - so the 1/8" step needs to be filled.  You could use lightweight filler but balsa is best. I cut some small triangular section to suit the 'gap' from 1/8" sheet.

Here, the first piece is glued in...

And after an hours work or so, the finished job front to back, both sides, requiring only the blending fillet (to be cut from some scrap aileron section) at the rear now.

Edited By Phil Cooke on 03/05/2016 23:40:14

[Peter Garsden]

Very interesting Phil. Do you recommend that we should make the sheet covering bigger than the plan? Or did you not follow the plan?

[Phil Cooke]

I followed the plan Pete. The gaps at the very front are inevitable if you are using 4" wide sheet as the 101mm width doesn't quite meet the part-circumference requirement of A1, its just too big. You could solve this by making a wider section of sheeting as we've done with the fuselage sides, but better I think the concentrate on getting the curvature right then finishing the gap with a slither of planking on the very top.

The little triangular wedges are needed to bridge the step between the 1/8" sheet and the fuselage side where they join at the oblique angle.

[lightning 759]

hi

hope you don't mind me asking but what would the chances be of converting it to electric with a pusher prop ,would it be possible

Andy

[Andy Meade]

Don't swear in here!

You could move the elevator servo forward and hide a small LiPo under the cockpit area I'd imagine. The tubular tailpipe could hide your sinful noise maker easy enough.

[lightning 759]

Posted by Andy Meade on 07/05/2016 18:26:15:
Don't swear in here!

Bit flat were I live unfortunately ,

thanks for the reply

andy

[Phil Cooke]

Hi Lightning - yeah I think it would take to electrifying ok - there's loads of room in the fuselage although you would have to make a hatch to get to the flight battery - in doing this I'd probably glue the wing on to stiffen it all up and then you could make the hatch and access as big as you needed with no worries.

You may also have to extend the prop shaft rear a little more than normal as the fin TE doesn't lend itself ideally to the prop arc position.

But I'm sure it would be a good little electric conversion - and built one piece I'm sure it would be robust and very swift!

If you do try a conversion, please keep us in the loop I'd like to see how you get on!

[Phil Cooke]

I got the air intakes finished today, bar final detail sanding.

First, the gaps left at the top between A1 and A2 were filled with tailored wedges of 1/8" balsa.

And at the rear, I sanded down some soft elevator stock to feather the intakes down from 1/8" to zero thickness to enable a full blend with the rear fuselage.

Next, I roughly shaped the intake front blocks with the razor plane catching the end of my thumb today in the process - which didn't aid proceedings!! Notice how, having aligned the woodpack blocks on the rear face (which will glue to A1) the outer most part at the front fell short by ~3/32" - I filled this with balsa and sanded the front face flush. Its a very good idea to sand these to shape as much as possible before gluing them onto the fuselage.

Once fitted with a blob of 5 minute epoxy, I sanded them up to the final profile and blended them into the air intakes. Note they are glued to A1 and to the fuselage at the bottom edge - the gaps on the top and the front face will again need a wedge of 1/8" balsa making up.

Bar the wedges and final detail sanding - that's job done! In fact once I've cut the slots in the fuselage top to enable the fin to be fitted, and added a little cockpit detail, that's the fuselage complete and ready for some serious shaping!

[Phil Cooke]

Prior to rounding the corners of the box fuselage its a good idea to mark up the 45 degree extents where to take the razor plane to, and before we can mark up the model with these guide lines its important to sand the fuselage to the correct 2D side profile. In particular - note how the bottom rear 1/2" sheet is only 1/4" thick at the front and the back.

Use the former guides on the plan as reference - here I've marked up how much material can safely be removed with the razor plane at a 45 degree angle before we round the corners with the sanding block. I've included here for reference, from front to back;

F1 - 15mm at the top, 14mm at the bottom... etc etc

[Phil Cooke]

Transfer all those dimensions to the model and draw in the lines to give you a confident reference guide for the 45 degree planing that lies ahead! Time to make some bedding for the pet rodent!

[Phil Cooke]

A hectic week at work has left little time or energy for A-4 modelling I'm afraid, the fuselage has been scalped with the razor plan to the 45degree datums and is ready for final sanding now as shown. Still quite a bit more rounding to do with the paper and block - again make good use of the fuselage section drawings on the plan to achieve the right shape.

I still need to add the gap filling wedges at the front and top of the intakes, and my 1/2" top sheet between the intake fronts is still a little high in profile - the intakes should blend seamlessly into the top sheeting on a curve with a constant radius.

It looks like a good day for sanding outdoors today so I'll be aiming to do more shaping after breakfast!

[Chris Barlow]

Yes, a good day for sanding outdoors Phil!

I did notice on my 1/72 plastic reference model that the intake radius blends into the fuselage and was wondering how close the sides were to allow it. Now yours is planed up I can see it will blend in nicely.

[Phil Cooke]

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.

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.

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

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

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.

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.