PSS A-10 Thunderbolt II - build blog

[Phil Cooke]

ahh yes the age old PSS debate, 'block 'em off or keep 'em open??'

I may be wrong, and some experimentation may be necessary on this particular model, but I thought as the engines increased in diameter beyond a certain (admitedly, unwritten) size then the advantages of blocking off the air intakes were overcome, something to do with the ratio between frontal surface area and the 'wetted' surface area on the inside of the nacelle or airframe when left open.

Definitely, on smaller models (like Matt's Hawker Hunter) and on models where the frontal surface area of the engine intakes it limited in size (like the 108" span B-52 you are building Tim, and Matts Vulcan) then blocking the intakes off has been proven the right thing to do - the Hunter in particular has been subject of a 'back to back' test proving the concept of the blocked passages, having flown with both configurations. Matt showed it was much improved (penetration, and top speed) with the intakes blanked.

However, those models with bigger engines pods, like Simon Cocker's An-225 and his larger span B-52 have all flown well with free flowing, open engine nacelles. In my head, I have put the A-10 into this larger engine category, based on the frontal surface area of the 2 engines combined.

Of course, if we do prove them to be too draggy, we can easily fit a ply disc to blank them off, but for now, Im hoping this logic is valid. Time will tell, of course.

Edited By Phil Cooke on 17/11/2012 12:51:08

[Phil Cooke]

So with my engine plans drawn up, it's at this stage I need to let you in on the fact that the development of some key components for the engines was a joint effort. Truth be told, there is actually 3 x PSS A-10s being built, all currently at various stages of build, all individually tailored by their respective builders, but importantly all based on the same Josh Harel plans.

PSSA members Simon Cocker and Phil Pearson are the other two builders, and our agreed intent to utilise ABS vac forms for the L/E intake and the T/E exhaust for our engines led to the work share.

[Phil Cooke]

Simon had acquired some huge pieces of Lime hardwood, and having the required access (and skills) on a wood lathe set about making a pair of plugs - one for the front 1/4 of the nacelle and one for the rear 1/4. These were turned to the plan profile on the outer surface, and an agreed profile on the inner to give the representative thickness and radius to the L/E lip and T/E chamfer to the nacelle.

Here's the front plug...

and here's the rear plug.

Simon did attempt to make some fibreglass moulds from these plugs at first, which was mostly successful, although a little damage to the plugs was incurred in trying to remove the moulds. It was having got that far that we agreed we should also try the vac-forming route.  I took these photos of the plugs whilst I was filling and sanding them up back to their original form prior to our first vac-forming attempts.  We were uncertain about how well a vac-form process would pull the ABS material over the lip and into the nacelle to form the inner wall.  We were in need of some expert opinion and I got in touch with Derek Gore at Sarik Vac-Forms in Bristol.

Edited By Phil Cooke on 17/11/2012 15:18:47

[Phil Cooke]

I sent Derek the pictures and outlined what it was we were trying to do.  I explained we were after an accurate profile on the outside with the process forming the L/E and T/E lip and running smoothly into the inner wall - at least 50mm of inner wall was required to meet my plans design intent - any less and the illusion of nacelle thickness would not be achieved.

Like us, he was unsure as to how well the inner wall would be formed, and we agreed to perform some trials with varying material thickness and process temperatures/speeds to see what we could generate.  I drove the plugs down to Derek in Bristol later that week where he continued to explain how the material inside the nacelle would reduce in thickness as a product of the process.  He was uncertain how much 'workable' material we would generate on the inner wall before it became too thin. We agreed to do trials in both 2mm and 3mm ABS sheet, I left the plugs with him and set off home back to Derby.

Derek called me later that day with the good news - the process was successfully pulling the material right down into the nacelle, in both material thicknesses - and he felt that a 60mm or 70mm inner wall was achievable before the material ran out paper thin. He popped the first test pieces in the post to me and within a couple of days I had these on my workbench ready for inspection!!

The T/E section is on the left, and the L/E section with its thicker lip is on the right.  You can see the material going paper thin towards the bottom of the inner wall.

Edited By Phil Cooke on 17/11/2012 15:42:27

[Phil Cooke]

The heat in the vac-form process had lifted the grain a little in the Lime plugs, but this could only just be seen in the vac-forms. Generally they were really impressive and the inner wall was certainly suited to my hand drawn plan intent.

There was very little difference between the 2mm and 3mm versions, other than weight and stiffness, and as I considered the lighter, 2mm forms to be more than adequately robust, conscious that the engines sit well behind the CofG on the model, I went with the 2mm sheet version.

I placed an order for 9-off fronts and rears, enough for the 3 models being built plus a spare front and rear for each of us should we damage anything either during the build or in future flight. The plugs would remain with Sarik Vac-Forms anyway so we could always make more whenever they are needed.

[Phil Cooke]

And just a few days after placing the order with Sarik Vac-Forms, this happened!

I'd like to thank Derek again for his help and excellent service in the development and manufacture of these bespoke ABS parts...top job Sir

Edited By Phil Cooke on 17/11/2012 15:58:37

[Tim Mackey]

Makes sense I guess Phil - those vac formed nacelles look the biz - I got my B52 ones off him too.

[Phil Cooke]

With my bench cleared of ABS nacelles, I set about making a kit of parts as per my engine plan. The spine and front and rear engine rings onto which the ABS forms would be mounted were marked out and cut from 3/16 ply.

Conscious of weight the ply components would be machined with lightening holes, so a bit of work with the scroll saw and the drill followed.

[Phil Cooke]

But eventually a kit of parts was achieved, seen here (one of the rings still to be lightened)

As well as the front and rear ABS nacelles, here trimmed up externally, we have a vac formed spinner which will be mounted onto a threaded bolt, and a clear exhaust cone (made from a supermarket soup container)

The spinner was actually copied from a nose cone designed for another one of Simon Cocker's models, it proved to be the ideal shape and size so we ran another batch off - thanks to Steve Davis at Vortex Vacforms for this one. A captive nut mounted centrally on the backing plate allows the spinner to screw on and off - to aid with with access when inserting your hands into the nacelle to tighten the winged nuts down onto the threaded rod in the pylons - my chosen engine retention method.

[Phil Cooke]

Each engine kit was then assembled. The central spine, the top and bottom braces and the front and rear cross members were glued together flat on the board. The front and rear ply engine rings were then glued square to this 2D assembly. Braces were then added at the 3 o'clock and 9 o'clock position to stiffen the structure.

Heres the engine 'skeleton' frame complete - with the spinner and exhaust cone temporarily fitted for the camera. The ABS vac-forms would slide over the front and rear ply engine rings, but before they could be fitted the inner section of the ABS had to be trimmed up and cut away, leaving my targeted 50mm of inner wall in place. Once done, they could be dry fitted - they formed a tight fit over the rings.

A ply engine mounting plate was added, carefully ensuring the alignment was in accordance with my drawing - 3 holes had been machined in each mounting plate to accept 2 location dowels and a single, central threaded rod for retention. As you can see, each engine is just over 1lb in weight before the central balsa planking is added.

[Bosh]

Awesome build of what's probably my favourite aircraft. I'm planning on building one in the future, Won't be anywhere near as big as this though

[Phil Cooke]

Hey Bosh, and welcome! Is your future A-10 likely to be a PSS model or EDF?

A few more pics of the engines before they are planked in 3/16th balsa and all that hard work on the skeleton frame is hidden forever The 3/4 front and rear views show the nacelle lip and the spinner and exhaust cone looking convincing, even in white!

The head on view from the front shows just how little 'line of sight' air gap there is - shouldnt look too 'empty' when flying around the skies.

[Bosh]

Posted by Phil Cooke on 17/11/2012 21:21:31:

Hey Bosh, and welcome! Is your future A-10 likely to be a PSS model or EDF?

I'm thinking EDF but this will be quite a way off I think. I need to get more build experience first though so I really do this model justice. Some flight time would be a good idea too

[Phil Cooke]

Before the balsa planking was added to the central section the inner skeleton was spray painted black.

Then the ABS nacelles were epoxied into place on the ply rings. Once dry, they too were sprayed black on the inner surfaces, then the planking began. The 3/16" balsa planks were positioned just proud of the circular ABS nacelles, that way they could later be rounded off down to their shape.

The balsa planking took me a while, it was pretty laborious work over a couple of nights, with each engine having 46 pieces to cut to size and fit. Here's the first planked engine before final sand to profile.

[MikeS]

Fantastic work and skill.

Mike

[Phil Cooke]

After a gentle sanding, the 46 balsa planks were soon conforming to the front and rear ABS nacelles and looking like a single rolled sheet. This surface will later be glassed, feathered into the ABS, but for now the engines were complete, bar a spot more painting.

With a short handled brush the insides of the planking was painted matt black.

The front 3/4 view shows the finished effect with the stub nosed spinner in place.

From the rear, the soup container exhaust cones looked a lot more realistic with a coat of paint!

This shot shows the engine pod mounting plates. The engines are designed to slip onto the pylons (which will be integrally mounted to the rear fuselage) - the 2 outer holes will accept a pair of 12mm wooden location dowels, the central hole will be for a short 10mm threaded rod, onto which a winged nut can be tightened down from the inside of the nacelle.

[Phil Cooke]

So with the engines complete, they were put aside for glassing along with the fins, the tailplane and the forward fuselage. These parts have actually gone into storage in the house to maintain sufficient work space in the garage.

The order I had placed a few weeks earlier for my foam wing panels from Barry Johnson at Foamwings.co.uk came through just in time, and with the bench clear I could focus on the wings next.

[Phil Cooke]

As previously described, the wing section as drawn on the power model plan was far from suited to life on the slope. Here it is for reference, I have no idea what the section is but you can clearly see the huge thickness and (like the full size) the massive under camber towards the T/E.

This view on the plan also shows the wing pod and wheel bay, which will of course form part of this PSS build thread in a while. One of the nice scale features of the A-10 is that the main gear doesn't fully retract, the wheel sits semi-recessed - and I aim to model this with proper rolling wheels which should work well for aerotow.

Edited By Phil Cooke on 19/11/2012 19:54:07

[Phil Cooke]

As a comparison, here's the wing section we've chosen for the PSS version, shown at the same section around where the wheel bay will be.

In building 3 x similar A-10s we were all after similar gliding performance, so once again there was some discussion and collaboration in the choice of wing section.  Simon Cocker has led on this, and the resultant design utilises an Eppler 374 for the centre panels which translates through the outer panels into an increased thickness (12%) Eppler 205 at the tip, in an attempt to keep that scale solid wing look when viewed head on. Both sections are well proven from the slope and should give the A-10 a steady going yet versatile performance.

The wing panels have been expertly crafted to our exact design spec by Barry Johnson at Foamwings.co.uk.  The sheer size of the foam panels meant I was wary about veneering them myself, besides Barry has the equipment and expertise to veneer to a very high standard using epoxy and vac bagging techniques.  The finish was superb, light and very strong.  Better still, he added a hardwood wing-joiner box right through the centre sections, holes throughout the cores for wiring looms (for the wing mounted servos and lights) and a pre-shaped Leading Edge in balsa.

Edited By Phil Cooke on 19/11/2012 22:46:08

[Phil Cooke]

So upon unpacking the big foam bundle received in the post from Foamwings, we have 4 huge foam wing panels, two inners, including the wing box assembly, and two outers.

The root section of the outer panel had been cut to include the required 7 degrees of dihedral as per the full size. With the inner and outer combined, each wing panel is 50" long, they will be plug mounted onto the side of the fuselage on a steel joiner and still require the characteristic curved wing tips adding before the full span is achieved.

[Phil Cooke]

Each wing half will be joined using a pair of ply dihedral braces, setting the 7 degrees as required between the inner and the outer panels. The bigger of the 2 braces will be married up to the ply wing joiner box for good load distribution throughout the structure. Each outer panel will house 2 heavy duty Savox servos, one each for the flaps and ailerons. There are also 2 LED lights planned for each wingtip, and the cavity for all this wiring plus the servo extensions was pre-cut in the foam by Barry to my spec.

The first task was to map all these parts out ensuring there were no clashes or thin sections before marking up the panels accordingly. The flying surfaces on the A-10 wing are massive, the ailerons are over 1/3rd chord at the tip, and Ive elected to reduce this a little for the model. The flaps have been simplified from true scale too - and Ive decided to maintain a common hinge line for both flap and aileron to ease the build - this way the hard balsa stock I use to line the wing T/E (along the hinge line) will also act as a full length spar for the outer panel.

You can see the aileron and flap marked out on the outer panels here. Servos will be mounted in balsa boxes identical to the fins and tailplane, exiting on the underside of the wing. In this shot I've added a 3/8th" balsa sub-tip on each panel and on the inner panels, Ive lined the surface that will mount up against the fuselage with 1/8th ply.

Edited By Phil Cooke on 25/11/2012 14:35:29

[Phil Cooke]

Here's the balsa sub tips sanded up to profile, ready to accept the curvy wing tips which will be made from balsa and blue foam. The hole is for the wiring to the wingtip strobes which I've had made to spec at BrainCube. More on this product later!

The outer panel is now fully marked up to start the cutting out of the ailerons and flaps. Once the nominal flying surface positions are marked out you can see the 1/8th" offset lines for the light ply facings required (plus a 1/16th" gap between each flying surface and the wing.) Once one panel is marked up satisfactorily then the measurements are transferred to the other panel. Servo box positions are marked up on the underside of each panel.

The 1/8th" light ply facings to the wing root (inner panels) have also been sanded to profile. This will give a robust wing/fuselage interface for use time and time again in the field.

The inner panels are also marked up for the ply dihedral braces which will be glued down and up against the wing box within the foam, I've sized these in 1/4" ply - 8" in the inner wing and 10" in the outer panel.

Edited By Phil Cooke on 03/12/2012 21:31:12

[Foamie Dave]

Cracking job !!!

[Olly P]

Lovely job, looking forward to seeing this beauty fly. My TB-3 has been delayed again this year, and is still not started. But the new shed means I will actually be able to build her!!

[Phil Cooke]

Before the outer panels were to be joined to the inners, I wanted to work up the flying surfaces to completion - by cutting them away from the foam wing and lining with balsa/ply. This did create a little issue in my head, as the flap runs right into the dihedral joint between inner and outer panel, so by removing it prior to joining you lose all incidence reference for accurate alignment of the last 6" of wing chord. But cutting the flying surfaces out and working them up after the wings are joined with the 7 degrees dihedral would be unnecessarily complex, so I proceeded to remove them up front whilst I could still lay them flat on the bench.

It's always a little unnerving cutting into a nicely finished foam wing, and the measurements were all double checked before a new blade was put into the knife handle. The depth of section was such that a fully inserted scalpel blade along the line from each side (top and bottom) still required a little working before the flying surface was free. Further material was removed from the released surfaces, separating the flap and aileron whilst allowing for the balsa and ply stock still to be added.