Robey Gunbus

It all started with an episode of Downton Abbey featuring a charabanc.  I remarked to my wife, “I’ve never heard of that word ≪charabanc≫.  It is a cool word and I wonder if there’s a model kit of a charabanc”.

There was a Pirate Models 1925 SOS Charabanc in white metal 1/76 but that was going to be hard to find.  There was, however, plans for a scratch build in an Airfix Magazine Annual, written by Gerald Scarborough.  The edition was a wonderful insight into the modeling world of my youth but with techniques for adults and hence beyond me at the time.  So, I decided to purchase all 7 volumes of the Airfix Magazine Annual and in Volume 5 I discovered a scratch build for the Robey Sopwith Gunbus – a Daimler lorry towing a trailer with a Gunbus without outboard wings or tail.

You can see the photo that intrigued me and Scarborough’s finished model here.

So, I thought this would be a good challenge for me – I would scratch build the Daimler lorry and trailer and then emplace a Gunbus kit, suitably de-winged and detailed. I could see that there were Gunbus kits by Pegasus and Merlin that would give me the parts I needed.

Whoops!  Those Pegasus and Merlin kits, which I did manage to track down, were for a Vickers FB.5 Gunbus which though sharing a resemblance to the Robey Sopwith Gunbus was decidedly not the prototype.  The fuselage was different and the Vickers Gunbus had a rotary engine whereas the Robey used an inline engine.  Plus, the kit parts in the Pegasus and Merlin were, for the most part overly thick and crude. Teach me to search on a single keyword “gunbus”.

So, it looked like I was going to have to replicate the scratch build instructions provided by Scarborough. Fortunately for me, Stephen Foster wrote and illustrated a comprehensive full Robey Gunbus build that I referenced many times.

Raw Materials

• The 1/72 plans in the Airfix Magazine Annual No. 5
• Airfix 1/72 DH4 which I used for the wings, propeller, manifolds, and wing struts
• Pegasus Vickers FB.5 Gunbus for the landing skid, cockpit seat (one) and wheels (all white metal) . I also cut up the Pegasus fuselage for bits to capture the fuselage top view curved surfaces but this could have just as easily been done with sheet styrene.
• Sheet styrene in thicknesses as recommended by Scarborough. This was used for the rudder, fin, and fuselage
• Brass wire for the boom Vees and boom cross bars (plus a soldering iron)
• Styrene rod for the boom vertical struts. Plus landing skid-to-fuselage/lower wing struts.
• Brass screen for the radiator (you could also grid score sheet styrene)
• Uschii vanRosten rigging line (0.5 mm)
• RFC/RNAF serials (decals)  from Pegasus Models/Freight Dog Models 72005
• Patience (and then some)

Spoiler alert – I ended up buying two DH4 kits because I messed up the wings on the first attempt; especially the struts and attachment points for the boom Vees.

This scratch build was an excellent lesson in the maxim – measure twice, drill, cut, or solder once.  The key for me was to build a couple of jigs to get the correct angle for the lower boom and to properly position the fuselage over the lower wing. I learned this lesson after jig-less hubris led to misalignment and a period of negative modeling. Note the fuselage doesn’t sit on the lower wing making for a tenuous connection point upper wing-strut-lower wing with the fuselage glued to the lower 10-40% or so of the strut (do your own measurements from the plans after you build the fuselage and radiator).

Paints Used

Sub-assembly	Paint
Upper wing surfaces	MRP PC-8
Underside of wings; Nacelle sides/underside	AK 2292 Clear doped linen V2
Top of nacelle; fin	Vallejo Model Air UK Azure Blue 71.108
Wheels; Tires	Model Master Acryl Flat White 4769; Lifecolor Tyre Black UA733
Rudder	Model Master Acryl Flat White 4769; Xtracolor RAF Roundel Blue X30 BS110; Humbrol 174 (red)
Struts/boom/skid	A five part application (all Vallejo): Base color: Model Color Tan Yellow 70.912 Model Air Golden Brown 71.032 Model Air Camouflage Brown 71.035 Model Color Transparent Orange Model Air (thinned) Burnt Umber reference: Encyclopedia of Aircraft Modeling Techniques Volume 3, Chapter 7
Engine	Vallejo Metal Color Burnt Iron 77.721
Coaming	Vallejo Model Color Leather Brown 70.871
Seats

Assembly Order

This is a model where you have to think hard about assembly order as the aircraft is inherently fragile in how the fuselage is attached to the wings, how the boom is attached to the wings, and how the fin/rudder is attached to the boom. You’re not building a 1/48 Mustang here. And, since you are scratch building everything, you can’t assume you’ll make the parts perfectly.

1. Nacelle/fuselage
2. Engine, radiator, and mounts. Once the mounts were added, holes were drilled in the back of the fuselage to secure the engine to the fuselage
3. Boom Vees and cross bars. Make the Vee longer than the plan as there’s an alignment step that requires the rudder/fin and wings to be done.
4. Rudder and fin.
5. Wings
6. Jigs for attaching boom to wing; fuselage to struts
7. Boom to wing
8. Fuselage to Wing; Wing struts
9. Boom struts
10. Boom and wing strut rigging
11. Jig for nacelle and boom for landing struts/skid
12. Landing gear rigging
13. Propeller
14. Final boom rigging
15. Wheels and tail skid

Step 1 – Nacelle/fuselage

I followed the instructions in the Airfix Annual and used sheet styrene to form the nacelle/fuselage. For the curved upper fuselage and openings for the two cockpit positions I attempted to use the overly thick fuselage from the Merlin and Pegasus Vickers FB.5 Gunbus. This was more trouble than it was worth as lots of sanding and scraping and thinning with my Proxxon was required and the results were at best adequate.

There was a decent white metal seat in the Merlin Fb.5 kit that I attempted to use as a mold using Blue Stuff and Plasmo’s technique using two-part epoxy but the results were unsatisfactory. I tried three times and gave up, salvaging only a lower seat out of the process.

Once all the sanding was done, I painted the fuselage. (see later steps)

Step 2 – Engine, radiator, mounts

There’s a grainy picture of the engine that wasn’t much help so I speculated a bunch and used the Airfix plans as a guide. I added some detail with Evergreen styrene and some wire. I made an assumption that the engine would have spark plug wires and added these. Once the engine is between the wings, it is too dark to even see these. The manifolds were from the Airfix DH.4 kit

The radiator was cut from sheet styrene and I used 1/350 photo etch screen for the grill. From the photos, it appeared that the engine was on mounts and the mounts were extended on beams to the fuselage. I built all this from scratch and affixed to the fuselage. The engine was cockeyed (which you can see in the pictures below) and I later ripped it out and redid it (without photos)

Sidebar – Workplace out-of-control

Step 3 – Boom Vees and cross pieces

My first attempt at doing this was to tape brass rods directly to the plans, file off ends at the vertex and then solder. Then I fitted cross pieces and soldered those. This was right for the top boom but led to an incorrect length for the bottom boom as the plans are top-down and I didn’t account for the angle from the lower wing to the tail of the lower boom. Of course, I didn’t realize this until I attached the booms to the wings and discovered that the two vertices weren’t aligned in the vertical plane (making it impossible to affix the tail). Negative modelling ensued and I ended up rebuilding the entire boom.

The vertical struts for the boom would come later once I had the boom attached to the wings and the tail attached to the boom (and everything lined up).

Step 4 Rudder and fin

This was straightforward. I traced onto styrene the template, cut it out, sanded, added ribs and then painted. Four colors (see above) and then I added the serials, one digit at a time from the Pegasus Freightdog Models sheet. The small hole at the bottom of the fin was used for a brass rod that would sit atop the lower boom Vee near the vertex and provide a secure gluing point (I learned this from the Foster article).

Step 5 Wings

I started with the Airfix DH4 wings and reshaped to match the plans. I did this wrong the first time and made the lower wing too wide. I also initially measured the boom cutouts to match the plans but this led to just not quite enough wing thickness and angle to insert the booms securely. If you build this model, you’ll know what I’m talking about. Don’t use these pictures as reference, I messed up and redid the wings but sadly, didn’t take photos (negative modeling had put me in a sour mood). Look at the plans over and over, measure twice, cut once.

Holes for the struts were marked (don’t forget the landing struts). It is critical that the inner strut holes be located so the forward strut can be glued to the fuselage.

Step 6 – Jig for boom and struts

You need something to support the fuselage as it doesn’t rest on the lower wing and you need something to rest the lower boom on at an angle. See step 8 for the jig I constructed.

Step 7 – Boom to wing

With the jigs built, I drilled holes in the wing edges for the boom. The holes need to be drilled at the appropriate angle and precisely placed so they are just big enough to fit the booms. You have less than 1mm of margin.     

I taped the plans down, then secured the jigs to the plans.  Dry-fit the booms to the wings. This is crucial as you want the vertex of the boom Vee to hit the center line on the top view.  You’ll also dry fit the rudder/fin to the boom vertex. The two vertices need to be vertically aligned so that the rudder/fin is as per plan. Inevitably, this will mean you’ll be trimming the open end of the Vee to make sure this happens. Some minor sanding/trimming might also be required in the upper or lower wing boom cutouts so the Vee clears the plastic.

Side note: The Foster build elected to put the booms atop the wings rather than into the wing edge. You’ll need to follow his advice if you want the boom recesses to match the width on the plans. I opted for extra strength via rod-into-hole and made the boom recesses wider by a bit than the plans. You’ll make your own decision based on what you end up using for your wings and how thick they are.

Step 8 Fuselage to wing; Wing struts
A recess was chiseled out from the front underside of the upper wing to match the width of the radiator. I used a .5mm chisel from Micromark. This provided a positive attachment point for the radiator as this is a critical joint.

Then, the lower wing was taped to the plans. A spacer plank of sheet styrene was used to hold the fuselage away from the lower wing at the proper distance.  This plank was affixed with sticky tack to the lower fuselage and everything lined up and centered. Now I could attach the upper wing to the top of the radiator while verifying alignment against the plans.

With all this lined up, it was time to measure the spots for the struts. This took some iterations. I used a pencil so I could erase mistakes.  Then, after measuring at least twice and making sure the markings were aligned, I chiseled out recesses for the struts.

 You need some positive attachment points when you emplace the struts for strength as well as proper placement.  Be sure to also do the underside of the lower wing and the underside of the fuselage.  These will be used for the landing skid struts.

Note that outer struts are going to be longer than the inner struts due to the upper wing dihedral.  Start by cutting the DH4 struts as long as possible, then as you dry-fit, shorten as required.  Properly measured, I took my time and did the inner struts that were affixed to the fuselage and lower/upper wings.  Clamp these to make sure the struts are securely glued to the fuselage. 

Then I moved onto the rear inner struts; and finally the outer struts (remember, these are longer). I used Tamiya Extra-Thin for all of this.

Step 9 – Boom struts

With the wing struts done and a major sigh of relief, next up was measuring and dry-fitting the vertical boom struts. I used styrene rod. This adds strength. Paint touch up will be needed.  It was at this point that I began to think I could actually finish this model successfully. Yes, there was a lot of rigging to do and I still had to measure and affix the landing gear struts.  

Step 10 Wings and boom rigging

I did most of the rigging before doing the landing gear struts as everything was going to be more stable while the lower wing was secured to the plans (and everything else was secured to the lower wings directly or indirectly. I started with too fine of elastic line (.01 mm) and decided to switch to .03 mm. You need to leave out some of the rigging near the trailing edge of the wing so you can add the propeller once you have the landing skid secured.

Taping the boom down to the jib made for a secure rigging platform. Rig from the inside out. Everything was attached with superglue.

Step 11 – Jig for landing gear; Landing gear
The Merlin model had a perfectly serviceable and correctly sized skid in white metal. This was painted to look like wood using the same techniques as used for the struts

Another jig was built to ensure the fuselage wasn’t skewed in either the x, y, or z axis for measuring / affixing the landing gear struts.

The first jig I built wasn’t right as the fuselage wasn’t horizontal.

My handy wood chisels solved the problem as you can see in the corrected jig below

Each strut was carefully measured and dry fit. Locking tweezers were crucial here. Having two (2) DH.4 kits to steal struts from proved essential as I used up all of them (if a strut got cut too short, it was useless). The 6th strut proved recalcitrant when inserting with the model upright but inverting the model made it easy to complete this.

Step 12 – Landing gear rigging

In inverted state, I added the landing gear rigging. No surprises here and all the rigging stayed put.

Step 13 – Propeller

I used the Airfix DH.4 propeller because it was 4-bladed. The fun part was painting it to look like wood. The picture shows one of the steps as outlined in the Encyclopedia of Aircraft Modeling Techniques Volume 3. Results were pretty decent in 1/72.

I had long ago dry-fitted the propeller to the engine as there would be no room for fiddling. The propeller can’t be added until the landing gear is on as otherwise it will hit the workbench surface.

Step 14 – Final boom rigging

Once the propeller was secured, I could rig the forward section of the boom. Thank goodness for elastic line. My stretched sprue skills are terrible at getting consistent diameters. All those pro modelers from the 1970s are scoffing at me.

Step 15 – Wheels and tail skid

At this point, horror-struck, I realized that I needed to test fit the landing gear to the trailer. If it were too wide, I’d need to rebuild the trailer. The wheels go in chocks so the hubs will clear the outer trailer bed frame but it was a close run thing (microns)

Ignore the boom not being square on the trailer rear support – I took the picture too soon. Trust me, it does fit.

Final thoughts

What would I do differently?  I’m not sure that all the effort at modifying the DH4 wings was worth it.  I could have built the wings from sheet styrene, then added ribs from half round styrene and do sanding to get the proper profile, leading, and trailing edges.  I couldn’t find any copies of the Airfix Avro 504 which was the basis for Scarborough’s and Foster’s builds. 

I should have airbrush shaded the ribs on the rudder and wings before I affixed them to the boom; instead, I ended up using AK Interactive weathering pencils to accent the wing ribs.  I tried hard to cast a suitable duplicate of the Pegasus seat but this failed and I ended up with something cruder for the front cockpit seat.

And of course, I was sometimes too clumsy with the superglue for attaching the rigging. This was finnicky stuff and it took me several days to rig it all – each day I might manage 4-8 strands before I could tell I was going to get sloppy and make a mistake.

I would definitely build the trailer after building the plane to make sure the landing gear fit inside the trailer bed. But, as this was a 100% scratch build project, I did the easier pieces first as a confidence-building exercise.