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1:8 Scale Wright-Martin H3 Engines (Hispano-Suiza 8F)


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I've wondered for a while where to put these pictures as they don't really fit in any single category. However, having read a number of build threads this morning with Roden and WNW Hissos in them I've decided to pop them here as the origins of the engine's design are resolutely aeronautical. They are also a little different from the normal fare and might be of interest. If the mods disagree, feel free to move it to where you think is appropriate. 

 

The models are scratch-built Wright-Martin H3 engines in 1:8 scale, but to further muddy the waters the engines I've modelled have undergone a marine conversion undertaken by the Auto Engine Works (AEW) of St. Paul Minnesota. Post war there were quite a few of these engines knocking about and AEW offered a conversion package that allowed them to be fitted in a number of very successful racing boats in the 1920s and 30s. I've produced both magneto ignition and coil ignition versions, the latter of which is destined to be installed in a model racing boat.

 

The H3 was a licence built version of the Hispano-Suiza 8F and manufactured in the US. This isn't the engine seen in the SE5a and various Spads. The 8F's capacity was increased to 18L from the 11.8L of the previous 8A and B engines resulting in an increase from 220HP to 300HP. I've seen it described as essentially a bored out version of the 8B, but that isn't really true. The engine is substantially larger in all respects as can be seen from this comparison of CAD versions of the 8Ab and 8F engine blocks side-by-side.

 

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The models were initially designed in CAD using original drawings and references, then 3D printed to produce mould masters. Silicone moulds were taken from these and the major parts either cast in resin and painted or 'cold cast', the latter being a posh term for resin casting loaded with powdered metal. All of the main 'aluminium' parts of the models were made with resin loaded with about 60-65% aluminium powder by weight. The same for the 'brass' parts, but with brass powder obviously. For those interested the build is described ad nauseam on my blog.

 

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The image below was taken before I put the exhaust header bolts in, which I almost forgot!

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As mentioned above, the red engine is destined to be installed in a boat model, so the fuel and water connections have been left unfinished at this stage.

 

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Edited by Twobad
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Stunning work! The details and mechanical representation are amazing, and I’m also impressed with the coloring and surface of the various parts. 

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I like something different and boy, they are that, top notch scratch building and modelling, just one point, where is the little puddle of oil found under all old engines....😉. Seriously though, awesome work.

 

I think a lot of people here would like to know more on the work process as it is so original, any chance of more?

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Posted (edited)
4 hours ago, Head in the clouds. said:

I like something different and boy, they are that, top notch scratch building and modelling, just one point, where is the little puddle of oil found under all old engines....😉. Seriously though, awesome work.

 

I think a lot of people here would like to know more on the work process as it is so original, any chance of more?

Yeah, I can do a quick precis here. There's a lot of info on my blog, but that would be a bit of a read for many.

 

The basic design is thrashed out in CAD. I still use AutoCAD because I've got a fair bit of experience in it, but were I starting again I'd use Fusion 360. Autodesk do 1 year free licences for non-professional users.

 

AutoCAD allows you to insert pictures into the work space and I usually start by obtaining various drawings of the engine I want to model. These are then scaled to full size and used to trace out the shapes required.

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Which leads to something like this.

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I then print that out on my home 3D printer. I have a Phrozen Sonic Mini4K, the 4K meaning that is has a 35um resolution. 3D printing is excellent when it works, which isn't all the time! It's a bit of a black art and I'd estimate about 60% of my prints work, the rest end up in the bin. In fact it feels sometimes like the main output of 3D printing , and resin casting for that matter, is alcohol soaked paper towels.

 

For reference, from top left: 35um 3D print, 50um print, cheapskate's postage stamp, 35um cleaned too vigorously before final curing. Some of the detail has been rubbed off. 

 

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How the prints emerge from the cleaning tank.

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The support structures are there to hold the pieces while they are being printed. The print comes out upside down hanging from a build plate immersed in the print resin. A UV light shines through an LCD array and transparent bottom of the resin tank and solidifies the resin in contact with the build plate. Then the build plate is withdrawn slightly and the next layer exposed, and so on. The prints in the photo above probably took about 4 or 5 hours to produce. Larger items like the cylinder block take about 7-8hrs. Parts are printed at an angle as it tends to hide the transitions between layers, especially on flat or nearly flat surfaces.

 

Then it's remove the supports and make silicone moulds from the cleaned up masters. I've tried several types of silicone and mainly use EasyComposites AS40 Addition Cure Silicone RTV. It is pretty thick stuff to pour and impossible to mix without getting a lot of entrapped air, but the big bubbles float to the surface quite quickly and the tiny ones seem to be absorbed during the curing process. I like the stuff because it is transparent so it's possible to see if any bubbles are trapped against the mould master. If there are (and there usually are) then you can help them out with a cocktail stick. You've got about 30 or 40 minutes to do that before the RTV starts to become too thick to play with. I do however, use a degassing vacuum chamber to pull the really large bubbles out quickly before pouring. Dedicated chambers are not cheap (£500+), but if you look around there are much cheaper ones about. You can also help to get rid of air by pouring the silicone from high up enough that it forms a very thin stream on the way into the mould.

 

Mould curing in progress.

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I use addition cure RTV (also known as platinum cure) because it has very low shrinkage (~0.1%). You can get condensation cure RTVs that are cheaper but they have 3 or 4 times the shrinkage which is enough to cause fitting problems for me.

 

After the moulds have cured (24hrs+) they are ready to use. 

 

I use a pressure chamber for my resin casting to crush down any residual bubbles entrapped in the casting resin. This is particularly important when cold casting as the mixture can be quite gloopy. The chamber I have is actually a converted paint spraying vessel. ( http://airtools24.com/pressure_tank_pt8,3,21206,27331 ). I also happened to have a spare air compressor with an 8Bar, 9L tank. That fully pressurized and vented into the pressure pot gives me an almost ideal casting pressure of around 57psi. Being able to charge up the compressor tank before hand also means that it only takes seconds to pressurize the casting vessel, which is useful as the casting resin I use has a 3 minute pot life.

 

The casting resin I use is Xencast P2 Fast Cast, primarily because it is err.. fast. 3 min pot life and demould after about 30 mins. It doesn't need degassing either. To cold cast with this I mix in 60-65% powdered aluminium by weight. In fact I put the aluminium in the mixing pot first, then dig a hole for each part of the resin. Then mix the whole lot up quickly, pour in the mould and then pressurize for 30 mins. More powder in the resin makes it thicker obviously, and I've found that 60-65% leaves it thin enough to flow, but have a good metallic effect. You will find that even with a 3 min pot life the powdered metal starts to settle in the resin and the bottom faces of the moulded part will have a noticeably more metallic finish than the tops, so position your part with this in mind. Brass is especially bad for settling out.

 

I use Lego bricks to make my mould bunds/setups, and also use them to support the moulds during casting to help keep things dimensionally accurate. I use a small piece of plywood to create a lip around the mould to stop resin pouring down the sides. Everything is done on a sheet of glass to keep things level and Pritt Stick used to secure the parts and Lego bricks to the glass.

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Before pouring the resin mix into the mould dust it liberally with powdered metal to further improve the surface finish.

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Et voila!

 

The finish is a matt grey fresh from the mould, which has its own attractions.

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To bring out the full metallic look a quick, and delicate, polish with some fine Scotch Brite is required. I've also found that the handling of the parts during the build process burnishes them up quite nicely and dampens down the fine scratches left by the Scotch Brite. Metal polishes do improve things a bit, but not as much as you'd hope.

 

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Astute readers will by now have noticed that these castings are for a Wolseley Viper which is in the build pipeline and may appear here at some point.

 

You can't get a truly polished finish on a cold cast part, but you can get quite a satisfactory metal look. Good enough for people to be surprised how light they are as they expect something heavier when they handle them. it is real metal though and will tarnish. I tend to polish the parts with a wax to try and delay this. 

 

I use quite a lot of photoetching to make parts too because I'm not very good at filing things out accurately. Rather than type out a great long speil on that I'll refer you to a post I did some time ago on my blog: Come And See My Etchings Upstairs. The results that you can get from home etching are quite surprising though.

 

This becomes...

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This...

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The artwork for the photoetch can also be used to make your own custom decals, which is what I did for the nameplate on the nose of the engine. That was made from some sheet brass with a decal on it. The decals themselves are simply printed out on decal paper using my inkjet printer. Then airbrushed with clear lacquer. When dry they can be used like normal decals.

 

The reason for the decal. My photoetching process isn't good enough for very small details. The top is etched. The bottom printed on an inkjet at 300dpi.

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I also do a fair bit of DIY plating with nickel, bright nickel and copper, but I'll leave that for now, unless people are interested in it.

 

 

 

 

 

 

 

 

 

Edited by Twobad
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I will second that, a very comprehensive write up ( for us anyway🤪).  I do find that people who have this ability make it sound simple but as you say there is a high failure rate at times.

 

Did you learn this purely for the hobby or is it part of your nine to five?

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My gob is smacked. 

Thanks for the mini-course in creating your own stuff btw, it shows more of what to do than many YT 'tutorial' out there :D

 

And nice touch on naming parts on the 3D print support base. That's attention to detail!

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I've dabbled in RTV/resin and as you say, it's definitely more magick than science.  3-d printing is a level of sorcery simply beyond my ken.

 

This is really some splendid work you're doing.  I doubt anyone will take offence to it being here, given that it is a) modelling, and b) at least tangentially related to aviation.  😎

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2 hours ago, Head in the clouds. said:

I will second that, a very comprehensive write up ( for us anyway🤪).  I do find that people who have this ability make it sound simple but as you say there is a high failure rate at times.

 

Did you learn this purely for the hobby or is it part of your nine to five?

Sort of both. I've been making models for almost 50 years, but I became medically retired over a decade ago and am housebound now. So I spend a lot of my time model making and designing things in CAD. I love trying new techniques too, so have gradually increased my repertoire over the years.

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Posted (edited)
1 hour ago, Jackson Duvalier said:

I've dabbled in RTV/resin and as you say, it's definitely more magick than science.  3-d printing is a level of sorcery simply beyond my ken.

 

This is really some splendid work you're doing.  I doubt anyone will take offence to it being here, given that it is a) modelling, and b) at least tangentially related to aviation.  😎

I had a load of problems with entrapped bubbles in my moulds and resin casts until I bit the bullet and bought a vacuum chamber and pressure tank. They make things so much easier. Basic resin casting is pretty run-of-the-mill for me now, but the cold casting stuff still tests me. You're constantly pushing the boundary of maximising metal content, but trying to keep the mix runny enough to flow in the moulds. Thin webs and small deep recesses need a helping hand with cocktail sticks. Even then I find it almost inevitable that there are tiny bubbles lurking just below the surface on sharp corners. Rubbing with Scotch Brite inevitable uncovers them and there is a need to mix up a tiny batch of cold cast to go around and fill them afterwards. I use a 7 minute pot-life resin for that to give me a little more time to mess about. The repairs are pretty much invisible after filing back flush though.

Edited by Twobad
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Posted (edited)
1 hour ago, alt-92 said:

My gob is smacked. 

Thanks for the mini-course in creating your own stuff btw, it shows more of what to do than many YT 'tutorial' out there :D

 

And nice touch on naming parts on the 3D print support base. That's attention to detail!

I can't claim the credit for that. The print files were prepared using some posh professional software and it did it automatically. At home I use the free Chitubox software which lacks such refinement. There's a Pro version, but it's £120/pa.

 

My home prints look like this.

 

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Edited by Twobad
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These are stunning pieces of work!  I am amazed at how you did this and the quality of your results.  This is amazing work.

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There's definitely room for subjects like this in my opinion. A part of modelling that most of us aspire to at some point is scratch building and this is how to scratch build, even the wire locking looks in scale. Very grateful that you've also taken the time to explain some of the processes involved. Superb work on every level from a master. More please....

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