Iconic Model Engines
by Stephen Wessel
Published 12 Aug 2021; Revised 23 Aug 2021

 

ENV Type F
60 hp Aero Engine (circa 1910)
Half-Scale Replica

This working true scale replica was constructed from the builder's drawings based on the one example in the United Kingdom and another at Old Rhinebeck Aerodrome, New York, U.S.A. The builder believes this model to be the only working ENV example anywhere.

The ENV Syndicate was a small company that began as the London and Parisian Motor Co. in 1908. By 1910 their water cooled V8 engines were in demand by pioneer pilots both in Europe and overseas. By the start of WW1 their designs were considered obsolete and usually replaced by the lighter rotary types. The company kept going however, producing specialist gearboxes, until the late 1960s.

The engine has several interesting features:

Model Specifications
Bore = 52.5mm
Capacity = 950cc
Compression Ratio = 4.8:1
Comfortable Running Speed = 2,000 rpm (maximum is possibly 2,500 rpm)
Propeller = Caudron Pattern 48" x 30"
Coolant = Evans Waterless

 

 

Video: ENV Running

 

Armstrong Siddeley Lynx IVc
215 hp Aero Engine (circa 1925)
1/3 Scale Replica

The Lynx was a one-row version of the more famous Jaguar, possibly the first truly successful radial engine designed in 1917. The smaller Lynx, offering around 200 hp, found much use in light aircraft such as the Avro 504 between the wars. Its final development resulted in the well-known Cheetah of which many examples exist.

There is one example of a Lynx kept in flying condition in the UK plus a couple of non-working ones in museums. No works drawings survived the blitz on Coventry in 1940 so the builder had to produce his own, making good use of the Handbook (pub. HMSO 1929) that is still available.

The builder believes this engine to be the only working Lynx model. It has won several awards.

Model Specifications
7-Cylinder Air-Cooled Radial
Bore = 1.67"
Capacity = 450cc
Compression Ratio = 5:1
Dual Ignition
Propeller = Avro 504N Type, 34" x 24"
Speed Range = 900 rpm (idle) to 2,750 rpm maximum

 

 

Video: Lynx Running

 

A Tale of Three Miniature Aero Engines and the Effort to Build Them
by Stephen Wessel

Introduction

For much of my life I have been bothered by the word ‘model’. Working as a professional mechanical engineer more years ago than I care to remember, the subject of ‘engineering modelling’ was and remains perfectly respectable with or without a computer, being an invaluable tool for solving all manner of technical problems, much as mathematical modelling is nowadays used to describe every human predicament or uncertainty from pandemics to climate change. However, admit to being a ‘modeller’ or ‘model engineer’ in some social circles and you risk being labelled as non-serious, amateur or worst of all, a maker of toys.

My current profession as a craft manufacturer of modern flutes, which has occupied me for nearly 40 years has, in recent times, lain comfortably alongside my hobby of model engineering, using the same workshop, the same tools and machinery, the same attention to precision, finish and artistry in overcoming technical problems, and very similar materials. I would go further to suggest that both aero engines and flutes are capable of producing the finest music!

The AEHS has invited me to write about the challenges involved in making miniature engines. There are many but for me the most fundamental one is this: How to justify the time, energy, expense and general solitariness on projects that may interest so few, have little or no practical use and ultimately very little value except as curiosities?

I shall have a little more to say about this later but suffice it to say that there is joy to be found in any creative pursuit, in between the bouts of technical troubles and guilt about being locked away in the workshop for days on end. Mountains are there to be climbed but although it may be vital to have an end in view I have found it a mistake to face all the likely problems at once.

Mountain ranges, when viewed from sea level, have a way of hiding even higher ranges beyond – if you could see them all at once you might never start. So one step at a time, savour each little creative success as it arrives but never forget to improve on the next one.

The Engines

To date I have built three small piston aero engines; the first was a Gnome rotary – the famous Monosoupape in 1/3 scale. This was suggested by a friend who planned to build and fly a replica of the one preferred by his father who had been a pilot during WWI. We think we may have been the first to fly a real scale rotary engine in the UK back in 2006.

The second engine was an Armstrong Siddeley Lynx radial, also in 1/3 scale. I had no plan to fly this one but chose it purely on the grounds that so few of the original 6,000 have survived; there is one left in the UK, kept in flying order by the Shuttleworth Collection, plus a couple of museum exhibits. It was without doubt the most complex and difficult project I had ever attempted.

The most recent engine turned out to be even more ambitious and certainly the most interesting from a historian’s point of view. The ENV Syndicate (as it was called in its second incarnation) was a small Anglo-French company set up in 1908 to produce many of the very first engines that were desperately needed by mainly European and British pioneer pilots.

For this quite small V8, I felt the larger scale of one-half might produce a sound somewhat like the originals, yet remain within the capacity of my modestly equipped workshop.

Three very different machines from three discrete eras in aviation history, each marked a technological leap and consequent obsolescence of those before. The early water cooled ENVs and others like them such as the Antoinettes, early Wolesleys, etc., lasted until about 1913, but were then rapidly overtaken by the French rotaries, revolutionary in more ways than one – air-cooled and super economical in materials, i.e., weight. During WWI, aluminium alloy research by the English Royal Aircraft Factory allowed development of the first practical radial engine. This was to become the 14-cylinder Siddeley Jaguar, fairly successful in the early 1920s. By then rotaries were practically extinct. The Lynx was a single row version of the Jaguar and appeared a few years later.

These are large models compared to the usual scales of 1/5 or 1/4, which means they run more slowly and produce a deeper, more satisfyingly throaty sound that it is all about. The ENV in particular, having a 48” prop, sounds very realistic indeed.

The Survivors

Of the three, the Gnome, or ‘Mono’, as we enthusiasts refer to it, is well known. There are many survivors; some are flying and some brand-new ones are even made to order. Models are also now quite common, some of my own design, published many years ago. People just love rotaries.

As far as I know there is only one Lynx that flies but several in museums around the world. I believe my model is unique.

There are a couple of ENV engines owned by the London Science Museum and two or three others in various states of disrepair around the world. None of them work or ever will. The company itself is almost entirely forgotten. Again, I believe my model to be the only example of a working ENV.

Difficulties

All of my engines had to be researched from scratch and working drawings made from what little remains in the various archives. I had to track down surviving examples, get permission to touch and photograph them in as much detail as possible. Bureaucracy and unwillingness on the part of some curators to help often got in the way. The ENV is kept in the Science Museum’s storage facility, no longer open to the public. I made two visits by appointment, after a long wait. I was allowed exactly one hour on each occasion before being turfed out (asked to leave immediately). On the other hand the RAF Museum at Hendon could not have been more helpful, providing a table, chair and good lighting for me to take details of their disassembled Monosoupape.

I do believe that suspicion is sometimes aroused if you mention the word ‘model’ in certain quarters but call yourself a historical researcher and nothing is too much trouble. This may well be a peculiar British phenomenon – I have no idea.

Work then began at home, sitting at the computer trying to make sense of all my sketches and notes, all taken in haste, with a large amount of muddle plus a few mistakes. In the case of the ENV, this was about all I had, apart from a few pages from Flight magazine written in 1910. The other two engines are well described, complete with small sectional arrangement drawings, in their manufacture’s handbooks, also now available. These handbooks, incidentally, are beautifully written in a simple, lucid style that puts to shame some of the long-winded, often ambiguous verbiage one finds in technical manuals today. Gradually I was able to construct my own detail drawings and plan how to turn them into metal.

Castings

By far the biggest hurdle concerning the Lynx involved decisions about casting methods and pattern making for the numerous alloy components such as crankcase, cover plates, cylinder heads, oil pump bodies and so on. I had some experience in the field but none in regard to lost wax casting, which looked like the only route towards the heads. This was at a time when 3-D printing of wax was in its infancy and not available to me. Nor had I any means of doing 3-D modelling on my computer. But back in the 1920s, they didn’t have any of that either and nor would they have used investment casting on a large scale. Pattern making was the most highly developed and artful kind of woodwork ever. One can only imagine the tensions between chief engineer, drawing office and pattern shop, as to what was considered possible or not, as for example the development of cooling fins, which needed to become ever thinner and closer together.

The first job was to seek out foundries that would be prepared to work with my patterns, for there was no way I should be attempting the hot part of the work; high-strength alloy casting is a highly specialised business and often very tricky. Furthermore, those castings would need subsequent heat treatment, another process impossible to do at home. The typical response from most was that they would only take the job on from the drawing stage and were unwilling to work with my patterns. Fair enough but I wished to make the patterns myself.

Eventually I found three; one for the investment casting in alloy, another for the same in stainless steel (valve rockers) and the third specialising in what they called RPM – rubber, plaster moulding. This allows complex one-piece patterns made in silicone rubber to be eased out of a plaster mould whereas the same pattern in wood would need many interlocking components.

It took two years of spare time to work out both how to cast wax cylinder heads and how to make rubber patterns for the three main crankcase parts. The wax became a waking nightmare. As a one-off model maker one constantly comes up against what I call the ‘small quantity syndrome’. If you only want ten of something, say, as opposed to many thousands, then you don’t feel like investing in all the proper equipment; you improvise instead, using whatever comes to hand, always trying to protect your wallet. There was a particular moment that remains vivid to this day when I hurled all my wax moulds into the bin, turned my back and vowed to give up the whole impossible project there and then.

Most thankfully, a week later they were out of the bin again, sitting on the bench and demanding a new approach. One must not be beaten by these wretched demons!

By comparison, the ENV crankcase, which admittedly is a large and complicated casting with a lot of interior ribbing, was straightforward. Conventional patterns made chiefly of PVC plate plus several very complex core boxes, were thankfully accepted by the foundry that did an excellent job using traditional methods and sand. I hope it’s merely a coincidence that just a short while later it went out of business! I know that were there to be a repeat order, it is unlikely I should find another foundry willing to take it on. CNC machining, or 3D printing would be offered instead, resulting in a very different look, somewhat at odds with my sense of what is appropriate for a scale model, all at a price which I could not afford.

Authenticity

Early decisions were made about how far I felt I could deviate from exact scaling of every component, bearing in mind that the models were always intended to run. There was also the related question of materials, the original specs often unknown or no longer available. For example I was entirely within the hands of the foundries where alloys were concerned. We have no idea what might have been used in 1910, but whatever it was, the modern alloys look the same, probably pour more easily and have improved mechanical properties.

Another example is the very important subject of seals and gasket materials. I doubt that synthetic rubber goes back that far, certainly not O-rings, while asbestos is often mentioned as a sealant. We have a host of both solid forms and liquid sealants, so it would seem masochistic not to use them, especially as they are hidden from view. Ball races have developed such that we now have angular contact types which can replace the traditional mixture of radial and thrust bearings with all their associated spacers.

Then there is the matter of scaling effects on fluid flow which generally lower efficiency as the scale becomes smaller. I guessed these influences would not amount to much in the scales I had chosen, except where carburettors were concerned; they would need much thought. Neither can you scale down electricity! High-tension circuitry also needs care to prevent flashover to ground at certain places.

There are aspects of casting that also introduce scaling problems. I found that the ENV crankcase needed to have a general thickness of about 4mm in order to get the metal to flow properly, rather than the preferred scale thickness of 2.5 - 3mm. So this would represent a large gain in weight. The surface finish was also over-scale despite the use of fine sand. I dealt with this by smoothing it all over with various sanders then blasting it with 400 grit.

Then there are the “missing” parts components hidden from view on the original (or perhaps actually broken off). The ENV oil pump, for example: I could see and measure the body while the scant description I found referred to a plunger type driven from an eccentric on the crankshaft. So I had to design it pretty well from scratch, working it into an extremely tight space; likewise the ignition distributor.

Ultimately a model can be what the builder wants it to be, perhaps a true scale ‘glass case’ type wherein every single nut and bolt, including the threads, is exactly scaled but where there is never an intention to run it. Or it might be true to scale but made in a completely different material such as plastic, gold or glass. But for those of us who at heart want to emulate the sound and fury of the real thing, a few small sacrifices to practicality may be quite in order as long as the appearance and overall design are correct. None of them will be truly 100% authentic.

Electroforming

The ENV engine is water-cooled and in common with most others of the period had separate cylinders, each surrounded by a thin jacket, in this case copper, deposited electrolytically. The company was proud of the fact that their jackets, being truly bonded to the various cylinder flanges, were not dependent on rubber seals and could not leak. The process they adopted was outlined in a publicity piece in Flight magazine, dated October 1910. I copied this, with some modern day adaptation, finding it perfectly feasible but extremely difficult and time-consuming. A particular issue concerned preparation of the steel cylinder surfaces which for copper plating requires the use of cyanide compounds. To avoid having to use these, I first had to attach thin brass extensions to the steel flanges by silver soldering. Copper-based alloys are easier to clean and plate on to. With hindsight, a better plan would have been to nickel-plate the whole cylinder beforehand and then deposit the copper onto this without the cyanide.

The main copper areas were laid down on to a temporary substrate of a low-melting-point white metal that was cast all around the cylinder (leaving those flanges exposed). This is melted out afterwards in a domestic oven to leave a ¼" gap all around between cylinder and the newly formed jacket of about 0.5mm thickness. I used the same technique to make the copper induction pipework, which comprises a lot of sharply curved thin-wall tubing.

There is no doubt that plating is something of a black art requiring much experience. One difficulty I had was to get a reasonably even deposit without being able to measure it directly. The overall weight deposited per cylinder was easy to compute, but would there be thin spots perhaps due to inadequate stirring of the electrolyte or placement of anodes? Well yes, there were a few but the jackets are so far holding up well and cyclic heating and cooling as a result of running the engine have not disturbed them. Nor do they leak!

Running the Engines

The Mono rotary is, like its full size brothers, harder to control than the later engines. It has no carburettor, so relies on fuel control and the blip switch. In the air it is very realistic and prone to reminding the pilot about gyroscopic forces. Although it is a four stroke, the cycle is unusual; the single valve operates for both inlet and exhaust; it opens early in the power stroke thereby sacrificing efficiency for simplicity. We found flying it hugely exciting but completely nerve-racking.

By comparison, the Lynx radial is much better behaved. It has the benefit of dual ignition and would probably fly very well were I to risk it. Early radials were notoriously dirty, burning their way through gallons of oil, and this model is no different; this is another plus on the authenticity scale but with the added risk of upsetting modern sensibilities.

I particularly enjoy running the ENV, partly because I think it’s the only working example in the world and partly because it is big enough to sound (probably) like the originals. It suggests a sort of steady reliability, which might, if I were about to launch myself across the Channel in 1910, as did Tom Sopwith in search of prize money, fill me with some confidence, although it is well to remember that most of those early engines were wrecked, along with the aircraft and the many aspiring pilots brave enough to fly them.

So here is, at least, one definite reason to build models – they can sometimes vividly bring history alive, reminding us of what went before. When there are few if any surviving documents and no one from the era still with us, perhaps it’s the only way. And yet there is a final problem, namely that model engineers tend to be somewhat publicity-shy. If we the makers are unwilling to show our work to as many people as possible, talk about it and dare I say it, write about it; if the models are allowed to fester in museums or collect dust on a back room shelf, then they will join the repository of forgotten archives and become irrelevant. So it’s up to us and thank goodness for YouTube!

 

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