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Before There Was Elfin.......the Aerol Engines
Indeed, it's actually a matter of some historical importance that I do so. The fact that the Aerol engines have hitherto been very sparsely documented has left many of those who are interested in the early history of British model engines blissfully unaware of the true significance of these designs. I hope that by presenting the Aerol models in the context of their time and place, I may be able to demonstrate that they were in fact highly instrumental in leading British model diesel design from the past into the future. The need to do justice to the memory of their designer and manufacturer demands no less.
Since Ron’s heavily encrypted site was left in early 2014 in an inaccessible state with no access codes, correction of this error is no longer possible. Indeed, further losses over time are inevitable given the impossibility of carrying out any of the required maintenance on Ron’s site. This fear is behind my ongoing efforts to transfer as many as possible of my earlier articles to my own site. The Aerol saga is far too interesting and important a story to allow it to become lost, hence my decision to re-mount it here on my own website. So here goes …………… Background
Finding this location today proved to be a challenge because it turned out that there were in fact not one but two streets of that name in Liverpool! The more prominent edition of Henry Street is located just up from the Salthouse Dock on the River Mersey. Today this street is dominated by hotels, restaurants and apartments which are an element of the trendy City Centre area of Liverpool, but it still retains hints of its former industrial character. I assumed initially that this must have been the location of the Aerol Engineering works. However, my valued friend and colleague Jim Woodside was able to confirm that the business of Aerol Engineering was actually conducted from an entirely separate edition of Henry Street located well to the east of the City Centre in Liverpool 13. This street still existed in 2009 in the form of a cul-de-sac leading south off Edge Lane, but it was quite hard to find since it didn’t come up in a Google search for Henry Street, Liverpool despite appearing on a then-current Google map of the Liverpool 13 area. It has subsequently disappeared completely. Reference to a currently-available historical street map of Liverpool shows that Henry Street was a very short street leading south off Edge Lane to connect to Lear Road just to the east of where the railway line passes under Edge Lane. The attached location map is Comparison with a present-day Google map suggests that the disappearance of this version of Henry Street resulted from its having been built over during the construction of the massive Liverpool Shopping Park which began in 2010 at this location. At the long-ago time of which we are speaking in the present article, the area was characterized by a mix of housing and industrial activity - for example, the factory where Meccano erector sets were made (under the direction of then well-known aeromodeller Doug McHard, now deceased) was located very nearby on Binns Road. The Aerol Engineering venture was the brainchild of the previously-mentioned Frank Ellis, who had worked during WW2 at the nearby Dunlop plant located at Speke in Liverpool 24 and had subsequently gone into the scrap metal business at Henry Street. The making of model engines was apparently a labor of love for Ellis, who had constructed a number of diesels in his spare time during the war. Moreover, there was a direct connection between his two business interests, since the Henry Street scrap-yard supplied the bulk of the raw materials for the new company's products. Start-up funding for the Aerol Engineering venture was provided by Frank’s friend Col. Thomas E. H. Davies of the well-known Liverpool cotton brokerage firm of Davies, Benachie & Co. Col. Davies was very impressed with the prototype model engines that Frank had developed in his spare time during the previous few years. The Aerol Engineering name was adopted to keep the model engine manufacturing separate from the scrap metal business. Col. Davies took on the role of distributor of the Aerol engines as a sideline to his cotton brokerage business. The original Aerol production facility was housed in a tin-roofed metal lean-to shed in a corner of the scrap-yard! The acquisition of all-new equipment was well beyond the financial capacity of the new venture. Accordingly, the business was started using a very basic inventory of well-used wartime machine tooling obtained at modest cost from Pratt's in Manchester. This original production equipment included a Herbert capstan lathe, a centre lathe, a drill press and a Delapena honing machine, plus a bench grinder for sharpening tools. All of this equipment had given its best years to the war effort before Aerol ever got hold of it.
However, all of these designs looked to the past, and Frank was far more interested in where model engine design was going than where it had been. He was keenly aware of the Arden spark ignition engines from the USA, having seen a picture of an example, although he apparently never saw any of the internal details of that design. At the time, America was recognized as the world leader in model engine design and production. Frank liked the look of the Arden, feeling that it pointed the way ahead in design terms rather than looking back over its shoulder. How right he proved to be …...! This led directly to the concept of developing a diesel along Arden lines as opposed to simply repeating the FROG, Mills, Majesco, Kemp or E.D. pattern. Frank recognized from the outset that the well-equipped American manufacturers with their huge market potential and consequent solid economic base could do better and more cost-effective work on a mass production basis. However, in the context of the far smaller British marketplace, Frank felt that he could design a diesel engine along similar lines that could be made to a sufficiently high standard in commercially-viable numbers with the very limited equipment available. Given the latter constraint, simplicity of construction would clearly be a major design factor in achieving this objective. The result was Frank's first commercial engine, the 2 cc (0.12 cuin.) Aerol Gremlin diesel, which appeared with little or no fanfare in around June 1947. At this point, some readers may be asking themselves - "Why 2 cc?!?" No doubt a rather odd displacement when viewed from a present-day standpoint, but this is a matter which has to be considered in a late 1940's context as opposed to a 21st century one. At the time in question, the "magic" displacement of 2.5 cc (0.15 cu. in.) was still some years away from being adopted by the FAI for international competition, resulting in the competitive use of engines covering a very wide displacement range in the same contest categories. Under these circumstances, no particular displacement really had much of an edge over another. The only issues were what size of model was desired and whether or not a given engine could do the job of flying that model to the standard required. This was particularly true from the standpoint of the sport fliers who then (as later) significantly outnumbered their serious competition-minded colleagues.
Aerol themselves continued this trend in mid 1948 when they released the first version of their iconic Elfin 1.8 model which replaced the Aerol designs which we are now discussing. Indeed, as late as April 1949 the “K” Model Engineering Co were to be found releasing their Falcon 2 cc diesel design, which they followed up even later with their Kestrel and Tornado models of similar displacement. Seen in this mid 1947 context, there was nothing at all extraordinary about Aerol Engineering releasing what was doubtless intended to be a "popular" new model in the 2 cc displacement category. It was only later, when the British Class 1 (later ½A) competition displacement limit was set at 1.5 cc and the 2.5 cc limit was formally adopted by the Society of Model Aeronautical Engineers (S.M.A.E.) for the Class A team race category (which later morphed into the FAI International team race class) that the attention of British manufacturers became progressively more focused on engines built to the new competition limits, leaving the 2 cc displacement category as something of an orphan lying between competition classes. Having briefly set the scene for the introduction of the Aerol Gremlin, let's take a close-up look at this very interesting design. The Aerol Gremlin Prior to the appearance of the Aerol Gremlin, British model diesel engine design had generally been somewhat stereotypically based upon the classic sideport arrangement which had been well established in spark ignition form prior to the onset of WW2. In large part this was due to the fact that model engine development and manufacture had ceased in Britain upon the September 1939 outbreak of WW2, not to be resumed until that conflict had ended. Not unnaturally, British manufacturers initially resumed their activities on the basis of the same design layouts with which they had become familiar before the war. Early British diesels such as the Mills 1.3, the B.M.P. 3.5, the Majesco 2.2, the AMCO .87 Mk. I, the E.D. Mk. II and Seen in the context of these "traditional" British designs, the physical appearance of the Aerol Gremlin was little short of revolutionary in a mid 1947 context - nothing even remotely like it had ever appeared previously on the British market. It's actually a little difficult today to fully appreciate the extent to which any modeller who had cut his teeth on the established design "formula" would have been totally gob-smacked upon sight of the Gremlin! The very sturdy engine featured updraft FRV induction in combination with a sleek, well-coordinated and undeniably futuristic appearance free from all extraneous elements, in sharp contrast to the knobbly look of all the other contemporary British offerings. Put simply, the Gremlin was a far more sophisticated and forward-looking design than any of its British contemporaries - in this respect, it represented a quantum leap forward. Full marks to Frank Ellis for leading the way!
The Gremlin was built up around a pair of very sturdy castings (crankcase and radial tank mount) which were gravity die-cast in piston alloy doubtless obtained from the scrapyard. In later conversations with family friend Jim Woodside which are summarized elsewhere, Frank Ellis recalled that he made the dies himself. However, the actual castings were produced by a local foundry in Liverpool, this being the sole aspect of the engines' production that was contracted out. Frank was very pleased with these early radial-mount cases because they were both strong and extremely resistant to distortion. He had far more trouble with durability and distortion when Aerol Engineering subsequently switched to the considerably less substantial pressure die-cast cases used on the later beam-mount Elfin engines.
At least, that was true as far as the engine's general external configuration went! Internally, it was a very different story. In particular, the cylinder design departed significantly from that of the Arden. The latter engine featured a screw-in steel cylinder having very thin integrally-machined cooling fins. By contrast, the Gremlin cylinder was an unthreaded steel component of plain tubular form having a location flange part-way down Apart from the different method of construction, the Gremlin's cylinder porting arrangements were very different from those of the Arden. It must be recalled that by his own later admission Frank Ellis never had the opportunity to examine an Arden personally, leaving him without guidance regarding the cylinder porting design. As a result, he was free to come up with his own concepts. The system actually adopted was a highly unusual form of reverse flow scavenging. The cylinder liner featured a tight group of three transfer ports which were drilled at an upward angle below the cylinder location flange. The exhaust chores were handled by a pair of radially-drilled holes which were placed above the location flange. Most unusually, these latter ports were radially centred at only 60 degrees in a counter-clockwise direction (viewed from above) from the central transfer port location. The accompanying view at the right should make this clear. The movement of incoming transfer gas was thus directed away from the exhaust ports rather than having a velocity component towards them - the fundamental principle of reverse-flow scavenging. The idea behind this is to limit the opportunity for incoming mixture to short-circuit the scavenging process by escaping directly through the exhaust ports. This goal is accomplished by having the transfer gas move more or less in the directly opposite direction from the escaping exhaust gasses as it enters the cylinder. The advantage of reverse-flow scavenging is that an upstanding baffle is not required to achieve such movement.
This step was apparently created by hand-filing, implying that at this stage the Aerol workshop still did not have a milling machine available. Indeed, the entire Gremlin design was clearly influenced by the evident fact that the Aerol company lacked milling capability at the time of its introduction. The three grouped transfer ports were fed by a shallow bypass passage formed internally in the crankcase at the front of the cylinder installation spigot. This dictated that the transfer ports be located at the front when the engine was assembled. In turn, it required that the piston step be formed at the front of the piston crown. In my example, the piston step is actually positioned a little to the left (viewed from the front) of the engine's axial centre-line. This may be an incidental error arising from the hand-filing operation, or it could be an intentional provision having the objective of directing the incoming transfer gas further away from the exhaust ports - in the absence of an opportunity to examine other examples, it's impossible to be certain one way or the other. The use of a piston having a step formed in a specific orientation eliminated any possibility of following the Arden lead by employing a ball-and-socket small end bearing for the con-rod, since this would have left the piston free to rotate during operation. Instead, the Gremlin used a conventional gudgeon pin small end bearing which maintained the correct alignment of the step.
All of this presented a significant assembly challenge to the manufacturers of the Gremlin. Oddly enough, no provision was made for the positive radial location of the cylinder liner during assembly. This is surprising given the fact that it is absolutely necessary for the transfer ports to be centrally located at the front in order to conform both to the internal bypass passage and the piston To make matters even more complicated, it's equally essential that the exhaust slot in the cooling jacket be in perfect alignment with the two exhaust ports when the system is firmly tightened down. Hence, both the cylinder and the cooling jacket have to be in one specific orientation relative to the crankcase and each other when everything is buttoned up tightly. The correct fitting of the cylinder liner and cooling jacket thus presented a significant assembly challenge. A thin gasket was included between the lower face of the cylinder location flange and the upper crankcase spigot, presumably allowing the adjustment of the system through the judicious use of gaskets of different thicknesses. However, it seems likely that the initial fitting was accomplished in a multi-stage process. First, the engine would have been assembled and tightened down with the cooling jacket completed only to the internal threading stage. The required location of the exhaust port in the cooling jacket would then have been marked, after which the jacket would be removed so that the exhaust aperture and cooling fins could be added (probably in that order) using a threaded mandrel to hold the workpiece.
For the most part, the rest of the engine followed completely conventional lines for an FRV diesel of the period. An alloy con-rod engaged with a one-piece steel crankshaft having an un-counterbalanced full-disc crankweb. The prop driver was mounted on a taper formed at the front of the main journal, with the prop being retained by an aluminium alloy spinner nut. The brass needle valve assembly fitted to the updraft venturi was also quite conventional, using a split thimble for tension. The needle was swept back to the left rear at a sharp angle, keeping the fingers well clear of the prop disc and also allowing convenient sidewinder mounting in a control-line application.
The resulting open rear end of the component was then sealed by a thin aluminium alloy disc which was pressed and swaged into position, very much like the approach used in the FROG engines at the time. Presumably some kind of sealant was used. Again as with the FROG, removable spigots were threaded into shallow bosses at the top and bottom of the tank to provide for refuelling and fuel pickup. The four holes for radial mounting were oriented in conformity with the alignment of these bosses. FROG influence is very noticeable here. As with the cylinder, the assembly of this item posed a challenge. The presence of the integrally-cast bosses for the fuel spigots dictated that the tank be oriented in one of two specific arrangements when fully tightened, such that the spigots were at top and bottom of the tank. This must have involved a degree of trial-and-error fitting, with a combination of selective assembly and variation of backplate gasket thickness presumably being employed. The assembly had to be really tight since a right-hand thread was employed to secure the backplate. The starting torque resulting from flicking the engine over would thus tend to cause the engine to unscrew itself from the backplate. Once running, however, operating torque would tend to keep things nice and tight. I have personally never experienced any problems with one of these engines actually unscrewing itself during starting.
Judged against the prevailing standards among British model diesel manufacturers, the resulting engine was unusually compact, tidy and light with a decidedly "futuristic" look about it. The Gremlin was undoubtedly one of the first, if not the very first, British model diesel engines to be designed with a forward view of where model engine design was heading rather than conforming to what had been done previously. Frank Ellis deserves far greater credit for his foresight in this regard than he has hitherto been accorded. Sharing his recollections many years later with family friend Jim Woodside, Frank stated his opinion that the Gremlin was a "terrific engine"! As part of their product testing program, Frank and his colleagues ran one for a week straight (some 180 hours) on a big tank at 8,000 rpm. Frank claimed that when dismantled after the test it was found to be as good as it had been when first assembled. The Gremlin seems to have caught on immediately in its target market area despite its somewhat steep asking price of 5 guineas (Ł5 5s 0d). At the time, model engines were undeniably expensive - the price of the Gremlin was at least equivalent to the weekly before-tax wage of many individuals in cash-starved post-war Britain. However, people would save up to pay such prices for model engines because they were still something of a novelty at the time. Prior to the original publication of this article on MEN, my greatly-missed mate Ron Chernich was kind enough to trawl through his collection of old British modelling magazines for 1947 and 1948. No advertising mention of the Aerol engines was to be found, either as a factory placement or in retail listings. Moreover, the Aerol engines did not feature in any contemporary model engine articles, most notably being omitted from D. J. Laidlaw-Dickson’s otherwise very comprehensive article entitled “British Commercial Diesel Engines” which appeared in the March-April 1948 issue of “Model Mechanic” magazine. This had us somewhat puzzled until matters became clarified through the emergence of a vital piece of additional evidence to be discussed in detail in the following section. The Aerol engines never appear to have carried serial numbers - I have yet to encounter one bearing such a number. This initially appeared to preclude any meaningful attempt to come up with a reliable estimate of production figures. However, thanks entirely to the invaluable assistance of my friend and colleague Jim Woodside, I now have a reference which (among other points of clarification) provides an invaluable insight into the issue of production rates at this early stage. This reference is of sufficient importance that I’ve elected to discuss it in a separate section of this article. Aerol in the News
First, the article seems to provide a complete answer to a question which had long bothered me - why are we unable to find any trace of evidence to suggest that the Aerol engines were ever advertised in the British modelling media or even included in the published advertising lists from British retailers, including those in the general vicinity of Liverpool? Moreover, why were they completely ignored by contemporary model engine testers and commentators despite their revolutionary design? The article clarifies this completely - all of the engines produced were destined for export, at least as of the date of the article! Hence it is in the modelling media (if any) of the countries to which exports were reportedly sent (including Australia, Canada, New Zealand, Africa and India if the article is correct) that one should look for any promotional efforts. I leave this as a challenge for others better placed than I to take up! It seems clear from this that Frank and his associates were taking full advantage of Government incentives then being offered to British industry to encourage the export of their production to overseas markets. The end of WW2 had left Britain with a serious foreign currency deficit as well as nursing a massive National Debt. The Government of the day quite logically saw the export of British goods as being a positive step towards the redressing of this situation, hence launching a major export drive under which a range of incentives were offered to companies who were prepared to commit to the export of the bulk of their production. Frank and his colleagues appear to have taken full advantage of this situation. Indeed, the major focus of the article was on the alleged success of the Aerol manufacturers in generating sales in export markets. Presumably the international business connections which Aerol distibutor Col. Davies had doubtless developed through his participation in the world-wide cotton industry were of material assistance in this regard. However, the engines themselves did draw their share of attention. The models mentioned in the article by name were the Gremlin and a previously-unrecognized Aerol design, the Elf of 0.8 cc displacement, which would appear to have been in effect a scaled-down Gremlin. The power output claimed for the Elf takes some believing - all of 0.70 BHP from 0.8 cc!! Frank was pictured holding an example of each of these models. The selling price of the Gremlin was quoted as 5 guineas (£5 5s 0d, or £5.25 in "modern" money). The article stated that the Elf was then set to join the Gremlin in production. However, the complete absence of any known examples or any further mention of the 0.8 cc Elf strongly suggests that production of such a model never commenced. Certainly, Frank Ellis never mentioned the Elf in his conversations with Jim Woodside. Strange, because he mentioned everything else!
It became clear immediately that, whatever it actually was, the illustrated "Elf" was certainly not a 0.8 cc model! In the photograph, it appears to be very little smaller than the accompanying "Gremlin". Put an Elfin 1.8 alongside a Gremlin, and the size relationship appears identical to that seen in the photo. Moreover, despite the poor quality of the greatly enlarged image at the right, a number of points of design similarity between the "Elf" and the later Elfin 1.8 are readily apparent. This leads directly to the almost inescapable conclusion that the true displacement of the "Elf" was 1.8 cc, not 0.8 cc as reported, and that the engine that Frank was holding for the photo was in fact an early prototype of what was to become the Elfin 1.8, complete with a partial rendering of the Elfin name! This seems far more likely than the notion of Frank developing a new 0.8 cc model to the prototype stage and then never producing it, subsequently forgetting about its existence altogether. The quoted displacement presumably arose either through a mis-communication between Frank and the reporter or as a result of a misprint which the Editor didn’t catch. The power output claim of 0.70 BHP certainly arose from a misunderstanding! The probability that this reading of the situation is correct is greatly reinforced by the fact that at no point during his recorded chats with Jim Woodside did Frank ever refer to a 0.8 cc model. Unfortunately, there's no longer anyone to whom we can turn for authoritative first-hand confirmation. We have to confine ourselves to noting the possibility that the genesis of the Elfin range dates from considerably earlier than we had previously believed. As if this wasn't enough, the article also clarified both the staffing levels and the production rates at Aerol Engineering as of the publication date. The article named one Harold Leahy as Frank's partner in the model engine side of the business at this time, going on to state that the partners were then completing around 20 engines weekly with the assistance of two other employees. It's clear from the claims regarding export sales up to that point that the Gremlin must have been in production for some time as of the date of the article, presumably at the same 20 engine per week rate or thereabouts. We probably wouldn't be far wrong in suggesting that for the export claims to be factually-based, the production of the engine must have commenced in around June 1947. If this is the case, some 400 or so engines would likely have been completed as of late October 1947. This was probably very close to the final figure, since indications are that the Gremlin was in fact replaced soon thereafter by the Hurricane, of which more below in its place. The cited staffing levels may seem very small for a venture of this kind, but such low staffing levels were by no means uncommon during this period - the contemporary Kemp Engines business of Gravesend, Kent is known to have been very similarly equipped to Aerol and to have operated initially with three full-time employees plus one part-timer. In light of this comparison, it's interesting to note that information derived from serial numbers implies that the 3½ man Kemp operation managed to produce around 150 engines per month as compared to the 80 or so managed by Aerol. A ready explanation for this apparent discrepancy is to be found in the realization that model engine manufacture at Aerol was not a full-time preoccupation for those involved - Frank and his associates were also running their scrap metal business at the same time. This would have limited the amount of time that they could devote to the manufacture of model engines. By contrast, the sole activity at Kemp Engines was model engine manufacture. Frank was later to recall that the maximum number of staff ever employed by Aerol Engineering was around seven, this being confined to a period when the much later Elfin 1.49 PB was selling as fast as they could make it and production figures occasionally reached as high as 200 engines per week. Although the absence of any authoritative references has rendered it impossible to establish definite dates, all indications suggest that the Gremlin remained in production for no more than 5 months or so before being replaced in around November 1947 by the Hurricane which will be covered in a later section of this article. If this is correct (and I admit that it is at best an educated guess at this point), then total production of the Gremlin was likely around 400 units. This relatively low figure is entirely consistent with the engine's extreme rarity today. The Aerol Gremlin on Test As noted earlier, neither of the models produced and sold under the Aerol banner was ever the subject of a published test in the modelling media. This being the case, I'll just have to fill that omission myself!
In addition to my existing test figures for the Mk. II and the Majesco, I actually had some previous numbers for the Aerol Hurricane which will be discussed in the following section of this article. This gave me a basis for selecting an appropriate array of test props which should yield the required comparative performance data for the Gremlin. Armed with these plus some suitable fuel, I headed out to the test area. The Gremlin proved to be a very easy starter indeed provided a small prime was first administered. This is a normal characteristic of updraft-intake engines in an upright mounting, since any liquid fuel which reaches the intake from the fuel line simply drips out of the engine rather than being absorbed. However, a light prime was the only necessary prerequisite to an instant start - the engine almost invariably fired up with one or two flicks, picking up right away on the fuel line. I found that a “dry” prime administered with the exhaust ports closed gave the best results.
One noteworthy characteristic was the engine's outstanding ability to "throttle down" through a combination of compression and needle adjustments. Most unusually, the motor simply slowed down when adjusted in this manner, with none of the usual "burp-burp" behaviour for which diesels tend to be known. It behaved far more like a glow-plug motor than a diesel in this respect. Power delivery remained very constant, albeit at a significantly lower level. This would have been a great asset for trimming and the like. Prior to the commencement of the tests, I was a bit worried about the possibility of the cooling jacket "doing a Sparey" on me by unscrewing during operation. Given the alignment issues described earlier, this would have created serious difficulties in the field! However, I needn't have worried - everything stayed nice and tight for the entire duration of a fairly arduous test. I was quite frankly rather astonished at the level of performance exhibited by this engine. I had been expecting a competent but not outstanding performance, but what I actually got was a truly excellent set of figures by the standards of 1947. The numbers recorded on test were as follows, together with the derived power curve:
The fractional diameter props are cut-down APC items created to fill gaps in my suite of calibrated test props. The power curve derived from these figures implies a peak output of around 0.133 BHP @ 9,100 rpm - an outstanding performance as of 1947, and one which was unmatched by any competing British 2 cc diesel of my present acquaintance. My earlier test of the small-bypass E.D. Mk. II indicated a peak output of around 0.086 BHP @ 7,000 rpm, while the 2.2 cc Majesco "22" proved on test to develop only 0.096 BHP at a little less than 6,000 rpm. In performance terms, it was no contest - the Gremlin simply buried the contemporary opposition, as well as being significantly lighter and more compact. How did Frank Ellis characterize the Gremlin - "...a terrific engine..." ? Seems entirely appropriate to me! It was certainly several years ahead of its time. The Aerol Hurricane
The Arden featured full 360 degree radial porting, in contrast to the Gremlin's far more restrictive form of reverse-flow scavenging described above. Apart from its more restrictive nature, the Gremlin's porting also imposed the previously-noted assembly difficulties, since the cylinder and cooling jacket not only had to be correctly aligned with one another but also had to be positioned relative to the crankcase so as to place the transfer ports in their correct location at the front. The piston crown also had to be stepped to create the desired transfer timing. Accordingly, a new cylinder design was developed which featured a form of radial porting that was far closer to the Arden set-up. There were three sawn exhaust slots equally spaced around the cylinder circumference. Directly beneath each of these ports was an internally-formed transfer flute which terminated just below the respective exhaust port. Since no overlap was possible with this arrangement, a relatively long blowdown period was unavoidable. The blowdown period was minimized to the extent possible by the fact that the exhaust ports were relatively shallow by comparison with the round holes of the former Gremlin component, thus minimizing the necessary delay in the opening of the transfer ports. Even so, the Hurricane was undoubtedly handicapped by having a longer blow-down period and hence a shorter transfer period than its Gremlin predecessor.
Naturally the re-design of the cylinder required a matching re-configuration of the aluminium alloy cooling jacket. The basic form of the jacket was unchanged, but it now had thee milled slots at exhaust port level to permit the egress of exhaust gasses from the cylinder (implying incidentally that by now the Aerol company had acquired milling capability). The size of the ports was such as to require that the jacket be aligned so that the cylinder ports corresponded to the slots in the cooling jacket when things were screwed down hard. Consequently, the former assembly issues were not completely eliminated - the tightened-down alignment of the jacket relative to the cylinder still had to be correctly established. However, the revised bypass arrangements relieved the makers of the need to ensure that both the jacket and the cylinder were correctly aligned with the crankcase as well as with each other - the engine would run in any alignment of the combined cylinder/jacket assembly. This eliminated one significant element of the former assembly problem, in particular the need for selective matching of parts. A further manufacturing simplification was the elimination of the need to create a step in the piston crown.
Apart from the cylinder design, the Hurricane was little changed from its Gremlin forebear. Bore and stroke were unaltered at 0.503 in. (12.78 mm) and 0.609 in. (15.47 mm) respectively for a displacement of 1.98 cc (0.121 cuin.). Weight too was virtually unchanged at 4.83 ounces (137 gm). The tank arrangements remained the same, along with the attendant assembly difficulties. A few detail differences were apparent. The design of the prop driver and spinner nut combination was amended slightly. In addition, the revised cooling jacket had fewer cooling fins. Finally, the angle at which the needle valve was swept back was substantially reduced. But none of these features had any functional effect on performance - in that respect, the difference between the two models was confined to the cylinder design. It's probable that production levels for the Hurricane were little changed from those achieved with the Gremlin since the Aerol company doesn't appear to have commenced the construction of the substantial brick factory which replaced their original metal lean-to shed until after the Elfin engines were introduced. The Hurricane may possibly have remained in production for a month or so longer than the Gremlin, but this is not certain. Regardless, the implication is that at the very most some 500 examples may have been made in total. The true number may well have been somewhat less. What is indisputable is that surviving examples of the Hurricane are almost as rare as those of the Gremlin. The Aerol Hurricane on Test As in the case of the Gremlin, there’s no contemporary published test report to which reference may be made. Accordingly, it was down to me to put the Hurricane through its paces!
The Hurricane had accompanied its Gremlin partner on my trip to the testing site, so as soon as the Gremlin test was done the Hurricane replaced it on the test block. The same props and fuel were used, making a direct comparison completely valid. The Hurricane proved to be just as user-friendly an engine as its Gremlin predecessor. Starting and running characteristics were pretty much identical. The major difference was the fact that the Hurricane lacked the outstanding ability of the Gremlin to run smoothly at reduced compression settings and richer mixtures. It exhibited the usual "burp-burp" behaviour associated with diesels when throttled back in this way. On that basis, it would have been slightly less user-friendly in a trimming context. This must of course be down to the significantly revised porting arrangements. Even so, the Hurricane ran every bit as well as its Gremlin counterpart. The following data were recorded.
The data presented above implies a peak output of around 0.120 BHP @ 8,800 rpm, slightly down on the 0.133 BHP @ 9,100 rpm recorded for the Gremlin. Since the two engines have completely different scavenging arrangements, there was no reason to expect them to perform at the same level, but it does seem a bit odd that the later design appears to have fallen slightly short of its predecessor in performance terms. I suspect that the difference is down to the Hurricane’s unavoidably longer blow-down period and shorter transfer period. That said, the difference in performance is sufficiently small that in practical terms the two perform at very similar levels - one would not notice much difference in the field. Even so, the Hurricane showed that it remained abreast of the improved competition represented by the E.D. Comp Special. My earlier test of that model indicated a peak output for my best unmodified example of around 0.121 BHP @ 8,200 rpm. This of course represents a very substantial increase over the earlier Mk. II. However, the Hurricane could match the Comp Special prop for prop while being considerably lighter and more compact into the bargain. The Aerol-Clan Connection - Did it Exist?
In a letter which was published in August 1994 in Volume 1, Issue 4 of the sadly long-defunct "Model Engine World" (MEW) magazine, the respected model engine builder Tom Crompton recalled an earlier conversation with a certain Mr. Arthur Whittle, former manager of Roland Scott's once-famous model shop in Bolton, Lancashire. The subject of the Aerol engines came up during that conversation, as did that of the Clan units. Arthur Whittle claimed that Roland Scott was a major distributor of the Clan engines in the English Midlands. If true, that involvement would necessarily have dated from the period mid-1947 to early 1948 when Scott still ran a small shop in St. Helens well prior to his commencement of national advertising in mid 1949 (see below) and long before his 1952 move to larger premises in Bolton, where Arthur Whittle worked as a manager. The short-lived Clan 0.9 cc diesel (their only model) was only on offer during that brief period. The notion that a lone individual trying to get a single small hobby shop off the ground (Scott's situation at the time) would secure an appointment as a "major distributor" seems highly improbable to me. Moreover, I’m sure that the Astral Aero Model Company of Leeds in Yorkshire would have been extremely surprised to hear Scott being cited as a major Clan distributor, since they were the official English Clan distributors of record as of that period! I’ve dealt fully with Arthur’s claims regarding the Clan 0.9 cc diesel in my separate article on that model. Suffice it for now to say that there are a few rather large holes in his story ………it's very difficult to accept its veracity. Roland Scott was doubtless well acquainted with Aerol Engineering founder and engine designer Frank Ellis, since his original shop in St. Helens prior to his 1952 move to Bolton was relatively close to the Liverpool workshop of Aerol Engineering. Scott promoted Aerol Engineering's later Elfin range very vigorously from the outset. Arthur Whittle actually recalled that Aerol Engineering made a very few examples of the Elfin 1.8 which were under-bored or under-stroked to 1.5 cc for Roland Scott's personal use in the S.M.A.E. Class 1 (1.5 cc) control-line speed category in which Scott was then competing. This part of Arthur's tale does have the ring of truth. But now we get to the meat of the matter in the context of the present article! Arthur Whittle also claimed that the Aerol Hurricane was based upon a Clan design which Aerol purchased from the Clan manufacturers! At first glance, there would appear to be a certain measure of internal consistency in Arthur's story. If (but only if) Scott was indeed a major Clan distributor, the fact that he was also closely associated with Frank Ellis would undoubtedly explain how Ellis might have been brought into contact with the makers of the Clan range.
According to a letter from one "Diesel Dave" which appeared in an issue of “SAM 35 Speaks” of which I’ve now mislaid the date, the appearance of that July 1949 advertisement resulted from Roland Scott helping out his friend Frank Ellis by buying up Frank's remaining stocks of Hurricanes, thus releasing some much-needed capital for Frank to use in the further development and promotion of his Elfin range which had replaced the Aerol marque by that time. Selling off these engines clearly took a while, because they were still being advertised by Roland Scott in the December 1949 issue of "Aeromodeller". The wording of Scott's July 1949 advertisement is undoubtedly consistent with the previously-discussed fact that the early Aerol engines (prior to the Elfin era) were targeted primarily at the export market. Evidently a number of those engines which were originally slated for export remained unsold when they were rendered essentially obsolete by the mid-1948 advent of the significantly improved Elfin 1.8 plain bearing model which was the Hurricane's direct successor. More of that model below. Obviously, once the superior Elfin 1.8 arrived on the scene, Frank would have been very reluctant to send any more Hurricanes overseas - he would naturally want to push his new Elfin range, since that was where his future lay. This would leave him holding a number of unsold Hurricanes. It seems clear that Roland Scott was helping his friend Frank to dispose of these surplus out-dated engines, which represented tied-up capital that Frank could ill afford. Note that the only spares said to be available were the spinner nut and the needle valve assembly - this confirmed that the engine was out of production despite its "new and amazing" advertising classification! In fact, Hurricane production is known to have ceased prior to mid 1948, when the Elfin 1.8 first appeared. A close Roland Scott-Frank Ellis connection combined with Scott’s alleged role as a Clan distributor could certainly answer the otherwise thorny question of how Ellis might have established contact with the Clan manufacturers in order to close the deal cited by Arthur Whittle. If Roland Scott was indeed a major Clan Well………. unfortunately there are a few very large stumbling blocks in the path leading to acceptance of the above scenario! Most importantly, the kindness of my valued friend and colleague Jim Woodside has put me in possession of an audio recording of a lengthy interview dating to c. 1990 which Frank Ellis (then over 80 years old but still as sharp as a tack) was good enough to grant Jim, who was (and is!) the son of a personal friend of Frank's. In that interview, Frank's first commercial model diesel, the previously-described Aerol Gremlin, and its direct successor, the Hurricane, were both discussed at length and in that order. Frank spoke openly of his admiration for the Arden engines from America and the influence which they exerted upon his own design efforts, for which he claimed full credit for himself apart from the openly-acknowledged Arden influence. That influence is readily discernable in the successive designs of the Gremlin and Hurricane.
The other glaring inconsistency with Arthur's story is that the Hurricane was unquestionably the direct successor to Frank's first model engine design, the Gremlin. Frank said as much, and you only have to look at the above descriptions and images of the two engines to see immediately that one is obviously a direct descendant of the other, right down to the use of near-identical working components along with a common crankcase casting and tank. In fact, the Hurricane with its more developed cylinder porting is very recognizably a transitional model standing in design terms between the original Gremlin and the Elfin 1.8 radial mount model that was soon to come. Given this very obvious design succession, if there had been any Clan influence it would have had to be related to the Gremlin rather than the Hurricane. Moreover, such influence would have to date to early 1947, since the Gremlin made its market debut in mid 1947. At that time, Clan manufacturers Kingdom Models were still hard at work developing their 0.9 cc sideport design - Roland Scott couldn’t have begun to distribute it until well after that time, even assuming that he ever did so. There’s no evidence that the Clan manufacturers ever commenced work on anything even remotely like the Gremlin. Moreover, by his own candid admission Frank Ellis was pushed to the limit financially in starting Aerol Engineering, hence having no spare cash to spend buying other people's designs. Indeed, Frank had no need to do so, being himself a very capable machinist and designer who had already designed and produced several successful prototype model diesels as of the latter half of 1946. It was in fact the demonstrated success of these prototypes which had placed him in a financial position to take the major step of entering the commercial model engine manufacturing field beginning in late 1946 with the backing of Col. Davies. In summary, Frank Ellis was an experienced and capable model diesel designer having his own ideas and already knowing exactly where he wanted to go as of early 1947. Why would he buy someone else's design, even if he could have afforded to do so?!? Given the above facts, it's frankly impossible for me to believe that the design of the Aerol Hurricane was influenced by any Clan design. To my present knowledge, there was never a Clan model that bore any resemblance whatsoever to the Hurricane, or for that matter to the Gremlin. By contrast, the Hurricane bears the clearest possible marks of being a direct design successor to the Gremlin, for which Frank Ellis no doubt legitimately claimed full credit apart from his open acknowledgement of the original Arden influence. Now I'm not in any way casting aspersions here upon Arthur Whittle's personal integrity. I'm certain that he related the facts exactly as he recalled them at the time of his discussions with Tom Crompton. It's simply that his version of the Hurricane-Clan relationship is not supported by any evidence of which I'm presently aware. Indeed, such evidence as does exist appears to directly refute his version of the story. To sum up, I’m sure that there was a close connection between Roland Scott and Frank Ellis, but I also believe that there may have been a certain amount of blurring of the details in Arthur Whittle’s mind regarding both Roland Scott's involvement with the Clan 0.9 cc diesel and the origins of the Aerol Hurricane. Given the passage of years between the hey-day of the engines discussed and the time of Arthur's conversations with Tom Crompton, this would be quite understandable. Unless some persuasive new evidence comes to light, I think that we'll have to leave it there ............. Enter Elfin Although the Hurricane represented a worthy attempt to deal with some of the assembly challenges associated with the Gremlin, Frank Ellis was clearly still not satisfied. A number of manufacturing issues remained unaddressed, while the Hurricane's evident failure to raise the performance bar must surely have rankled. I previously noted the possibility that as far back as October 1947 Frank may well have produced a prototype of an entirely new design to replace the Aerol models, although developing the tooling and arranging for the required castings for this model would of course have required both time and a capacity to invest further funds, luxuries which Frank clearly didn't have in abundance.
The result was the mid 1948 appearance of an engine that was destined to become the stuff of legend - the original radial-mount plain bearing (PB) Elfin 1.8. In his discussion many years later with Jim Woodside, Frank was to recall that following its mid 1948 introduction he took one of the new Elfin 1.8 cc models to a major contest at Leighton Buzzard in Bedfordshire, strictly to promote the engine. This appears to have been Frank's first significant move towards domestic promotion of his engines - previously, export sales had evidently been the focus.
Frank recalled that on first starting the engine he immediately attracted a large crowd - that famous Elfin "crackle", no doubt! One of the individuals in that crowd was none other than Henry J. Nicholls, who told Frank on the spot that he would take as many as Frank could supply! Nicholls' initial advertisement for the Elfin 1.8 appeared in his July 1948 advertising placement in "Aeromodeller" magazine. Nicholls actually appears to have replaced Davies, Benachie & Co as the sole domestic distributor of the Elfin range - certainly, the Elfin box labels from this early period identified Nicholls' Mercury Models division in this capacity. However, it's worth noting in passing thatDavies, Benachie retained a connection with Aerol despite Nicholls' intervention, since they continued to be identified in Elfin As previously noted, the first advertising reference to the new model was the announcement of its appearance in the Henry J. Nicholls placement in the July 1948 issue of "Aeromodeller", with Aerol Engineering beginning for the first time to place their own national advertisements soon thereafter. However, it seems almost certain that the engine actually appeared in around June of 1948. This implies that the manufacture of the Aerol Hurricane had ceased as of May 1948 after some 5 or perhaps 6 months in production. As noted earlier, it's highly unlikely that total production of the Hurricane exceeded the 500 unit mark - in fact, it way well have been somewhat less. The engine is certainly elusive today. Although the manufacturer of the new 1.8 was still Aerol Engineering, this was the first model to be sold under the soon-to-be-famous Elfin trade-name. No more engines were destined to be sold as designated Aerol products - that line ended with the Hurricane. Anecdotally, the Elfin name arose because Frank was quite taken by the result of combining a pair of names - Frank's family name of Ellis and the family name of one of his associates, a certain Mr. Finn. The slightly reduced displacement of the Elfin 1.8 was achieved through a reduction in the stroke from 0.609 in. (15.47 mm) to 0.562 in. (14.27 mm). The bore remained essentially unchanged at 0.505 in. (12.83 mm), yielding a marginally reduced displacement of 1.84 cc (0.112 cuin.) as opposed to the Hurricane's figure of 1.98 cc (0.121 cuin.). The major consequence of this change was that the 1.8 was considerably less under-square than the Hurricane, reflecting an emerging trend in model diesel design as peaking speeds went up.
In very broad terms the Elfin 1.8 showed clear evidence of its Hurricane ancestry, including the ongoing influence of the Arden engines. The revised cylinder featured exactly the same porting arrangements as on the Hurricane apart from the use of four exhaust and transfer ports as opposed to the former three. In addition, the cylinder was externally threaded above the port belt to allow the use of a screw-on cooling jacket which did not obscure the exhausts. Below the port belt, the cylinder was externally threaded to match a female thread formed inside the upper crankcase casting. This completely eliminated the assembly issues which had been inherent in the Hurricane design - the engine would work perfectly well regardless of where the components ended up when tightened down. No doubt this too was a major design objective. The tank arrangements too were very different. The 1.8 used a conventional screw-in backplate together with an open-backed conical Arden-style radial mount which was cast in unit with the crankcase. The free-flight version had provision for a plastic tube tank (for which end plugs with spigots were provided), while the control-line Overall, the design of the Elfin 1.8 was functionally very similar to that of the Hurricane apart from the use of four ports in place of the former three. In addition to the slight reduction in displacement, the main effect of the changes was the elimination of some weight and bulk along with the improvement of the transfer arrangements through the addition of an extra transfer port. Perhaps equally importantly, the revisions eliminated several assembly challenges. A very clear case of a capable designer learning from experience. A test of an example of the Elfin 1.8 free flight model undertaken on the same day as the previously-documented tests of the Aerol models demonstrated in no uncertain terms that the Elfin represented a very clear step forward in performance terms. The figures obtained were as follows, together with the derived power curve:
The implied peak output of 0.156 BHP @ 9,500 rpm puts the 1.8 streets ahead of both the Hurricane and the Comp Special on a performance basis, while in terms of its lightness and compactness it was simply no contest. A very significant achievement on the part of Frank Ellis! It's noteworthy that the specific output of 0.085 BHP/cc is well within range of the figure of 0.097 BHP/cc reported by Peter Chinn (and subsequently confirmed by my own testing) for the later Elfin 2.49 PB radial mount model. It's also worth mentioning in passing that the above figure for the engine's peak output substantially exceeds that obtained by Lawrence Sparey in his July 1949 test of the Elfin 1.8. Sparey reported figures of 0.114 BHP @ 12,100 rpm, results which are in no way supported by my own testing reported above. It appears that my example developed significantly greater torque than that tested by Sparey. Mind you, the Elfin engines were to become notorious for exhibiting vastly different levels of performance between different examples of the same model. This was almost certainly a result of the ongoing use of worn-out WW2 surplus machine tooling. The Elfin 1.8 was the first of a series of engines to be released under the Elfin banner. The marque was destined to enjoy considerable sales and competition success, although as time went by it became increasingly bedevilled by quality control problems arising from the company's enforced dependence upon the clapped-out wartime machinery which they could not afford to replace. In the end, this proved to be the company's downfall - by early 1957 the quality control issues had reached the point where the manufacture of the Elfin engines by Aerol Engineering had to be suspended and the company wound up. But that's another story, which I’ve already recounted in detail elsewhere on this website........ Conclusion Neither the Gremlin nor the Hurricane are commonly encountered today. As a result, when they do appear on eBay or elsewhere they tend to sell for relatively high prices, especially if in complete and original condition.
All of this having been said, the Aerol engines are highly significant in being the first British model engine designs to make a clean break with the past and steer British model diesel manufacturers towards the design path of the future. Their designer Frank Ellis deserves far greater credit for this than he has hitherto been accorded. The Aerol designs were also the direct precursors of one of the most influential British model engine ranges of the middle 20th century - the Elfin marque. Consequently, they richly deserve both our remembrance and our respect. I hope that you've enjoyed our shared look at these highly influential motors! As always, if anyone out there knows more or can set me straight at any point, please get in touch! All contributions gratefully and openly acknowledged... _________________________________ Article © Adrian C. Duncan, Coquitlam, British Columbia, Canada First published on MEN 2013 Revised edition first published on this website September 2021
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The 
Elsewhere I’ve recounted the story of 
By early 1947, Frank Ellis and his associates were ready to commence the development and limited production of a commercial range of model engines. When considering his first commercial design, Frank would naturally have taken note of the competition that he would face in the form of the popular 
Engines of 2 cc (0.12 cu. in.) displacement or thereabouts were actually quite common in Britain during the period in question. Beginning in early 1947, E.D. had enjoyed considerable sales and competition success with their 
In terms of its general layout, the Gremlin clearly reflected Frank Ellis's admiration for the 
its length. This cylinder was retained by a screw-on alloy cooling jacket of elegant "beehive" form which was internally threaded at its lower end to engage with a male thread formed around the exterior of the upper crankcase casting.
The vertical port positioning enforced by the presence of the cylinder location flange placed the transfer ports well below the exhaust ports, necessitating the provision of a sizeable step in the crown of the cast iron piston to reduce the blowdown period very significantly, thus achieving a more appropriate transfer timing. The step also directed the incoming gas flow upwards towards the combustion chamber. 
The need to align the three grouped transfer ports with the front bypass passage in turn dictated the positioning of the two drilled exhaust ports on the left-hand side of the engine (looking forward in the direction of flight) at a 30 degree forward angle - a highly unusual configuration! The exhaust ports discharged through an oval slot formed by drilling and filing through the alloy cooling jacket at the appropriate elevation above the assembly threads. If you ever encounter an example of the Gremlin with its exhaust ports in any other orientation, you can be sure that it's been incorrectly re-assembled at some point and probably won't run.
step. A location pin in the crankcase combined with a suitably-positioned slot in the cylinder location flange would have sufficed to ensure the correct alignment, but it wasn't incorporated. This would actually have been a very rational after-market modification for any owner who was experiencing problems with maintaining the correct alignment of the various components. It would be very easy to add.
The engine would then be ready for final assembly with the cylinder liner correctly oriented. The ultimate goal would be to ensure that with the cooling jacket firmly tightened down, the exhaust ports were in alignment with the slot in the cooling jacket and in the correct alignment relative to the crankcase to ensure that the transfer ports corresponded to the bypass passage which fed them. As I know from personal experience, this isn't easy! Once correctly assembled, this engine is definitely best left alone if misalignment issues are to be avoided.
The one remaining slightly unusual feature was the tank mount. As with the Arden which inspired a number of key elements of the design, there was no provision for beam mounting. Instead, a separate casting was machined to form a combined radial mount, fuel tank and screw-in backplate in a single component. The front installation spigot was externally threaded to engage with the female thread at the rear of the crankcase, while the casting was internally machined at the rear to accommodate the fuel. 
Once the tank was correctly assembled, either spigot could serve as the fuel pickup, with the other used for refuelling. As supplied, the engine could thus be used either upright or inverted, the latter being a very popular mounting orientation at the time of which we are speaking. In control-line service using a sidewinder mounting configuration, a separate tank having more appropriate venting for that application would generally be used.
That said, there may be far more to the story than this! Jim Woodside set the ball rolling by suggesting the possible significance of the name Elf - this seemingly still-born project may in fact represent the initial inspiration for the name "Elfin"! This thought led me to a closer perusal of the photograph of Frank and the two engines which accompanied the article. 
It's always valuable to have a basis for comparing a given engine's performance to that of its major competition. In this instance, I had a reliable set of previous measurements for perhaps the most popular 2 cc unit in Britain at the time - the 1947 small-bypass E.D. Mk. II. At the time when the Gremlin appeared, the Mk. II was E.D.'s sole 2 cc model - the more powerful Comp Special with its sub-piston induction and larger bypass had yet to appear. The comparison thus seems entirely fair. As a bonus, I also had figures for the contemporary Majesco "22" model of 2.2 cc displacement.
The contra piston in this example proved to be rather more tightly fitted than ideal, but the compression setting remained fully adjustable at all times despite this. Needle response was excellent - very positive without being overly sensitive. Optimum settings were thus easy to establish. Running qualities were first class, with no trace of misfiring and a very steady speed being held on any given prop.
Although the Gremlin embodied a number of features which were to be enshrined in the British model engine design genre for many years to come, it still fell somewhat short of meeting Frank's expectations in a number or areas, particularly given its Arden heritage. Since the Gremlin's major design departure from the Arden pattern (apart from the elimination of the ball-race shaft and the ignition system) was its cylinder porting configuration, it was inevitable that this feature would soon come under scrutiny by its designer. There were also a number of production difficulties which merited correction.
The Hurricane’s three exhaust ports undoubtedly had a larger total area than the two holes of the Gremlin. This might be expected to assist the clearance of exhaust gasses quite materially. The scavenging was still of the reverse-flow variety, but this was now achieved more conventionally by directing the incoming gas towards the axial centre of the cylinder and thence upwards in a column inside the surrounding "tubular" downward escape path of the exhaust gasses.
By the time the Hurricane made its appearance, E.D. had come out with the improved 
Before I embark upon a brief account of the mid-1948 replacement of the Aerol engines by the Elfin range, I feel it to be necessary to deal with what I personally believe to be something of a red herring in connection with the Aerol story. This is the alleged existence of a connection between the Aerol engines and the 
The closeness of Roland Scott’s association with Ellis is underscored by the fact that Scott's very first national advertisement ("Aeromodeller", July 1949) specifically referred to the availability at a very competitive price of examples of the "Super Hurricane" 2 cc diesel which were "surplus to export requirements". This advertisement is reproduced at the right. A glance at the image of the engine confirms that the unit to which the advertisement refers is unquestionably the Aerol Hurricane 2 cc diesel which has been fully discussed above. It is this particular engine which Arthur Whittle specifically claimed to have been a Clan design which was purchased by Frank Ellis.
distributor, and if a Clan design (the Hurricane) was in fact available for sale to others, their Lancashire agent Roland Scott would likely have known about it and could have put Frank Ellis in the running to purchase it. It all seems to hang together, doesn't it?!? 
However, Frank did not mention the Clan engines at any point in his lengthy "tell all" interview with Jim Woodside. If there had been a connection, it seems inconceivable to me that Frank would not have recalled it - the level of detail which he did recall about the early years at Aerol Engineering was truly impressive. No hearsay here - I have the recording.
However, as 1948 drew on, it must have become increasingly apparent that a change was necessary. Apart from the fact that other manufacturers were catching up to the Aerol designs in performance terms, it's highly likely that a contributing factor here was the ongoing assembly challenge posed both by the Hurricane's cylinder design and its tank mount assembly. There also appears to have been a desire to shave some more weight and bulk off the engine, thus enhancing one of the major competitive advantages enjoyed by Frank's designs.
version had no tank provisions at all. Again, this eliminated the selective assembly requirement imposed by the design of the Hurricane. However, it did have the disadvantage of requiring the manufacturer to produce two distinct crankcase castings. 
In large part, the rarity of these engines is doubtless due to the fact that not that many were manufactured. However, paradoxically it may also reflect the fact that both models were very serviceable and durable engines which would have served their owners well. This may have led to them being simply run into the ground before being discarded when worn out or damaged. That happened a lot in those far-off pre-collector days. It's the good engines that get the air time and take any lumps that are going! Any limited-production vintage engine that still shows up in significant numbers in near-mint condition probably isn't much of an engine ……...!| |