The BWM Engines
by Adrian Duncan
|Click on images to view larger picture.|
Here we summarize our present knowledge of a short-lived and somewhat obscure range of engines that were manufactured "within the Wall" in the West German sector of Berlin during the Cold War era of the 1950's but did not survive the decade in production terms and are seldom encountered today. These are the BWM model diesel and glowplug engines.
The BWM company initials are sometimes confused with those of the far more prominent BMW car and motorcycle manufacturing concern which was founded in Munich in 1916 as the Bayerische Motoren Werke or Bavarian Motor Works. This similarity in the initials has understandably led people at times to assume some sort of connection. In point of fact, no such connection exists. The model engine range with which we are concerned was established after WW2 in Berlin, and the BWM initials stand for Berliner Werkstätten für Modellmechanik (Berlin Model Engineering Workshops). The company was founded in 1952 by one Horst Freise but reportedly got into difficulties in 1954, at which time the business was acquired by Freise's works manager Manfred Göcking and the range was significantly downsized.
Reports from Germany indicate that the BWM engines were designed by Manfred Göcking and Franz Freise. The latter was presumably a relative of managing director Horst Freise, perhaps a brother or a son.
Contemporary English-language documentation regarding most of the BWM models is in rather short supply. Although the final 2.5 cc (.15 cuin) model, the 250D, was exported to several European countries as well as the USA, albeit in relatively small numbers, and was accordingly covered in some detail, the marque never achieved a high profile outside its native Germany. Consequently, the development and eventual demise of the range were not followed in detail by the International modelling media.
This being the case, we have only a few published descriptions as well as a single published test to inform our quest for knowledge. We'll just have to do the best that we can based on these sources as well as a direct examination of several available examples. Our hope is that the publication of this article may stimulate further research or at least the sharing of additional information. So if anyone out there knows more, let's hear from you! All contributions gratefully and openly acknowledged, as ever.
In the meantime, we'll begin by reviewing the presently-available English-language information sources (apart from the engines themselves). We apologise in advance for the quality of a few of the scanned media images which illustrate the following text, but since these are the sole images of the engines in question which are presently available to us, they'll just have to suffice until we're in a position to do better!
The BWM range in the English-Language Modelling Media
The earliest and at the same time most comprehensive English-language reference to the BWM range of which I'm so far aware is to be found in a highly informative article by the late Peter Chinn which appeared as part of Chinn's ongoing Accent on Power series in the December 1952 issue of Model Aircraft This series of articles today constitutes one of the most invaluable archival records of model engine history from the standpoint of a well-informed and knowledgeable contemporary observer. We are forever indebted to Peter Chinn for much of the available contemporary information on the BWM engines as well as numerous other makes.
The particular article in question was entitled Engine News from Germany It appears to have been the first detailed survey of German model engine manufacturing activities published in English since the end of WW2. For some years following the conclusion of that conflict, model flying activities in Germany had been severely curtailed by the occupying powers and German modelling progress had suffered as a result. However, by 1952 these restrictions had been relaxed and Germany was in the process of re-emerging as a force to be reckoned with in International competition. New German model engine manufacturers were appearing to meet the consequent increase in the domestic demand for suitable powerplants. Chinn's article was a praiseworthy attempt to summarize their progress as of the latter part of 1952.
The soon-to-be-famous Webra range had already assumed a leadership position in the German model engine manufacturing field, and Chinn described the then-current Webra models in some detail, noting that they appeared to be well up to par with contemporary engines from other countries, including Britain. Chinn then described the 2.47 cc Metro 52 diesel (a rare bird today!), after which he turned his attention to the BWM range.
At the time when Chinn was writing (the latter part of 1952), the BWM engines were described as "entirely new", indicating that they had only just been introduced. The company had evidently published a prospectus of their planned range, which included both diesel and glow-plug models in all of the "popular" displacement categories from 0.5 cc to 10 cc as well as a pulse-jet unit. However, Chinn reported that many contemporary observers were somewhat sceptical of the ability of a brand-new manufacturer to actually deliver such an ambitious production program. He openly admitted that he was uncertain regarding how many of the listed models had actually entered production at his time of writing.
Chinn noted that the BWM shaft-valve models appeared to be rather Frog-influenced, specifically citing the similarity of the BWM 150 to the Frog 150. An illustration of the glow-plug version of this engine was included to underscore this point. Chinn reported the claimed output of this engine as 0.12 BHP at 12,500 rpm and the weight as 3.18 ounces—all very comparable with the corresponding figures for the Frog.
There were also 1 cc and 2.5 cc models of very similar design, designated as the BWM 100 and BWM 250 respectively. Chinn included an image of the 250 diesel model, which confirmed its similarity to the smaller 150. All of these models were reportedly to be offered in both diesel and glow-plug configuration. There had been talk of a similarly-designed BWM 50 model of 0.48 cc, but no details had yet been released at Chinn's time of writing.
All of the above models appear to have featured plain un-anodized crankcase castings and were in essence conventional shaft-valve radially-ported sports engines by the standards of their day. However, BWM clearly had ambitions in relation to the all-out competition market as well and were touting a twin ball race rear-disc induction twin-stack 2.47 cc diesel called the BWM 251 for some obscure reason. This was a short-stroke design which appeared to owe a lot to the contemporary E.D 2.46 Racer. It reportedly weighed 5 ounces and produced a claimed 0.27 BHP at 15,000 rpm. Unlike the other contemporary BWM models, it featured a black crackle-finished crankcase. Only 50 of these ultra-rare engines were reportedly produced.
There was also a BWM 500 glow-plug model for which an output of 0.45 BHP at 12,000 rpm was claimed. However, no details were as yet forthcoming as of late 1952 and no illustration was apparently made available. Lastly, there was a 10 cc racing glowplug model known as the BWM 1001, which featured a ringed alloy piston, twin ball races and rear disc induction. It was reportedly styled along very similar lines to the twin-stack 251 diesel model mentioned earlier. This unit had a bore and stroke of 24 mm and 22 mm respectively, and produced a claimed 1.00 BHP at 12,000 rpm. Room for improvement there in a contemporary competition context.
Apart from the conventional two-stroke model engines listed above, the company was also promoting a pulse-jet unit, the BWM "RT 2000" model, using glow-plug ignition instead of the more usual spark plug and trembler set-up. This unit was reportedly very much like the Dynajet in appearance and had a similar claimed performance.
Seldom can a new manufacturer have entered the model engine market with such an ambitious production program, and it's scarcely surprising to learn from Chinn that there was much contemporary scepticism regarding the new company's ability to actually deliver. The start-up investment alone in terms of dies and tooling for so many models would have appeared daunting to most new manufacturers, who generally started small and built up their ranges as their market presence expanded. It appears that the scepticism of the doubters may have been justified, since nothing more was to be heard in the future regarding several of the models included in Chinn's very informative article.
Three of the BWM engines made their next appearance in Chinn's Accent on Power article in the May 1953 issue of Model Aircraft This article included a listing of a number of International-class engines in the context of a discussion regarding airscrew selection. At this time opinion was still divided regarding the optimum engine size to use for International free flight competition (then one of the "glamour" classes) and a number of very successful competitors were still using 1.5 cc motors despite the fact that the 2.5 cc upper limit had by then been adopted by the F.A.I. Accordingly, Chinn's listing included the previously-mentioned BWM 150 along with the long-stroke BWM 250 model and the short-stroke disc-valve BWM 251. None of the other BWM models was mentioned in the context of this discussion.
The next appearance of the range of which I'm presently aware is to be found in the October 1954 issue of Model Aircraft. In an article entitled New Engines Peter Chinn mentioned the fact that the BWM concern had then recently passed into the hands of former works manager Manfred Göcking, who had apparently rationalised the company's production by eliminating a number of models and focusing on the BWM 250D model of 2.47 cc displacement. Chinn specifically stated that the 250D was "an improved version of the B.W.M. (sic) shaft valve diesel", almost certainly indicating that the 250D was a replacement for the 250 mentioned earlier. Sadly, the disc-valve 251 was apparently one of the casualties of the retrenchment program.
It seems that Göcking had targeted the US market as a prime sales area for the 250D, which now sported a handsomely black-anodized case. Peter Chinn's Foreign Notes column in the January 1955 issue of the US magazine Model Airplane News included a photograph and description of this model. Mention was made of some unspecified difficulties in early 1954 which had led to the change of ownership. Chinn specifically commented on the engine's relatively low weight, which he felt would appeal to purchasers in export markets who had tended to resist diesels on account of their higher weight compared to their glow equivalents. His main complaint was with the standard of the English used in the accompanying "English-language" instruction leaflet!
The range was next mentioned in Peter Chinn's International Engine Review in Model Aircraft for April 1955, again with an illustration of the BWM 250D. The 14 mm bore of the engine was incorrectly quoted as 13 mm in this article! Presumably this was a typo, since Chinn had previously reported it correctly elsewhere. Quoted output was 0.20 BHP @ 11,000 rpm, which was presumably an estimate based on first impressions since it is less than the manufacturer's claim but also differs slightly from the published test figures which appeared in the following issue of Model Aircraft
The 250D was the only BWM model included in this article—in the text, Chinn noted once again that the 250D was the only BWM design then available out of the five said to have been previously listed. The very specific figure five is instructive—it certainly implies that a number of the models originally announced by the company back in late 1952 had never actually reached production. It seems not unlikely that the difficulties experienced by the company in 1954 under Freise's leadership may have resulted from the manufacturer having over-reached himself in terms of the scale of his production ambitions.
The following month of May 1955 saw the publication in Model Aircraft of the one and only test of a BWM product that was destined to appear in the English-language modelling media. Inevitably, the subject of the test was the 250D, and this test coupled with the availability of several examples for study makes this the sole BWM model which we are able to describe in detail at first hand. We will do so in a later section of this article.
At this point, the BWM range appeared to drop out of sight. It's true that the listing of the world's engines in the early 1958 first edition of Model Aero Engine Encyclopaedia included the 1 cc, 1.5 cc and 2.5 cc BWM shaft-valve models, but this is no guarantee that they were still in production at that time. It actually appears likely that the range never re-expanded beyond the 250D after the focus was placed upon that model in 1954, and production appears to have ended without fanfare at some indeterminate point in the latter half of the 1950's.
We're now in a position to summarize the five production models which are attested in the above references. If any reader can fill in the admittedly large gaps in our knowledge, we'd be most grateful! In particular, we'd very much appreciate any additional images which could be tied into the following text.
This model is presently known to us solely from its appearances in Peter Chinn's December 1952 article and in the early 1958 listing of the world's engines in Model Aero Engine Encyclopaedia According to both listings, it was a plain bearing crankshaft rotary valve design which was similar in appearance to the other contemporary BWM shaft-valve models and (like them) had provision for either beam or radial mounting. Other than that, we have no specific design details at present. The basic specifications derived from the above references are as follows:
|Reported weight:||71 gm|
|Performance:||(no specific figures available)†|
† The quoted speed range was 8,000 to 12,000 rpm, and the Aeromodeller power classification was D.
This model appears in three of our references; Chinn's December 1952 article and his May 1953 listing of International-class engines as well as the 1958 listing from Model Aero Engine Encyclopaedia Once again, it was a plain bearing crankshaft rotary valve design in the established "house style" which was arranged for either beam or radial mounting. The basic specification data from all sources are mutually consistent, as follows:
|Reported weight:||92 gm (3.25 ounces)|
|Performance:||0.12 BHP @ 12,500 rpm†|
† (manufacturer's claim). The Aeromodeller power classification was E.
The existence of this model is only attested (as far as I am aware) in Chinn's original December 1952 article as well as his subsequent listing of International-class engines which appeared in the May 1953 issue of Model Aircraft Despite the name, the engine did fall within the then recently-adopted FAI displacement limit of 2.50 cc for International competition. The "1" was presumably added simply to distinguish this model from the companion 250 design.
The BWM 251 was the "performance" model in the original BWM range. Its design departed significantly from that of the better-known 250D or the earlier BWM 250 (see below). For one thing, it was a short-stroke engine in contrast to the 250 and the 250D. It featured twin ball races allied with twin exhaust stacks and rear disc valve induction. As one would anticipate from this specification, it also weighed substantially more than its FRV counterparts.
The basic specifications given in the two cited listings are as follows:
|Displacement:||2.47 cc (0.15 cuin)|
|Weight:||142 gm (5.0 ounces)|
|Performance:||0.27 BHP @ 15,000 rpm†|
† (manufacturer's claim)
Once again, this model is presently known solely from its appearances in Chinn's December 1952 article and in the May 1953 listing of International-class engines mentioned earlier. It was a crankshaft rotary valve engine of similar design to the other BWM shaft-valve models and was mainly notable for its very light weight. Based on the images presently at our disposal, the 250 appears to have featured a metal tank with a cap for the filler hole. It also had a transverse needle valve set-up which extended at right angles to the axis of the engine. The manufacturer's claimed performance was significantly down on that for the 251 model.
It seems almost certain that it was this model which was replaced by the 250D in 1954, with the 251 being simultaneously dropped from production. Published specifications for the BWM 250 are as follows:
|Weight:||111 gm (3.90 oz)|
|Performance:||0.23 BHP @ 12,500 rpm†|
† (manufacturer's claim)
This is by far the best-known of the BWM models. According to Peter Chinn, it dates from 1954 when it replaced an earlier BWM 2.5 cc crankshaft rotary valve design, almost certainly the 250 model noted above. It shared the same bore and stroke as the 250, and indeed these were the only long-stroke models presently known to have been produced by the company.
The 250D was a plain bearing FRV sports diesel of essentially conventional design. Although far from common, it is the most frequently-encountered BWM model today, likely because when Manfred Göcking took over the manufacturing operation from Horst Freise in 1954 he reportedly elected to focus on production of the 250D to the exclusion of the other models in the range. The 250D thus had a significantly longer production life than the other BWM models and was marketed far more widely during the mid 1950's. Even so, it remains a relatively rare engine.
Despite presenting a very similar external appearance, the 250D was somewhat heavier than its 250 predecessor, suggesting some significant design differences. From an examination of the respective images, the only externally-apparent differences are in the cylinder porting, the needle valve alignment, the tank and the crankcase finish.
The 250 seems to have had only three radially-disposed exhaust ports as opposed to the four ports used in the later 250D. It's possible that the transfer arrangements were somehow different as well, but in the absence of an actual example we have no basis for further comment at this time. The reason for the increased weight of the 250D must therefore remain an open question at present. However one suspects that the difference must have been somehow related to the visible change in the cylinder design.
The needle valve of the 250D was aligned at a dramatic upswept angle to the main axis of the engine, in contrast to the right-angled alignment which had been used in the earlier 250 model. The metal tank of the earlier model had been replaced by a plastic item with no filler cap. In addition, the case and backplate were now anodized black. In other respects, the 250D appeared little changed from its predecessor.
A detailed description of this model will appear below. For now, it's sufficient to quote a few vital statistics for comparison purposes:
|Weight (with tank):||131 gm (4.6 oz)|
|Performance:||0.19 BHP @ 11,500 rpm†|
† Model Aircraft test report. This was of course considerably below the manufacturer's claim of 0.23 BHP @ 12,000 rpm.
The Aeromodeller power classification was F.
This concludes our summary of the five BWM models which appear to have actually entered production. Apart from Peter Chinn's original article from December 1952, we have been unable to find any further references to the BWM 50, BWM 500 or BWM 1001 models. It appears that they either never entered series production or were withdrawn before they could make any impression upon the contemporary marketplace. We are however very much open to informed correction on this point, as always.
We are now ready to describe the sole model for which we have both test data and several actual examples available for study. This is the BWM 250D model just summarized. Let's have a closer look at this engine.
The BWM 250D Described
The main features of the BWM 250D are very clearly depicted in the excellent Ken Carter cut-away illustration which accompanied the published test of the engine in the May 1955 issue of Model Aircraft The 250D is a well-made long-stroke FRV diesel engine of generally conventional design. The use of long-stroke geometry results in a rather "tall" engine for its displacement. Nonetheless, the engine is quite attractively proportioned and finished overall.
The 250D is based upon a cleanly-formed die-cast aluminium crankcase which is anodized black. Contacts in Europe have confirmed that the original BWM models had plain cases, while the later ones had black-anodized cases. In both instances, the cases were of aluminium alloy, not magnesium as might otherwise be assumed from the black coloration. A word of warning—make sure that the bearers are protected when mounting these engines, otherwise the black finish on the bearers will be marred for sure.
The serial numbers of the 250D engines were most unusually stamped on top of the screw-on cooling jacket in a circular layout around the comp screw. A bit sketchy—change the cooling jacket and you've changed the engine number! The numbers begin with the ciphers BWM 250D followed by a number in the form 15-xxxx. I have numbers 15-2133 and 15-2188—apparently only 55 units apart in the production sequence. The latter number is actually the highest that I have so far encountered among the relatively small number of examples that have come to my personal attention over the years. If higher numbers exist, as they well may, I'd be most grateful for an update!
The plain bearing is bronze-bushed and of generous length for good shaft support. In both of my examples, the bearing is very well fitted to the 8 mm diameter shaft journal. The Model Aircraft tester reported that the fit on his example was a little loose, but this was apparently of no significance in performance terms.
The one-piece crankshaft is of generally conventional design, with a round hole serving as the induction port and a full disc crankweb. The one somewhat unusual feature is the fact that the crankweb disc is counterbalanced by being drilled through at 3 mm diameter to lighten on both sides of the 4.5 mm diameter crankpin rather than having a counterweight machined onto it. Given the fact that the piston is rather on the heavy side, the counterbalancing is a good feature. It seems to be effective also—the engine vibrates far less than one might expect from a long-stroke design having a relatively heavy piston.
The con-rod is a turning from high-strength aluminium alloy. It is mated to the piston by a press-fitted gudgeon pin having a rather skinny diameter of 3 mm. The press-fitting is necessary with the type of porting used, since a fully floating gudgeon pin could well foul the internal transfer flutes. No attempt has been made to lighten the piston by internal milling.
The steel cylinder is again of conventional design, having four exhaust ports sawn into a port belt which also serves as the cylinder locating flange. The cylinder is externally threaded at both ends and screws into the crankcase casting with a gasket for sealing, with the aluminium alloy cooling jacket screwing onto the upper cylinder in the conventional manner. A standard T-shaped compression screw of sturdy construction completes the assembly.
Beneath the exhaust ports are four internal flute-type transfer ports, very much along the Arden pattern. The arrangement does not allow for any overlap between the exhaust and transfer ports, and the blow-down period is accordingly rather lengthy. The exhaust port opening period of some 140 degrees of crank angle is actually fairly generous for a sports diesel of this type, but the transfer porting arrangements still force a rather conservative transfer period of around 90 degrees.
The timing of the crankshaft induction port is also on the conservative side, extending from 70 degrees after bottom dead centre to 20 degrees after top dead centre for a total opening period of only 130 degrees. The fact that both the induction port and the intake venturi are circular in section means that opening and closing of the induction system is far less rapid than it could be, and the system is fully open for substantially less than the 130 degrees between opening and closing. To offset this, the engine is provided with a lengthy sub-piston induction period of some 80 degrees (40 degrees each side of top dead centre).
So far, it's all pretty much conventional stuff. But now we get to the first of the features that sets this engine a little apart from the rest. The needle valve assembly is most unusually aligned, being located on the right-hand side (facing forward) and sharply angled both rearwards and upwards. This alignment places the control arm at a very convenient location well aft of the propeller disc and clear of the exhaust, but also means that the needle cannot be switched to the left side for sidewinder control-line use. If such use were contemplated, it would be almost mandatory to make up a stub needle which did not hang out in the breeze to be bent or broken during every landing!
Another unusual and rather neat feature of the spraybar is the T-piece that is soldered onto the pickup end. This serves as a 90 degree connection point to keep the fuel line close to the engine without kinking. In fact, the needle valve lay-out on this engine is strongly reminiscent of that used earlier in the British E.D. Baby 0.46 cc diesel.
The engine is equipped with a long externally-threaded needle which provides excellent mixture control but appears highly vulnerable to crash damage. Early examples (including the one tested by Peter Chinn—see below) simply used a lock-nut for maintaining the needle setting, but this would have required constant adjustment—Chinn actually recorded some difficulties in connection with the setting of the lock-nut system on his test engine. The manufacturers must have learned from this, and later examples (including both of my own engines) have a knurled barrel soldered to the needle with a wire spring ratchet secured between a pair of nuts which retain the spraybar. It must be said that the second system represents a far more practical solution.
Perhaps the most unusual feature of the engine is the means of securing the propeller. The front of the crankshaft has a 6 degree taper, onto which a conventional aluminium alloy prop driver fits in the normal manner. The interior of the shaft is hollow at the front and is tapped 5 mm. This accepts a brass stud, which should bend in a hard crash and thus minimize the possibility of crankshaft damage. Replacement of a bent stud after a crash would be a very simple matter. The end of the brass stud is slotted for a screwdriver to facilitate extraction.
So far, it's all familiar. The interesting part of this set-up is the actual means of airscrew attachment. The BWM uses an aluminium alloy spinner of conventional form. The major Achilles heel of this form of airscrew attachment is the well-known tendency of the threads in the spinner to strip with repeated propeller changes. The BWM uses an unusual and seemingly very effective approach to avoid this problem.
Most spinners of this type feature internal threads which start in the same plane as the rear of the spinner itself. In the case of the BWM spinner, the rear of the component is quite deeply recessed and incorporates a relatively short threaded internal boss which occupies only the forward portion of the component. The brass stud screws into this truncated boss as far as it can go without fouling the cross-hole for the tommy bar and is then locked in place by a thin hexagonal nut. Reference to the Carter cut-away view will clarify this assembly.
The consequence of this arrangement is that the spinner and the stud become effectively a single unit, and the use of a suitable tommy bar in the spinner cross-hole to tighten the prop turns the stud in the shaft rather than the spinner on the stud. This transfers any thread wear to the brass stud and steel crankshaft rather than the aluminium spinner and minimizes the possibility of stripping the spinner thread. A stripped stud is of course easily replaceable. The stud has ample thread to accommodate the full range of propeller sizes likely to be used.
The die-cast aluminium alloy backplate is anodized black to match the case and is secured to the crankcase by a pair of 2.5 mm round-headed steel bolts with brass nuts. These could be replaced with longer bolts if radial mounting were desired. A gasket is used to ensure a seal. The backplate incorporates a 2.5 mm tapped central boss for tank mounting. The engines came equipped with a moulded clear plastic back tank suitable for free flight use, but this is usually missing or cracked on surviving examples.
The quality of manufacture of the engine is extremely good. Both of my examples are very well fitted throughout, with outstanding compression and perfectly-fitted contra-pistons.
The BWM 250D On Test
A test of the BWM 250D (almost certainly by Peter Chinn) was published in the May 1955 issue of Model Aircraft Chinn's description tallied completely with that just given above. He did make the comment that although the con-rod bearings in his example were very well fitted and were drilled and reamed parallel to each other, they were not formed exactly at right angles to the rod. This was clearly a manufacturing error, and it might be expected to result in accelerated rod bearing wear as well as a certain loss of performance. It was most likely a fault of the particular engine tested—the standard of manufacturing was reported as being generally extremely good, which tallies with my own observations.
In terms of performance and utility, Chinn (assuming it was he) commented that the engine was "a sensibly arranged motor of pleasing appearance and, it would seem, with no marked vices. It starts quickly, handles easily and runs well over a useful range of speeds." He commented that "the engine started easily and without recourse to priming and without the control settings being critical." The engine's response to both controls was said to be good, although Chinn did experience a little initial difficulty in setting the lock-nut which maintained the needle setting on his example. It may have been this published comment which led the manufacturers to make the switch to the ratchet system described earlier.
Chinn was unable to match the manufacturer's claimed performance figures of 0.23 BHP @ 12,000 rpm, but he did extract a useful performance (by the standards of the day for a plain bearing sports engine) of 0.19 BHP at 11,500 rpm. It is of course possible that the manufacturer's figure would have been more closely approached if the con-rod bearings had been correctly aligned. Chinn commented that the engine delivered "a most useful and economical performance. in the 8,000/10,000 rpm bracket". Presumably he was referring to the most appropriate static loading speed range, since the engine actually peaked at 11,500 rpm and this would therefore have been the target airborne rpm figure.
The summary comment with which the test report was concluded was quite favourable to the engine. Chinn stated that "the general running characteristics of the 250D were found to be entirely satisfactory, being free from excessive vibration, and no signs of stress were exhibited on inspection after test". The latter comment is interesting insofar as it implies that the misalignment of the rod bearings in Chinn's example had had little or no effect upon the engine's observable wear patterns.
I can only comment that my own impressions of the engine based upon present-day bench tests entirely support Chinn's reported findings. The engine is indeed an easy starter with or without a prime, and responds to the controls very well. It's no earth-shaker as regards performance, but it does shift a useful amount of air when suitably loaded. Torque rather than ultimate horsepower is its strong suit, and it's definitely an engine for the larger props by 2.5 cc standards.
My own bench tests of number 15-2133 yielded the following rpm figures, with power figures derived from estimated propeller power coefficients:
|Test Prop||RPM||Power (BHP)|
|Taipan 9x6 GF||8,900 (8,600)||0.177|
|Taipan 9x4 GF||10,500 (10,300)||0.208|
|Tornado 8x6 nylon||10,100 (10,000)||0.202|
|Taipan 8x6 GF||10,400 (10,400)||0.206|
|Taipan 8x4 GF||12,400 (12,400)||0.183|
These imply a somewhat higher power output than that achieved by Chinn. A Taifun Rasant 1 tested at the same time as a control yielded almost identical figures, as shown in brackets beside the figures for the BWM. Nothing there to come between Webra designer Gunther Bodemann and his beauty rest, but there's no doubt that this would make a more than acceptable power unit for general-purpose sport flying as opposed to all-out competition. It was clearly more or less on a par with the German plain-bearing competition at the time of its introduction.
I subsequently ran a few additional rpm tests using APC props for which reliable BHP coefficients are now available through the good graces of Gordon Cornell. Results as follows:
|Test Prop||RPM||Power (BHP)|
|APC 9x6 GF||8,600||0.155|
|APC 9x4 GF||10,300||0.192|
|APC 8x6 GF||10,600||0.210|
|APC 8x4 GF||11,900||0.182|
These results are generally consistent with those obtained earlier using the Taipan and Tornado props, and once again suggest a somewhat higher output together with a slightly lower peaking speed than the figures measured by Peter Chinn. Allowing for the inevitable "scatter" due to missing the perfect setting on some props, the admittedly incomplete "power curve" derived from the combined figures set out above suggests a peak output close to the maker's claimed 0.23 BHP, with peak power being found somewhere in the vicinity of 11,300 rpm. The engine has clearly passed its peak by the time the upper 11,000 rpm range is reached, which is consistent with Chinn's findings.
The End of the Line
As stated earlier, following the publication of the May 1955 Model Aircraft test the BWM range pretty much fell below the radar, at least as far as the English-language modelling media was concerned. The appearance of the range in the early 1958 listing of the world's engines in Model Aero Engine Encyclopaedia is no guarantee that production was still ongoing at that time—a good few engines on that list were unquestionably out of production as of early 1958.
There is actually no present indication that the range ever re-expanded following the 1954 take-over by Manfred Göcking and the effective reduction of the range to the single 250D model. It's actually far more likely that the 250D failed to make the necessary impression upon domestic and overseas markets to justify the continuation of the range, which had already experienced difficulties in 1954 as reported earlier. It seems highly unlikely in fact that the 250D survived much if any past 1957.
A number of potential reasons for this failure are readily apparent. It was probably a combination of lack of resources coupled with ever-increasing competition that caused the downfall of the BWM range. Even in 1954 when it first appeared, the 250D was a rather "old-fashioned" design with its long stroke, conservative timing and Arden-style porting. Competing domestic models from the likes of Webra and Taifun were more than a match for the BWM in performance terms, as were a number of contemporary British 2.5 cc diesels.
Another more insidious factor which was not recognized at the time but is readily apparent today in hindsight was the fact that the world market for model engines had already peaked by around 1957 and was even then poised to enter the long and slow period of retrenchment which has continued to this day. In such a market, only the best would survive. The BWM engines were very well-made, but they were yesterday's technology in design terms. In order to survive, the range would have had to be substantially updated and aggressively marketed, but it appears that the resources were simply not there to allow this to happen. Pity...
The relative scarcity of surviving examples of the BWM range implies rather low production figures during the approximately five year existence of the marque. If the serial numbers of which I'm so far aware are any indication, perhaps 3,000 examples of the 250D were made in total. Even so, the 250D is a relatively rare beast today, and the other models in the range seem to be like rocking horse droppings!
Still, you may well run across a 250D on eBay or elsewhere. If you do, remember that this is a very user-friendly sports diesel that is well made and looks great either in a collection or in a classic model. So if you ever get the chance, one of these would be a very worthwhile acquisition for any lover of "classic" model diesel engines!
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