Webra 2.5R glowplug model

The Webra 2.5R Glow-Plug Model

This will be the fourth in my series of tests of classic 2.5 cc competition glow-plug engines from the 1950’s. I've previously tested the 1956 Super Tigre G.20 S, the Vltavan 2.5 and the MVVS 2.5R-58. Now it's the turn of a German entry into the 2.5 cc high performance glow-plug sweepstakes – the Webra 2.5R glowplug model of 1956.

This engine was unusual among then-contemporary competition glow-plug engines in that it retained reverse-flow scavenging as opposed to the cross-flow loop scavenging which was by then almost universal in competition glow-plug circles during that pre-Schnürle era. The only other notable contemporary design to retain this form of scavenging, albeit in a very different configuration, was the Barbini B.40 TN, the remarkable story of which has been recounted in a separate article on this site.

The Webra 2.5R was in effect a straightforward glow-plug conversion of the far better-known Webra Mach I diesel which had first appeared in 1953 and had subsequently acquired a formidable international reputation as a contest engine in its own right prior to the appearance of the glow-plug version. This being the case, I’d better go back to the beginning and trace the introduction of the Mach I diesel.


The early history of model engine development in post-WW2 Germany is greatly obscured by the fact that  for some years following the conclusion of that conflict, model flying activities in Germany were severely curtailed by the occupying powers. Moreover, Germany was barred from participation in International competitions, presumably to allow time for festering post-war resentments to simmer down. This had undoubtedly applied the brakes to the development of German aeromodelling technology as well as restricting the visibility of German modelling activities to the rest of the world.

Of course, the enthusiasm of German modellers was by no means diminished by these restrictions – you can regulate peoples’ activities, but not their enthusiasm!  Consequently, modelling activity become re-established relatively quickly in Germany following the conclusion of the war, albeit at a somewhat constrained level. A number of new German model engine manufacturers soon emerged, although at the outset they were small-scale operations which were limited to serving only the German domestic market as well as the odd member of the occupying forces. The illustrated 1950 RGU Universa 2.65 cc diesel is a typical example.

These early products escaped notice elsewhere due in significant part to the ongoing restrictions against the participation of German model fliers in International competition. Indeed, the restrictions upon model flying in early post-war Germany make it seem likely that many of the German model engines produced during this period were used in model boats and cars, the operation of which was not restricted. 

One of these early post-war German ranges was the Webra marque, which appeared in 1950 with no international fanfare. This soon-to-become iconic range had its origins in a 1949 decision by two partners in a German export agency, namely Walter Weichler and Martin Bragenitz, to enter the model engine business. Foreseeing an eventual relaxation of the model flying restrictions then in place as well as a softening of the somewhat negative international attitudes towards German products arising from the recently-concluded hostilities, they decided to develop a range of model engines to offer competition to the British designs which then dominated much of the European market. The name of their fledgling range was Webra, derived from the first few letters of their respective last names.  

Not being precision engineers themselves, the partners naturally needed help. They formed a partnership with a precision engineering company called Fein und Modelltechnik located at 5 Genestrasse in Berlin-Schöneberg, which was part of the Western zone of Berlin. The individual responsible for the design of the Webra engines was Günther Bodemann (1921–1971), who was destined to become arguably Germany’s foremost model engine designer of the "classic" era prior to his untimely demise on October 21st, 1971 from cancer.

According to Peter Chinn, at some later date Martin Bragenitz assumed sole ownership of the Webra venture under the trade name of Bragenitz & Co., although he naturally retained the by then familiar Webra trade-name. Later still, in 1962 the Webra marque was acquired by Martin Eberth, who reorganized the whole operation and worked very productively with Günther Bodemann to revitalize the range.  But that’s another story ………..

Returning to our main thread, the first Webra production model, a neat and compact 2.46 cc radially-mounted plain bearing diesel, appeared in late 1950. Initially, the company focused on the more or less continuous development of this model, which appeared in a number of variants over the following few years. In mid 1952 the Webra range was expanded with the introduction of a very light, compact and powerful radially-mounted 1.48 cc model built along more or less identical lines to the 2.46 cc offering.

By this time the Webra range had already assumed a dominant position in the domestic German model engine marketplace and was beginning to make inroads into the international sphere. Arthur Mullett of Brighton in Sussex soon became the British distributor for the Webra engines. 

It was also in 1952 that the remaining restrictions relating to model aircraft operation in Germany were finally relaxed. At the same time, Germany was re-admitted to the International competition arena, leading to the country’s progressive re-emergence as a force to be reckoned with in such competitions. Several German competitors showed up at the 1952 World Free Flight Power Championship meeting held in September at Dubendorf Aerodrome in Switzerland. Using then-current Webra 2.46 cc plain bearing diesels, they finished 6th and 7th respectively, thus making a very respectable come-back showing for Germany. Webra engines also powered around half the entries in the 1952 German national free flight power championships, including the winner.

The Webra’s excellent showing in the 1952 World Free Flight Championships naturally awakened a high level of international interest in then-current German engines. This resulted in the publication of the first English-language survey of German model engines of which I'm presently aware. This was a highly informative article by the late Peter Chinn (1921 – 2005) which appeared as part of Chinn's ongoing “Accent on Power” series in the December 1952 issue of “Model Aircraft” magazine.

This long-running series of articles today constitutes one of the most invaluable archival records of model engine history from the standpoint of a well-informed and extremely knowledgeable contemporary observer. We are forever indebted to Peter Chinn for preserving so much contemporary information on the model engines of the “classic” and post-classic eras. I routinely refer to Chinn’s work when researching almost all of my own articles, including this one. I often wish that someone would collect all of Chinn’s writings into a searchable DVD or book form …………I’d do it myself if my “Model Aircraft” and "Aeromodeller" magazine collections were anywhere near complete.

The particular article in question was entitled “Engine News from Germany”. It represented a praiseworthy attempt to summarize the progress of the various German manufacturers as of 1952 following Germany’s re-admission into the international competition fold. By the time that Chinn was writing this article, I noted previously that the Webra range had already assumed a leadership position in the German model engine manufacturing field. Consequently, Chinn began by describing 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.

At this time the 1.48 cc and 2.46 cc Webra engines were both radially-mounted plain bearing diesels built to a more-or-less common design. They were both light and powerful by the standards of their day. Chinn published a test of the 2.46 cc model in the February 1953 issue of “Model Aircraft”, citing the manufacturer's claimed peak output of 0.22/0.23 BHP @ 11,000 – 12,000 RPM. He commented that his test example did not quite reach those figures, managing "only" around 0.21 BHP @ 10,900 RPM. In his view, the shortfall was due in large part to a sharply tapered bore which gave rise to the infamous “Continental squeak” due to excessive piston tightness at the top of the stroke. Translation - it needed more running-in time! Even so, Chinn characterized the 3.6 ounce Webra as having “one of the highest power-to-weight ratios realized among F.A.I. Class “1”  diesels”.

The success of his relatively unassuming 2.46 cc plain bearing diesel against international opposition evidently set Günther Bodemann to thinking about what could be accomplished with a reconfigured model designed from the ground up as an all-out competition engine. No sooner envisioned than realized! The result was the June 1953 appearance of one of the most iconic classic Webra models of them all - the Mach I diesel.

This twin ball-race diesel featured disc rear rotary valve (RRV) induction through a very large venturi which used a racing-style surface jet fuel delivery system. A screw-in cylinder was employed, with a copper gasket inserted between the cylinder location shelf and the upper crankcase to ensure a seal. The screw-on cooling jacket was anodized red on a few of the very early examples, but the vast majority of these units featured green anodizing.

Cylinder porting included three radially-disposed sawn exhaust slots with eight internally-formed transfer flutes below them. This was a broadly similar arrangement to that introduced in the 1940's by the Kemp Eagle Mk. I and AMCO 3.5 cc models from England and the far more obscure Pepperell engines from New Zealand. 

This arrangement provided an unusually large bypass area, its limitation being that the need to terminate the internal flutes below the exhaust ports in order to maintain a crankcase seal necessitated a long blow-down period with a relatively short transfer period of only some 80 degrees. Doubtless the very large combined area of the eight bypass/transfer flutes did much to mitigate any negative effects arising from this cause.

The design of the Mach I was undoubtedly successful in meeting Günther Bodemann’s goal. Peter Chinn’s test report which appeared in the September 1954 issue of “Model Aircraft” cited a peak output of 0.291 BHP @ 15,000 RPM, an outstanding performance for a 2.5 cc diesel by 1954 standards. The Mach I was immediately adopted by the international modelling community as one of the engines to use in free flight power competitions, retaining this position for some years.

Meanwhile on the control line speed front, things were moving. After contesting the title in both 5 cc and 10 cc classes in previous years, the FAI announced that beginning in 1955 the World Control Line Speed Championships would thenceforth be contested exclusively in the 2.5 cc displacement category. The first such contest to be run under the new displacement rule took place in early July 1955 at Croix-de-Berny, a few miles south of Paris, France.

At the time in question, readily-available commercial 2.5 cc control line speed engines were relatively thin on the ground - most manufacturers' design efforts had hitherto been focused upon the larger displacement categories. The most prominent 2.5 cc offering was the Super Tigre G.20 S from Bologna in Italy. Super Tigre designer Jaures Garofali was instrumental in ensuring that a strong Italian team equipped with well-prepared examples of the latest Super Tigre G.20 S lapped-piston ("Lappato") powerplants appeared at this meeting. Their goal was to sweep the results by taking both the individual and team titles, thus making an unmistakable statement regarding the superiority of the Super Tigre G.20 S.  

For contestants who could not or did not wish to use Super Tigre power, the choice was relatively limited. Frankly, nothing else that was commercially available in early to mid 1955 could match a well-prepared Super Tigre for all-out performance in a control line speed context. People showed up with whatever they could find to run and hoped that the Super Tigres would run into trouble. 

They did indeed run into trouble, but not of the sort that would give other commercial engines a chance. Unfortunately for the Italians, this meeting coincided with the emergence of a new threat from the other side of the Iron Curtain in the form of the State-sponsored MVVS tool-room specials from Czechoslovakia (as the Czech Republic was then).

Jaures Garofali’s showcase ambitions almost succeeded, but the top prize eluded him by a narrow margin. Josef Sladký of Czechoslovakia won the individual title with a speed of 179.11 km.hr (111.30 mph) using one of the MVVS one-off powerplants, the SK-25 “Broucek” (Poppet) design. Italy's Amato Prati came in a close second with his G.20 S powered model at a speed of 176.05 km/hr (109.39 mph), closely followed by his fellow Italian team members S. Monti and C. Cappi.

Together with G. Gottarelli’s sixth place finish behind Mir Zatońćil’s fifth-placed MVVS, this was easily enough to give Italy the team title. However, the fact that the individual title had eluded them cannot have sat well with Signor Garofali!

Sladký’s success signalled the beginning of what might be termed the “works team” or “special” era in International control-line speed. Thenceforth there were in effect two distinct tiers of competitors – those using well-prepared examples of commercially-available engines and those using non-commercial individually-made “specials”. As might be expected, the "specials" generally ruled the roost ...........

The breakdown in the numbers of commercial engines other than the Super Tigre which appeared at the 1955 Paris meeting is interesting. Apart from the five Super Tigre entries, the four MVVS units and the lone Carter Special of Britain’s Ray “Gadget” Gibbs, there were no fewer than eight contestants using Webra Mach I diesels. This represented almost exactly a quarter of the field of 33 entries. There could be no clearer demonstration of the prominence which the Webra Mach I had achieved within the international competition community.  

The highest placed of the Mach I contestants was Olle Ericsson from Sweden, who placed a very respectable seventh as the highest-placed diesel with a speed of 169 km/hr (105.0 mph), only some 10 km/hr behind the overall winner and ahead of Gibb’s Carter Special as well as all the K&B's. Other Mach I-powered models finished in twelfth, sixteenth through nineteenth inclusive, twenty-third and twenty-fifth places. The nineteenth-placed entrant was Günther Bodemann himself.

Other engines appearing at the event included five K&B .15 glow-plug units (the best of which finished eighth behind Ericsson's Mach I with a speed of 163.02 km/hr (101.3 mph)) , five E.D. 2.46 Racer diesels, one E.D. Racer converted to glow-plug operation (which had won the previous year’s non-Championship international meet for Pete Wright at a higher speed than the 1955 World Championship winning mark!), one Micron .15, one Oliver Tiger diesel and one Aero 250 diesel from Yugoslavia.

However, for the purposes of the present article, the most significant powerplant to appear at the meeting was a Webra Mach I which had been converted to glow-plug operation by future British team-race great Dick Edmonds. Pete Wright had a similar engine which he used in the final round without improving on the speed achieved with his trusty E.D. Racer glow-plug unit. Edmonds and his converted Webra ended up in a very creditable ninth place with a speed of 162.05 km/hr (100.70 mph).

Ron Moulton’s report on this meeting which appeared in the September 1955 issue of “Aeromodeller” magazine specifically mentioned that Günther Bodemann (himself an entrant) had taken a considerable interest in the glow-plugged Mach I’s used by Edmonds and Wright. He had apparently provided the two Brits with a considerable amount of assistance at the meeting. A photo of Bodemann with his own Mach I diesel entry was captioned with the comment that he was now a “glowplug fan”!

It seems abundantly clear that the subsequent appearance on the market of a factory-built glow-plug version of the Mach I stemmed directly from this experience. Bodemann evidently returned to Berlin determined to try such a conversion for himself. The result was the appearance of our subject test engine – the Webra 2.5R glow-plug model.

The first few examples of this engine were effectively factory “specials” expressly intended for field testing through some serious competition work. These were evidently in circulation among selected competition modellers as of early 1956, to be followed by a production version of the 2.5R glow-plug model which appeared later in 1956. It is this production model which is the central subject of the present article.  

The Webra 2.5R Glow-Plug Model – Description

The Webra Mach I diesel is so widely familiar to most model engine aficionados that this part of the article need not detain us long. A general description of the Mach I was included in an earlier section of this article.

The 2.5R glow-plug version of the Mach I was basically identical to its diesel counterpart apart from the upper cylinder components. Bore and stroke were unchanged at 15.5 mm (0.610 in.) and 13.0 mm (0.512 in.) respectively for a displacement of 2.45 cc (0.149 cuin.). The glow-plug model weighed in at 133 gm (4.7 ounces) - a very reasonable figure for a twin ball-race disc valve glow-plug motor. Checked geometric compression ratio of my illustrated example is 8:1.

The engine featured a barrel-type crankcase of the usual “racing engine” pattern, with a detachable front cover incorporating the crankshaft in its twin ball races and a detachable rear cover accommodating the intake and disc valve. Both covers were retained by four hex-head machine screws apiece - a quality touch. In both cases, thin paper gaskets were used to ensure a good seal.

The cylinder porting was also identical to that of the Mach I diesel, retaining the same three radially-disposed sawn exhaust slots with eight internally-formed transfer flutes below them. The piston, rod and crankshaft were all identical to those of the diesel model.

By the time that the 2.5R glow-plug model appeared, a few changes had been introduced to both the diesel and glow-plug models. The external diameter of the formerly oversized intake spigot had been reduced, with a spraybar needle valve assembly now being used in place of the former surface jet arrangement. This move was presumably aimed at improving operational flexibility. The rotary disc was now made of thermoset plastic instead of the aluminium alloy used previously. This was a very good move in terms of longevity.

The internal diameter of the intake was now 6 mm, which is about as large as it could have been made with the available metal. The spraybar major diameter was 3 mm, but this was neatly turned down to only 2.4 mm at the jet location in the section which traversed the intake. According to Maris Dislers' extremely useful Choke Area Calculator, this gives an effective choke area of 14.268 mm2, which is good for a practical minimum operating speed of 11,939 RPM with useable suction. It seems safe to assume that this engine would normally be operated at speeds well above this figure.

The major changes required to create the glow-plug version were of course centred on the upper cylinder. The contra piston was naturally omitted, its place being taken by a deep spigot protruding from the underside of the aluminium alloy cylinder head. This head was centrally tapped for a long-reach glow plug. A separate internally-threaded open-ended finned cooling jacket was screwed onto the externally-threaded cylinder and turned down until its top surface was just below the top of the cylinder proper. The head was then installed using a fibre gasket to seal against the top of the cylinder liner and was secured by five slot-head screws which threaded into tapped holes in the upper cooling jacket.

This was a very clever arrangement, because the tension of the head installation screws ensured excellent contact between the cylinder and cooling jacket threads, thus promoting first-class heat transference. It also strongly discouraged any tendency for the cooling jacket to unscrew during operation. I have never experienced any unscrewing problems or head seal issues with one of these engines.

One oddity associated with many examples of the glow-plug model was the retention of the diesel’s very tight piston fit in the upper cylinder. Several contemporary commentators noted this factor, and I have experienced it myself with several examples. One would have thought that engines intended for high-speed glowplug operation would feature somewhat relaxed piston/cylinder fits. As supplied, many examples of the engine evidently required a fair bit of running time to deliver their best performance.

Another feature worthy of comment was the means by which the con-rod big end was retained on the crankpin. The crankpin was hollow, and the rod was first installed on the crankpin, after which a keeper was pressed into the hole in the crankpin centre. This keeper had a thin central shoulder at greater than crankpin diameter which retained the big end on the crankpin and prevented it from moving back to foul the rotary disc valve. A small-diameter rearward extension protruded from the centre of the keeper to drive the disc valve.

This poses a challenge when one wishes to replace the con-rod. It's necessary to unscrew the cylinder and then remove the front housing with the piston and rod still attached. One then has to remove the shaft from the main bearing to allow access to the front of the central hole in the crankpin.  One can then drive out the keeper from the front using a drift of suitable diameter. Please do not try to remove the keeper from the rear using pliers or vice grips - damage is inevitable!!

The Webra 2.5R Glowplug Model in Competition

As far as I can determine, the first competition appearance of the factory-made Webra 2.5R glowplug model came at the Criterium of Europe meeting held at Brussels, Belgium between April 30th and May 1st, 1956. The 2.5 cc control line speed category at that meeting was won by J. Battlo of Spain (manufacturer of the Byra engines) using a modified Super Tigre G.20 S to record a speed of 201.97 km/hr (125.5 mph) just ahead of Britain’s future World Champion Ray “Gadget” Gibbs with his Carter Special-powered "Nipper". Standards had clearly taken a massive leap forward in the nine months since Paris 1955!

A number of Webra-powered entries appeared at the same meeting. Interestingly enough, the best of them was a Mach I diesel entered by Huppertz of Germany, who came in fourth with a speed of 168.98 km/hr (105.0 mph), thus matching Olle Ericsson's seventh-place speed from the previous year's World Championships. Huppertz was closely followed by two Webra 2.5R glowplug-powered models in fifth and sixth places with speeds of 164.15 km/hr (102.0 mph) and 163.34 km/hr (101.5 mph) respectively. These speeds would have placed their entrants in the top eight at the previous year’s World Championship meeting at Paris, but both Jaures Garofali of Super Tigre and Fred Carter had clearly stepped up their games in the interim while the Webras had stood still.

Unfortunately, these results sent out the message that there was little point in anyone continuing to use the Webras in international control line speed competition. Only two Webra-powered models showed up at the 1956 World Control Line Speed Championships held in Florence, Italy in October 1956 - one glow-plug and one diesel. These placed 26th and 27th respectively, with the 26th placed 2.5R glow-plug model only managing a speed of 158 km.hr (98.8 mph) – close to the fifth place performance from the earlier Brussels meeting, but a very far cry from Ray Gibbs’ winning speed of 211 km/hr (131.1 mph) at Florence with his Carter-powered "Nipper".

As far as I can determine, this was the final appearance of the Webra 2.5R glow-plug model in the World Control Line Speed Championship. The emgine's presence in the International control line speed field was thus very short lived. However, the 2.5R continued to enjoy considerable success in the free flight power field, dominating the 1956 German National Championships and placing well in other contests for some years to come.

Despite its success in the free flight field, the 2.5R never achieved the widespread and enduring popularity of the Mach I diesel model. It is consequently a significantly rarer engine today. Production seems to have been terminated before the close of the 1950’s after perhaps 3,500 examples had been made based upon known serial numbers. The highest serial number of my present acquaintance is 3188, pictured in Jim Dunkin's invaluable book on the world's 2.5 cc (.15 cuin.) engines. 

The Webra 2.5 R Glow-Plug Model in the British Modelling Media

Although it appeared on the German market in 1956, the production version of the Webra 2.5R glow-plug model did not find its way to Britain until early 1957. Its existence was first revealed to British enthusiasts in  the “Motor Mart” feature which was included in the February 1957 issue of “Aeromodeller” magazine. It was mentioned again in Peter Chinn’s “Engines Only” article which was published in the March 1957 issue of “Model Aircraft”, being referred to again by Chinn in his “Motors of the Moment” piece in the June 1957 issue of “Model Aircraft”.

The 2.5R made no fewer than three appearances as a test subject in the British modelling media. The first of these was Peter Chinn’s test report which was published in the August 1957 issue of “Model Aircraft”. Chinn described the changes which had gone into the conversion of the Mach I to glow-plug ignition, commenting upon the engine’s relative compactness for its specification and also praising its starting and running qualities very highly. Using a fuel containing 30% nitromethane and 25% castor oil, he measured a peak output of 0.270 BHP @ 16,100 rpm, a very good figure for a commercial 2.5 cc glow-plug motor of the period. By way of comparison, Chinn's test of the O.S. Max-I 15 which won the 1956 World Free Flight Power Championship for Britain's Ron Draper ("Model Aircraft", December 1955) recorded a peak output of 0.268 BHP @ 15,000 rpm. On Chinn's evidence, the Webra was well in the hunt!

In his test report on the Webra 2.5R, Chinn included the comment that the manufacturers had published claims of up to 0.33 BHP for certain examples of that model. He recognized the possibility that careful matching of such factors as fuels, plugs and compression ratios to the prevailing atmospheric conditions might result in higher performance figures than those which he measured. To add my own comment here, I would personally expect that a degree of relaxation of the unusually tight piston/cylinder fit (by glow-plug standards) would also improve performance to a measurable extent. Evidence for this will becoma apparent later ..............

The 2.5R made its next media appearance as one of a series of comparative tests of various 2.5 cc competition engines which was conducted during 1957 by Ron Moulton and published in that year’s “Aeromodeller Annual”. This was not intended as an in-depth performance analysis but rather as a snapshot of the relative prop-swinging capabilities of a sizeable sample of the higher-performance 2.5 cc engines then commercially available. Both diesel and glow-plug models were included. Strangely, both the Webra Mach I diesel and the Super Tigre G.20 S were omitted from this comparison.

The article stated that all diesel models were tested using the same fuel of unspecified formulation. The figures for the glow-plug models were reportedly obtained using nitromethane contents which were “adapted” to the engines in question, whatever that means. The airscrews used for all tests were the same family of hand-finished wooden props, the sole exception being a FROG nylon 9x6 airscrew which was included.

As can be seen from the above table of results, the Webra 2.5R glow-plug model acquitted itself well in this comparison. Wisely, no attempt was made to try the engine on the 9x6 prop. On other props, the Webra slightly outperformed most of its glow-plug competition, including the Barbini B.40 TN (which had finished third at the 1956 World Control Line Speed Championships) and the O.S. Max 15 (which had won the 1956 World Free Flight Power Championship for Britain's Ron Draper). The Webra only lost out to the MVVS-based Vltavan 2.5 from Czechoslovakia on the 8x3½ free flight prop. Admittedly, it fell a little short of a number of the tested diesels, but not by much in the majority of cases. It outperformed the Alag X-03 and the D-C Rapier, also coming very close to matching the highly-regarded Enya D15-I as well as the Zeiss Aktivist IV and V models. Overall, a solid performance in comparative terms.

The 2.5R’s final appearance as a test subject came in January 1958, in which month it was one of two featured test engines in “Aeromodeller” magazine. This test is a bit of a puzzle, because tester Ron Warring was only able to extract a peak output of 0.202 BHP @ 13,200 rpm. This is way below the result obtained earlier by Peter Chinn as well as falling far short of the manufacturer’s claims. The peaking speed in particular seems suspiciously low given the high-speed nature of the design. Moreover, the results seem highly inconsistent with the engine’s showing in Ron Moulton’s earlier comparative test, where it held its own against engines such as the Enya 15D-1 and the O.S. Max-I 15 for which far higher figures had been obtained on test.

For these reasons, I have always considered Warring’s results for this engine to be highly suspect. As we shall soon see, my own example of the engine performs far better than this! The reasons for Warring’s unflattering results are no longer discernable. I can only speculate that Warring may have had a particularly tight example. There’s no doubt that the fuel that he used was a little odd, containing 25% nitromethane and no less than 35% castor oil. The latter figure suggests that Warring may indeed have detected a high degree of tightness in the piston/cylinder fit and taken precautions accordingly. If so, he failed to mention this in his report, although he did specifically comment upon the excellence of the engine’s internal fits and general workmanship. Chinn’s significantly higher figures were obtained using more nitro and less oil.

The upshot of all of this is that we have a range of published opinions regarding the performance of the Webra 2.5R glow-plug model. Only one way to sort this out – conduct yet another test!  Let’s get straight to it ……………

The Webra 2.5R Glow-Plug Model Reappraised

Having a fine example of the subject engine on hand in the shape of engine number 2313, I was in an excellent position to test the above findings for myself. My example of the engine is in near-mint and completely original condition, although it has had some bench running in the past and seems to have seen a certain amount service in a model if the witness marks on its mounting lugs are anything to go by. Either way, it is very well freed up, with no trace of the squeak-tightness that appears to have troubled other examples - just a very slight (and desirable) pinch at top dead centre. I would objectively rate all fits as exceptional.

I had bench-run the engine myself in the past, finding it to be an excellent starter and a fine, steady runner.  However since it had sat idle for some years, I elected to give it a few shake-down runs on a fuel containing 15% nitro before turning it loose on a 30% nitro blend. I used an APC 8x4 prop for these initial runs.

One aspect of the Webra's reported performance that was amply confirmed by this test was its incredibly easy starting. Both Peter Chinn and Ron Warring praised this characteristic. A single choked flick on a full fuel line followed by a small open-port prime, and the engine started first flick almost every time. Quite a treat for this long-suffering tester - my flicking finger virtually got the day off! Once running, response to the needle valve was very positive, with settings being held firmly at all times. 

For the fully leaned-out test runs using the hotter fuel, I switched to a cold glow-plug. On the first few props tested, the most noticeable characteristic was a slight crackling misfire which couldn't be eliminated using the needle. However, it turned out that this was confined to speeds below 13,000 rpm. Once speed climbed above that point using the 8x4 and smaller props, running became absolutely smooth and mis-free. This is definitely a high-speed engine! When fully leaned out on the faster props, running became absolutely smooth and steady, with no tendency to sag during the run.

This issue may have been a reflection of the fact that below 13,000 RPM the engine was operating in the vicinity of its minimum operating speed with adequate suction, as calculated earlier using Maris Dislers' choke area calculator.  It would be only reasonable to expect that around this lower limit carburetion might become a little inconsistent.

The results of my testing proved to be so surprising that I actually went back and repeated the entire series, reproducing the same results more or less exactly! The engine performed at a far higher level than I had been anticipating based on the previously-published figures cited earlier. Check out the following data: 

APC 7x6 WB  11,700  0.242
APC 71/2 x4 WB  12,800  0.283
APC 8x4  14,400  0.325
APC 7x6  15,300  0.358
APC 7x5  17,100  0.397
APC 7x4  18,200  0.385






The two WB props are wide-bladed items created by cutting down and calibrating larger-diameter APC props to fill gaps in the torque absorption figures for my standard test set.

As can be seen, this example of the Webra 2.5R glow-plug model really did itself proud! The above figures imply a peak output of around 0.400 BHP @ 17,200 rpm, leaving both Chinn's and Warring's figures in the dust and actually beating the factory claim by a wide margin. I have no explanation for this - I can only report the figures obtained on test, all of which were essentially duplicated during the second round of testing. Anyone doubting my veracity is welcome to visit and join me in repeating the test for a third time!

I'm reluctant to disturb an engine that is as well settled down as this one appears to be. As far as I can determine through external examination, the tested engine appearsto be in completely stock and original condition. The two major factors that I can think of which might have affected the measured performance relate to the piston/cylinder fit and the plug heat range.

Looking at the first of these factors, this example appears to have an absolutely perfect piston/cylinder fit, with none of the tightness reported by other commentators. The engine does appear to have seen some use in a model, and it's possible that a knowledgeable previous owner has taken the trouble to optimize that fit through careful lapping. If so, he did a great job!

The other possible factor relates to the reported fact that the engine was originally supplied by the factory with a "hot" Webra glow-plug. Both Peter Chinn and Ron Warring used this plug for their tests, although that in Chinn's test example burned out prior to the conclusion of the testing. Based on my experience, this engine really needs a cold plug when operated at high speed on a high-nitro fuel. This factor too could have a considerable effect upon measured performance. As I've noted in my separate article on glow-plug heat ratings, changes to the plug heat rating can affect engine speed by hundreds of rpm.

Be that as it may, my testing certainly demonstrated the potential that was there in the design. In fact, my results don't fall that far short of those obtained during my earlier test of the contemporary 1956 Super Tigre G.20 S, which delivered some 0.418 BHP @ 17,400 rpm usng the same 30% nitro fuel. They both certainly leave the diesel Mach I model in the shade!

The engine came through its quite arduous double test completely unscathed, with no evidence of any mechanical issues developing. The cold plug used for testing survived the entire series, which involved a lot of high-speed fully leaned-out running. All bearings remained very closely fitted, with an outstanding piston/cylinder fit being maintained throughout. Overall, a real quality product!

A Further Test

I have to admnit that the test results for engine number 2313 really knocked me sideways - how could this example be so much more powerful than the examples tested years ago by others, even exceeding the factory claim?  Moreover, how could an engine featuring the relatively unsophisticated early 1950's cylinder porting of the Webra (by evolving standards) give the competition such a close run for its money? Finally, if the engine was that good, why wasn't it used by more International contest modellers in 1956-57?

I had an uncomfortable feeling that my engine might in fact be a rather "special" example, perhaps tweaked either by the factory or by a competent owner. If this was the case, its performance could hardly be taken as representative of the design. There were plenty of suspects - apart from Gunther Bodemann himself, there were those German free flight competitors who used the engine to such good effect against all comers.

The only way to resolve this nagging issue would be to test another example of the same engine. It was at this point in my thinking that my valued Danish friend Luis Petersen came to the rescue, as he has so often done previously. He had a well-used and somewhat "scruffy"" example of the Webra 2.5R bearing the relatively late serial number 2757 which had been given to him some years ago by a friend who was a former free flight modeller. He was happy to pass this example along to me to serve as the subject of a second test. Needless to say, I'm extremely grateful to Luis for his timely assistance in this regard.

When it arrived, the engine proved to be in very good mechanical condition despite showing some external evidence of an "active" life. It was well freed up and only needed some cleaning to restore it to a pretty good appearance. As received, it had the wrong needle valve assembly, which I quickly replaced with an original Webra component of the correct type. The venturi was externally trimmed diametrically (don't ask me why!), but this did not affect the venturi bore or length in any way. 

Once set up in the test stand, engine number 2757 quickly proved itself to be just as user-friendly as its remarkable companion number 2313. It invariably started first or second flick with just a preliminary exhaust prime. Response to the needle was gratifyingly positive, while running was very smooth at all speeds tested apart from a very slight periodic misfire at the lower end of the tested speed range. This precisely mirrored the behaviour of number 2313 tested previously - these are definitely high speed engines. 

I conducted this test using the same batch of 30% nitro fuel that I had used in the previous exercise, even going so far as to use a "cold" glow-plug of the same make. Despite this, it quickly became apparent that this example was not going to come anywhere near matching the performance of its predecessor in the test stand. Its measured performance was actually quite close to that reported by Peter Chinn in his previously-cited August 1957 test of the engine. The following data were recorded:





APC 8x4

APC 7x6

APC 7x5

APC 7x4

APC 6½ x4 WB












The 61/2x4 WB (Wide Blade) airscrew is another of those modified props which I have created to fill glaring gaps in my test prop suite with respect to power absorption coefficients. This one is a trimmed APC 7x4. I didn't go beyond that prop in this instance because it was apparent that I had already passed the point of peak output.

As can be seen, the second example was in the order of 2,000 rpm plus or minus down from engine number 2313 on the same prop. The figures reflect a peak output of some 0.265 BHP @ 15,200 rpm. This of course is quite comparable with the output of 0.270 BHP @ 16,200 rpm reported by Peter Chinn.

One point of interest which I noticed was the fact that both engines more or less peaked on the same airscrew - the APC 7x5. The same props appeared to be applicable to both units - it was just that the second example turned those props at considerably lower speeds. The engine felt really good when turned over and showed no signs of mechanical distress at any time - just like its more powerful stablemate, in fact. Therefore the lower performance was not down to any mechanical deficiencies.

Speaking of the other example, I put engine number 2313 back into the stand during the same test session and re-tested the APC 7x5 and 7x4 props using the same fuel and plug. The engine turned those two props within 100 rpm of the remarkable figures measured earlier. To me, this completely validates my test results for the two engines - the differences are real. 

The performance differential between these two examples is far too large to be the result of the usual incidental differences between two basically identical production-line engines. It seems clear that engine number 2313 has been the subject of some very well-focused modifications to improve performance. I'd have to take the engine apart to determine what may have been done, a process to which I'm reluctant to subject the well-settled engine. No-one's using the Webra in competition any more, so there's little point in pursuing this line of inquiry as far as I'm concerned - no-one would apply any lessons learned. 

I would hazard a guess that the rather heavy piston may have been considerably lightened and the plain disc crankweb may have been counterbalanced. In addition, the induction timing may have been somewhat adjusted. It's also possible that a revised cylinder head having a more efficient combustion chamber may have been created. Finally, that superb piston fit likely has a lot to do with the matter. However, that's as far as I'm able to speculate.

In summary, this second test confirmed that Peter Chinn's test results were likely representative of the proformance of a standard over-the-counter example of the Webra 2.5R glow-plug model. My originally tested example number 2313 is clearly an exceptional unit! This finding makes much more sense of the reluctance of contest modellers outside of Germany to use this engine in competition - a typical example was a capable but unremarkable performer in competition terms.   


In terms of its ultimate potential, the Webra 2.5R was clearly a highly under-rated racing engine of its day. Its reputation has hitherto suffered greatly from what appears to have been a drastic and in my view inexplicable under-reporting of its performance potential. Indeed, those published figures (particularly those appearing in "Aeromodeller" magazine) may well have contributed to the seeming reluctance of many competition modellers to consider the Webra 2.5R as a potentially suitable engine to use. Only the German free flight enthusiasts appear to have realized the engine's true potential. Perhaps my very powerful example number 2313 originated as one of their competition engines.

The figures which I repeatedly extracted from that example are well above those reported in the contemporary modelling media, presumably because that particular unit is better fitted than those tested previously, either through extended use or expert preparation. It also appears that it may have received some internal attention which I have not evaluated directly. My use of a cold plug for testing may also have helped.

Regardless of the reason for that engine's performance, it's clear that a well-prepared example could give other highly-regarded mid-1950's racing 2.5's a very good run for their money, which doubtless explains the Webra's considerable but largely unheralded success as a free flight competition powerplant in its native Germany.

The 2.5R is also significant in that it was one of only two International-class racing engines originating in the mid 1950's to retain reverse flow scavenging. The other such unit was the Barbini B.40 TN, the story of which has been recounted elsewhere. The Barbini enjoyed considerable success in the control line speed field in 1956 and 1957 despite the fact that its published performance figures were actually no better than those for the Webra and fell far short of the figures which I reported above. It seems that the main reason for the Webra's failure to match the Barbini in this field was the fact that after early 1956 the more serious competitors simply stopped using it for that purpose. As a result, its successes were essentially confined to the free flight power arena, in which it enjoyed considerable if somewhat unheralded success.

I hope that this article has done something to restore the reputation of what must surely be one of the hottest radially-ported glow-plug engines ever produced. In my view, the Webra 2.5R stands as a testament to the abilities of one of the world's great model engine designers, Günther Bodemann. I hope that I've convinced a few of you!


Article © Adrian C. Duncan, Coquitlam, British Columbia, Canada

First published August 2018

Updated October 2018