The Vltavan 2.5
by Adrian Duncan
The story behind the Vltavan engines begins in 1953 when the Czechoslovakian state authorities decided that there was international prestige to be gained through Communist success in the field of model aeroplane competition at the World Championship level. To further this aim, they established a state-sponsored model development centre at Brno. This facility was known as the Modelárského Výzkumného a Vývojového Strediska (Modelling Research & Development Centre), better recognized by its initials of MVVS. The express intention was the development of model aero engines which would power Czech modellers to the achievement of World Championship honors. Further details regarding the establishment of this facility may be found in our focused article on the MVVS engines elsewhere on this site.
Success was not long in coming. After only two years of development work, the 2.5 cc MVVS engines had progressed to the point where speed flyer and chief MVVS designer Josef Sladký showed up at the control line World Championship meet in Paris and walked off with the top honors in the Speed category at a then very respectable speed of 112 mph. On top of this, the Czech team took second place in the team category, bested only by the favoured Italians.
The engines that the Czech team members were using at this contest were in essence conventional racing motors of their time—lightweight ringed aluminum piston, twin ball-bearing shaft, oversized transfer passage, etc, etc. Their one departure from the established "racing engine" formula was their use of a crankshaft rotary valve (FRV) instead of a rear disc valve (RV), probably influenced by the success enjoyed by the Italians up to that time with their FRV Super Tigre G.20 engines. However, Sladký's 1955 MVVS motor certainly offered something extra by way of performance and left the Super Tigre opposition in its wake!
During this period, the MVVS engines were made in very small numbers. They were not developed as a commercial venture but were instead created simply to provide selected Czech experts with engines that would give them every chance of success against international opposition. Cost was no object! The Cold War was at its height, and international prestige in all spheres of activity was everything!
The engines used by Sladký and his compatriots were individually built at the MVVS workshops to the very highest standards. It would clearly have been impossible to produce equivalent engines commercially at anything like a reasonable price, and no attempt was made at this time to do so, at least at the MVVS facility. However, a need was perceived for the production of more affordable engines of similar design for the use of a larger spectrum of aspiring flyers in order for them to gain the experience necessary to challenge for positions on the Czech team.
Accordingly, a decision was taken to establish a separate factory in Prague (very distant from Brno) for the purpose of manufacturing "consumer" versions of the 2.5cc and 5cc MVVS racing engines in sufficient quantities to make them more readily available to modellers in the Iron Curtain countries. This decision was acted upon in late 1956, and the Vltavan engines were born. Series production of both models seems to have commenced in early 1957.
The facility at which the Vltavan engines were manufactured was known as the Praha-Modrany factory. The engines were named for the river Vltava, which flows northward through Prague. In effect, the Vltavan engines were "the engines from the banks of the Vltava"! Sort of like calling a line of engines made in Newcastle the "Tyne" series...
Reports from Czechoslovakia at the time suggested that a production target of 5,000 engines annually was planned. Information from behind the Iron Curtain was scanty and unreliable during the era in question, but the evidence suggests that such figures were never achieved or even approached. What can be said with certainty is that the highest serial number for a Vltavan 2.5cc model which has come to my attention in many years of looking is 3142, seemingly confirming that at least that many were made in total. It seems doubtful that there were many more.
The Vltavan engines were never offered for sale outside their political zone of origin. However, a few examples did get past the Iron Curtain to the West one way or another, and more have since followed. Ron Moulton somehow obtained access to a Vltavan 2.5 quite early on for inclusion in a 1957 test (of which more below), while another one found its way into the capable and well-informed hands of Peter Chinn in late 1958. The latter engine was loaned to Chinn by well-known British free-flight competitor Ron Draper, who had previously contrived to obtain an example of the companion 5cc Vltavan model as well. Draper's example of the 2.5cc Vltavan was illustrated in the Latest Engine News feature in the November 1958 issue of Model Aircraft, being more fully described in the Latest Engine News column of the January 1959 issue.
After one gets beyond the very attractive red anodizing on the head and front bearing housing (either a concession to "consumer appeal" or a party political message), the first thing that strikes one is the spectacular swept-back exhaust stack. This is copied directly from Sladký's 1955 World Championship motor which was illustrated earlier. While the similarities between the two will be immediately obvious, there is also one equally obvious difference. This is the fact that while Sladký's 1955 world-beater used the FRV induction that had been successfully pioneered by the Super Tigre G.20 models, the MVVS designers had already decided by 1956 that the future lay in a return to the rear disc valve (RV) induction of the "traditional" racing engine.
In large part, this decision was influenced by the success achieved by MVVS co-designer Jaroslav Kocí, who rebuilt one of the 1955 FRV engines in RV form and won with it at the 2nd annual International People's Democratic States contest in Vrchlabi, Czechoslovakia in September 1955 at a speed of 111.84 mph—very close to Sladký's World Championship-winning speed of a few months earlier. Kocí subsequently achieved a significantly higher speed of 126.47 mph on thinner lines to establish a new 2.5cc world record.
Given this latter success, it was hardly surprising that the MVVS design team took the decision to adopt RV induction. Their competition glow-plug designs from 1956 onwards were to reflect this approach for some years to come. The introduction of the Vltavan models was sufficiently delayed that advantage could be taken of the very latest thinking emanating from MVVS. Accordingly, as can be seen, the Vltavan sported RV induction, just like the new competition designs then emerging from the MVVS center. The Vltavan was in fact pretty much a direct copy of the MVVS 2.5/1956-D design with which the official Czech team had challenged the rest of the world in 1956.
In most respects, the Vltavan 2.5cc model is a very typical racing motor of its day—twin ball-bearing shaft, ringed aluminum piston, rotary disc valve, offset plug, etc. The engine features the standard Continental bore and stroke of 15mm by 14mm for a displacement of 2.47cc. It weighs a mere 4.2 ounces.
The Vltavan's MVVS heritage is clearly reflected in the fact that it is very well engineered and displays some neat features. The design of the disc valve is particularly worthy of comment—it is extremely well mounted on a steel shaft of generous length which runs in a close-fitting bearing in the centre of the backplate and is secured at the outer end by a steel collar and grub screw. This arrangement provides outstanding support for the rotary disc, which can thus be maintained in very accurate alignment with the face of the backplate. The disc shaft is drilled to allow adequate lubrication to reach the bearing. However, the effect of all this clever engineering is somewhat marred by the fact that the disc in the Vltavan is of cast aluminum alloy rather than some more durable material such as Tufnol, moulded plastic or cast iron. Presumably, cost considerations were at work here.
Interestingly, a standard spray-bar type needle valve was used in conjunction with a nicely-shaped venturi intake. The spraybar represents a significant reduction in the cross-sectional area of the venturi throat, but it does provide a very good level of suction for a racing motor. This was important given the fact that the Czechs were using chicken-hopper tanks with suction feed at the time. In any case, the spraybar could of course be thinned if desired to increase the effective venturi throat area.
The transfer passage is cast into the side of the crankcase opposite the exhaust in the usual way. It has a conventional point of entry at the lower end and is extremely generous in size, but even this was not enough for the MVVS designers—the piston features two large skirt ports which register with corresponding ports in the lower cylinder liner a la Dooling and in fact take up much of the piston skirt on that side.
Two potentially negative consequences arise from the use of this set-up. Firstly, the crankcase volume is rather larger than might be considered desirable. Secondly, the piston ports significantly reduce the skirt area on the transfer side, which has to resist the major lateral loading during the power stroke. Mechanically speaking, this is not an ideal arrangement—there simply isn't much skirt left to take those loadings! However, compromises are inevitable in matters of this nature when all-out performance is the sole imperative.
On the positive side, the presence of these ports has three clear benefits. One, they improve gas access to the bypass passage; two, they promote the flow of cool incoming mixture through the piston interior, thus improving piston cooling; and three, they doubtless enhance the lubrication of the more heavily-loaded transfer-side area of the piston skirt. The very slight reduction in piston weight may also be viewed as a minor additional benefit—even a small reduction in reciprocating mass is a step in the right direction for a high-speed engine!
Quite apart from the above-mentioned skirt ports, the cast and machined piston is a work of art! It has two rings and an intricate domed baffle crown which conforms to a contoured combustion chamber shape formed in the cylinder head. The plug is well offset to the transfer side to accommodate these arrangements. The gudgeon pin bosses are beautifully fitted to the pin itself, and the alloy rod is also very well made and fitted. The pin is equipped with aluminum end-pads. Reciprocating weight is minimal—a very good feature. The extremely generous cylinder ports consist of multiple square openings separated by thin pillars in the normal fashion when using a ringed piston.
One point worth noting is that if you ever find an unmodified example of one of these engines, a standard glow-plug won't fit! The thread of the Czech glow-plugs for which the Vltavan heads were originally tapped was M6x0.75. This thread was used in the 1950's by MVVS in addition to Vltavan, presumably to accommodate the thread then in use on standard Czech glow-plugs. A quick calculation reveals this thread to be pretty close to the Western 1/4-32 standard for glow-plugs. Expressed in metric units, this latter thread would be 6.38mm x 0.78mm pitch, which is slightly larger but sufficiently close that a 1/4-32 tap will open out the metric thread very easily with no preparation, making the conversion to a standard plug very simple indeed. Any Vltavan (or vintage MVVS) engine that has been used by a modeller in the Western countries will generally be found to have been re-tapped in this manner. The illustrated example retains its original metric thread and Czech glow plug.
If you wish to run an engine having a metric plug thread, a far better option from the standpoint of conservation is to use a M6x0.75 die to re-cut the threads on the desired standard plug. I've tried this myself with good results. All that is necessary is to ensure that the re-cut threads are correctly aligned to the plug body—easy to do if you have a lathe and a tail-stock die holder.
Returning to our description of the Vltavan 2.5, the heavily counterbalanced crankshaft is very well supported in two ball bearings which appear to be of adequate if not spectacular quality. The prop mounting thread on the shaft of the pictured engine (serial number 1225) is a rather fine 5.0mm by 0.5mm pitch—not your everyday metric propshaft thread! An earlier example in my possession (number 663) also has this thread. By the time that they got to serial number 2436, they had changed to a far more standard 5.0mm by 0.8mm pitch thread.
Less worthy of praise is the fact that the mounting lugs are left "as cast" on both upper and lower surfaces. This would be OK if they were flat, but they're not—anyone planning to use one of these engines in anger would have to set the case up in the milling machine and true up the underside of the lugs for mounting in a speed pan. And frankly, there's already a minimal amount of metal in the lugs—they could have been made a lot sturdier in my book.
All that aside, how did the Vltavan perform? Over the years, the engine has maintained a reputation for having been a considerably less-than-stellar performer by comparison with its MVVS progenitors. To what extent is this view justified?
The only published "test" of the engine of which I'm currently aware appeared in the 1957 edition of "Aeromodeller Annual". The Vltavan was one of the engines included in a comparative evaluation of a number of then-contemporary 2.5cc competition engines, both diesel and glow-plug. This comparison was conducted by the late Ron Moulton. The means by which Ron got hold of a Vltavan 2.5 so soon after production had commenced are unknown, but the tested example may have been borrowed from British speed flier Pete Wright, who is known to have secured an example as of mid 1957.
Each engine was tried on the same series of standard test airscrews, thus allowing a direct performance comparison. No actual power figures were reported, but the results are nonetheless highly informative. As one might expect from a racing glow-plug motor designed primarily for all-out speed work, the Vltavan was a very poor performer indeed on the larger airscrews. It's apparent from this that low-end torque was relatively minimal, as we might expect. However, it's equally clear that torque was unusually well sustained at higher speeds. Consequently, on an 8x3-1/2 prop typical of those used in competition free-flight applications at the time, the Vltavan pulled itself up to the top of the heap, turning this prop at a highly respectable 15,400 rpm. In this it was matched only by the Oliver Tiger Mk. III, although Ron commented that the Vltavan ran more steadily than the Ollie at this speed and appeared more than willing to go faster if desired.
Published tests on the Mk. III Oliver by Ron Warring and Peter Chinn were quite consistent in finding an output for the Oliver of some 0.32 BHP at this speed. It seems safe to conclude that the Vltavan was developing a similar output in this range. However, the difference was that the Oliver was already past its peak at 15,400 rpm, whereas the Vltavan was clearly still working up to its peaking speed, with more yet to come. The Vltavan also turned a 6x9 speed prop at a quite creditable 14,000 static rpm, which would doubtless rev up to something approaching the engine's peak in the air. Ron Moulton commented that this latter figure matched those for "many reworked specials" on the same prop, also noting that the engine was an easy starter. Overall, he was clearly most impressed with the Vltavan.
I've never run the example pictured earlier which appears to be pretty much unused and still has its 1950's-vintage Czechoslovakian plug installed. However, I recently tested my well-used but still highly serviceable Vltavan 2.5 number 2436. The compression ratio seems relatively low, and I would guess that the engine is expressly intended for operation on a very high-nitro fuel, which was of course allowed in International competitions when these engines were current. Accordingly, I used a fuel containing some 30% nitromethane along with a healthy dose of castor oil.
Using this fuel, hand-starting was perfectly straightforward using conventional methods. The two piston rings do not seal perfectly, but there's more than ample compression for easy starting. Once going, the engine ran extremely steadily with unusually low levels of vibration. It also needled very well indeed, with adequate suction. I had no difficulty holding 13,600 rpm on an APC 8x4 and 17,100 rpm on an APC 7x4. Based upon established power absorption coefficients for these airscrews, the corresponding power outputs are 0.27 BHP and 0.34 BHP respectively. By no means a discreditable performance by 1957 standards, and highly consistent with the far earlier findings of Ron Moulton. Although the three results reported above are from two different engines tested at different times, one can't help noticing that they lie on a very smooth curve which implies a peak of around 0.35 BHP @ 18,000 rpm or thereabouts.
Based on this test in conjunction with Ron Moulton's reported results, I have no hesitation in saying that while the Vltavan 2.5 did not perhaps come up to the performance levels of the individually-prepared MVVS factory originals, it did provide ample performance for aspiring Czech modellers to gain valuable competition experience. It would have made an excellent "clubman's" speed motor—certainly far better for that purpose than anything then commercially available in Britain. Indeed, at the time of its release the Vltavan was almost certainly one of the most powerful commercially-available 2.5cc racing engines worldwide—as of 1957, the only competing commercial products that might have beaten it in standard form were the evolving Super Tigre G.20 models. It would probably also have acquitted itself well in free flight competition if fitted with an appropriate airscrew.
The only reference that I can find to a Vltavan 2.5 appearing in International competition was an official flight by Britain's Pete Wright at the 1957 control line speed World Championship meeting held at Mlada Boleslav near Prague, Czechoslovakia. Wright switched to a Vltavan (probably obtained at the meeting given the location plus the fact that production had only recently commenced) after his Barbini B.40 TN expired, but his speed of just over 100 mph was no threat to Josef Sladky's winning 134.2 mph figure using the latest MVVS prototype. As noted earlier, it may well have been Wright's engine that was tested by Ron Moulton in 1957.
By 1958 the MVVS Centre had further refined their 2.5cc design to produce the outstanding 2.5R/1958 model. This engine performed at a far higher level than either its MVVS progenitors or the Vltavan clones. It didn't take long for the Czechs to recognize that the strategy of putting large numbers of increasingly uncompetitive and out-dated MVVS clones into the hands of ordinary modellers was not perhaps the best approach to the development of a broader talent pool. Indeed, it must have become apparent that if a way could be found to increase the availability of genuine MVVS engines, the modellers who were facing an uphill struggle trying to overcome the growing shortcomings of the Vltavan models would have far better equipment and would thus achieve higher standards of performance with a greatly reduced frustration factor.
Accordingly, it was in 1958 that MVVS took the first steps towards becoming a major supplier of high-quality competition model aero engines to modellers in general rather than to a few selected Czech experts. They took on a number of extra workers at the centre and added more production equipment. Once this was done, several of their engines were put into small-scale production for wider distribution than had formerly been possible.
The first engine to be produced in this way was the MVVS 2.5-D, a plain-bearing development of the 2.5/1958 ball-bearing diesel. In 1959 it was joined by a second series production, the aforementioned 2.5R/1958 racing glow-plug motor. The general release of this latter model rendered the Vltavan 2.5 obsolete in terms of model aircraft applications. The factory carried on for a year or so making car versions of both the 2.5cc and companion 5cc models, but production seems to have ceased altogether at some point in 1960.
Still, as my own experiences and those of Ron Moulton show, the Vltavan 2.5 was a useful enough engine, with which an aspiring Czech speed flyer could undoubtedly have gained valuable experience. In that context, the engine probably served its intended purpose quite well enough. It was certainly a far better performer than its "legend" suggests!