The K Vulture
by Adrian Duncan
Click on images to view larger picture.
In other articles on this website we have summarized the history of the British range of model engines from Gravesend, Kent which were initially produced by Kemp Engines and later by the "K" Model Engineering Co It's now time to take a closer look at one of the better-known products of the latter company—the K Vulture diesel of 4.98 cc displacement.
This design has carried a less-than-stellar reputation down through the years, being noted for its handling challenges as well as a few structural issues which have reared their ugly heads on occasion. This however has not prevented it from becoming quite a sought-after collector's item.
Here we'll look into the basis for the Vulture's chequered reputation through first-hand evaluations of a number of actual examples to which we have access as well as our own experience in using these engines. We'll also try to sort out the production history of the engine in terms of the various forms in which it appeared. Finally we'll offer such advice as we can to present-day owners who may wish to give one of these cantankerous beasts a run. It can be done; trust me!
Before getting started, I'd like to thank David Owen for supplying both images and serial numbers to add to the data upon which this study was based. I'd also like to thank our Editor Ron Chernich for his usual assistance in seeking out those elusive advertisements of over 60 years ago. Much appreciated, mates!
Following the initial publication of this article, I also heard from several more individuals who had further material to contribute. This is exactly the kind of response that we most hope for! Brian Cox was able to supply some additional serial numbers which extended the known production figures for the Mk. I Vulture. Perhaps even more significantly, Paul Rossiter supplied an actual example of the Mk III Vulture which added greatly to my understanding of the changes embodied in that model. Finally, Alan Strutt kindly supplied an interesting variant of the Mk. I Vulture which contributed greatly to our understanding of the issues of production figures, factory upgrades and serial numbering. The additional information has been incorporated into the text below. Our very sincere thanks to Brian, Alan, and Paul!
Our previous articles on the Kemp and K engines from Gravesend, Kent, have traced the story of the range from its establishment in 1946 down to the point in mid 1948 at which the founder of the original Kemp Engines company, Harold Kemp, sold or perhaps partially sold that company and its designs to Len "Stoo" Steward, who carried the business forward under the name of the "K" Model Engineering Co Ltd Steward was an active and highly practical aeromodeller who was a leading member of the prominent West Essex club and was one of Southern England's foremost control-line stunt fliers during the early post-war years during which that branch of the hobby was being developed.
As one might expect from an individual with such a background, Steward's influence over the affairs of the new company was very much directed towards the development and manufacture of engines suitable for use in control-line stunt flying. This made complete business sense given the fact that control-line stunt was probably the fastest-growing branch of aeromodelling in Britain at the time. The sideport engines which had been the primary focus of the former Kemp Engines company in no way met the requirements of this burgeoning field of activity, and their production was quickly terminated once Steward took over.
Steward clearly and correctly saw that the future lay with engine designs which were both lighter and more efficient than the sideport models which up to 1948 had constituted the mainstay of the British model engine manufacturing industry. The introduction of control-line created a need for higher airspeeds, and this in turn dictated a requirement for higher engine revs per minute during flight. In Steward's view, crankshaft front rotary valve (FRV) induction best met the dual requirements of higher speeds and lower weights and was thus the layout of the future for general-purpose engines suitable for control line use.
Accordingly, Steward immediately set about the development of an updated range of engines featuring this form of induction. The sideport 4.4 cc K4 model was dropped immediately upon Steward taking over, and plans went ahead at full speed for the replacement of both that design and the sideport 1 cc Eagle model. The little sideport 0.25 cc Kemp Hawk Mk I, which was always aimed primarily at the free-flight market, was still selling well at the time and its production was continued for some months until it too was replaced by a 0.20 cc FVR Mk II version in May of 1949. More of this may be found in our companion article on the Hawk.
The first hint of what was to come was dropped in an initial advertisement by the new company which appeared in the 1948 issue of the compendium Model Aviation. The exact date of this publication is uncertain, but the advertisement must have been placed shortly after the change in the ownership structure of the company. It focused upon the little sideport K (formerly Kemp) Hawk which remained in production, presumably to maintain a cash flow while new models were developed. However, there was also a rather cloak-and-dagger reference to a pair of new models which would shortly be appearing—the 1 cc Eagle in its FRV form and the 5 cc Vulture. No details were provided, but the advertisement claimed that these two forthcoming designs would be "the talk of the model world!" Quite an advance billing to live up to!
Unlike some other announcements of a similar nature for models which never materialized, this one proved to be well founded. Both the Vulture and the FRV Eagle duly made their appearances in the marketplace prior to the end of 1948. The Eagle was a replacement for the discontinued sideport 1cc diesel of the same name, while the Vulture was clearly intended as a updated replacement for the old sideport K4 diesel of 4.4 cc displacement. From this point onwards, we will focus entirely upon the Vulture.
The Mk I version of the K Vulture was a radially-ported 5 cc FRV diesel of comparatively lightweight construction by the standards of the day. The engine's design embodied a number of unique features and was in many ways quite revolutionary for its time in a British context. For one thing, at the time of its release in October 1948 it was the first British diesel of its displacement to feature what was to become the "standard" 1950's general-purpose combination of radial porting, FRV induction and variable compression.
Nominal bore and stroke of the Vulture were 0.750 in. (19.05 mm) and 0.6875 in. (17.46 mm) respectively for a displacement of 4.98 cc (0.304 cu. in.). This put it just over the 0.299 cu. in. limit for US Class B competition, but the engine complied with the British Class B rules and there's no evidence that the "K" Model Engineering Co ever seriously contemplated exports to the USA. These figures also put the Vulture well and truly into the "short-stroke" category at a time when the British market was still dominated by long-stroke designs.
The Vulture was somewhat lighter (at just over 7.5 ounces) and more compact than the average 5 cc diesel of its day. Although the short stroke had much to do with these attributes, the simplicity of the engine's construction also made a significant contribution. The design utilized no screws at any point, being based throughout on threaded components. The entire engine consisted of only 18 individual components, including the off-the-shelf prop nut, prop washer and spraybar nut!
This simplicity of construction also allowed the makers to offer the engine for sale at a very competitive price of £3 19s 6d, which presented a notable contrast with the whopping £7 19s 6d price tag of the ETA 5 diesel and even the £4 19s 6d cost of the DC Wildcat Mk II, to take just two contemporary examples.
The steel cylinder was of basically tubular form with a step down in outside diameter just above the exhaust ports. The upper portion above the step was externally threaded to accommodate the screw-on machined alloy cooling jacket. The lower end of the cylinder below the ports featured a base flange which served to locate the cylinder vertically on top of the crankcase.
Unusually, there was no "spigot" length below this flange to locate the cylinder laterally on the crankcase. Instead, the cylinder was both secured and laterally located by an internally-threaded ring which bore on the top of the cylinder flange and screwed onto an externally threaded portion of the upper crankcase. This threaded ring quickly became known colloquially as the "dog collar", a highly descriptive term which has stuck to this day! The dog collar on the Mk I Vulture was made of brass and took the form of a simple knurled ring with an internal shoulder above the threaded portion. A gasket between the cylinder base flange and the upper crankcase ensured a good seal.
The upper cylinder was equipped with a conventional contra piston made of hardened steel. This gives rise to one of the Vulture's more annoying handling characteristics—the steel contra piston tends to stick in the bore when the engine warms up. This is not good, because with this design it's generally necessary to reduce compression considerably as the engine warms up. It appears that some owners have taken steps to relieve the fit by lapping or honing, and examples with very loose contra pistons are not infrequently encountered as a result, even in engines which have clearly done relatively little running. A cast iron contra would have been much better, and I have in fact fitted such an item to my usual "running" Vulture, which had an unusably loose contra piston as received and which now handles far better as a result of the replacement.
The fully machined cylinder jacket featured both barrel fins and head fins combined in a single light alloy component. Unusually, the thread for the compression screw was not formed in the centre of the head—instead, the cylinder was topped by a hard brass disc with a central spigot which protruded through an oversized hole in the centre of the head. This brass disc was sandwiched between the head and the top of the cylinder when the jacket was screwed down. The 1/4-26 BSF thread for the comp screw was formed in the protruding spigot rather than in the head itself—a good feature since a worn thread could be rectified simply by replacing the brass insert rather than the entire cylinder jacket.
The design also had merit in that it made the provision of an optional glow head a very simple matter indeed. All that was necessary was to omit the contra piston and replace the brass comp screw insert with an alloy head button threaded for a glow plug. The engine could then be assembled in the normal way. This option was offered by the makers from the outset at a additional cost of 10 shillings. It appears however that few conversions were sold, since glow versions of the Vulture are relatively infrequently encountered today.
The arrangements described above had the further advantage of allowing the cylinder to be tightened in a radial configuration which ensured that the head fins were in their correct fore-and-aft alignment when the cooling jacket was tightened down. The cylinder alignment was easily re-adjusted using the dog collar if the relationship between the jacket and the cylinder changed over time due to repeated tightening. Do not decide against buying a Vulture simply because the head fins are misaligned!
Turning now to the cylinder porting, this again possessed some distinctive features. The exhaust ports were essentially conventional for a radially-ported engine, consisting of a ring of four sawn ports separated by relatively thin pillars. However, the transfer arrangements were rather more unusual. The actual transfer port itself consisted of a continuous annular channel machined around the entire circumference of the inner cylinder wall just below the exhaust port ring—an example of true 360 degree porting! This annular channel was fed from the lower crankcase by six shallow bypass flutes machined internally at a uniform spacing into the cylinder wall below the channel.
This arrangement was perhaps less than ideal in that it provided relatively poor directional control of the incoming charge and must have resulted in significant losses of unburned mixture out of the exhaust ports. You can actually both see and smell this when the engine is running.
The design must also have resulted in reduced transfer gas velocities due to the extreme circumferential length of the transfer port. In addition, it militated against the retention of a prime in the cylinder since there was no opportunity for a prime to "pool" on the piston crown. These issues undoubtedly played a part in creating the notorious starting challenges which plagued these engines, of which more in their place below.
The piston was again of hardened steel and was used in conjunction with a hardened steel connecting rod. Unusually once again, the piston and rod were connected by a ball-and-socket joint rather than the more usual gudgeon pin arrangement. This system had in fact been introduced to British model diesel manufacture in the former Kemp Eagle 1 cc sideport diesel and was thus something of a Kemp/K trade-mark.
The use of a ball-and-socket small end was probably a good move in view of the bypass arrangements used—unless it was well secured, a conventional gudgeon pin might well have tended to foul the bore at the top of the bypass flutes. The downside was that since both components were hardened after installation, replacement of the rod was impossible—if that requirement arose, one had to replace both piston and rod, and that would mean a rebore or a new cylinder into the bargain. The system also prevented the re-swaging of the ball-and-socket joint if it became loose—any attempt to do so would likely result in a fracture of the hardened socket. Be warned!
The hardened steel crankshaft was of conventional form but was somewhat more lightly constructed than would have been ideal for a big-bore diesel of this displacement. The crankweb was relatively skimpy and featured only a small counterbalance. The shaft ran in a well-fitted steel bushing which was pressed into an alloy housing cast integrally with the main crankcase. A round induction port was used in conjunction with a venturi base of similar shape formed in the bushing.
Crankshaft journal diameter was a nominal 0.375 inch (3/8") with a 0.250 inch diameter central hole for the passage of incoming mixture. This left a shaft wall thickness of only 1/16"—a very marginal amount of metal to absorb the relatively high operating loads to be anticipated with this large short-stroke diesel design. However, the crankpin had a very sensible diameter of 0.250 inch
The steel prop driver was secured to the shaft by being pressed onto a short splined section of shaft forward of the locating shoulder. A standard prop nut and prop washer were used to secure the prop against the driver, although a spinner nut could be purchased as an optional extra.
Looking now at the main crankcase casting, we find ourselves confronted with a few more weak points in this engine's design. The 26 TPI threads on the upper part of the case onto which the dog collar screwed were apparently formed using a large Coventry die-head. This forced the designer to keep the rest of the casting below the level of these threads so that nothing would foul the die-head. Accordingly, the downdraft intake venturi was not cast integrally with the main case but was instead a separate component of turned alloy which plugged into a stub boss on top of the integrally-cast main bearing housing and was retained in position by the spraybar.
The spraybar was a very solid piece of work indeed, having an outside diameter of no less than 0.186 inch (nominally 3/16 inch or 4.76 mm) with unusually large jet holes. This unit combined with a venturi throat diameter of 0.325 inch (8.25 mm) to give quite good suction. However, a lot of owners seem to have fallen into the trap of trying to extract more power by "waisting" the spraybar and boring out the venturi. This is counter-productive in the extreme in that it makes the engine almost impossible to start! More of this in its place...
It must be said that the venturi mounting arrangements look extremely prone to damage in a crash. For some inexplicable reason there was no fillet between the rear of the venturi boss and the front of the main case. Even a moderate frontal impact on the top of the rather long venturi (as in a typical crash) would at first sight be almost guaranteed to break the mounting stub.
A fillet between the rear of the stub and the front of the case would have greatly increased the strength of this arrangement and would in addition have stiffened the main bearing quite appreciably. It's a complete mystery why such a fillet wasn't provided. Having said this, it's only fair to point out that examples with the boss broken in the anticipated manner are very rarely encountered. There may have been reasons for this, of which more later.
The main bearing was braced at the sides by forward extensions of the mounting lugs and was also provided with a long bracing web on its underside In addition, the front face of the crankcase below the centre line was plentifully equipped with cast-on bracing/cooling fillets. It's odd that a similar level of concern for strength wasn't displayed above the centre-line.
The mounting lugs were another weak point. They were far thinner than ideal for an engine of this displacement and were in fact less substantial than those on many engines of half the displacement. There was a token effort to strengthen the lugs through the provision of centrally-located stiffening webs, one to each side on the upper sides of the two lugs. However, the lugs remained very marginal for the job, and cases with damaged lugs are not infrequently encountered. The same weakness was in fact carried over to the company's 2 cc FRV models, and those models did not even have the stiffening webs.
Finally, the backplate was a simple screw-in item turned from solid aluminium alloy bar stock. It sealed to the rear of the case with a gasket. The engines all carried serial numbers which were stamped upon the right front surface of the main bearing housing just behind the prop driver. The numbering appears to have followed the usual procedure for the Kemp and K companies by starting at engine number 1. The serial number of the illustrated example is 282, and the highest presently-known serial number for an authenticated Mk I Vulture is 959. No four-digit serial number are known for this model, and it thus appears that no more than 1000 examples of the Mk I Vulture were produced.
It is not uncommon to encounter Mk I Vulture cases which have been retro-fitted with Mk II components—engine number 867 is an example of this, although there is evidence to suggest that in this case the retro-fitting may have been done at the factory. The key to recognizing an original Mk I case is the magnitude and location of the number. If it's a three-digit number and appears at the right front of the main bearing, the case was originally fitted to a Mk I.
And it didn't stop there - once the Mk II Vulture with its far stronger working components was introduced in around February of 1949 (see below), it appears to have become normal practice for the company to fit Mk II internal components as replacements for any failed Mk I items. Hence examples of the Mk I Vulture with Mk II internal components are quite frequently encountered.
Some additional confusion has arisen from the fact that it is also possible to encounter correctly-numbered examples of the Mk. I Vulture which have been retro-fitted with the far stronger Mk. III crankcase, of which more below. Engine number 311 which was kindly made available to me by Alan Strutt is an example of this—Alan obtained it from the original owner, who had evidently returned it to the factory for a case replacement following a breakage of the kind mentioned above. The factory fitted a Mk III case which was however stamped with the original three-digit serial number in the correct Mk I location. Company practice seems to have been to transfer the original serial number to any replacement Mk III case which may have been fitted to a Mk I or Mk II engine—a unique approach in my experience.
Apart from the serial number, the only other identification to appear on the engine was the company's well-known "K in a circle" trade mark. This appeared on both sides of the crankcase above the mounting lugs at the rear.
The result was a handsome-looking unit which was both compact and relatively light for a diesel engine of its displacement. It was well out of the contemporary British design rut for engines of its displacement and also sold for a most attractive price, as noted previously. Not only that, but Len Steward got it off to an excellent start from a publicity standpoint by personally using it to win the first contest in which it was entered. This was the stunt category of the London Area Control-Line Competition held at Fairlop Aerodrome on October 10th, 1948.
Small wonder that the Vulture quickly attracted the attention of the resident engine tester for Aeromodeller magazine—the redoubtable Lawrence L. Sparey. Let's see what he had to say about the new model.
The Mk I Vulture On Test
Lawrence Sparey's test of the Vulture appeared in the January 1949 issue of Aeromodeller magazine as number 9 of the series of 47 tests that Sparey was destined to carry out for the magazine over a four-year period beginning in May of 1948. Allowing for editorial lead time, the publication date of this test reflects unusually quick work by Sparey, since the Vulture had only been on the market for a few months at the time of the actual test. Sparey must have been remarkably quick off the mark in obtaining his test example, or perhaps the publicity-minded Len Steward ensured that he got one of the first engines off the line.
The test report is interesting insofar as it reflects the difficulty that Sparey was having adjusting to the emerging design trends which were then gaining momentum among British diesel manufacturers. He spent the first part of his report expressing surprise that the Vulture had confounded one of his earlier pronouncements to the effect that as displacement increased, the speed at which maximum BHP was realized went down! The Vulture rocked him back on his heels by delivering its peak power at 8,900 rpm, a quite useful speed by the standards of the time, especially for a 5 cc diesel.
Sparey comforted himself by noting that there was a price to be paid for this level of performance in terms of the engine's handling qualities and its flexibility. He made several references to difficulties in establishing an optimum needle setting. However, he stated that when loaded for its optimum speed range, the Vulture ran "quite steadily and evenly". He also reported that with such a load hand starting was "fairly simple".
However, Sparey's further remarks are quite illuminating as regards his basic understanding of the business of testing model engines. He stated that the engine became increasingly difficult both to start and to set when the load was reduced to allow operation beyond the peak, to the point where even running could not be obtained beyond 14,000 rpm. Really! Why, one may ask, would a tester expect anything different? And why test an engine at speeds so far beyond its peak in the first place? Engines peak at a given speed because that's where their various processes combine most efficiently, and it's completely unreasonable to expect anything other than reduced efficiency beyond that point and unfair to criticise an engine on this basis. Sparey appeared to be unable to grasp this fact.
Actual torque readings were taken over a speed range extending from 5,000 to over 13,000 rpm. Again, one wonders why the latter figure, given the fact that the engine peaked at under 9,000 rpm. All that operation at speeds significantly beyond the peak would accomplish would be to impose unnecessary stresses upon the engine!
Despite what would appear to have been an excessively severe test in Sparey's hands, the test Vulture evidently survived unscathed—at least, Sparey did not report any failures, and other tests show that he generally did so when they occurred. The final power figure arrived at was 0.246 BHP at 8,900 rpm—not very impressive by later standards, but considerably better than the contemporary sideport opposition as of late 1948. As an example, the far heavier and more costly ETA 5 tested by Sparey only a few months previously (presumably using the same methods) had developed only 0.1805 BHP at 6,250 rpm. So the Vulture came out of this test looking pretty good.
The Mk II Vulture
Based on known serial numbers and estimated production rates for the Mk I Vulture, it appears that it didn't take long for the "K" Model Engineering Co to become aware of some of the weak points noted above in the detailed description of that model. It's particularly evident that problems must have been encountered with the shaft and con-rod pretty early on.
The company's response to these issues appeared in early 1949 in the form of a revised version of the K Vulture which was advertised as the "Vulture Mk II Special". The fact that this was only a few months after the engine's initial appearance is a pretty clear indication that the Mk II represented a quick reaction to some significant problems with the Mk I version.
Every confirmed Mk II serial number reported to date is within the 2xxx range. Based on this fact, there appears to be no doubt whatsoever that the serial numbering sequence was restarted at 2001 for the new model. Presumably the 2 was inspired by the Mk II designation of the engine. The number was relocated from the right front of the main bearing to the left side of the case beneath the mounting lug on that side. This location plus the four-digit number makes the differentiation between Mk I and Mk II cases a trivial matter.
One interesting exception to this is engine number 867, which has excellent provenance since it was owned until quite recently by its original purchaser. He has confirmed that the engine remains in the same configuration in which it was sold. The engine is clearly based on a Mk. I case but has the Mk. II dog collar fitted. The internal components are unknown at this time, but it would appear that when the Mk II was introduced the company may have had a number of unsold Mk I's still on hand and in effect converted them to Mk II's to make them more saleable. Only a detailed internal examination of the engine in question would further clarify this point, but at present this appears to be the most probable scenario. Thanks to Kevin Richards for making us aware of this possibility!
As with the Mk I, enthusiastic collectors have taken a hand in the game here, and examples of Mk II Vulture cases fitted with Mk I dog collars (often repros) are by no means uncommon. Such engines are unquestionably collector-assembled hybrids—both of Clanford’s illustrated Mk I Vultures are examples of this kind of conversion.
Bore and stroke of the new Mk II variant were unchanged, as indeed they were throughout the production life of the Vulture. The only external change was the replacement of the former brass dog collar with a light alloy item having an upward tubular extension to a point just below the exhaust ports. This extension carried a set of shallow cooling fins. Given their location, these could have had little practical effect and were doubtless more for show than anything else.
On a more functional note, the lower end working components were significantly strengthened in this variant. We noted previously that the shaft and rod in the Mk I Vulture appear rather skimpy for their allotted tasks, and it seems that experience in the field had fully justified this impression. In the revised model, the company took steps to remedy this by designing a sturdier shaft featuring a full-disc crankweb which incorporated a substantial crescent-shaped counterbalance. This was used in conjunction with a far stronger rod having a considerably greater column diameter.
The same relatively skinny crankshaft journal diameter of 0.375 inch was retained, but the diameter of the internal gas passage was reduced from 0.250 inch to 0.228 inch, creating a modest increase in the shaft wall thickness of some 0.011 inch (from 0.0625 inch to 0.0735 inch). This represented an approximately 15% increase in the sectional area of the crankshaft wall which had to resist the shear and torsional stresses imposed by the operating loads transmitted through the con-rod to the crankweb. Still a bit marginal, but an improvement nonetheless.
The extra metal represented by the full-disc crankweb, the thicker crankshaft walls and the more substantial rod caused the weight to rise a little to around 7.8 ounces. However, the additional space occupied by the more massive components also resulted in a reduction in the already-small crankcase volume. The additional base compression resulting from this may well have contributed to a useful increase in transfer gas velocities, something which the Vulture could certainly use.
The serial number on the Mk II variant was relocated from the right front of the main bearing to the side of the case beneath the mounting lug, usually (but not always) on the left-hand side. In all other respects, the engine was more or less unchanged from the Mk I.
The "Special" hyping of what was in reality the same engine with stronger working components was likely motivated by a desire to obscure the fact that the most significant changes had been forced on the makers by design flaws in the original model! The engine was promoted as a "Special" rather than as a response to original design shortcomings—an understandable marketing ploy.
Nonetheless, the company seems to have accepted the fact that the word was "out" in informed modelling circles regarding the strength issues associated with the Mk I Vulture's shaft and rod. Their advertising of the engine in the early part of 1949 expressly stated that the Mk II Special version of the Vulture featured "improved crankshaft and con rod design". This was as close as they felt they could come to admitting that the original design had been seriously flawed in those areas.
Known serial numbers for this variant start at 2076 and go up to a present high figure of 2863. It thus appears that around 900 examples of the Mk II Vulture were produced in total.
As with the Mk I Vulture, examples are encountered which have been retro-fitted with the far stronger Mk III crankcase, presumably following a breakage or perhaps as a measure by the factory to improve the sale-ability of the engines—my own engine number 2863 is an example of this, as is engine number 2804. As noted earlier, the company appears to have adopted the unique practice of transferring the original serial number to replacement cases.
The Mk III Vulture
It will be appreciated that the design revisions noted with respect to the Mk II Vulture did nothing to address the evident weaknesses in the main crankcase casting. Chief among these were the very thin mounting lugs, which even a cursory examination shows to be quite inadequate for an engine of this weight, displacement and torque.
The "K" Model Engineering Company apparently came to agree with this assessment and showed this in the clearest possible way by quickly releasing yet another variant of the Vulture which they marketed as the "Vulture Mk III Competition Special". Note that once again they dressed up a response to a design issue in the guise of a "special" product! The initial advertisement for this new variant appeared in the May 1949 issue of Aeromodeller
The most immediately obvious feature of this variant which set it apart was the fact that the cooling fins which had been milled into the cylinder head on both previous models were no longer incorporated. Doubtless the company would have had some performance-related explanation for this, but the plain fact is that it was most likely a cost-cutting measure. The only practical effect which it had was to reduce the height of the engine a little, since the smooth head was machined off level with the top of the brass head insert which continued to be used. The resulting finless look gave rise to the name by which this engine is informally identified today—the "Bald Headed Vulture"!
Another externally-visible change on the Vulture Mk III was a slight reduction in the length of the venturi insert together with a significant steepening of the rake angle at the intake. This may possibly have been intended to reduce the stress imposed on the venturi mounting boss during a crash impact, as discussed earlier. It had little functional effect.
Of far greater importance in a practical sense, this variant of the Vulture did much to address the chronic weakness of the mounting lugs. The lugs on the earlier models had been a mere 0.108 inch thick, but those on the Mk III were a far more rational 0.165 inch thick—more than a 50% increase. Oddly enough, there were no moves to address the apparent ongoing weakness of the upper main bearing and the venturi spigot, the configuration of which remained unaltered. It is possible that experience had shown that the seeming weakness of this set-up was not in fact the issue that it would appear at first sight to be; or perhaps they were relying upon the shortened venturi insert.
A further change was the provision of three lugs at the rear of the crankcase for the purpose of allowing for radial mounting of the engine. At the time in question (mid 1949), radial mounting was enjoying something of a vogue among British modellers, with several manufacturers producing models with this type of mounting. The "K" Model Engineering Co went one step further by including a neat Arden-style conical mounting of cast alloy to go with this variant of the Vulture.
Oddly, the Mk III Vulture illustrated in "K" Model Engineering Co advertisements from May 1949 right though to the final ad in late 1950 did not feature the thicker lugs and integrally-cast radial mount lugs described above. Instead, it had a different type of machined screw-in backplate which incorporated a radial mounting flange. It also retained the thin mounting lugs of the earlier models—the location of the "K in a circle" trade mark on the case confirms this.
It seems likely that the illustration was based upon a prototype unit made up from a Mk II case which was fitted with a Mk III cooling jacket and venturi as well as a screw-in radial mount back plate. This was most likely assembled purely to serve as a model for the illustration, or perhaps this was the intended configuration at the outset. If so, it clearly didn't last! This being the case, it's a little odd that they didn't change the illustration at any time—presumably this was a budget decision. The hybrid Mk III was still featured in the final "K" advertisement of them all in November 1950.
We must concede the very slight possibility that a few Mk III engines were in fact sold in the illustrated configuration, but if so the number must have been very small indeed. Even if they had been, it's likely that the screw-in radial mount would have unscrewed under the considerable rotational stresses imposed by flicking the engine over to start!
Apart from the changes already noted above, a number of other alterations were featured in the Mk. III Vulture. Thanks to the kindness of Paul Rossiter in supplying a somewhat ratty but eminently restorable example of this latter model for direct examination, we're now in a position to document these changes.
One externally-visible change was the elimination of the steel prop driver in favour of an item machined from aluminium alloy. This was in all probability intended as a weight-saving measure. The same change was also applied to the companion 2 cc Kestrel diesel, presumably at the same time and for the same reasons. The illustrated engine with this type of driver bears the serial number 3057. We are also aware of engine number 3047 showing the same modification.
The most significant changes, apart from those to the crankcase, were internal and were related to the bypass arrangements. Previous models had all had six internal bypass flutes, but the revised cylinder was provided with no fewer than 12 such passages. Moreover, these passages were considerably deeper than their predecessors. The result was a substantial increase in the cross-sectional area of the bypass pathway.
To allow for the greater depth of the revised bypass flutes, the top flange of the crankcase casting was given an internal chamfer to eliminate the "step" in the bypass pathway which would otherwise have resulted.
In addition, the "K in a circle" trade mark which had appeared on each side of the case at the rear above the mounting lugs was relocated to the front, still on the sides above the mounting lugs. But that was all-apart from the above changes, the engine was essentially identical to its predecessor. The rod and shaft remained unchanged, indicating that the strengthened components had been found to be satisfactory.
The serial number on the Mk III was relocated yet again, this time to the right side of the case beneath the mounting lug on that side. Logically enough, the serial numbering for the Mk. III Vulture evidently started at 3001, the 3 presumably denoting the Mk. III designation just as the 2 had done with the Mk II version. We saw earlier that engines with Mk. III cases are encountered with lower serial numbers (in some cases far lower), but these appear to have been retro-fittings following the failure of the far weaker Mk I and Mk II cases. The company's evident habit of transferring original serial numbers to replacement cases has done much to confuse the issue here.
As stated earlier, known serial numbers for this variant start at 3009 with the highest Mk III serial number of which we are presently aware being 3383. It appears from this that perhaps 600 or so examples of the Mk III Vulture may have been produced before the design was finally abandoned. This made the Mk III the least prolific variant of the Vulture in production terms. A pity, because it was also the best.
Serial Numbers and Production History
Because the company survived for such a relatively short time and never really achieved mainstream status, there's actually not that much contemporary written history on the Kemp and K engines. Accordingly, our best guide to what happened and when is the series of advertisements which appeared in the British modelling media during the period 1948 through 1950. Thanks to some sterling work by our friendly Editor Ron Chernich, combined with an assessment of known serial numbers with the invaluable assistance of David Owen, we are able to follow the fortunes of the Vulture and the company which manufactured it with some confidence.
The best information at our present disposal seems to indicate that the Mk I Vulture first appeared in October of 1948—the engine certainly made its contest debut on the 10th of that month. We saw earlier that authentic surviving serial numbers for this variant point to around 1000 examples being produced before the Mk II version appeared. Examples of Vultures in Mk I configuration with serial numbers in the 2xxx range are not uncommon, but these are collector-assembled hybrids using Mk II cases—the one illustrated in Clanford's "A-Z Pictorial" bearing the serial number 2330 in the wrong location for a Mk I is an example.
Present evidence (to be summarized below) suggests that the Vulture was produced at a relatively high rate when it was first introduced. As we shall see, it appears that initial production rates may have been as high as 250 or even 300 units per month at the outset. Based on these figures, the Mk I Vulture was likely in production for no more than 3 or 4 months at most before being supplanted by the Mk II variant, probably in February of 1949.
We saw earlier that the Mk III Vulture made its initial appearance in May of 1949. This leads us directly into one of the real oddities connected with the Vulture story. This is the fact that in May of 1949 the company began to advertise both the Mk II and Mk III Vultures simultaneously! We saw earlier that they were in effect the same engine, the main practical differences being the strengthened beam mounts and the added radial mount option on the Mk III. However, the notion that they were produced simultaneously is persuasively refuted by the fact that their serial numbering is consecutive. This bears closer examination.
Here I have to emphasize that what follows is in many ways conjectural and cannot be read as established fact. I merely claim that the following scenario explains all of the observed facts, and I have been unable to think of another scenario that does so as well. And it's surely better to lay out one possible chain of events that explains the available facts than to do nothing at all?!? At least it opens the door for further discussion. With this caveat in mind, let's proceed...
The first point of interest arising from this situation relates to production rates. We've seen that the confirmed serial numbers for the Mk I variant imply a total production figure of around 1000 units, while the numbers for the Mk II suggest that some 900 units of that model were manufactured up to the point at which the Mk III entered production. Since the Mk III model appeared in May of 1949, it follows inescapably that a combined total of 1900 examples of the Mk I and Mk II Vultures must have been produced prior to May of 1949 as a minimum.
Since the engine was apparently introduced in October of 1948, this works out to the quite impressive average monthly production rate of some 250 engines referred to earlier, and it may have been higher at the outset. This was of course in addition to the production of the Hawk and Eagle models which were being manufactured concurrently. No wonder that they needed those expanded production facilities at Darnley Street!
This unquestionably implies that the company must have foreseen a strong immediate demand for an up-to-date FRV 5 cc diesel among contemporary modellers. We may safely assume that "Stoo" Steward had taken the pulse of his fellow flyers regarding this point, and as an active and very prominent modeller himself he was in the best possible position to do so. He must surely have demonstrated prototypes to his immediate colleagues, who may well have been quite impressed. It seems clear that the high initial production rate of the Vulture was tailored to meet an anticipated sales boom for the design immediately following its release.
However, it appears that this boom, if it occurred, didn't last long. Most likely the early buyers soon found that the engine was somewhat less than user-friendly to operate, and the built-in engineering design weaknesses of the Mk I Vulture may have manifested themselves quite rapidly as well. Word of this would have spread very quickly within the tight-knit aeromodelling community, with an immediate negative effect on the marketplace view of the engine (and perhaps the manufacturer). Such impressions are hard to overcome, and the task of doing so becomes more difficult with the passage of time.
It was likely for this reason that the "K" Model Engineering Co moved so quickly to introduce the Mk II version with its beefed-up bottom end, likely within three or four months of the engine's initial release if the above production figures are anything to go by. But why, we may ask, didn't they fix the crankcase as well?
The most probable answer is that they had a stockpile of cases for the Mk I and wanted to realize on the investment that these represented. The numbers suggest that they may have contracted for the initial production of 2000 cases, with perhaps 100 either flawed in some way or designated as un-numbered spare parts. Engines with un-numbered cases are encountered from time to time, showing that some Mk I cases were indeed used up as spare part replacements without the serial number being transferred as they were later with the Mk. III cases.
The first 1000 cases had gone into Mk I Vultures, but that still left them at the beginning of 1949 with 1000 paid-for cases waiting to be turned into complete engines. So they dressed the engine up a little with its revised dog collar, installed the stronger working parts, promoted it as a "Special" and hoped for the best.
But the weak mounting lugs remained and likely continued to break, as presumably did the lower end components of the surviving Mk I Vultures which remained in circulation. This would undoubtedly have contributed to ongoing sales resistance, and all the available evidence suggests that the rate at which the Mk II was manufactured far exceeded the demand from the marketplace. Consequently, the company soon found themselves with a significant inventory of unsold Mk II Vultures.
This of course represented precious capital which was tied up in an over-large inventory of unsold engines and hence was unavailable to the company for other purposes. This problem was further compounded by the fact that the companion 2 cc Falcon model (of which more in a separate article) also apparently failed to meet sales expectations, creating an additional tie-up of capital in the form of unsold Falcons.
It must have become glaringly apparent from this that further steps would have to be taken to remedy an increasingly unfavourable market position. So the company developed a stronger case for use with their already beefed-up working components. By demonstrating their speedy responsiveness to design issues identified in the field, they must have hoped to re-build consumer confidence. It's clear in addition that they fully intended to use the stronger cases as replacement parts for broken Mk I and Mk II engines, as documented earlier. Presumably they arranged for the production of additional Mk III cases to allow for this.
However, it would seem that they still had quite a few unsold Mk II engines on hand at that point, and they understandably wanted to liquidate that asset as well. Accordingly, they decided to release the improved Mk III version immediately to demonstrated their ongoing commitment to the improvement of the design, while at the same time continuing to offer the Mk II in order to liquidate their unsold stocks of that model.They may in fact have fitted Mk III cases to some of their remaining stocks of Mk II engines—this would explain the existence of engine numbers 2804 and 2863 to which reference has already been made.
It’s worth noting in passing that a similar pattern was to be followed with the 2 cc models. From August 1949 onwards for many months, the Falcon and the succeeding Kestrel were offered concurrently despite the fact that Falcon production had almost certainly ceased by August of 1949. As with the Vulture, this dual marketing strategy almost certainly represented an attempt to liquidate an inventory overstock of Falcons and thus free up capital which was desperately needed for production improvements.
If this scenario is correct, then the Mk III Vulture may be seen as a somewhat desperate attempt by the makers to respond to the mounting lug weakness, to improve the power output with the revised porting and thus rehabilitate the Vulture's (and perhaps the company's) presumably-fading reputation. But they also had to deal with the fact that they had a good few unsold Mk II's lying about for which the anticipated buyers had failed to materialize. The recovery of the capital which this represented was doubtless a matter of critical importance.
To manage this situation, the company continued to offer the Mk II at its original price of £3 19s 6d, but increased the price of the Mk III to £4 15s 0d at the same time, thus giving the Mk II a consumer price edge over the Mk III. Presumably their hope was that word of the improved durability of the Mk III might get around and possibly persuade some more customers to get in on the act and at the same time save a few bob by purchasing one of the left-over Mk II's. The latter model did after all have the same strengthened lower end components as the Mk III and should have been just as reliable as long as it didn't hit the ground too hard.
If this is the way that things went, the plan does not appear to have succeeded. We've seen that although around 900 examples of the Mk II seem to have been made during the "gold rush" production period prior to May 1949, we have evidence for only around 400 examples of the doubtless superior Mk III having been produced before production of the Vulture was finally suspended. This was less than three month's production at the former rates. The implication is that Vulture production likely ceased well before the end of 1949.
However, even at the point where Vulture production had ceased it appears that the company had substantial stocks of unsold engines on hand. Advertisements for both the Mk II and Mk III Vultures continued into early 1950, indicating that they still had unsold Mk II's on hand at least 10 months after production of that model had evidently been suspended in favour of the improved Mk III. This is about as clear an indication of ongoing sales resistance as one could find—it took them 10 months to sell 800 engines! Overall, the structural shortcomings of the Mk I and Mk II Vultures may have done as much as any other single factor to undermine the company's market position and financial viability through an erosion of consumer confidence in their products and a consequent tie-up of their capital.
By May of 1950 it appears that all of the 900 examples of the Mk II Vulture had finally been sold, since from that point onwards the company's regular advertisement in Aeromodeller offered only the Mk III variant. It's actually likely that production of that model too had not continued into 1950 and that they were simply selling off unsold examples at this stage. The far more user-friendly 2 cc Kestrel diesel and Tornado glow models were the only other K models offered at this point—the FRV Hawk models had been dropped in 1949 and the Falcon made its final appearance in a February 1950 advertisement, ironically just after the appearance of its only published test in the January 1950 issue of Model Aircraft
The farewell appearance of the Vulture in an advertisement placed by the "K" Model Engineering Co came in the November 1950 issue of Aeromodeller Indeed, this was the last advertisement placed by the company as far as we are presently aware. But the truly astonishing thing is that this was some 19 months after the introduction of the Mk III, and they had still not sold off all of the 400 or so examples that they had manufactured during all of that time!
The fact that production had long ceased and the company was simply liquidating its assets is pretty clearly demonstrated by the fact that the price of the Mk III Vulture had dropped from its former £4 15s 0d to a mere £2 19s 6d. See our companion article on the K Hawk for a detailed analysis of the reasons for the company having been wound up.
Based on the known serial numbers, some 2300 or so examples of the Vulture had been manufactured, probably in a period of less than a year. The fact that quite a few of these still appear to be in existence and in relatively unscathed condition may well reflect the possibility that most of them received very little use once their handling difficulties became appreciated and word got around regarding some of their structural issues. Certainly, it's almost unheard-of to encounter a Vulture that is anywhere near worn out!
It's interesting to note that the value of the Vulture as a second-hand engine plummeted rather quickly following the cessation of production. As of November 1951, Roland Scott's advertisement in Aeromodeller was offering good second-hand Vultures in both diesel and glow configurations for only 35 shillings! Dream on.
So You Want To Run a Vulture?
Well, fair enough—a worthy and entirely achievable ambition for any diesel aficionado, albeit not one for the faint of heart! But first, to paraphrase Mrs. Beeton in her famous Victorian cook-book, you must catch your Vulture! Fortunately, these engines still seem to crop up fairly regularly on eBay and elsewhere, selling for moderately high but not completely unreasonable prices for a reasonably intact example in working order.
As usual, how much you'll pay will be extremely dependent upon condition. In that regard, it's important to be clear from the outset regarding what you want to do with the engine once acquired. If you simply want a Vulture for display purposes, there's little problem—if it's complete and looks good, buy it! As noted earlier, it's almost unheard-of to find a Vulture with poor compression, so that's generally not an issue.
However, if you want to actually run the thing, that's another matter! The Mk I is undeniably a bit fragile as far as its internals go, and I personally wouldn't recommend doing too much running with one unless you have a really good feel for diesel management, have access to good fuel and can keep the stress levels down through appropriate handling of the old comp screw. The Mk II and Mk III seem far less fragile. You may also encounter a Mk I Vulture that has been upgraded to Mk II specification, presumably through the replacement of the original working components following a failure. I have such an engine myself, and it's a dependable runner.
Apart from the basic design, there are a number of issues that can affect the ability of a given example to serve in an operational capacity. Here's a summary of the most commonly-encountered problems:
- Stripped prop driver splines. The splines on the Vulture shaft are pretty fine for a diesel engine of this displacement, and it's not uncommon to find that the prop driver is free to turn on the shaft. If this is the case, you'll either have to make (or have made) a replacement driver (assuming the crankshaft splines are still OK) or make up a split collar mounting for the existing driver, modifying the centre accordingly.
- Wrong spraybar. The stock Vulture spraybar has an unusually large diameter of 3/16 inch, and if this diameter has been reduced or a skinnier spraybar fitted, suction will be adversely affected, quite likely to the point that the engine won't start. The Vulture needs its monster spraybar and large jet holes, make no mistake! If the spraybar is not original, by far the best plan is to make (or have made) a replica of the original unit.
- Bored-out venturi. The above problem is further exaggerated if the venturi has been bored out, as many of them seem to have been in a misguided attempt to extract more power. Even with the correct spraybar fitted, the reduced suction can have disastrous effects on handling. The only remedies are either an internal sleeve to restore the correct internal diameter or a replacement component. Fortunately, an accurate replica is easily machined from bar stock if you have the facilities.
- Bent con-rod. This problem is most prevalent on the Mk I models, but has been encountered on the Mk II and Mk III engines as well. It reflects the efforts of frustrated previous owners to start the engine by cranking on excessive amounts of compression or by using an electric starter. The usual symptom is a certain stiffness around bottom dead centre, where the engine should turn freely if all is well. This stiffness is caused by the big end bearing being no longer at right angles to the upper con-rod column. Once the rod starts to bend, it only gets worse with time and will eventually break. And because it's hardened, a bent rod can't be dependably straightened.
- Cracked crankshaft. As noted above, the strength of the shaft is a bit marginal for a 5 cc diesel. I have personally encountered two Vultures that looked fine on the outside but proved on close examination to have cracks in the shaft just forward of the crankweb. There can be only one result if an attempt is made to start an engine with this problem. The causes are the same as for the previous issue, and the only cure is an original or replica replacement.
None of the above points are necessarily a reason not to buy a Vulture—it all depends on what you expect of the engine. Just be aware that any of these problems can occur—they've all been encountered on more than one occasion! If you want a Vulture to run, the safest thing to do is to buy an engine that has been test-run by the seller. A Vulture that has been successfully run will generally keep running as long as it's handled properly.
The other operational point that may affect your decision to buy is whether you want to merely bench-run the engine or actually (gulp!) try flying it! If your ambitions extend to in-flight use, I'd strongly recommend that you stick to the Mk III Vulture, preferably an example that already shows some signs of previous use. The other two variants retain those rather skimpy mounting lugs, so unless you're pretty confident of your ability to keep it out of the ground, the Mk III is the safest option—it seems to take a "rough landing" pretty well.
OK, let's assume that you have a Vulture which is mechanically sound and has carburettor components of the correct dimensions. Let me assure you that you do have a working engine in such a case! It may just take a little time and effort to prove it.
Before attempting to run the engine, make sure that the dog collar is tight. It's never happened to me, but two of my friends have had this component come loose while running. The results can be unfortunate. Also, check that there are no crankcase leaks past either the cylinder base or the backplate rim. Replace the gaskets if necessary. Good crankcase compression is a must for successful operation of a Vulture!
Next, set yourself up with the necessary mounting hardware. That pesky web in the upper middle of each lug is a problem—the top clamp of a typical test stand bears upon the web rather than the surface of the mounting lug. This explains why so many of these webs are well chewed up. No sense adding to the number of surviving Vultures with damaged webs—make up a pair of alloy plates roughly the same size as the mounting lugs and then cut or file an angled slot in the centre to allow the plate to sit flush on the mounting lug with the web inside the slot. The test stand clamps now bear on the surface of the plates rather than the webs—end of problem!
Mount the engine in the stand with the top of the fuel tank as close to level with the spraybar as possible. This will ensure that fuel supply during starting is not an issue and will make it easier to find the correct needle setting. The engine has pretty good suction once running, but first we have to get it running! A fuel line that stays full is a great help here.
Next we get to the first big secret when it comes to starting a Vulture! Ron Warring actually commented on this point in retrospect in his March 1955 Aeromodeller test of the 5 cc Miles Special diesel. He noted correctly that the larger diesels of the pioneer era had a tendency to "brake" themselves when flicked over due to the high compression loads and bearing friction involved, to the point where it was often difficult to get such an engine to carry over from the first firing stroke to the next. This is certainly true of the Vulture. Warring commented that this as much as anything else was the reason why few of the early 5 cc diesels survived long in production.
Warring also noted (again correctly) that one aid to improved starting was to reduce bearing friction through the use of ball race shaft support, as seen in the Miles Special and the Mk II Drone 5 cc diesels, for example. However, the Vulture was not so equipped. Therefore, the cure in this case is to use a big prop with plenty of flywheel to help the engine carry over from one firing stroke to the next. I've had good results with an 11x7 wood prop, and I don't recommend anything smaller for the initial starts. A 12x5 is also good.
Next important point—use decent fuel! The Vulture likes a fair bit of ether in the fuel, and also responds very well to a modest dose of ignition improver (amyl nitrate or equivalent). It's also wise to use plenty of castor oil—that ball-and-socket joint is pretty heavily loaded! I find that a fuel blend of 35-35-28-2% ether-kerosene-castor-ignition improver works very well in these engines.
If you don't know the settings, you're undeniably in for a bit of a struggle! The engine needs quite a bit of compression for starting—this is a typical characteristic of large diesels. At the correct cold-starting compression setting, the resistance around top dead centre is considerable. However, it's always best to start low and move high—set the compression by "feel" so that the engine fights back a bit when flicked, but not unduly so. You should be able to snap it over quite vigorously without dislocating your starting finger!
The needle setting is more problematic—with the stock system in place, somewhere between 1 ½ and 2 turns usually seems to work, but that's assuming it hasn't been tampered with, as many of them have. The mixture needs to be slightly on the rich side for cold starting, and you'll just have to find out where that is by trial and error. When choked, the fuel should jump along the fuel line pretty smartly when the engine is turned over slowly. Choke to fill the fuel line—no more. We don't want to flood the crankcase at this stage.
Now it's time to go for a start! Get yourself a good finger protector—you'll need it! Not because the engine is vicious—it isn't in the least. Merely because it takes some effort to flick it over, and you may do that for a while before you achieve that first start. A piece of rubber hose works great. Chicken sticks are both useless and unnecessary.
Next secret of starting Vultures—a hefty prime! But there's a problem here—with that 360 degree transfer port at the bottom of the bore along with those long radial exhaust ports, there's nowhere for the prime to pool in the cylinder—it all either goes back down the transfers or gets shot out of the exhaust! Sparey actually noted this latter tendency in his test report. So a prime doesn't stay in the cylinder for very long!
To get around this, set the piston so that the transfer port is closed on the upstroke (piston top just below the bottom of the exhaust ports), prime, give one slow turn to ensure that you're not in a hydraulic lock and then immediately start flicking hard and don't stop! Don't give the cylinder any time to drain! If the compression is anywhere near right, a quick series of hard flicks should produce some reaction in the form of a few pops and bangs. If not, we have to try again.
If there's no reaction, increase the compression about 1/8 of a turn and repeat the procedure. Keep doing this and sooner or later the thing will fire up (honestly, it will!). But don't increase compression to the point where the engine is becoming really hard to flick over. With a big prop, these beasts are actually not that hard to flick over at normal starting settings. If it's not firing at what definitely feels like a high compression setting, suspect the fuel. Got enough ether? Got some ignition improver in there?!?
If the engine fires in energetic bursts but won't keep running, try opening the needle. If it fires up and then smokes and blubbers to a stop, it's too rich—close the needle and try again without priming (it's probably flooded!). If none of this helps, re-check those fuel system components! We repeat that if the venturi bore and spraybar diameter are not somewhere near right, you're doomed, as C-3PO would have said! Same goes for any base leakage.
Above all, do not allow frustration to tempt you into using an electric starter! If you do, you will end up with a bent rod and/or a broken crank—it's that simple. Once you get the hang of them and have the settings more or less correct, these engines are perfectly straightforward to start by hand provided the prop is reasonably large. Again, Sparey commented in much these terms and he was quite right.
However, getting the hang of this may take some time. On the first occasion on which I tried to start a Vulture, I gave up after 1 ½ hours. Boy, was I mad—me, a veteran diesel "expert", defeated! Imagine how a relative newcomer to diesel operation would have fared? I could get it to pop and bang, but not to keep running. And my flicking finger was done! The problems were of course that I was using a fuel with not enough ether, had too skinny a spraybar fitted and in addition was using too small a prop to give me the required flywheel. For the next attempt, I made and installed an accurate repro spraybar, fitted a hefty 12x5 prop, added some extra ether and had it going within a minute of getting stuck in!
Once running, the Vulture is very responsive to the controls, and the optimum settings are easy to determine. It will be found necessary to lean out the needle and reduce compression quite a bit once the engine is running—usually you can back off the compression screw up to 1/4 of a turn as the engine warms up. And it's wise to do so, otherwise pre-ignition will set in, with dire effects upon stresses in the rod and shaft. As the cylinder gets hot and the mixture leans out, reduce compression until a misfire just creeps in, then go back up a hair to eliminate the misfire. Then fine-tune the needle, and there you are! Sparey's reported difficulties in establishing the optimum needle valve setting are not typical in my personal experience with these engines.
The Vulture seems to run best on a very slightly rich mixture. You'll notice that there's quite a bit of exhaust smoke when the engine is running properly. If you try to lean out to reduce this, a misfire creeps in which can't be cured with the compression control. In reality, a fair bit of the smoke is unburned fuel resulting from the rather inefficient transfer design employed. You'll be able to smell this in the exhaust if you have a well-developed "diesel nose"!
A problem that you'll almost certainly encounter, particularly when first running a Vulture that has just been rebuilt, is the need to re-tighten the dog collar. The heat cycles allied to the vibration tend to cause the cylinder base gasket to compress, and this process generally continues for some time. It's wise to re-tighten the dog collar between runs until it no longer wants to move by "fair" means. If the collar becomes visibly loose while running, the engine should naturally be stopped immediately for correction. Please use methods that protect the knurling on the dog collar—no direct contact between metal jaws and the component!
As noted earlier, steel contra pistons tend to stick when the engine gets hot, and this may leave you in a situation where you know that the engine is running hard and over-compressed but can do nothing about it because the dratted contra won't shift! In this condition, the engine will be pre-igniting and stresses on the rod and shaft will be greatly increased. Accordingly, if this happens the engine should be stopped at once (preferably by pinching off the fuel line), re-primed with the comp screw backed off and then immediately re-started. The first firing stroke with the hot prime will usually knock the contra piston back to the screw, and hopefully all will then be well.
Failure to heed this advice may well result in damage to the rod and/or shaft if you keep running in an over-compressed condition. I would guess that many of the damaged engines that crop up have been mismanaged in this way. A cast iron contra piston would have been a far better choice, since a well-fitted item of that material seldom sticks in a steel bore. My own example which has been fitted with such a component is completely trouble-free in this regard, being fully adjustable at all times while holding its settings firmly.
If you persevere, eventually you'll arrive at good running settings for both needle and compression. When you do, you'll find that Sparey was right yet again—the Vulture runs very steadily indeed, never missing a beat. It's also relatively insensitive to artificially-induced fuel level changes, indicating that operating suction is excellent. This is backed up by the flight performance—the Vulture is a very steady airborne performer. There's no question that it makes a good stunt diesel, which is exactly what it was designed to be.
Hot restarts are very easy at running settings—just fill the tank, prime, and off she goes. But the prime is essential—the Vulture must surely be the team-race pit man's worst nightmare! The reduced compression at running settings is a bonus here—the engine becomes quite easy to flick over. To restart from cold, though, you'll have to pull the needle a little and crank the compression back up to that initial starting level. When cold, the thing won't even fire at running settings.
Once you get the hang of all this malarkey and have the settings nailed down, starting on a 10 inch prop becomes perfectly straightforward—I even got one of mine going on a 9x6 with little trouble, just to see if I could do it! But that's far too small a load—a 10x8 is probably the best flight prop for this engine, and indeed that's the size recommended by the makers for control-line stunt use. It will turn this on the ground at around 7,500 rpm and speed up to somewhere near the peak (approximately 9,000 rpm) in the air. At those revs, the 8 inch pitch and fairly large blade area give both good pulling power and a quite reasonable airspeed. For free flight, the maker's recommendation of a 10x6 appears entirely appropriate.
In summary, a well set-up Vulture belies its reputation when it comes to starting—it's actually not that difficult to start provided it's properly set up in the first place, is supplied with appropriate fuel, is loaded for its design operating speed and has its controls set more or less correctly. The problem is that establishing the correct starting settings may prove to be a tedious and at times frustrating experience. Once you have established the settings, I'd recommend making a note of them!
Despite this, once you sort it out the Vulture is a perfectly useable engine. However, there's no doubt that it's a "real man's motor" and getting on top of starting one of these brutes is not an easy chore. It's not at all hard to see why these engines have carried a reputation for cantankerousness down through the years—they rate it!
We hope you've enjoyed this in-depth look at a brave but flawed attempt to produce the first British second-generation 5 cc diesel. The Vulture was (and is!) an interesting design that certainly looked to the future but fell somewhat short in a number of areas. Despite this, it has a quite useful performance if you can get the hang of starting it! And the satisfaction that you feel when you achieve that goal is well worth the aggro involved in getting there! So go ahead, give it a go—interesting times and plenty of finger exercise are guaranteed!
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