Our heading photo shows Ron Checksfield's answer to the oft-repeated demand
far a British stunt 35 in this new engine, known as the Merco 35.
The prototype was used by Bill Morley to gain 2nd place in this year's Gold.
The big news from the home front is that, with a bit of luck, Britain is going to have a genuine stunt 35 at last. It has been designed by Ron Checksfield (whose past work on racing motors is well Itnown) and is known as the Merco 35, Only one prototype has so far been built, but this was used to good effect by Bill Morley in his Thunderbird to win second place in the Gold Trophy at the Nats. this year and it has now been passed on to MODEL AIRCRAFT for testing.
As will be seen (heading photograph), the engine is of the favoured stunt-35 layout: shaft valve, plain (bushed) bearing, loop scavenged cylinder, compact design and moderate weight. It is, in other words, closely modelled on the best American 35s but few stunt enthusiasts should grumble about that. Past efforts to produce a stunt 35 of original British design have met with only lukewarm reception, the general feeling being that what was wanted, in order to achieve American standards of stunt performance, was an engine modelled on proven American practice and that attempts to be more original could follow later. It is, therefore, to be hoped that, if and when the Merco gets on the market, it will gain the full support of those modellers who have hitherto complained long and loud about the lack of a British engine of this type.
In general, the Merco resembles a mixture of Fox and Series 100 Veco with a few features of its own thrown in. A one-piece casting, incorpating crankcase, cylinder barrel and main bearing, generously webbed, on the Veco pattern, is used. Like Fox, the Merco employs a Desaxe (i.e. offset) cylinder, an unusual feature in a producti9n model engine, but one which has definite theoretical advantages, as have been enumerated in these columns in the past. Stroke/bore ratio is also approximately the same as the Fox, which has a nominal bore and stroke of 0.800 x 0.700 in.
The crankshaft has a 7/16 in dia. journal - the usual size nowadays with 29 arid 35 engines - and a 7/32 in. crankpin. As on thc Fox, and unlike other 35s, - which mostly use crescent counterweights on a full disc web, the Merco has the web flanks cut away on the crankpin side as a means of balancing rotating mass. The shaft has the usual rectangular pattern valve port, 3/8 in. long and occupying 3/8 in. of the shaft circumference. It is fed from a circular intake port in the main bearing and conveys mixture through a 5/16 in. dia. passage.
The piston is of typical design, with a nicely filleted baffle and the lower third of the skirt relieved approximately 0.0005 in. The gudgeon pin is of 3/16 in. dia., fully floating, with brass end pads A drop-in cylinder liner, flanged at the top and having a wall thickness of 0.054 in., is used. The flange fits in a channel in the cylinder head, which is of a hemispherical pattern with centrally located long-reach plug and is held down with six screws.
The port timing of the Merco is fairly conventional, except, of course, for the fact that, in terms of crank angle, the opening and closing of the cylinder ports, due to the Desaxe layout, are not symmetrically disposed either side of b.d.c. The actual effect of this is that the ports open and close about 5 degrees later in the cycle, while the piston velocity is speeded up during the compression stroke and slowed down during [lie power stroke, thus giving more time for the expanding gases to do useful work. Rotary valve timing ts normal, with a total induction period of about 185 degrees.
The prototype Merco 35 weighs a little over 8 oz. - fractionally more than the average American 35. It is possible that this will be s!ightlv reduced in the produetiuri version, especially if a pressure-cast case is adopted, although, personally, we would see no objection to the engine remaining at its present weight, retaining a gravity or sand casting and thus gaining in durability from greater physical strength and better thermal conductivity. Among the detailed fittings we hope to see retained is the excellent needle-valve assembly with its angled fuel-line nipple.
We have intentionally omitted to emphasise the all-round excellence of the finish of the various parts. It is obvious that these have profited, in this hand-made prototype, from the individual attention that it is seldom possible to lavish on a quantity-production engine. If, however, the Merco should display equivalent high standards in its production version, purchasers may rest assured that it will be generally superior to current American mass-produced 35s.
Since last month's interim report on the Merco 35, we have had a chance of running some tests on this motor and are also able to give a few details regarding its background.
As is known, the prototype Merco was used to gain second place in this year's Gold Trophy by Bill Morley and it is to Bill that much of the credit is due for the existence of this, the first British made true stunt-35 motor. Previously, on several occasions, he had endeavoured, unsuccessfully, to interest British manufacturers in producing engines of this type. Then, when the A.M.A. schedule was adopted by the S.M.A.E., he teamed up with Ron Checksfield, noted tuner of racing motors, to design and build a private venture prototype.
There is only one way to be sure of turning out an item - no matter what it is - that is as good as the best that the opposition has to offer, and that is to have had some experience of the opposition's products. This, Morley has and he was thus able to bring the practical user's requirements to bear when Checksfield began the design work.
From the time that castings were obtained to Checksfield's pattern, it was a race to get both a prototype engine and a model built for the Nationals. We don't know how confident Morley and Checksfield were at that point, but it seems clear that they were pretty sure they were on the right track. The engine emerged 10 days before the Nats., and went through a series of bench tests and modifications during the next week. The model (a Thunderbird) was finished only 24 hr. before the Gold was due to take place and, when it was taken to Waterbeach, the whole outfit had not even been flown, It speaks volumes for Checksfield's engine and Bill Morley's flying ability - not to mention Bob Palmer's excellent Thunderbird - that, with only two preliminary test flights, second place was obtained, only three points behind the winner.
Bench tests are virtually of academic interest only when an engine has already proved itself conclusively under actual flying conditions and our own tests have, therefore, only served to confirm b.h.p. figures and prop/r.p.m. speeds comparable with the recognised top American 35s. More important is the engine's unfaltering pulling power; its ability to pull the motlel through the tightest manoeuvres without "sagging."
Production Merco 35s should be reaching the market during late December or early January and every effort will be made to ensure that these will be just as good as the prototype. The main difference will be in appearance only, due to the adoption of a pressure diecast crankcase, cylinder head and rear cover in place of the present sand cast and machined components.
We shall be giving further news of the Merco 35 and its companion 29 model as soon as production versions become available.
Parts of the Marco 35 glowplug stunt engine.
A 0.29 cu. in. version to the iame basic design will also be available.
My thanks to Terry McDonald for supplying copies of this material.