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Published in 'Model Aircraft' October 1964.

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One day, during the summer of 1961, we were driving John Maloney (the U.S. Merco distributor at that time), and his wife, to London Airport, in company with Bill Morley, who, with Ron Checksfield, was responsible for the original Merco engines. Back at the Maloney's hotel, Bill had just been showing us the first prototype Merco 49 and the conversation now turned to the question of whether R/C engines would get any bigger than the then new 45/49 class. John, already selling Super Tigre 56's in the U.S., thought this might well happen, unless the R/C equipment manufacturers further reduced the size and weight of transistorised equipment and models got smaller. We ventured the opinion, having examined the parts of the Merco 49, that it ought to stand boring and stroking to 10 c.c. if need be. Bill then confessed that, when Ron Checksfield designed the 49 crankcase, just such a possibility had been borne in mind and that the engine could, quite easily, be enlarged to a "60" without overstressing the main casting or imposmg undue limitations on the dimensions on the shaft, bearings, etc.

We mention this conversation to emphasise the fact that the new "big" Merco is not, strictly speaking, merely an enlarged 49, but that the original design was envisaged as a 10 c.c. engine right from the start. At the time, these facts remained unpublished at the particular request of Bill Morley and the Merco's new manufacturers, D. J. Allen Engineering Ltd., for fear that interest in a projected "Merco Sixty" would be aroused prematurely. Obviously, time was required to develop and test a 10 c.c. version and, as things turned out, demand for the 49 was such that nearly three years were to elapse before the 60, or "61" as it was to be called, was released for general sale.

For twelve months before its release date, however, the 61 was actually flown in competition. In May 1963 Dennis Allen flew a prototype at the second R/C Trials at Odiham and demonstrated, with fast vertical climbing rolls, the usefulness of those extra few hundred r.p.m. In all, six prototypes were built for the 1963 season and received a valuable workout in the hands of such notables as Frank Van den Bergh, Chris Olsen and Ed. Johnson. These prototypes had a nominal piston displacement of 0.604 cu. in., the bore of 0.937 in. being dictated by the use of American Hornet 60 piston-rings, since no ready-made British piston rings of the approximate size required were available. The production models, however, have a bore of 0.940 in. and the rings are especially made for the engine by Messrs. Wellworthy, the well-known manufacturers of full-size rings. With a stroke of 0.875 in. (bore and stroke are therefore precisely the same as for the McCoy 60) the swept volume is 0.607 cu. in., hence the "61". Prototypes had plain cylinder heads, but the current production model has a finned head like the 49. In fact, the external appearance of the two engines is virtually identical and the only outward difference is that the 61 is about 1/16 in. higher.

Internally, there are a number of changes quite apart from the obvious dimensional differences occasioned by the increased bore and stroke. For example, cylinder port timing has been modified quite considerably. The exhaust ports now remain open for 136 degrees of shaft rotation while the transfer duration has been extended (by some 16 degrees) to 120 degrees, thereby reducing exhaust lead by approximately five degrees to eight degrees. On the other hand, the actual exhaust and transfer ports (four exhaust and four transfer all exactly the Same) are identical in size and number to those of the 49, so the 61 has, in proportion, smaller port areas. On the induction side, there are no changes, the carburettor is identical with that fitted to the 49 and rotary-valve timing and intake passage dimensions are precisely the same.

The Merco 49 and 61 are, of course, ringed engines, using machined aluminium alloy pistons. A recent modification, which is also being applied to the 49, is the addition of skirt transfer ports in the piston, two in number, which register with similar ports in the cylinder liner as the piston approaches the bottom of its stroke. The purpose of these is to assist the passage of gas from the primary compression chamber to the combustion chamber and it is also considered that, in avoiding the collection of stagnant gas below the pistol crown, better cooling of the piston occurs. Incidentally, the fully floating tubular gudgeon-pin, unlike the original 49, does not have end pads and the connecting-rod (identical with the 49 rod) now has large slots at both ends for lubrication instead of the small holes previously used. The piston has a flat crown with a nicely filleted straight baffle. The cylinder head has a deep hemispherical internal shape, interrupted by a slot for the piston baffle. Six screws attach the head to the cylinder, three passing through the fins and into the main casting to tie the complete cylinder assembly to the crankcase.

The 61, like its smaller brother, is beautifully made. Its mounting dimensions are identical with those of the 49 and it is only about 1/4 oz. heavier, thereby making is easily interchangeable with the 49.

Specilleation

Type: Desaxe single-cylinder, air-cooled, loop-scavenged two-stroke cycle, glowplug ignition. Crankshaft type rotary-valve induction. Baffle piston and central ignition plug in hemispherical combustion chamber. Coupled carburettor throttle and exhaust restrictor.
Bore: 0.940 in. Stroke: 0.875 in.
Swept Volume: 0.6072 cu. in. = 9.950 c.c.
Stroke/Bore Ratio: 0.931:1.
Weight: 12.7 oz.

General Structural Data

Pressure diecast L.33 alloy crankcase and main bearing housing with sand blasted external finish. Counter-balanced, hardened and ground steel crankshaft running in one 1/2 x 1 1/8 in., and one 8 x 22 mm. ball journal bearings. Piston machined from low expansion aluminium alloy with reamed gudgeon-pin holes and two Wellworthy piston-rings. Hardened, ground and honed leaded-steel cylinder liner. Connecting-rod of forged RR.56 aluminium alloy. Cylinder head and cylinder fins machined from high quality duralumin. Pressure diecast L.33 alloy crankcase backplate. Machined alloy prop driver on split taper collet. Machined alloy carburettor body with barrel type throttle and separate screw adjustments for idling speed and idling mixture. External swivelling steel plate type exhaust restrictor coupled to carburettor throttle. Beam mounting lugs.

Test Conditions

Running time prior to test: 2 hours.
Fuels used: (a) 75 per cent methanol, 25 per cent Duckham's Racing Castor oil (running in); (b) 70 per cent methanol, 25 per cent Duckham's Racing Castor oil, 5 per cent nitromethane (performance tests).
Ignition plug used: Makers' A-M long-reach as supplied.
Air temperature: 70 deg. F.
Barometer: 30.00 in. Hg.

Performance

Power curves

One of the big advantages of using a ringed piston in an engine of this size is that it drastically reduces the running-in time usually found necessary with large lapped-piston engines. We kept our test 61 running on the rich side for the first hour, but, even after about 30 minutes it was obviously sufficiently free to allow normal use in a model. As is our normal practice, running-in consisted of a series of short runs with cooling-off periods between each run.

After about 1 1/2 hours, r.p.m. checks yielded the following figures on typical props: 8,400 on 14 x 6 Top-Flite wood, 9,700 on 13 x 5 1/2 Top-Flite wood, 10,400 on 12 X 6 Power-Prop wood, 11,200 on 12 X 5 Power-Prop, 12,100 r.p.m. on 11 X 5 Top-Flite wood. The pulling power on the bigger sizes was clearly evident. The high power developed at moderate speeds was even more obvious on the torque-reactor dynamometer where a maximum approaching 100 oz. in. was obtained at the lowest speed tested. We have tested three other 10 c.c. R/C engines during recent months (including one not yet in production) and this is the highest torque figure obtained thus far. Equivalent to a b.m.e.p. of approximately 66 lb./sq. in., it is also better, relative to the swept volume, than the Merco 49 R/C previously tested in this series.

As load was reduced, torque dropped off at a fairly even rate and peak b.h.p. was realised at around the 11,500 mark, the actual recorded output between 11,000 and 12,000 r.p.m. being slightly over 0.88 b.h.p. This is very good indeed and, again, is slightly better, on a specific output basis, than the 49.

The Merco carburettor is of the conventional barrel throttle type with an air-bleed screw adjustment for controlling the idling mixture strength. With our test engine, we thought that finding the best throttle settings needed a little more care than with the 49. Incidentally, with the throttle properly adjusted, the engine can be started easily from dead cold with the throttle closed. Minimum practical idling speeds would appear to be in the region of 2,500 - 2,800 r.p.m.

Starting was excellent at all times, using standard procedure of an exhaust prime when cold and one or two choked flicks when the engine was warm, and the Merco ran smoothly and steadily over the entire load range tested, The best prop size for most applications would, we suggest, be a "fast" 12 x 6 or a 12 x 5. These are well-matched to the peak of the power curve, but a 13 X 5 1/2 might be a good choice for a large and heavy model. Eleven inch props, however, will allow the engine to run well past its peak in the air.

Power/Weight Ratio (as tested): 1.11 b.h.p./lb.
Specific Output (as tested): 88.5 b.h.p./litre.

Drawing

My thanks to Terry McDonald for supplying copies of this material.

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