Quarter-Scale Merlin Details


The Quarter-Scale Merlin is based on the Rolls-Royce Merlin, and there many things about it which are exactly like the real Merlin.  However, it is not an exact scale model in every respect.  The original focus was on creating an engine which looks and sounds like a Merlin, is capable of flying a scale model plane, and which can be built as easily as possible.  Therefore, the original prototype was quite simplified.  Even so, there was no doubt it looks like a Merlin.  As design progressed on the 'real' version, more and more scale detail was included.  However, the idea of simplification has not been completely abandoned.  Like the prototype, the current design uses two valves per cylinder, instead of the Merlin's four.  This greatly simplifies construction, and realistically will have no effect on performance.  Instead of the Merlin's complicated tower gear camshaft drive system, a chain drive is used.   In the Quarter-Scale Merlin's simplest version, glow plugs are used, avoiding the complication of spark ignition, and there is no supercharger.


There are two reasons why high-performance aircraft engines like the Merlin have superchargers.  One, of course, is to increase power, particularly at high altitude.  The other is to improve mixture distribution, which is always a problem for inline engines fed by a single carburetor.  Even a slight pressure increase seems to help even out mixture distribution among the cylinders, but much of the improvement is simply due to the supercharger's agitation of the fuel/air mixture.  These factors should work equally well on the Quarter-Scale Merlin, but the downside of adding the supercharger is complication and extra weight.  The wheelcase/supercharger section is almost as complicated as the rest of the engine, and some of the castings are fairly challenging to work with.

The supercharger designed for the Quarter-Scale Merlin is based on the early Merlin's single-stage version.  After consulting with supercharger experts, it was decided to gear the impeller to run at approximately 10 times crankshaft speed, or 36,000 rpm at the target 3,600 rpm engine speed .  Calculations indicate that this will provide a small but significant pressure boost, sufficient for the target horsepower/rpm goal.  Of course, boost pressure would be considerably higher if the impeller were geared to the maximum possible rpm for its size.  Based on the general rule for centrifugal superchargers, that the impeller tip speed should not exceed roughly the speed of sound, this would be on the order of 160,000 rpm.  Since the supercharger design is completely untested, even gearing it at 10:1 seems  risky enough.

Those primarily interested in scale appearance will naturally want the supercharger, but those looking basically for a running, flyable engine may wish to consider alternatives.


One way to help the mixture distribution problem is to increase the number of carburetors, and position them evenly with respect to the intake ports.  Since each cylinder will have its own running characteristics, a good solution is to provide each cylinder with its own carburetor throat, with individual mixture adjustment.  Fine tuning can be done by measuring each cylinder's exhaust temperature.  This sounds complicated, but compared to the supercharger approach, it is vastly simpler.   Without a supercharger, the engine will have to be run somewhat faster to produce enough horsepower to fly a plane, but the increase shouldn't be too great.  After all, the engine does have quite a large displacement.

The Quarter-Scale Merlin cylinder head was not designed with this approach in mind.  Except for having two valves per cylinder and allowing a chain camshaft drive, everything else about the head is exactly like the real Merlin.  In the case of the intake ports, the dividers between adjacent ports do not extend all the way to the manifold flange.  On the real Merlin, a flame trap (basically a long rectangular metal filter, which prevents backfires from propagating) is held to the head by the intake manifold.  If each port were completely separate, it would receive charge only through the portion of flame trap directly in front of it.  By joining the ports in front of the flame trap, flow through it is shared by all cylinders.  On the Quarter-Scale Merlin, a flame trap should not be needed, and it certainly will not be needed with separate carburetor throats for each cylinder.  In order to allow separate carburetor throats and individual cylinder tuning, the space between ports will have to be blocked, but this should not be difficult.

During machining and assembling of the complete casting set, it was found that a considerable amount of ingenuity and care (and building of fixtures!) was required to machine the scale intake manifolds and get them to fit the heads.  The multiple carburetor approach eliminates this particular challenge.


A basic set of castings for building a runnable, scale-looking Merlin, using the no-supercharger, multiple carburetor approach, consists of the following:

Crankcase, gearbox front cover, oil pan, cylinder blocks, heads, valve covers, and exhaust pipes.  Optional are the coolant inlet and outlet fittings.  Actually, of course, the exhaust pipes are optional too.  The particular style of exhaust currently offered was used on later model Merlins, used in the Mustang and Spitfire Mk IX.  At some point the earlier-style three-port exhaust manifolds may be produced, but they would be fairly difficult.  The multiple carburetors are not available yet, but a design which will retain as much scale appearance as possible is in work.


The additional castings required for adding the complete supercharger section are:  center intake manifold, two side intake manifolds, wheelcase, supercharger front case, supercharger rear case, supercharger rear cover, supercharger exit elbow, supercharger impeller, and scale carburetor.  Optional are the magnetos, and the coolant pump, which mount on the wheelcase, and the coolant inlet and outlet pipes.