We start with an all moulded Model Composite MX2, this model being produced in Czech Republic & not to be confused with the similar sounding company of China. The model is to be powered by a 3W 170cc Twin spark engine, the engine is brand new & comes with an optional Pearl blue painted cylinder heads. Twin spark, in 3W terms is to have an additional plug per cylinder & an additional ignition system, the benefits are two fold, with an additional ignition system there is a redundancy element to the ignition system of the engine, & the dual spark system adds approximately an additional horsepower to the performance of the engine. The power output of this engine is 19 horse power, the owner of this model will have power to spare! as this model performs well on the 15 horse power of a DA150. There is a lot of talk about light weight engines that have come onto the market & the advantage of light weights, unless airframes are designed specifically for very light weight engines then you will end up having difficulty obtaining a correct centre of gravity. We have witnessed people purchasing these engines & then having to shift all batteries into the cowl fixing them as forward as possible then having to add lead weight as well. This design has been flown with standard 150 sized engines & the 3W was specified as the engine of choice & the model came with the engine mountings predrilled for the engine. Not only this, the mounting system had machined to length, alloy stand offs to ensure the engine fitted to model perfectly. Further more, the mounting system that Model Composite has devised allows for right & down thrust adjustments that maintains the engines centre line position.

Over the last forty years of building model aircraft, we have adopted certain preferences in putting models together, one of these preferences has to been to mount the throttle & choke servos as close to the engine as possible. As the 3W 170B2i has the carburettor mounted under the engine in between the two exhaust manifolds, I made a plate up to mount the throttle & choke servos directly behind the carb. This means the push rods are short & direct with no slop. Ball-raced rod ends were used on the push-rods to further enhance the slop free connection between the servo & the carb.

Why use a servo on the choke? We are often asked this, first & foremost, it is an additional safety feature. If your throtle servo was to malfunction & perhaps jam at full throtle, which would be a big problem for a model with this power to weight, the choke can be activated to stop the engine & allow you to land. The second feature of a servo operated choke is that you would not be putting your hands near the engine to operate the choke with the ignition on, it does not take much movement for the engine to fire, so that’s our reason for the two servos on the operation of the engine. Something we will deal with latter on the build is we also install a electronic switch which connects to the radio which switches the ignition power on & off as required via the transmitter.