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REMOTE CONTROL BY BOB YOUNG Internal combustion motors for R/C models; Pt.2 Last month, we explored some of the basic concepts applied in internal combustion motor design for models of various kinds. This month, we will look at some of the practical applications of these motors and the basic rules for their successful use. Fundamental to the discussion of motors is the understanding and use of some sort of fuel. Model fuels typically fall into two categories: (1) petrol based and (2) methanol based. Petrol based fuels use a mineral oil as the lubricant, while methanol based fuels use a vegetable oil, typically castor oil or a synthetic upper cylinder lubricant. There are exceptions. For example, Castrol M is pure castor oil whereas Castrol R40 is castor oil with modified viscosity due to the addition of a synthetic component. It corresponds to a 40 grade oil. Castrol R30 is a lighter oil of 30 grade. Fuel/oil ratios Very few model motors are fitted These days, most people elect to use an electric starter for their models. Typically, they run from a 12V supply (ie, a car battery) and draw about 6A. with a sump and thus the oil must be mixed into the fuel as with mower or chainsaw fuel. The ratio of the power component (P) in the fuel to the lubricant (L) is quoted as P:L. Thus, a 3:1 fuel has three parts of the power component to 1 part of lubricant. Note that the lubricant takes up volume in the cylinder and this affects the burn characteristics of the fuel, so there is a definite performance benefit to be gained from a reduced oil level. In contest work, this is all important and when combined with other factors, the quest is on for the lowest possible oil ratios. In some MAAA contests, the fuel is supplied so that all contestants are on an equal footing with regards to fuel. All unburnt fuel is ejected from the exhaust system and this includes the lubricant. Thus , the higher the ratio of oil to power components in any fuel, the more residue on the model after the run is completed. Herein lies the problem with castor oil. In the case of an aeroplane, this sometimes thick, smelly and heavy deposit can affect flying characteristics and, in contest work, this is not good. Burnt castor oil has to be one of the most obnoxious substances known to mankind. When using 3:1 fuel, the model lands after a 20-minute flight literallysoaked in a thick black slime. It is difficult to clean off and in time ages into a sticky, gumlike substance that turns glorious paint jobs a dirty grey. It soaks right into models and renders them almost impossible to repair. It also coats everything in a model box - tools , batteries and test equipment - with a mixture of gum and dust. SEPTEMBER 1992 93 A tachometer is invaluable for engine tuning. Commercial models are readily available or you could build the popular unit that was published in the May 1988 issue of SILICON CHIP. Finally, it soaks into the very pores of the modeller himself, rendering him instantly recognisable to other modellers. So there is considerable incentive to reduce the castor oil content in modern fuels, for two reasons: (1) motor performance; and (2) the messy business of castor oil residue. Thus, the stage is set for the argument we will now be examining in some detail. P:L ratio is a very important factor when using model engines and is the subject of endless argument and discussion. There are no hard and fast rules and therefore every modeller has his own pet theory, a situation in which the tyro modeller can be very easily led up the garden path. Engine tuning Central to the argument is the longevity of the model motor. Model engines are very expensive and they can be ruined in one run if the wrong advice followed. Once again, it must be remembered that the lubricant comes into the motor with the power component and at this point a brief explanation of tuning a model engine is in order. The most popular method for start94 SILICON CHIP ing model motors is with an electric starter. This is fitted with a rubber cone which is pressed firmly against the prop spinner. Electrical power is then applied to the starter motor. Thus, to start a glow plug motor, for example, a 1.2 volt battery (usually a 4Ah nicad) is connected to the glow plug located on the top of the cylinder head. The usual method here is a 2-pronged clip which makes contact with the central insulated core of the plug. One end of the platinum glow element is connected internally to this core and the other is connected to the motor body. Current consumption of a typical gl'ow plug is around 4-5 amps although it varies depending upon the heat range of the plug. If it is the first start of a new motor, disconnect the glow plug battery and close the needle valve fully shut by screwing it in the clockwise direction. Now open the needle valve by turning it anticlockwise for two and one half turns. If the needle valve was set correctly last flight, then do not fiddle with it. The motor is then primed by placing a finger over the carburettor air intake and turning the motor over slowly by hand. Watch the fuel inlet tube for signs of the fuel moving through to the carburettor. Once fuel is up to the needle valve, two or three turns are sufficient to prime the motor. Care must be exercised here for several reasons. One is that motors can sometimes start spontaneously, even without the plug connected. It is rare but most experienced modellers have had it happen at least once in their flying careers. Secondly, if the motor is flooded and a hydraulic lock exists, serious damage can be inflicted upon it, especially with small motors. Conrods can be bent or broken very easily with rough handling. For the same reason, when using an electric starter on small motors, use the minimum amount of pressure as the rubber cone then acts as a slipping clutch should the motor hydraulically lock. Once the motor is primed, turn on the R/C set, set the throttle to about one third and connect the glow plug battery. This done, hold the model in one hand, apply the starter cone to the spinner and switch on power to the starter. The motor should spin freely and start within 10 revolutions or so. If it does not start, check that the fuel is flowing through the intake line, that the glow battery is charged and that the plug is glowing. Checking the glow plug In the old days before we used mufflers, one just simply looked inside the exhaust port for that nice rosy red glow inside the cylinder. It was eerie to note that the glow still existed once the motor was running and the glow plug battery had been disconnected. These days, one must remove the glow plug from the motor and connect a battery while viewing the element. The element should glow a healthy cherry red. Once the motor is running, disconnect the glow plug battery, open the throttle to . full and turn the needle valve in the closed (clockwise) direction. As the needle valve is closed, less fuel is delivered to the motor and the revs will gradually increase. Continue th.is process until there is a noticeable drop in rpm; ie, tune for maximum rpm. A phototachometer is a very useful item at this point, giving a clear and accurate indication of the performance changes between different combinations of glow plugs and fuels. Now here is the tricky part and the core of the argument in regard to fuel/ shift in trim as the fuel load is burned off. Running rich This was the first series target aircraft built for the Australian Army by Silvertone Electronics. Called the "Ugly Stick", it was fitted with an OS 40 Schnurle Ported motor. Note that it had plenty of ventilation. If it had been housed inside cowling, ventilation could have been a serious problem. oil ratios. You will remember we established that the oil comes into the motor with the fuel. Therefore, as we tuned the motor more and more in the lean direction, we were delivering less and less oil to the motor and all this at a time when rpm was increasing. If this process is continued on after peak rpm is obtained, then the motor is running with a much reduced oil level and the danger of the motor seizing is very real. This is compounded in flight when the rpm increases due to the prop being unloaded, or the mixture leans out when the nose of the model is raised during a climbing manoeuvre. This is called a "lean run" and if the motor is poorly ventilated, it can easily overheat, thus dramatically increasing wear and tear. In some situations, it can even seize completely. For this reason, once the motor is tuned to peak revs, open the needle about one eighth of a turn in the anticlockwise direction to allow for the increase in revs in flight. Also, hold the nose of the model vertically to ensure that the fuel will continue to be drawn into the motor without the motor leaning out or stopping completely. Fuel tank In the days before nicad batteries, glow plugs were energised with a large 1.5V cell. This particular unit has been fitted with a switch & a meter to check the current. One word here on the mounting of the fuel tank, a vitally important point for a reliable engine run. The tank centreline should be level with the needle valve and as close to the motor as possible. Always use muffler pressure to pressurise the fuel tank, or better still a fuel pump. The latter ensues almost faultless motor runs once set up correctly. There is another benefit with fuel pumps in that the fuel tank may be mounted on the centre of gravity of the model, eliminating the annoying Now the complexity of the argument becomes a little clearer. Typically, a modeller might tell you that he flies with 5% Gloglide (a synthetic oil) and that to add more oil is just a waste. What he probably will not think to mention, for the simple reason that he is probably not really aware ofit, is that by habit, he runs his motors uncowled, slobbering rich, and uses exhaust pressurisation of the fuel tank or a fuel pump; so there is always ample cooling air and ample oil under all circumstances. In other words, from past experience he has found that he does not need more than 5% Gloglide in his models, because of the way he sets them up and operates them. The missing factor is , however, his limited experience, and everybody's experience is limited. New situations are constantly arising and that same modeller may be in for a shock one day when he moves from his old uncowled, fully ventilated motbr installations into a fully cowled scale job fitted with a brand new $400 motor. I have seen many a modeller scratching his head and muttering about the quality of modern motors because his brand new $400 dollar motor welded the piston into the cylinder on its very first run. The key factor So you see the key factor in this argument is the effective level of oil delivered to the motor. One modeller running 5 % oil in a rich mixture is probably delivering as much oil to the motor as a modeller using 10% oil and running at the correct mixture setting. Unfortunately, there is no simple method of establishing what this effective level is. Just be very careful about the advice you accept in this area and as a general rule err on the conservative side until your experience indicates otherwise. My own considerable but still limited experience always dictated 20% castor oil and every time I tried less oil or synthetics, I came unstuck in some way. But then I was always told that I ran my motors too lean (trying to get the maximum rpm) and that I was ultra conservative. SC SEPTEMBER1992 95