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REMOTE CONTROL BY BOB YOUNG Internal combustion engines for model aircraft The subject of this month's column was prompted by a reader's letter enquiring about the possibility of converting a glow plug model aircraft engine to operate on petrol. Inevitably, as part of the conversion, the reader wants a design for an ignition system to complete the conversion. Why would you want to do such a conversion? The reader, Leon Burkamshaw, of Coffs Harbour, NSW, is quite succinct when he states that the cost for methanol-based fuel is becoming prohibitive. He is right. The short answer is to his query is "yes but with some difficulty and great care". However, I am not renowned for my short answers so sit back while I endeavour to fully explain this subject. First though, I will give a brief This is particularly true of 4-stroke motors which can run too hot when fuelled by petrol. This excess heat can cause severe damage to the valves and head. And, as a secondary effect, the spark plug element can begin to glow under the heat and compression and cause severe pre-ignition, with attendant problems to the conrod and bearings. While fuel prices are important, so is safety and petrol is a very explosive "Anyone attempting to convert a modern motor to petrol ignition is advised to proceed with caution as there are vast differences between motors designed for methanol and petrol fuels". explanation and a word of warning on operating petrol motors. Anyone attempting to convert a modern motor to petrol ignition is advised to proceed with caution, as there are vast differences between motors designed for methanol and petrol fuels. The main areas of difference are in compression ratios, timing and carburation; in other words, in virtually every area of the motor design. 58 SILICON CHIP chemical when compared to methanol. Handling methanol is a fairly laid back business and in over 40 years of hanging around methanol powered models, I have rarely seen a model catch fire and I have never seen anybody seriously burnt in an accident. Some of the fuel additives are, however, · very toxic when inhaled and great care is called for when using all chemicals. Nitro-benzene was a very . popular additive in my early years and was particularly dangerous. Fortunately, it is rarely used these days. I must add here that one should never run motors or fly indoors and for those who think I am mad for even suggesting it, I once flew a helicopter inside my factory and nearly gassed myself. To go one better, a friend of mine once flew a helicopter in his hotel room and suddenly found himself and his helicopter on the footpath outside the hotel; along with all of his luggage! Fuel dangers On a more series note, there was an article in an American modelling magazine some time ago, written by an American modeller who was also a trained military aviator. He had a methanol fuel-can fitted with an electric pump explode and catch fire under his face. He was immediatelv engulfed in flame from the chest up. His survival training had taught him never to breathe in until the flames went out if caught in this situation, as otherwise one's lungs and throat can be badly seared. He survived to write the story. An untrained person may not have been so lucky so please be careful, even when using methanol based fuels. As a personal idiosyncrasy, I will not use an electric fuel pump. I realize that such accidents are rare but they do happen. The situation with petrol is potentially much more dangerous. Again, I have not heard of any accidents in the modelling field but stories abound in general society of the dangers of petrol. Carrying petrol in a. model box Speed Control Still Coming This month, I must apologise to those who have been eagerly awaiting the speed control construction article. Judging by the response to the speed control series, there are many more interested in this subject than I first anticipated. Unfortunately, pressure of work has prevented me from devoti ng the amount of time necessary to complete such a complex project. And here I must point out that I am quite surprised at just how complex this project has become. When Leo Simpson first asked me to do a construction article on a simple speed controller, I had exactly the type of circuit I wanted in mind. Leo applied only one constraint - that the design was to be based on commonly available components. The NE544 servo amplifi er ch ip is no longer current and other chips can suffer the same fate at any moment, so I considered this a reasonable request. It did, however, introduce containing batteries is just not on and it should always be contained in a well sealed can, painted red and clearly marked PETROL - INFLAMMABLE. Tanks should be fi lled in such a way as to prevent spillages an d the motor sh ould not be started over any area where a spillage has occurred. Even a small spark from a glow battery or starter battery can start a petrol fire and the really wise will carry a small powder fire extinguisher in their flight box as a matter of course. I might add that electric fuel pumps and petrol sounds a really dangerous combination to me. Thus endeth today's sermon . Motor types The motors most commonly employed in models fall largely into two categories: single cylinder 2-stroke and single cylinder 4-stroke. But, in keeping with all fie lds of modern human endeavour, the recent technological revolution has resulted in a proliferation of very expensive multicylinder motors. These days , it ap pears that the sky is the limit in regards to cost, with radios selling for some unexpected complications. This constraint also had a second and less obvious consequence in that it 'also precludes a microprocessor based system on the grounds that the software also becomes a single source component and therefore difficult to obtain . But because the speed control ler lends itself so well to a micro-based system, I have finall y managed to twist Leo's arm into a painful enough position for him to agree to let me do a second construction article at a later date on a more specialised unit. The good news is that as a result of the enormous amount of work put into analysing the prevailing speed contro l design philosophy, we have come up with a very interesting new wrin kle to the design approach, again using readily available corn: ponents. Prototypes are currently under construction for testi ng and more will follow shortly, so stay tuned . up to $2000 and multi-cylinder engines and the n ew gas turbines exceeding even that figure. Prices of this order were unheard of in th_e late 1960s but th ey do give the manufacturers a chan ce to recover costs so that they can develop .newer and even more specialised units. The two categories may be broken down again into three sub-categories: Diesel, Glow Plug and Spark Ignition. Each type has its own advantages and disadvantages and each has its own particular set of design rules, all of which are too complex for a full analysis in a column such as this. There are , however, many books covering engine theory and operation for those interested in this fi eld. The names "4 -s tro ke" and " 2stroke" derive from the number of basic operations required to complete one cycle of operation. In a 4~stroke motor, there are inlet and exhaust valves to enable the fuel mixture to get into the cylinder and the exhaust to get out. In a 2-stroke motor, by contrast, there are no valves and the crankcase is used to transfer the mixture into the cylinder and to scavenge SOLDER HEATSHRINK TUBING Big savings on quality 60/40 1in/Lead, resin-cored solder. Perteet tor electronics. Long lasting 1/2 kilogram rolls. Diameter 0.56mm was $23.50 NOW $14.50 0.71mm was $22.88 NOW $13.50 1.0mm was $21.00 NOW $11.50 16mm was $19.25 NOW $12.00 Upon application of heat, this tubing shrinks to approximately half normal size· and our prices have shrunk to approximately half normal size!! Per metre: , 1.6mm was $1.95 NOW $1.20 2.5mm was $2.95 NOW $1.50 3.5mm was $3.45 NOW $1.BO 4.8mm was $3.65 NOW $1.90 6.4mm was $4.75 NOW $2.40 9.5mm was $5.25 NOW $2.70 12.7mm was $5.85 NOW $3.00 OR - try one of our handy packs of assorted sizes. Only $6.50. 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Great for energy storage for PanciJ1t 94x2.4mm Cab~ ties. memories, etc. Black or white. $1.50 per 100 were $2.52 NOW50C CATALOGUE APOLOGY Our sincere apologies to all our customers awaiting copies of our catalogue We are having to endure a systems error at our typesetter He 1s 1nstalhng a new system and production should be finished shortly MAJOR CREDIT CARDS/CHEQUES WELCOME Orders under $50 · $4,00 P&P - Orders over $50 · $6.00 P&P ALL ELECTRONIC COMPONENTS 11 8· 1ZZ Lonsdale Street, Melbourne Vic. 3000 Telephone: (03) 662 3506 Fax: (03) 663 3822 A UGU ST 1992 59 Protect your valuable issues MODEL AIRCRAFT ENGINES - CTD Silicon Chip Binders the exhaust products out. The all important factor in any engine is the timing of these operations and much energy has been expended on developing better methods of timing. In keeping with all technological achievements, the early beginnings were very quaint and somewhat primitive but the fundamentals were there. Glow plug motors These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold uo to 14 issues & will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l r----------1 Enclosed is my cheque/money order for $_ _ _ or please debit my II I I II O Bankcard O Visa O Mastercard Card No: Card Expiry Date __/__ Signature _ _ _ _ _ __ _ __ Name _ _ _ _ _ _ _ _ __ _ II Address- - - - I _______ P/code_ __ 1,, _ _ _ _ _ _ _ _ _ _ _ .. 60 SILICON CHIP The invention of the modern glow plug model motor is attributed to an American engine enthusiast, Ray Arden, and the idea swept the world, pushing the diesel and spark ignition motors into the background. Here it seems was a great new idea, yet the Wright Brothers flew at Kittyhawk with a glow motor of their own design. The old adage of "there is nothing new under the Sun" still holds it appears. The method of generating the ignition is the next most important factor. Diesel motors use a self-ignition process which relies on the heat generated in the compression process to raise the fuel/air mixture to flash point. Diesel motors used in model work have a sub-piston fitted above the piston and a screw on top of the cylinder head to adjl.).st the compression ratio whilst the motor is running. The compression is reduced for starting and then increased for maximum revs. They are very difficult to throttle effectively and do not run well at low speeds. Fuel is commonly a mixture of ether, kerosene and castor oil. The piston/cylinder seal must be good. Glow plug ignition is again a selfignition process, in which a glow plug which consists of a wire element, usually made of platinum, supplies the ignition. To start this type of motor, the plug is raised to bright red heat by connecting a battery across the element. Once the motor begins to run, the platinum wire element will continue to glow under the heat from compression and the chemical effects of the methanol fuel. At this point, the battery is disconnected and the motor will continue to run. As in the diesel motor, the piston/cylinder seal must be good which these days means piston rings. · If there is a noticeable drop in en- gine revs when the battery is removed, the plug is "too cold" and a hotter plug must be fitted. What this means is that a more active element must be fitted for that compression ratio. The art of motor tuning is very subtle and such things as plugs, porting and fuels make a very big difference to engine performance. These motors throttle extremely well and are very popular in R/C work as a result. The fuel is usually methanol and castor oil with extra oxygen supplied sometimes in the form of nitro-methane. Glow fuels make excellent paint strippers and many paint jobs have been destroyed completely because the model was not fuel proofed or the incorrect paint was used in the first place. Spark ignition And now to get to the question which started this article. Spark ignition is the oldest form of model engine, the first engines being petrol powered "gas jobs". And very quaint they were. The compression ratio in these motors is usually much lower and the spark is supplied by a true spark plug as used in cars. This plug is connected to a coil, condenser and a set of points and the spark ignites the fuel -air mixture. Because of the lower compression ratio, the piston/cylinder seal does not need to be as good as for glow plug motors. A good bright spark cures a lot of the ills encountered in model motors, such as worn pistons and cylinders. The early motors did not throttle well, however, and a 2-speed set of points was often fitted, to provide spark advancing and retarding. Modern spark ignition motors are now fitted with electronic engine management systems and many of the old ills are now cured. The fuels used are most commonly petrol and mineral oil but methanol-based fuels can be used. Once again though, the dreaded space restriction brings us to the end of this month's column and we must await next month's issue for a more detailed analysis of the ins and outs of modern spark ignition and 4-stroke engines. SC