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RADIO CONTROL BY BOB YOUNG Jet engines in model aircraft This is the first in a series of articles covering the technical aspects of jet engines for model aircraft. In this coming series we will look at engine theory, engine management systems and fuel theory. For too long, modellers in general have been locked out of modelling modern jet aircraft due to the lack of a suitable power plant. Today however, we stand at the dawn of a new and tremen­dously exciting era in R/C modelling with the recent introduction of the pure turbine engine. In my opinion, the jet engine will do for R/C modelling what proportional control did back in the early 1960s. In the 1960s, we made do with reeds which did the job re­ markably well and it would have been difficult for a bystander in those days to tell the difference between a well-flown reed system and a proportional system. However, this was more to do with the skill of the pilot than an attribute of the R/C system. Reeds really were the sort of system that only the truly dedicat­ed modeller could warm to. There is an exact parallel today with the ducted fan model and pure turbine. The ducted fan model has been developed to a remarkably high This beautifully finished model of an F-20 Tigershark was built by Brett Davies. It is powered by an OS91 engined fitted with a Ramtec fan. It is 1.82 metres long and weighs just on 6kg. 70  Silicon Chip level and performs exceptionally well in the correct hands. But at no time can you ever forget that you are watching a model powered by a piston engine. In fact it is abso­lutely impossible to forget that fact for the simple reason that a ducted fan sounds like it is powered by the controlled fury of one thousand caged banshees. After eight hours of sharing the pits with this incredible din, one’s ears are begging for mercy. Again, the ducted fan system is the sort of system that only an absolutely dedicated modeller could develop a liking for. Turbine engines How delightful it is then to hear the soft pop of a turbine igniting and the gentle whine, or more correctly, whooshing of the turbine as the pilot runs it up prior to takeoff. And the differences do not end there. In flight the turbine pushes the model effortlessly and quietly (75dB) with that characteristic rumble that jets develop at a distance. Most jet model pilots only run their engines at about 60% power as the thrust on some model turbines is absolutely staggering. The overall effect is to produce a flight with a rock solid, very smooth and realistic sounding characteristic. In contrast the ducted fan model in flight is constantly screeching out a reminder that inside this machine is a very large reciprocating racing engine, worked up to the nth degree and being pushed to its limit at all times. Whilst there is little difference in the measured speed of both systems (at the moment, that is), the turbine engine produces an infinitely superior result. By now you may have noticed, I am hooked on the turbine powered model, especially now that kerosene is replacing propane gas. There is of course one proviso in all of this and that is the cost. The turbine at the moment is ferociously expensive ($5000-$10,000) and my bank manager was decidedly guarded in his response to my request for a loan of that magnitude, especially for an item that may disappear in a mushroom cloud at any moment! Be that as it may, progress will follow rapidly now that the initial breakthrough has been made and prices will fall as more manufacturers enter the field and volumes and production techniques improve accordingly. One other drawback will also succumb to the relentless march of progress and that is the question of fuel consumption. Turbines are notoriously thirsty and a typical fuel load current­ly is around 1.5 - 2kg for a 15-minute flight. So how do these wonderful gadgets work and why has it taken so long for the turbine to finally make its appearance on the model scene? This Mirage was built from a Jet Hobbies Hanger kit. It is powered by a Golden West Models FD-3/67LS turbine and controlled by a Silvertone transmitter. It has a wing span of 1.09m, length of 1.56m and a weight of 5kg. It carries 1.75kg of kerosene. Brief history In the “Aeromodeller” annual published in 1954 there appeared the most wonderful article on turbine-powered jets. The author, Mr W. Ball, claimed he had flown turbine-powered deltas in England as early as 1947 and gave details of some of his early flights. The lead photo in the article (p87) showed the author proudly posing beside a very modern looking delta model with his ground based transmitter at his side. Page 88 showed a cutaway drawing of a turbine engine featuring a 3-stage axial compressor with annular combustion chamber and a single stage turbine. The figures quoted are interesting and we will come back to these shortly – length 28 inches (711.2mm), diameter 6.5 inches (165mm), weight 3lb (1.36kg), static thrust 10.8lb at 26,000 rpm. The article went on to give scanty details of high speed flight (100 mph) with rudder and trimmable wing tips combined with 3-speed motor control. Sadly, in common with a lot of preco­ cious inventors, he suffered a terrible loss in the form of floods which swept away his entire workshop (and all evidence of his experiments). Nowadays they usually have a fire in the work­shop, a visit from the oil companies, the CIA or even the “men Chris Mounkley built this Star Jet which is powered by a JPX-260 turbine. Note the maze of wiring in cockpit. in black”. Thus at the time of writing he was only flying a ducted fan delta which could be adapted to take a turbine “if required”. Did it exist? So did this motor ever exist and did those models fly? Interestingly enough, I never forgot those articles for they had stirred my imagination and that of my friends and despite an intense search we could never find any evidence of those models being flown with turbines. Ever hopeful, I even asked David Boddington about this article on his recent visit to Australia but he could never find any evidence either. Today the consensus is that the whole thing was a fabulous hoax. Upon re-reading that article for this column, I even discovered one of the photos of the delta in “flight” was upside down. But we were young and we lapped it up for it articulated the dream. And anyway, who could ever January 1998  71 Kevin Dodds of Tingalpa, Qld built this semi-scale A-10 “wart hog”. Powered by a JPX-T-240 turbine, the model weighs 7kg empty and 8.5kg fuelled. Maximum engine speed is 122,000 rpm! system for its safe operation. Finally however, somewhere around the late 1980s, model turbines began to make their appearance on flying fields. Kurt Schreckling is credited with being the first person to construct very small, lightweight turbines using amateur means. To date there is no evidence to suggest that an axial flow turbine could run successfully at model sizes even today and all successful engines so far have used centrifugal compressors. This results in a shorter, more rotund engine than the axial flow engine but still of practical size. Kurt Schreck-ling’s motor was 235mm long, 110mm in diameter, 1.14kg in weight and produced around 30 Newtons of thrust (about 8lb) at approximately 100,000 rpm. At this thrust these engines will push models along at more than 320km/h. Compare this data to that of the Ball engine. Did those motors exist? I genuinely doubt it, especially when you consider that ceramic bearings give the best results at the RPM encountered in these engines. Having suggested that turbine engines would make a good series for SILICON CHIP, Leo Simpson sent me off to Leeton (the premier jet gathering in Australia) to gather first-hand data for the series to follow. So let us look now at what I found there. Leeton 1997 This is a closer view of the A-10 engine installation. The amount of plumbing in these models is amazing. doubt such an eminent authority as “Aeromodeller” magazine? The dream took a very long time to become a reality however and proved to be a fearsome task, taking even longer than the model helicopter to master. The engineering and metallurgy are quite demanding and the major difficulty facing the manufacturers of these engines is in matching components in one engine. Quite often motors will not run successfully until all components are correctly matched and that is 72  Silicon Chip with components manufactured with modern machine tools. RPM can be down, tailpipe temperatures up and in the worst case, the turbine can drip out onto the tarmac if local hot spots develop. An even distribution of temperature inside the engine was one of the major difficul­ties and can still cause problems. We will examine these points in detail in coming articles. More importantly, the successful engine relied upon a very sophisticated electronic engine management The Leeton Jet fly-in, hosted by the Leeton (NSW) Model Aircraft Club, is the longest running jet event in Australia and attracts fliers from all over Australia. Due to the increasing popularity of jet aircraft there are now many such events being staged in other localities and as a result numbers were down at Leeton this year. But Leeton was the first and is still consid­ered by many as the premier event. Certainly there was no lack of enthusiasm and the standard of models present staggered me – from electric ducted fans to swing-wing F111s fitted with turbines, they were all there. Fliers from as far afield as WA and Queensland were present in numbers and the sky was never clear of these daring young men and their flying machines. Basically the models are now divided into two classes, the older ducted fan system and the newer turbine engines. As the name suggests, a Starting a turbine Whilst starting a ducted fan model is a fairly laid back affair, starting a turbine takes on a more serious air. Compressed air is used to spin the compressor up to speed prior to ignition. This usually comes from a blower or compressed air bottles, while a helper stands by with a fire extinguisher. The propane gas used as fuel in the early turbines does present some element of risk and caution is the order of the day. The more modern turbines are gradually changing across to liquid fuels and this is where the future lies. Once started, the turbine settles down to be just like other motors, with throttle control providing a complete range of thrust from idle to full power at will. In flight, the turbine-powered model presents a glorious sight and sound. The dream has finally become a reality and whilst Ball may have taken some poetic licence in his presentation of the facts, he provided the spur for SC it to finally become a reality. SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM PRICE ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏  3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my ❏ Bankcard   ❏  Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________  Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Street ___________________________________________________________ Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). ✂ “ducted fan” is a system whereby a reciprocating engine, usually a very highly developed racing motor, is used to drive a fan inside a close fitting, carefully designed duct. The ducted fan is still the predominant system and these were present in great numbers at Leeton. Turbines were not as well represented but there were at least six or seven in attendance. A striking feature of the models at Leeton was the amazing internal complex­ ity. There were tubes, pipes and cables in vast numbers. It has taken a long time from Charlie Peake’s .15 powered, catapult-launch­ ed “Screaming Mimi” delta in the early 1960s to the .91 powered missiles of today but the ducted fan system has finally come of age. Capable of speeds in excess of 320km/h, these models are impressive performers indeed. Usually fitted with retractable under­ carriages, these models can take off with­out the assistance of the catapults that were used in the early days of ducted fans. Several examples of ducted fan models are shown in the accompanying photos and externally there is nothing to suggest any difference between the turbine and the ducted fan models. It is not until the motor is started that the real difference is apparent. January 1998  73