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RADIO CONTROL BY BOB YOUNG Jet engines in model aircraft; Pt.4 This month we will look at the turbine, shaft and tail cone of a model jet engine and discuss an Australian-made turbine designed for home construction. I am absolutely fascinated each month by the uncertainty of outcome which each column will have due to factors outside my control. Reader feedback takes some really interesting turns and can lead to all sorts of unforeseen results. The Mk.22 transmitter series was a classic in this regard and the Speed1B controller even more so. The Mk.22 system just kept growing and developing due to reader demands. Just recently, I have put a programmable AM-FM transmitter module (a world first to my knowledge) into production. It came about solely as a result of reader feedback. The Speed1B speed control module continues to amaze me, even though it was done nearly seven years ago and is now quite old by electronic standards. The latest adventure for that little device is to power full-size electric bicycles in Asia. The same thing is now happening with the gas turbine ser­ies. As a result of reader feedback, I learned of an Australian turbine for the home This is the turbine end of the shaft in Ken Jack’s motor. Note that the blades have been profiled in a definite aerodynamic shape. constructor, designed and developed by Ken Jack, a very long time modeller and a professional pattern and model maker by trade. Ken has spent a considerable amount of time and effort in developing this engine and has arranged for an associate to make the parts available. One of the photos in this article shows the major component groups of one of Ken Jack’s motors. In the foreground is the shaft with turbine and compressor fitted. Immediately behind is the inlet, diffuser combustion chamber, nozzle guide vanes (NGV) and tail cone. In the background is the outer hous­ing. Another photo clearly shows the turbine with the blades profiled in a definite aerodynamic shape. A very complex machin­ing operation is need to achieve this. On a different note, Fig.1 shows an exploded view of the Golden West Models FD/67 turbine which is available fully assem­bled and tested from Klaus Breitkreutz, in Sydney. This is a popular American engine which runs on kerosene. It is the engine in the Mirage featured in the January 1998 issue of SILICON CHIP. Excitement is mounting in modelling cir­cles in regards to turbines and all that remains is for the price to fall to a more accessible level. Turbine stage Now to get back to the subject under discussion, last month we looked at the combustion chamber of the model jet engine. Following the combustion chamber is the turbine stage. This works in exactly the opposite manner to the compressor. Its purpose is to extract work from the hot exhaust gas from the combustion chamber and reduce it to rotational kinetic 70  Silicon Chip Fig.1: an exploded view of the Golden West Models FD/67 jet engine. This American engine runs on kerosene.   1   2   3    4    5   6    7    8  Front cover Compressor Diffuser Shaft support Bearing bushing Shaft Inner combustion chamber Fuel vaporiser energy. This rota­tional kinetic energy is then transferred via the shaft to the compressor. The turbine stage consists of fixed nozzle guide vanes (NGV) and a rotor. The gases from the combustion chamber flow through the turbine’s NGVs where the blade ducts act like small jets, accelerating the gases in the direction of turbine rota­tion. At the same time, the gases expand. As pressure and temper­ature fall, the speed rises rapidly, reaching about 1620km/h, even in model engines. Once again we encounter these phenomenal operating condi­ tions, all of which have served to place the model turbine out­side the realms of possibility until recent times. The photo of Ken Jack’s jet engine shows quite clearly the complex shape   9  10  11  12  13  14  15  16  Outer combustion chamber Outer housing Turbine wheel Exhaust nozzle Ball bearings (steel) Heavy-duty E-ring M4 flat washer M4 hex nut of the connecting shaft between the turbine and compressor. This shaft is subject to severe dynamic bending stresses as it approaches critical rotational speed. If there is even a minute imbalance in the system, then as the rotational frequency approaches the resonant frequency of the shaft, oscil­ lations may set in and the shaft may be completely destroyed or at the very least, bent permanently out of shape. Worse still, the turbine blades may come into contact with the outer casing, with severe damage the certain result. What must be borne in mind at all times when dealing with a jet turbine is that it spins at about 120,000 rpm while subject to very high temperatures. Any imbalance, casting or machining flaws can lead to a catastrophic 17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  M4 hex nut, centred Pitot tube Nipple gasket ring M4 x 25mm set screw M3 x 8mm shcs Shim spacer M4 x 10mm shcs Oil feed tube assembly Bearing preload spring Combustion chamber spring Exhaust nozzle spring Tachometer assembly Ext. retaining spring EGT sensor assembly M4 Nylock nut failure which could result in a blade penetrating the outer casing and causing injury to bystanders. For this reason, the golden rule of rotating engines ap­ plies with a vengeance. Do not stand in line with the propeller or any rotating parts, which in this case are the compressor and turbine. And while we are on this subject, this is one of the nice things about operating model jets. There is no whirling propeller to stick your hand into; a very common cause of injury to model flyers. One very prominent modeller recently lost his thumb in a ducted fan, so even these propulsive units are not without their dangers. Care is the order of the day in all modelling activi­ ties, especially when dealing with high-powered motors of any kind. April 1998  71 Fig.2. this diagram shows the typical exhaust temperatures behind the engine. If the engine is not carefully mounted it can easily set fire to the tailplane. Careful design of the outer casing of the model turbine renders these devices relatively safe from blade failure. The diffuser shrouds the compressor and the NGV housing can be ex­tended back to double the thickness of the casing shrouding the turbine. But the golden rule should still apply. Do not let people stand in the plane of rotation. Another problem in regard to the turbine is that machining tolerances must be tight. This is to minimise air bleed past the turbine blade tips; excess air bleed greatly reduces the effi­ciency of the turbine. When you consider the temperature, rpm, metal creep and expansion, combined with the bending and flexing of the main shaft, this becomes a major compromise. Once the gases have left the turbine, they are relatively free of swirl and with little energy left to convert into thrust. For this reason the design of the tail cone is extremely import­ant in a model engine. The correct design can result in an in­crease in thrust of 15-20%, a worthwhile improvement to chase after. So there is much to look forward to in the develop­ment of the model turbine. Thrust will go up, fuel consumption will go down, the size and weight of the engines will be reduced, and their reliability increased. Yet over all this development hangs the spectre of a model axial flow engine making its appearance. This will indeed revolutionise the fitting of turbines into slender airframes with the consequent increase in flying speeds. One wonders where it will all end. We certainly do live in exciting times. Operating a gas turbine Fitting a gas turbine into a model 72  Silicon Chip aircraft is a completely novel experience for most modellers and there is much to learn. That pool of know­ledge regarding most modelling activities, available at the local model club, is not available to the pioneer turbine flyers so they will be very much on their own for some time. First of all, in place of a dangerous whirling propeller there is now a very hot exhaust to burn the unwary. More impor­tantly, it can burn the model as well. The diagram of Fig.2 shows the typical exhaust temperatures behind the engine. Fortunately, there are simple fixes for these problems. The most simple is to mount the motor outside the fuse­ lage, as on the A-10 Warthog shown in the January 1998 issue of SILICON CHIP. This is the recommended installation for your first jet-powered model. This type of installation places no demands on your knowl­edge of intake and tailpipe aerodynamics and provides easy access to the engine for servicing and adjustment. And it presents the least fire hazard during starting. Burying the engine inside the fuselage introduces a myriad of problems and is best undertaken after you have made yourself comfortable with the vast differences between operating a jet engine against a normal motor or ducted fan. Once the engine is buried inside the fuselage, internal aerodynamics become almost as important as the external aerody­ n amics. To begin with, the air intake should act as a diffuser, slowing the incoming air and increasing the pressure in front of the compressor. This establishes a dynamic pressure in the model fuselage which varies with the square of the model’s speed. At the same time, the energy of the inflow air is dimin­ished, thus reducing the effect of internal fittings. Provided these fittings do not reduce the cross section to any great extent, they will not have an undue effect on engine performance. The ideal intake has gently rounded intake lips and a ven­ turi-type duct with the sides widening and opening out as they approach the engine intake, at an angle of no more than 10 de­grees. The size of the air intake can be much smaller than for a ducted fan without loss of thrust and should be matched to the maximum speed of the model for maximum pressure transfer. Running a duct directly to the motor is of no value. Most important is the locking down of all nuts and screws in the intake area. A single nut or screw going into the motor could completely ruin the internals. Likewise, dirt and rubbish must be very carefully removed after a rough landing. Small tools and especially rags and papers must not be left in front of the model. These things work like a giant vacuum cleaner and anything left in front of the model will immediately fly into the compres­sor, so you have been warned. Cooling the fuselage The engine itself presents few problems as it stays rela­tively cool. The compressor area runs at around 120°C and up to about 200°C at the rear end. The only parts which become extremely hot are the turbine enclosure, mounting flange and the exhaust cone. The greatest problem is ducting the exhaust gas out of the tailpipe whilst minimising the duct losses. A thrust pipe which acts as an injector is the best solution here. This type of duct draws in cooling air and increases the total throughput of gases, thereby increasing thrust as well as cooling and protecting the tailpipe. The increased throughput must be calculated into the air intake which will need a correspondingly larger cross-sectional area. As I said before, having the engine out in the open places no demands upon your knowledge of duct aerodynamics. It’s not as pretty to be sure, but is a lot easier for your first model. Balsawood is very susceptible to hot exhaust gases as the wood contains plenty of oxygen. An imperceptible glow is quickly fanned into life when you open the throttle and it spreads This is a very exciting development in the use of jet-powered models: an Australian designed engine developed by Ken Jack. In the foreground is the shaft with turbine and compressor fitted. Immediately behind is the inlet, diffuser combustion chamber, nozzle guide vanes (NGV) and tail cone. In the back­ground is the outer housing. over the wood in long snaking lines. A few seconds at full throttle can be enough to have the tailplane engulfed in flames. Aluminium foil glued on with thinned white glue provides a good protective barrier against the less severe gases while thin aluminium sheet (0.3mm) can be reserved for the hotter areas. You can refer to the diagram of Fig.2 for a guide to the temperatures at various distances from the tail cone of the motor. Starting the gas turbine Starting a fully enclosed motor presents additional prob­ lems. The starting fan may not provide sufficient air to cool the ducting as well as start the motor. Flames coming out of the motor before it settles into normal operating revs and tempera­ture can very quickly raise the tailpipe ducts to red heat. Thus, two of the requisite items for jet starting operations are a very strong fan or air source (compressed air bottles) and a fire extinguisher. As soon as the engine is running, turbulence causes cooling air to be mixed into the exhaust stream and half a metre down­stream the temperature is low enough that it will not burn ply­wood. The hot core of the exhaust stream extends to a point approximately three times the diameter of the tail cone. I should make one more point while on the subject of hot exhaust gases: they can start grass fires. The strips used for jet operation often feature long brown strips of dead grass, so watch out. Ancillary equipment Unlike its piston-powered equivalent, the model gas turbine is not a self-contained unit. There are several support items which need to be mounted in the model for the unit to operate satisfactorily. Of these, the two most important are the fuel pump and oil reservoir. Most model turbines use a total-loss oil system where oil is either placed under pressure or pumped into the bearing shaft and the oil circulates through the bearings and out of the engine. Typical oil consumption can be as high as 5ml a minute but is usually lower on most motors. On early experimental jets the throttle drove the fuel pump and the supply of fuel determined the engine rpm. However, this is not very satisfactory and more sophisticated commercial engines such as the Golden West FD/67 use an engine control unit (ECU) which monitors exhaust gas temperature and RPM. The throttle channel is hooked directly into the ECU and special software algorithms compute the acceleration requirements of the turbine. The ECU then drives the fuel pump and monitors the safety aspects of the engine. If any parameters move outside the safe zone the engine is automatically shut down. The ECU is mounted in the aircraft. By now the reader should be aware of the high level of technology inside a gas turbine model and the precautions SC necessary to operate it. April 1998  73