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RADIO CONTROL BY BOB YOUNG Jet engines in model aircraft; Pt.2 While the jet-powered model has been like the “Holy Grail” to aircraft modellers, there have been intractable prob­lems to solve in scaling down the jet engine to make it fit into typical model aircraft. This month we look at the fundamental principles governing the design of jet engines for model applica­tions. Jet propulsion of a body such as an aircraft is quite simply explained. The propulsive force is developed in reaction to the ejection of a high-speed jet of gas. In other words, it is action and reaction. The action is to squirt a lot of gas out at high velocity and the reaction is that the aircraft zooms off into the distance. The jet-driven turbine or turbojet, consists of four basic parts: compressor, combustion chambers, turbine and propelling nozzles. Fuel is burnt in the combustion chamber, after being mixed with air coming from the compressor. The combustion process generates expanding gases which spin the rotor of the turbine. The shaft of the turbine is connected directly to the axis of the compressor so the turbine drives the compressor. After passing through the turbine, the gas is exhausted to the at­mosphere at high speed through a nozzle. In the propeller-driven turbine or turboprop, the turbine not only drives the compressor but also drives a normal pro­peller. A ramjet engine relies on its own forward motion to com­press the air that enters it. The Turboprop and ramjet have no model equivalents and thus will not feature in this series. Fig.1 shows the basic layout of a typical jet engine. The development of jet engines for use in models has proven to be a very difficult task, largely because of “scale effect”. Briefly, there are two separate Fig.1: this shows the basic layout of a typical jet engine. Air enters the compressor at left and is mixed with fuel which is burnt in the combustion chamber. The expanding waste gases then drive the turbine before being exhausted. The turbine is directly connected to the compressor. 80  Silicon Chip Fig.2: these are the three basic forms of jet engine compressor. Because of its lesser sensitivity to scale effect, the radial compressor is most suited for use in jet engines for models. problems relating to scale ef­fect. First, we have the problem of machining tolerances. For example, as the compressor and turbine are reduced in size, the gap between the rotor and its housing becomes more significant when expressed in terms of a percentage of air leaking past the compressor/turbine relative to the volume flowing through the compressor/turbine. Compounding this are the problems of metal­lurgy and expansion due to heat. Second, we have the problem of the loss of aerodynamic efficiency as the compressor/turbine blades are reduced in size. The engine designer would refer to the latter problem as “diffi­culties with Reynolds numbers”. In plain English, this simply means that as the size of a wing, propeller or turbine blade moves closer to the size of air molecules, the laws of aero­ dynam­ics start to break down. Now of all of the modern propulsion units, the jet engine is perhaps the most reliant upon aerodynamic theory for its successful operation. We have all heard that in theory the bumblebee should not be able to fly. Among the reasons that aerody­ namicists would give for this, Reynolds number is high on the list. Without going too deeply into the complex mathematics of Reynolds numbers with their strange units (slugs), it is suffi­cient to state for this series of articles that the Reynolds number is given by the formula: R = Density x Velocity x Size/Viscosity. The higher the Reynolds number, Built by Chris Patterson of Brisbane, this superb 1/7th scale F18 carries the colours of 75 squadron of Williamstown, NSW. It is powered by two OS91 motors driving a Ramtec fan unit. The model has a length of 2.49 metres and a wingspan of 1.82 metres. the greater the efficiency. Reynolds numbers for full size flight vary from about 2,000,000 for small slow speed aircraft up to about 20,000,000 for large high speed aircraft. Combine this with the fact that lift increases with the square of the velocity and the large high speed aircraft becomes very efficient indeed. This is largely the reason that a modern jet fighter can carry much the same load as a World War II bomber Thus it is quite clear that as size decreases and the velocity falls to model speeds, the Reynolds number falls away rapidly and the efficiency of any aerodynamic device tumbles. By the time we arrive at turbine blades of a size suitable for model engines, efficiency is very low indeed. As a result, the design of successful turbines for models has centred around components which are the least sensitive to scale effect. This has lead to the almost universal adoption of the centrifugal compressor for model aircraft jet engines. Fig.2 shows the three basic forms of jet engine compressor in order of common full size usage. Fig.2(a) shows the axial compressor, Fig.2(b) February 1998  81 Fig.3: gap losses increase as the gap between a compressor and its housing are increased. These effects are magnified in jet engines for model use. shows the centrifugal (or radial) compressor and Fig.2(c) shows the diagonal compressor. Early full size jet engines tended to favour the centrifu­gal compressor for a variety of reasons but the resulting engine is shorter and greater in dia­ meter than the axial flow type and thus not the ideal shape to fit into a slender fuselage or engine nacelle. However, for model size engines the centrifugal (radial) compressor is the ideal choice. Once again we must consider scale effect in the choice of the compressor. Referring back to Fig 2, note the gap between the tips of the compressor blades and the housing in the axial and diagonal compressors. No matter how tight the machining tolerances, there will always be some leakage between the blade tip and the housing. Fig.3 shows gap losses at various gap widths. Now look at the situation for the centrifugal compressor. By the very nature of the design all of the air is thrown off the tip into the collector (diffuser) ring. True, there will be some leakage past the compressor face but that is more than made up for by the much larger size (higher Reynolds number) of the centrifugal compressor blades. Also it is possible to curve the blades as in Fig.5 or even fit a cover plate, reducing leakage losses even further. By virtue of these facts the model engine designer has almost been forced into using the centrifugal com­pressor. However, this choice is not as one-sided as it would first appear. There are other good reasons why a radial compressor is a wise choice for a model jet engine. As we have already noted, the Reynolds numbers are higher and the tip losses are less. In addition, they are easier to construct, are much more robust and therefore more reliable in operation. Constructing a model size axial compressor with its rows of tiny compressor and diffuser blades would be a very difficult and tedious task. Then there is the problem of anchoring the blades solidly enough to withstand speeds in excess of 100,000 rpm and possible ingestion of foreign matter. What must be borne in mind at all times is the very high rotation speeds encountered in these engines. Shaft speeds in excess of 100,000 rpm are routine in model size turbines. When combined with very high temperatures there is a real danger of compressor or turbine failure and this must be guarded against at all times. Fig.4 shows the typical operating conditions for a model jet engine. There is also a more subtle consideration to the radial compressor and we will deal with this shortly. The downside of the radial compressor Fig.4: typical operating conditions for a model jet engine. Note also that the engine may be rotating at up 100,000 rpm! 82  Silicon Chip Fig.5: leakage effects in a radial (centrifugal) compressor can be minimised by various curvatures of the blades or by fitting a cover plate. SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. P.C.B. Makers ! • • • • is the more rotund appearance of the com­pleted motor. It is nowhere near as slender as the axial flow engine. Notwithstanding this, the final size of a successful centrifugal compressor type of engine is well within the limits available in a reasonable size modern jet fighter model. Automotive turbo chargers All that aside, the most important factor in the choice of centrifugal compressors in model engines is the fact that turbo superchargers for cars use radial compressors which are an ideal size for model work. Now automotive turbo superchargers are very highly devel­oped devices. Just what drove the turbo designers to radial compressors is not known but the preceding considerations prob­ably played a large part in the development of these devices. Whatever the reasons, the automotive turbocharger provided a perfect jumping off point for early experimenters and radial automotive turbo-compressors found their way into many an experi­mental model jet engine. As supplied, turbocharger compressors are accurately dynamically balanced, a very important point. They achieve efficiencies of between 70 and 80%, depending upon their size; the larger the compressor, the higher the efficiency. The radial compressor can be built in many configurations, all with widely differing characteristics. First­ ly, there is the matter of cover plate or no cover plate, the former being known as an “enclosed wheel” compressor. • • • • • If you need: P.C.B. High Speed Drill P.C.B. Guillotine P.C.B. Material – Negative or Positive acting Light Box – Single or Double Sided – Large or Small Etch Tank – Bubble or Circulating – Large or Small U.V. Sensitive film for Negatives Electronic Components and Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 • ALL MAJOR CREDIT CARDS ACCEPTED February 1998  83 Fig.6: under certain conditions the airflow from the compressor can collapse, leaving a lower pressure at the compressor than inside the engine. A reverse flow of air begins which continues until the internal pressure falls below that of the compressor. This cycling effect can destroy the engine. Secondly, there is the matter of blade curvature. Fig.5 shows radial compressors with various configurations. Fig.5(a) shows radial tipped blades, Fig.5(b) shows slightly retro-curved blades and Fig.5(c) shows an enclosed wheel with highly retro-curved blades. Throttle response Experiments have shown that the compressor with retro curved blades is more efficient overall than the straight blade compressor. However, more subtle effects of blade curvature are to be found in the very important feature of throttle response. In aircraft work, it is imperative that throttle response be as close to instantaneous as possible. The Me262 was very vulnerable during landing and takeoff due to poor throttle response and allied airmen exploited this weakness to the full. Kurt Schreck­ling’s FD 3 84  Silicon Chip model engine uses a retro angle of 45 degrees and responds to the throttle almost as quickly as a well-adjusted piston engine. I could write an entire chapter on throttle response, throughput and blade curvature as it really is at the very heart of the jet engine and it is here that we encounter the dreaded surge line. The “surge limit” of a compressor refers to the tendency to supply the working medium cyclically instead of constantly. This may sound a little innocuous but to the full size aviator it is viewed with considerable alarm, since the usual result is damage to the engine which may progress to the very serious. In model size engines the effects are not as dramatic but the compressor can still be damaged if the surge limit is exceed­ ed. To simplify an exceedingly difficult subject, compressor surge is often the result of mismatched components at the design stage, particularly too small a turbine which restricts the airflow through the engine. Under certain conditions the airflow from the compressor can collapse, leaving a lower pressure at the compressor than inside the engine. A reverse flow of air begins which continues until the internal pressure falls below that of the compressor and the compressor begins to deliver air again – see Fig.6. In a model jet engine, the cycles follow on so quickly that all you hear is a loud unmistakable growling sound. If this occurs, then you need to close the throttle immediately for the condition will not clear itself and the end result is overheating and engine damage. Once the air leaves the compressor it passes through a diffuser which straightens the flow and slows the air in order to raise the pressure in accordance with Bernoulli’s Theorem. In the streamline flow of an ideal fluid – ie, one which is not viscous – the sum of the Energy of Position (Potential Energy) plus the Energy of Motion (Kinetic Energy) plus the Pressure Energy will remain constant. In other words, the residual speed energy of the air is converted into pressure energy inside the diffuser. In this case the energy of the gas is proportional to the square of its speed. Therefore if we can halve the gas speed we have already converted three-quarters of its energy. It is here that the radial tipped compressor blades vary from the retro curved blades. The radial tipped blades use the diffuser to raise the pressure whereas the retro-curved compres­sor begins the process inside the compressor itself. Thus the losses are higher in the radial compressor. Once the air passes through the diffuser it enters the combustion chamber and then the hard part begins. Burning the fuel/air mix evenly and efficiently, avoiding overly long flames which result in localised hot spots on the turbine, and preventing raw fuel pooling in the engine or running out onto the tarmac are all very difficult tricks to master. A model jet belching a metre-long flame may look spectacu­lar but it ain’t gonna last long! Next month, we’ll talk about taming the combustion chamber and SC turbine.