Home Built Model Turbines (WWW - Asec.ir) [PDF]

Zitiervorschau

BY KURT SCHRECKLING

THE MODELLER'S WORLD s

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Home Built

MODEL TURBINES

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MODEL TURBINES

BY KURT SCHRECKLING

www.ASEC.ir

© Auflage 2004 by Verlag Fi.ir Technik und Handwerk

Posrfach 227"1, 76492 Baden-Haden

Translated from rhe original German by Nigel Price

© 2005 Traplet Publications Ltd

All rights reserved. All trademarks and registered names acknowledged. No part of this book may be copied, reproduced or transmitted in any form without the written consent of the Publishers. The information in this book is true to the best of our knowledge at the time of compilation. Recommendations are made without any guarnntee. implied or otherwise. on the part of the author or publisher, who also disclaim any liability incurred in connection with the use of data or specific information contained within this publication.

Published by Traplel Publications Limited 2005 Traplet House. Pendragon Close, Malvern, Worcestershire. WR14 lGA United Kingdom.

ISBN I 900371 37 5

Front Cover: "KJ66 engine designed by Kurt Schreckling and manufactured in kit.form hy Jesus Artes". Back Cover."KJ66 engine 11iewedfrom the rear".

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Printed by Wa Fai Graphic Arts Printing Co., Hong Kong

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Contents Foreword ..................................................................................................... ...... .... ............... ................................................... 9 Chapter I. Physical-Technical Principles of Mcxlel Jet Turbines ................ ......... .... ..... .... .. ............................................... 11 What are we talking ahout? ................................................................ .................... ....... ............. ...... ..... ............... ........ ... 11 How does a model jet turbine work? ....... ...... ........................................ ....... ........... ....................... ............................... 11 The most important physical parameters ............................................. .... ... ......... ........ ....................... .... ........ ............... 11 Suirahle fuels ........... ... ................. .... .............. ........... .. ........................ ....................................... ........ ................ ....... ..... ... 13 Description of components ..................................................... ...... .................................................................................. 13 The compressor stage ...................... ......... ......... ....................... ........... .... ................. ............................................. .... . 13 The turbine stage ............................................................................................. ......... ...................... ........ .................... 15 Hearings, counteracting resonance vibration and lubricating the rotor. ........... .. ..... ..... ... ........... ............................. 17 Combustion chamber and ignition ................ .............................. ............ ....... ........... .. .................. ............................ 20 The thrust nozzle ................................. .................................................. ................... ... ............................................... 21 Operational performance ............................................................................................. ... ........ ....................................... .22 Correlation of rotational velocity, air mass flow, compressor pressure, thrust and temperature .......................... 22 Reactions to changes in fuel flow 1 ............................................................................................................................. 24 Fuel consumption .. .............................. ................... .. .. ....................... ..... .................. ................... ...................... ......... 25 Influence of weather and altitudc ............................ ............................................................................... ................... 26 Correlation between thrust and flight velocity ................ ...... ........... ......................................................................... 27 Noise Jevelopment ......................................................... .................... .................. .......... .................... ........................ 27 Chapter 2. Necessary Accessories ..................................................................... ................. .. ................................................ 28 Different iypes of st:arrer.. ....................................................................................................................... .................. ....... 28 An essential: a fire extinguisher .............................. ................................ ......... ....... ............. .. .. ........ ....... .......... ... ........... 29 TI1e fuel pump .................................. ......................... .............................................. .............. .................. ........... ..... ........ 29 Fuel tank with feed lines .................................................................... ........ .. ............. .. ..... ........... ............ ...... ... ............... 30 Cartridge-fed auxiliary gas ............... ........................................................................................................ ....................... 30 Electrically-powered glow plug ........................................................... .... ... ........ .. ........... .... ...... ..... .... ...... ............. ......... 30 Calibrating of the restrict or for the supply of lubricant.. ............................. ................... ................................. ...... .... .... 30 Electronic regulation and control .................................................... ...... .... ................... ........ .... .. ... .... .................... ......... 30 Chapter 3. Test Stand and Measuring Equipment ............................. ............... ............ ..... ................. ......... ............ .... ....... 32 The engine on the test stand ................................................................................................................. ................. ........ 32 Measuring the 1hnist ......................................................................................... .... ....................... ....... .. .... .. ... ...... ............ 33 Mea.o;uring rotational velocity and pressure .................................................................... ...................... ... .............. ........ 33 Temperature measurement ..................................... ....................................................................... ............. .... ................ 34 Chapter 4. Which Turbine, Which Model? ........................ ....................... ................................ .................. ........ ................. 35 Chapter 5. Home-Built Jet Engines ............................. .......................... ...... ........................... .............. ................ ................ 40 Comparative technical dara .......................................................... .... ............... ................ .................... ...... ...................... 40 Kit version of the l:lehotec J-66 jet turhine ..................................... .............................................................. ............... ... 41 The construction kit - a detailed look .................... ................................ ....... ...................... ...... .................. .............. 41 General point-; on the constru(1ion of the KJ-66 and TK-50 .. ...................................................................................... +i Ha lancing the rotor ......... .................................................. .... ................................ ............................................. ........ .44 The home-built TK-50 jct turbine, made from a thermos flask ................................................................................... .44 How it evolved ............. .... ........... ................................................................. ... ......... .. .... .... .... .................................... 44 Construction requirementr guide system ......... ........ ........... ................... .... ..................... .... ......................... ............ ..... .... .. .............. 6 1~ Intake nozzle ................. .. ........................... ........................................................................... ...................................... 65 Compressor guide vanes ..... .. ........... ................... ......................................... ............................. .................. ...... ......... 65 Connecting the compressor guide vanes to the guide vane holder .......................................... ............ ..... ....... ...... 66 The lid ....... .................................................................................................................................................................. 66

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Aligning the intake nozzle with the compressor wheel and adjusting the size of the gap ... ............................... ..66 O,mbustion chamber. glow plug .... ..... ............... ..................... ........... ................ .... ...... ..... ............ ............ ... ............. 67 Supply lines for fuel, auxiliary gas and lubricant ......... .. ........... ............... ............. .... ................. ................ .............. 68 Exhaust gas nozzle ........... ..... ... ..... ....... ........ ... ......... .................. ... ........ ..................... ................. ........................ .......69 Calibrating the diameter of the turbine wheel .............. ...................................... .............................. ................ ........ 70 Connecting the exhaust gas nozzle to the housing ... .............................. .. ............................................................... 70 Final assemhly ...................................... ..................... .............................. ......... ................ .... ....... ...... .................... .. .... 70 Home-built KJ-66 jet turbine .......... ........... .......... ........... .. ......... ....... .... ........... ...... ........ ...... ............... ............ ............... .. 71 How it evolved .. .... ............................. ........ ... ....................... ..... ......... ................. ............... .............. ... ............. ...... .... 71 Demands on the home builder ..... ..................... ..... ...... ............... ..... ......... .. ................................ .............. ................ 72 Parts list and drawings ... ................. ... .......................... .. ........ .... ..................................... ...................................... ..........73 Constructing individual components .......... .. ...... ........... ... .......... ....... ... ...... .......... .... ........ ......... ......... ... ....... ..... ...... ....... 92 Rotor (I.I to l .9) ......... .... ............................. ........................ .................................................... ... ..... ....................... .... 92 Shaft tunnel (21 to 2.3) ..... ...... ..................... .......... ....... ............ .......................... .. .... ................ ........... ....... .. ............. 92 Intake nozzle and lid (3.1 to 3.3) ............................................................................. ................. ................................. 92 Compression guide system ... ................ .... ............ ................ .. ............. ............................................. ................ .......... 92 Combustion chamber (5.1 to 5.9. 6.1, 6.2) ....... ..................... ... .. ................ ..................... .......................................... 92 Fuel supply system (7.1 to 7.4) .. ........................................... .............. .... ................... ...................................... ... .......93 Auxiliary gas system, oil supply system (8.1 to 8.4. 9.1 to 9.3) ........... ............................. ....................................... 93 Turbine guide system (IO.I to 10.4) .......................... ........... ... ... .............. ... .. ....... ... .............. .... ............ .. ............... ... 93 Housing (11) ...... .. ............... ...... ........ ..... ................. ......................................... .......................... ......................... ... ..... 93 Flange A, flange B (12.1. 12.2) ... ................. ............... ................... ............ ... ................ .............. ............. ................... 93 Exhaust gas nozzle ... ......... .................... ... ........ .................... ........... .... .......... .. ............ ....... ... ........... ........ .. ...... ..... ..... 93 Final as.-;embly ................... ................................................ ........................................... .... ............ ... ............................ 94 Chapter 6. Important Safety Instructions ........................................ .................................. ............ ................................... .... 95 Danger of fire ....................... .......... .. .. ................. ............ ............. ....... .............. .......... ... ...... ....................................... 95 Danger of sucking in foreign ohjects ..... ...... .............. ............... ... .......... ... ............... ... .......................... .... ......... .. ..... 95 Danger due to exhaust gases ........... ................ ...... ........................... .. ........... .................................... ........................ 95 Danger of rotating parts .. ............ ........ ...................................... .............. ....................................... ............................ 95 Danger of insufficient proficiency in flying models .......... ... ..................................... ....... ............ .. ......... .... .... .... ..... 95 Chapter 7. What Does The Future Have in Store? ................... ............................ ................... ............................................ % Appendix ............... ...................................................... ... .... ...... ..... .. ........... .......... ..... ........... ............... .................... .. .. 98 Potential suppliers ............ ................ ............... ............... ................... .......... ................. .................. ............. ... ............ 98 Refe rences and sources of information ................ ............. ................ ... ....... ....... .................. .......... .. ......... ......... ..... .... .. .... 100 Books. Magazines ... ................. .............. ................ ..... ............... ...... ...... ..... ......... ............ ............ ..... ... ............... ...... 100 Organisations ...... ..... ...... .............. ........ ....... .......... ... .. ........... .............. ......... .. ......... ........... .. .................................... .......... . 10 I

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About The Author iri Schreckling was born in 1939. His first practic;d experience of model aircraft came at the age ,f five, when he transformed a tangled-up kite into an ;ieroplane. Over the years th:ll followed, not only did he build a great numher of model :,ircr:1ft, he also developed several of his own remote controllers. He was aged eleven when he first saw Vampire jet planes in the sky. That turned out to he a defining moment in his life. Study of a small encydopeclia gave him the belief that he could come up with a simple way of constn1cting a turbine. However. it took almost four decades before he finally started work in earnest at fulfilling his youthful dream. Today, people like tu refer to him as the "inventor", the ··pope of turbines•· or also as the "father of turbines ... None of that is really true. lnsteacl. he prefers to see himself as one in a long line of fathers of mcxld jct turbines . He received a basic technical education and went on to study physics at university. He then worked for 32 years as a qualified engineer for a large Rhine-hased chemical company. In his job he gained experience of different technical areas. none of which ever involved turbines. He was happy to rake early retirement in 1999. Having completed his hook about turbo props, he feared that he might have to retire as turbine developer as well. However. as it turned out. this was not the case. Ku1t Schreckling cannot deny his special love or good food. Indeed, flying mcxlel aircraft is not his only vice. He was also tempted into off-the-wall experiments with skis in the snow, exploring the effects of gravitational forces. l~p until now he h:1.~ managed to come through ii all unscathed.

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By Farshad at 6:32 pm, Oct 27, 2010

Foreword

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he inventor, Or. Hans Joachim Pabst von Ohain ( l911-19H8), develof)t:'J the first jet turbine which flew for the first time in August 19.W. I w:1s only three months old at the time and still in nappies. As far as I am concerned. the history of scale models of jet turbines began at the end of the 1980s. In 1988 I hegan putting some ideas into practicc that were to produce the first simply constrnctt:d model jet turbines capable of flight. I Sr wheel the air flows wi1hou1 swirling into the channels crea1ed by 1he compressor vanes and

leaves them. as imlkated above, with a high degree of swirl. Once in the compressnr guide system. the high velocity of 280 m/s is transformed as effectively as possihle into pressure. The photos show that several differenl designs of compressor guide system are possihle. A common feature they all share is first that the guide vanes are localed in the rotational plane of the wheel. The flow on 1he outside is redireclecl in the direction of the combustion chamber. To the best of my knowledge there has been no suhstan1iated analysis as to which design is 1he most efficienl. Un the basis that all the de:,igns shown here work effectively, it can be concluded thal the differences in terms of efficiency are negligibk· for our purposes. As a result, you are free to choose a design that is hest suited to the production facilities you have at your disposal. It follows, therefore, that the compressor guide sys1em for 1he KJ-66 and olher similar turhines cm be nwchined on a CNC lathe in one single piece. You can identify wedge-shaped primary vanes and curved St'.condary vanes. The Kamps design provides ;1 good solution for home builders. h has curved steel vanes which are fixed into a guide vane holder using :1dhesive. 111e primary and secondary vanes are cons1ructed in one piece. I 'nfortunatcly, this melhod has ils limitalions. When air is compressed, there is an inevitable rise in lemperature. This rapidly reduces 1he c:1pacity of the adhesive to withstand higher loads. As an ahernalive to fixing the guide vanes wilh adhesive, for example, you can use screws to secure them to 1he guide vane holder. Tests that I undertook wi1h the guide system for 1he design of my new TK-"i0 engine showed that the axial length of the secondary vanes is nol al all critical. You will find a detailed description or this system in the construction manual. Each closed channel that continuously expands has the effect of decelerating the flow. This leads to the intended increase in pressure. Such a system is called a diffuser. A diffuser lakes the energy of motion, or, to be more precise. the kinetic energy out of the air and transforms it inlo pressure energy. Accordingly, the cross-sectional shape of the channel is of lesser significance. In praclice the guide systems used can he understood as a

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ring of diffusers that capture the flow of the compressor wheel. It is imperative that the diffusers are arranged in such a way that their axes are aligned as closely as possible with the tlow of air from the compres:mr wheel. Otherwise, the result will he a loss of thrust, i.e . a reduction in efficiency. This implies less pressure, with some of the kinetic energy in the air being transformed instead into additional heat energy. In a diffuser. once the flow of air detaches itself from the wall ii begins to swirl. This tendency is greater, the larger the expansion angle of the diffuser. If you construct the diffuser to be particularly long and thin, then friction loss on the necessarily large wall surfaces additionally reduces efficiency. Consequently there's no sense at all in constructing particular!)' long extended diffusers. The efficiency of our compressor stage or, more to the point. the total efficiency of the compressor wheel and guide system, is between 65 and 75%. This value is also dependent on the working condition of the engine. Thl' compressor wheel will only work properly if the air really does flow through the vanes. TI1is is ensur0.000 revolutions min. Clearly, standard hall bearings are no longer suitable for such applications.

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To ensure that the balls do run truly in the bearing carrier, it is imperative that they are axially pre-loaded. Failure to do this leads to rapid destruction of the bearing carriers, whether they are well lubricated or not. When you look for practical ways of calculating the preload force for our application, you will find that none exist. Specialist catalogues and books on the subject fail to provide any answer. The simple fact is that we have insufficient experimental data to develop adequately robust calculation models. However, in Germany we are blessed, for example, with GRW (Gebri.ider Reinfurt, Wi.irzburg). This company has addressed this problem and now manufactures bearings in a range of sizes that are appropriate for our purposes. Versions without a cage, so-called full complement ball bearings, are particularly interesting ta cage that does not exist, cannor go wrong!). In this regard we reproduce the following exchange of letters between myself and Mr. Sender, the engineering consultant at GRW:

Shaft tunnel

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Spacer bush Shaft Drawing I: the necessary pressjit connection between turbine wheel and shaft is via both the spacer bush and the internal race of the bearing. At a load of around 1,000 N does this cause undue deformation?

Dear .\1r. Sender

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As you are aware, I am writing a nell' hook on the subject of model jet turhines. Naturally. this includes a chapter on the subject of bearinf!,s. I would be very grateful ifyou could help me by answerin[? the following questions to tbe best of your knowledge and providinP, additional information where applicable. What types of hearing does your company produce for the above application? Do you have catalogue sheets available that include all the most important data such as dimensions, with details of tolerances for the bearings and their fit, permitted rotational velocities, minimum/maximum pre-loading, lubrication and lubricant, method of assemh(y? Prices and terms and conditions of delivery. In the cast! uf cageless hearings it is not possihle to prevent the balls touching during operation. I have been told that this causes micro-damage to the hearings that leads to rapid wear of the hearin[< carrier. What is your position on this point? The maximum shaft pou•er of our engines. tbat is the power transmitted }rum the turhine wheel to the compressor wheel, is in the order of magnitude of several tens of kilowatls. For example, the smallest engine has a shaft power ofapprox. 10 KW at 150,000 revolutions/min. This results in a torque of around 0.64 Nm. To transmit tbis torque via the internal race of the bearing, it must be force-fit to the shaft (see drawing 1 {jig 5}). I estimate tbat a force of around 1,000 N is required between the face side of tbe internal race and the corresponding contact areas uf the shaft. Is there a risk that these forces will cause the internal rings to unduly deform? Do you need to use the rather more complicated construction as shown in drawing 2

{Fig 5}?

Without dismantling tbe assembly, what is the easiest way of identifying whether or not a bearing is already damaged? Do you have any other information that you believe could be important for users? Thank you very much in advance for your answers. Yours sincere(y, Kurt Schreck/ing

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Spacer bush

Drawing 2: press.fit connection between turbine wheel and shaft is via the spacer bushes. The on{y wad on the internal race comes from the i,ulependent{y adjustable axial pre-wading. Fig. 5 Drawings relating to the enquiry sent to GRW.

And this was the reply I received:

Dear Mr. Schreckling We are pleased to answer your questions as fol/oil's: Re: Q 1. and part of Q 2.- see the attached fl received data sheets detailing a total of six different bearing types/. With regard lo the issue of pre-loading, we can provide the following infonnalion.- as far as miniature bearings are concerned, the rule of thumh is that the hare diameter in mm equates to the pre-loading in N. This force is sufficient to reduce the extent to uihich the ball~ in the bearing slip and spin. so as lo ensure that they last for an acceptable length of lime. For example, a preloading of 8 N is sufficient for the 608 bearing Admitted(y, a much higher pre-/oadinf!, is necessary to give the hearing a higher level of resilience. The standard pre-load values are hetween approx. 20 lo 50 N. We believe that this Jorce is su{ficient. Instead of employing higher forces, ii lt'Ould be heller to devote greater effort to balancing the turbines. Whilst higher pre-loading can be used to conceal the tendency of the turhine lo vibrate, the vibrations are ullimate(y still there and they still add load to the bearing What is nwre, the high pre-loading artificially creates a still greater load.

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Fur all hearings designed e.,pecia/11•.for 111udel jet turbines, the hore and tbe external diameter are co11str11cted to a tu/erance o_f 0/-q µ,m. Tbe sba/i should be constructed to a tolerance of approximatel1• -5/-9 JJ.111 and +II+ 5 for the> housing. Precision-built cu111pu11ents should 1101 he press:fitted. That is tbe principle. Tbe illfention is to a1 1oid nm1-centricities and tensions in the bea1i11gs. Ve,y t(qht, hut J,-eefitt i ng components necessitate 1•er1-' high le1•e/s of precision. Furthermore. a tightfittin!!, ojfen, the best protectiun against cunvsiun. Should this rust occw: it can be counteracted brfitting or coating the bearings appruprimelr or bv 111m111ti11P, them 011 O-rings. Tbe bearings rotate at such high 1•elucities in the t11rhi11es, that ii is scarce!)' possihle or practical to estahlish ill tbeory their 111a:1:i11111111 rotatiuua/ 11elucities. Some ralues acbie1•ed in practice.T1pe

111i11-J (approx.)

D60R/602 60.! 1)608/60.! 839 D60R!60.! 976 D/1.!!6()3 0R9

15'>.()()0 155,000 .200.()00 90.000

With feu• exceptions, the most co111111011l1' used luhricants are standard 1111-hine oils. £1,en though the!' cume ji-0,11 different 111c111u/i1cturers, mos1 oils are basical~v similar in type and 11iscosity. Since ll'e do not ba1 1e a preference for une 111a1u~facturer in particular, ll'e wuuld pre.fer not tu reco111111e11d a specific hrand. A, far as ll'e are a!l'are, neither of the fll'0 standard methods of luhrication (separate luhrica1ion or as an addition tu_(lie/) appears tu hm•e anr negatil 1e e.Oect un the bearing According.Iv, ll'e are prepared to ad1•ise that butb method, can he used. Q(a/1 the bearings u•e produce, uur AC (f11ll-cumple111ent) hearings are the 111os1 popular. When _fill in!!, these bearings, you should onlv load them a-..:ial~v Otherwise, you 111av end up with the hearings alrea~r_(alling apart e1'e11 as yuu _fit them. The ultimate rule is that furce-flull's u 1i/l soon lead to indentations in tbe hearing grom·es and should be m•uided. Re. Q 3: See attached. Re: Q --1.- With rep,ard tu full co111pleme111 bearinp,s it is defi11itelr the case that the halls ll'ill tuuch each other repeated/1•. Furthermore, u•ear is caused to the ceramics at the poi111s u•here the halls 111h agai11s1 each mher, i.e. these hearings are in principle also sul?ject to a process of u•ear. Nel'ertheless, these 11ersions ha1 1e more ad1•a11tages than disadrantages. The ceramic halls ll'eigh 1•ery little compared to steel bearings. Consequent~v the11 exert comparatil'e/1• little centrffugal force or load on the outer races. Furthermore, they are che111icall1' inert in re/atinu In the steel races. Com 1e11tional ball bearings are almost inel'itah~l' suhject to 111icm-11'elding caused hy direct material contact. This effect dues not occur bet11•ee11 ceramic balls and steel races, ll'hich eliminates one of the main causes of ll'ear. The absence u.f a cage is also heneficial. A ca[(

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J1iflue,1ce of weather a11d ahitude Air pressure and temperature are derendent on the height above 1-.ea level and they in turn affect air density. The atmosrheric pressure at a particular location can vary hy ± 5% from the mean, depending on weather conditions. Variances in air temperature from the standard value of l 5°C can cause changes in density in the opposite sense. A1-, a nilc of thumb you can note that: A variance of plus or minus 1 °C from the standarwer of the exhaust g;1s. Therefore: However, air temperarure also has a hig effect on the temperature of the exhaust gas and, therefore, likewise Heating rower: 0.24 x 555 x 1.005 kW= (j--1 kW on the criticd temperature of the turbine vanes. A 1 °C plus change in rhe temperature of the intake air results in a 0.2--1: 2 x 3152 W = 12 kW corresrxmding 2°C to 3°C change in the temperature at Jet rower: equals: 1-16 kW. the inlet to the turbine. This change is equally apparent in the temperature of the exhaust gas. The turbine is This 146 kW of power must I~ met by burning fuel. designed to operate at a temperature of I 5°C. At 30°C in Burning one gram of kerosene per second, produces the shade the air temper.Hure in the sun. close to the 4-l.000 watts of heat. Therefore. it is necess:1ry to hum at ground, can he 40°C or higher. As a result, the vanes of the turbine wheel are then around 7C:,°C hotter than they least 3 .:32 grams. the equiv:1lent of 4.15 millilitres per second. This equates to a fuel now ,~1te of 249 ml/ min . would he in normal conditions. Measuring a particularly low exhaust gas temperature in winter should not According to diagram 7 the KJ-66 uses 260 ml/min, i.e. the measured data value is only slightly higher rhan the fool you into making the nozzle narrower so as to value that we calculated. On this basis it could not he achieve a greater thrust. Things will get critical in the expected that an improvement in the combustion summer. process would hring an appreciable reduction in fuel Driving a jet turbine without being able to regulate consumption. Differences in consumption between difrotational vekx:ity is more complicated. This could involve the use of a limiter tu 1-,et a maximum rate of fuel ferent fuels such as diesel, kerosene, paraffin or hiodb,el are also negligible. flow or a control device for compressor pressure. Therefore, if the air pressure drops and/or Diagram9 the temper:1ture increasThrust correction at con~tanl rotational "cloctty, lndc_pcndc..-nt or air pr~urc and tempcrarur._.. es, the engine will turn appreciably faster, due to the correspondingly lower air density. Accelerating the engine to a cunstant pressure. while the ambient pressure is less than standard pressure, will inevitably result in a higher compression ratio. However, this also implies a higher rutational velocity. An increase in rotational velocity can lead to an increase in temperature at the turbine stage due 900 950 1000 1050 750 800 850 Air Pressure [mhar] to the intake air being

Diagrams

Correlation between specific consumption. stepped etficit'ncy and relati~e rotational velocity

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hotter and it would not take long to exceed the maximum loading on turhine wheel. For a more precise investigation 11 1s necessary lo measure the air pressure :md temperature on site and work out the actual air density hy means of the gas bws. It has been possible to fly model jet turbines at some high-altitude locations e.g. close to Mexico City at a height of 2.500 111 above sea level. Before take-off the oper.ltors were fully aw:ire of the aforementioned rules and actu:dly put them into rrnctice. There is a sometimes quoted theory that model jet turbines cannot operate at high altitudes . Clearly, there is undeniable evidence to the contrary. Of course the reduced air pressure, or more precisely the reduced air density. also means that the aerofoil protluces less lift. As a consequence. a correspondingly greater take-off sreed is required, even though at the same time one h:1s less thrust. This inevitably results in the need for a longer ground nm to take off. As a rule of thumb it can he stated: for every 100 m increase in :ilritude you need 2% to 3% more ground run. This rule applies similarly ro both large and small aircraft. Modern jet turbines have considerable thrust reserves to completely overcome this rrohlem. Corre/at/011 betwee11 thrust and flight rielocity l 1p until now we have only looked at the engine's static thrus1. This equals outflow velcx:ity times air mass flow. However, for a mcxld in flight we h:1ve to use a value of velocity that is cakulated from outtlow velocity minus flight velocity. We already t..now that the olHflow velocity e.g. for the _J-66 is around 315 111/ s , or 1.134 km/ h . Let us say that the mcx.lel is flying at a very high fli!J.ht velocity of 300 km h. the equivalent of 83.3 m 1s. /\ simple calculation shows that thrust is reduced to 73.5%. Instead of 75 N. we arc left with only 55.2 N. The net rower of the jet turbine is then thrust [NJ x tlight velocity [nvsl = '1 598 watt' In point of fact. a higher llight velocity increases the impact pressure and this has the effect of slightly improving the efficiency of the compressor stage. Likewise. this results effectively in a slightly higher outflow velocity and consL'quently :1lso marginally greater thrnsl. To achieve the :1forementioned flight power with a propeller or impeller drive, the engine required to do so would require a higher shaft power. This is because a propeller or impeller is inherently less efficient and resulcs in a partial loss of thrust. At n30%, which is the minimum rotational velocity that is possible in practice, the outflow velocity is arproximately 50% of the maximum velocity. i.e. around 340 km. h. However, it will generate thrust as long as the outflow velocity is higher than the flight velocity. This means that when a jet model makes it:s approach for landing, the jet turbine still provides minimum forward propulsion. In contr~1st. when you throttle hack the rotational velocity of a propeller engine, it act:s a:s an air brake.

Diagram 10 J •66 jt:l turbine - increa..w in the sound prc.~-urc lt.~el. relative to thrust 40 ·

.5

10

.,

20

,.

40

..

Tluusl(N)

.

TO

. .

noise, or, technically -;peaking, the sound pressure level, increases proportionally to the air mass tlow. However. any increase in outflow velocity cau:ses a much bigger increase in the sound pressure level. Consequently. for any particular engine the sound pressure level increases in line with thrusl. Diagram 10 shows the relative noise increase in relation to thrust. The scale for the sound pressure level at a minimal rotational velocity is arhitr.trily set to zero. from this you c;m cle;irly see hem the sound pressure level incrl.'ases more steeply at higher levels of thrust. It can he assumec..l that anyone who rem:1ins in close proximity to the test stand for any length of time. with the turbine running at full thrust. would suffer hearing damage due to the high sound pressure. Consequently, in :such conditions it is strongly recommended to wear hearing protection . The data reading of the sound pressure level depends on both the distance from the jet turbine and the propagation conditions. To determine the sound pressure level at a particular location it is necessary to perform complex measurements with precise measurement equipmelll and to calculate propagation using a recognised method.

Noise development If a jet turbine emits a high-pitched whistle, it should he switched off immediately. This kind of noise is caused by badly-balanced wheels and ·or hy the wheels rubbing. Air flowing through the turbine ;,ilw:1ys makes a rushing sound. This is unavoidable. The volume of this

27

Home Built .'vlodel Turbines

www.ASEC.ir

Chapter 2

Necessary Accessories Di.fferent types of starter

compressor o r a bottle of compressed -air. you decide to use a h__,,, / ;1 , ' \\ 11 i ~ I I the jet turbine 's rotor. A small oute r race with an internal rubber ring \ ' ~~/ I I I 'l.. ~ serves as a dutch; the / I I I principle i.s similar to \ ~ ; I I I that of piston e ngine \ ~/ I starter mot o rs. If the rotational velocity of the --, _ __ ., I motor at idle is not sut~ ~ ficient, e. g. to start a Main body of tl.1e blower wheel sm,tll je t turbine. vou

,.-------P?~'

~

-+-~-~--j- --: -)----!\}--~ ~

------1----·'

//

i

Home Built .11odel Turbines

28

www.ASEC.ir

Starter blower working with a Kamps turbine. can replace the race clutch hy a disc with an external rubber ring. l11e diameter of the ring should he approximately twice the size of the compressor nut. In spite of the relatively low rotational velocity ot thr geared pumps to generate adequate pressure or convey a sufficient quantity of fuel. The required maximum delivery pressure is around 4 bar. Take care! If the fuel flow is blocked, very good, i.e. hermetically-sealed. geared pumps can cause a very large buikl-up of pressure. In the worst case scenario this cm result in damage to connecting lines or seals inside the pump. The standard drive for a geared pump is a small electric motor. Like all electric motors on hoard a model aircraft. it must be installed with noise suppression. It is imperative that it is not installed in close proximity to the receiver. The best position would be for the pump to he lower in the model than the fuel tank. This would avoid any possibility of suction problems. should the pump run dry. There are many sources that can provide ready-toinstall fuel pumps complete with drive motors that arc fitted with noise suppression.

Fuel tank with feed lines A good solution is to use a plastic tank with a felt ·clunk'. Its vr,Iume should he based on the size of the engine and the desired flight time. One thousand millilitres is adequate for small jet turbines. Larger ones would require a minimum volume of 1.500 ml. Some moc.lels are designed in such a way that the total volume of fuel has to he divided between sever;il different tanks. The fuel lines must be made of materials that are resistant to hoth petrol and pressure. On the pressure side, that is tu say between the pump outlet and the engine, you shoukl estim;tte a rressurc of around 5 bar. You can use thick-walled hoses with an internal diameter of 1. 5 to 2.0 mm, the type that is used for petrol engines. Of course, it is also possible to use thin metal pipes. You can make the hose/metal connections sufficiently pressure-resistant by winding wire around the connection. Rubber is never permanently resistant to both petrol and pressure. As a consequence, it is necessary to replace the hoses every two to three months. Polyurethane connecting pipes are not particularly heat-resistant and therefore are not really suitable for use in proximity to the jet turbine. Our model colleagues from the steam guild use metal fittings and glands to create pressure-resistant fuel lines th;tt are totally plastic free. A turbine manufacturer will also have a stock of suitable fuel hoses, the corresponding quick-release couplings as well as T-pieces, magnetic valves and shut-off valves. Likewise, it is possible to use fuel filters designed for comhustion engines as well as shut-off valves with a Teflon seat. It should be noted that fuel filters have a high through-flow and therefore tend to get clogged in n0 time at all. The solution is to fill up with pre-filtered fuel. The restrictors are m;ide from approx. 60 mm long capillary tubes ( injection needles) with an internal diameter of 0.5 to 0.6 mm. A steel wire is used to adjust the flow resistance. Adjustment to the required resistance is achieved by inserting the wire, to a greater or lesser distance. inro the capilh1ries. In order to connect the feed lines, the capillaries arc soldered into tubes with an external diameter of 2 to 3 mm, corresponding to the internal diameter of the connection lines.

Cartridge-fed auxiliary gas l'sing handy gas cartridges for gas torches is a good idea. Cartridges filled with a propane/ but:ine mixture will also gu:iramee sufficient auxiliary gas pressure even when the weather is cold. You need to fit a nipple where the hurner would normally he in order to enable connection tu the jel turbine. The volume of auxiliary gas required is minimal. providing that you do not forget to dose the valve aher starting the jet turbine.

Electrically-powered glow plugs l11e best ones to use are srandard quality ·cold' glow plugs. These have a spiral-wound filament which you need to pull out slightly. To bring about ignition the filament must glow bright yellow. It is possible to check the ignition properties before fitting the plug. This requires the use of a gas lighter. You light it, hlow out the flame and while the gas continues to flow, you bring the glowing filament towards the gas. If the glow temperature is set correctly, it will ignite the gas emanating from the lighter. If you do not have an ECU (elecrronic control unit see the section helovv· on 'electronic regubtion and control') with a glow plug output. it would be sufficient to use ;1 2 volr lead hanery or two NiCd cells with a capacity of at least 1.5 Ah connected in series. If you use NiCd cells, you will need to incorporate a pre-resistor to reduce the voltage slightly. An insulated two-core cable made from 0.5 mm" wire would do the job. You just need to find the right length hy a process uf trial and error. As a guide you can assume a length of around 1 m. It goes without saying that you can also use an adju::.table current glov.· rlug driver which you can buy from specialist ret:tilers.

Calibrating of the restrictor for the supply of lubricant It is essential that the fuel contains turbine uil at a level of approx. 5%. Hetween 3 and 5% of this mixture should be fed as a lubricant through the restrictor. Calibration is performed at :1 pressure of approximately I bar. This is achieved with the pump under partial load. The easiest method of calibration is to use two measuring cylinders. Failing this. you can use a letter balance to weigh how much fuel you collect. After the pump has been running for I minute you should measure 100 g for the main fuel tlow and 3 to 5 g through the resrricror.

Electronic regulation and control The first models that were built incorporated nothing more th:m a simple drive controller. This was all that was needed to remotely regulate the fuel pump and hence the thrust. The pilot had an accele1~1tion lever. He always had to take the working condition of the turbine into account. Too much or too abrupt acceleration leads inevitably to destruction of the engine. Moreover. it is dangerous. In this case, hy accelerator lever we mean the control stick found in most motorised models and used to adjust the fud flow. In a turbine the operation

30

//0111e

www.ASEC.ir

Built Model Turhines

of the fuel pump is regulated by the accelerator channel, in a similar way to controlling electric model aircraft using so-called drive controllers. There are a few model turbine enthusiasts who know a thing or two about electronics. So this really gave them something worthwhile to sink their teeth into. Unfortunately, what first emerged appeared to seasoned campaigners to he some kind of computer game. Anyone who tlew turbines, hut did not happen to be an electronic engineer at the same time, had no chance. However, it was not long before things changed for the bener. Today I have no compunction in recommending that everyone. old-hands included. uses a modern regulation and control unit. We have adopted the abbreviated term for this, an ECU, from the English 'electronic control unit'. Modern ECUs are connected at the very least to a temper,tlure and rotationa I velocity sensor. Rotationa I velocity sensors almost always consist of info1-red photoelectric barriers. For temperature measurement, thermocouples with a thin protective tube (approx. 1.5 mm diameter) have proven a reliable solution. The safety imperative is such that. should the sensur or parts of the electronics fail, the pump is automatically switched off and with it the engine. To provide 1he fuel pump with power, you need a separate battery. The capacity and number of cells is dependent on the drive motor for the pump and the type of ECl: fitted. ECUs perform the following basic functions: To be able to start the fuel pump, the rotor must be turning and the temperature at the engine outflow must have reached its minimum setting. In the start phase the pump is automatically adjusted in line with acceleration. The rotational velocity of the engine is regulated according to the setting on the acceleration lever within the permitted r.mge, i.e. the minimum and maximum permitted rotational velocity can he set electronically. During operation, both the temperature of the exhaust gas and the rotational velocity are monitored. If limit values are exceeded. in some cases the supply of fuel nm be cut. The temperature regulator works in the same way, by stopping the flow of fuel. This can occur, for example, when there are air pockets in the fuel line that lead to combustion being extinguished. TI1e engine is accelerated or decelerated by changing the position of the acceleration lever. This automatically regulates the flow of fuel to prevent any risk of overheating or flame failure. To programme and read the data generated by the ECUs. you need to have an additional data terminal. This does not have to be, but can be, installed on board. Before running the jet turbine for the first time, you should adjust the ECU nor only to the turbine, hut also to the remote control equipment. This involves a rather complirnted technical procedure. However, for the ECUs with which I am familiar, supplied by Cat/Jetrunic and Orbit, the process is largely automated. This includes provision of the corresponding data terminals and, providing rhar you follow rhe operating insrrucrions, even those of us who are not electronic engineers, can usually get it right first time. In any case, they are foolproof in so far as it is impossible to destroy anything by incorrect operation. Once they are programmed and adjusted, the ECUs work on rheir own, i.e. you do nor necessarily have to switch on the data terminal when starting the

engine. Nevertheless, I would recommend that you do so. The respective manuf:icturers have gone on tu introduce some additional functions. When the model is not in operation, the supply of fuel to the engine is safely shut off by means of a magnetic valve. This reliably prevents the engine being flooded unintentionally when filling up with fuel. The Orbit electronics measures the time it takes for the rotor to stop turning afrer it has been deliberately switched off. This is used to monitor the condition of the bearings. What is more, both manufacturers enable data terminal read-outs to he taken. even retrospectively. so as to he able to identify operational problems such as an unexpected engine shut-off. There is the option of fitting the ECU with a chip for a maximum of 120,000 or 160,U00 rev/min. The 12U,000 rev/min are perfect for use with engines that have a whL"el diameter of 66 mm or greater, while the 160,000 revlinin are intended for smaller jet turbines, whose maximum permitted rotational velocity is, of course, higher. However, on no account should you misuse this higher velocity chip. Getting an engine designed for 120.000 re\·/min to turn faster is not a good idea. TI1e Jemmie has an extremely praccical range of additional functions. For example, the system enables the fully automatic start of a jet turbine by means of its own on-board starter motor. This includes the power supply to the glow plug. Then:" are also the options of using a speed sensor to control the tlight velocity or also to take a read-out of the maximum flight velocity after the flight. Another magnetic valve can be used for the remote activation of a smoke generator.

31

Home Built Model Turbines

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Chapter 3

Test Stand and Measuring Equipment The engine on the test stand If you build .i copy of a proven jet turbine and operate it using modern electronics, there is no real need to go to the effort of building a te:-.t stand. Having said th:1t. you should in that case already have some experience of working with jct turbines. Of course. a trainer model, designed so th:11 there is open access to the engine, can :1lso he used as a test stand. Providing that electronic measurement of rotational velocities and temperatures gives satisfactrny results. the jet turbine 's thrust will not fluctuate hy itself. You c:.111 read more about changes to the thrust brought about by atmospheric conditions in the chapter entitled ·physicalte c hn ical principles· in the section ·operating performance'.

However. for anyone who wants to trial hi:-. own development work or test improvements to a jct turbine, constructing a test stand is a must. As a general rule. you will want to measure thrust. rotational velocity. exhaust gas temperature and fuel consumption. The connection diagram for the operation of an engine is shown (Fig. 6) in the above- mentioned chapter under th e section headed ·description of components·. Please note a few important safety rules for :Ill test stand trials: If you use a normal t:,ble for a test stand, RO N of thrust are more than capable of turning it over. This can h:1ppcn without any warning. You need to determine the required level of o.;tahility before you start. A jct turbine of the size of the KJ-66 requires approximately 200 litres of air per second. Y. It is e xpected that this engine will achieve at least 40 N of thrust.

Technical data for jet turbines Units g

We ight Max. diameter Length Wheel dfam1eter Weight of compressor wheel Weight of turbine wheel Combustion chamber volume Intake diame ter Ring cross-sectio n turbine wheel Ring cross-section thnisr nozzle

g g

Max. pe rmined rotational velocity Data at max. rotational velocity TI1rust Exhaust gas temperature Pressure ratio Outflow velocity Kerose ne consumptio n Lubricant consumption Air flow rate Recommended max. thrust Rota tional velocity at reco mmended max. thnist Minimum roratio nal velocity Residual thrust Exhaust g;is temp. at min. rotational velocit y Acceleration : ic.lle to full load Deceleration: full load to idle .\fax. te mperature during accelerntinn

111111 111111

lllm

ml mm cm cm '

FD-3 64 870 1 IO 265 64 30 40 '-05 33 18.1

Beho tec J-66 1,040 113 230 67

KJ--66 930 112 2.30 66 67

68

68

6h

1'

19 18.5

(30 :i6 19 17.9

rev.min

75.000

120.000

120,000

N

24 6,iO 1., 209 160

84 .2 575 2.19 350 300

84.5 'i80 2.15 365 300 10 0.23

oc m .1 s ml; min ml/ min kg/s

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2

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0.115

0.24

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24

rev; min

7"i,OOO

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N

oc

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oc

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1

1.95 270 120 7 0126

75 114.600 '!li,000

60,000

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4

550

"i8S

3 3 750

2.5 2 '50

Home Built Model Turhines

40

www.ASEC.ir

Kit version of the Behotec J-66 jet turbine Behotec is based in Hergkirchen. It supplies products to the engineering industry amongst others and is equipped with modern CNC machine,. Thi:-enables them to build small jet turbines, hut the story does not end there. More important is the fact that within their midst they have someone who is a model jet turbine and model aircraft enthusiast, Thomas Berktokl. He also happens to he the boss of the company and is definitely the right m:m in the right place. He developed his first This is the cu11structio11 kit fur the J-66. Nut shou•11 are the small ,:umpo11e11ts flight-worthy jet turbine a11d the Orbit electronics that are supplied as part ufthe kit. hack in 1992 and presented it at the Whinle Ohain Trophy. This engine had :1vaibble to fit the engine. Mr. Herktolt decided that they 100 N of static thrust and weighed 2.5 kg. At that time it were not good r wheel Spacer hush Compressor hearing Slide bush Pre-load spring Shaft Shaft tunnel Turbine hearing Space bush Turhine wheel Turhine nut Intake nozzle Lid Front screw Guide , ·ane holder Connector Screw Tooth wheel Screw Sooth wheel Rivel Hive1

No.

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55

Home Built ,'vfode/ Turbines

www.ASEC.ir

TK-50: Sheet 10

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- - - - - - - - - - - - - - - - - - - - - - 2 5 7 , 6 - - - - - - - - - - - - - - - - ~\

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www.ASEC.ir

P.41 Tab

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Detail Y and Z. side view Scale = 2.5:1

Detail X Scale = 2.5:1 Brass - plugs

www.ASEC.ir

TK-50: Sheet 15

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61

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TK-50: Sheet 16

P.11 Turbine wheel

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,4

17,8

------'26

------~o --------Pos. 2 Compressor wheel

62

Home Built J1ode/ Turbines

- - -- - - -- - - --

www.ASEC.ir

Constructing individual components Rotor, beari11gs and shaft tunnel The compressor ,vheel (part 2) is a Rnished component. Whilst ii may require fine balance correction, it does nut require further reworking.

o,, the le.fl, a hand-ma,le turbi11e u,/:,eelfor the prototype ,.if the TK50; on the right, a professionall__v cast wheel, based on Martin Lambert's plastic model

We begin with the turbine wheel ( 11 ). The cast hlank must he bored to a nominal bore diameter of 6 111111. First you centre it on the lathe, then hore a through-hole using a 5.5 111111 carhide drill hit (or alternatively a carbide reamer, if you have one availahle). With the wheel still clamped in the same position on the lathe you Gm finish the bored hok. The nominal bore diameter is 6 mm. You need to use a manually-operated mini grinding machine and a small conical milling tool, followed by a cylindrical grinding tool with a diameter uf 5 mm. The lathe needs to he set to a low speed. The bored hole should be slightly conic:il in shape. starting with the nominal bore diameter at the front of the hole. l11e turbine wheel should he loosely press-fit to the shaft, naturally with the pre!->:-,-point at the narrowest part of the bored-hole. The external diameter is finished at the same time as the turbine guide system as pan of the final assembly. The spacer bushes (.~ lU) provide traction between the compressor 'turbine wheel respectively. This ensures that the inner races of the hall hearing are not crushed when the shaft is clamped tight. It requires the bearing seat to he made approximately 0.02 mm larger than the length of the internal race. Each of the spacer bushes can be manufactured in a single damping position. The hole is bored with a centre drill, pre-drilled to 5.8 mm and finished off with a 6H7 reamer. The bearings (4 ·9) are carried in a diameter of 8 111111. You can use a hall hearing with an internal diameter of 8 mm to calibrate this cross section. The bearings (4/9) intended for this application are in fact not suitahle, since rhey are extremely fragile and c.111 easily fall apart. The shaft (l) is first pre-turned. Precision work is carried out between centres to fit the spacer hushes (3, 10!, compressor wheel and turhine wheel. An alternative to employing a precision gauge is to use standard hall bearings with the same internal or external diameter as the hearing. The spacer hushes are constructed to loosely press-fit the shaft. Special care should be taken to ensun: that the front faces of the spacer rings are coplanar to each other. Next, finish the shaft-seating for the turbine wheel or compressor wheel to provide a

lorn,e pre!->:-,-fit for both wheels. lbe adjustment b facilitated by heating the wheels to l 50°C. At this temperature: the wheels should slide easily into position. The shaft-seating for the compressor is only required to fit at the hack of the bore hole. The compressor wheel ( 1) is pre-turned on the lathe and a left-hand M5 thread cut. The external shape is not significant. The maximum diameter should be adjusted to the diameter of the compressor wheel. Consequently, the nut should only be finished to its final shape once it has been assembled with the shaft and the compressor wheel. Instead of the stated turhine nut ( 12) you can also use a hexagonal M5 left-handed nut. The push sleeve ( "i) serves to take the compressor hearing (•1). The bearing should fit firmly in the push sleeve. The external diameter of the push sleeve is polished. The shaft tunnel (8) is machined from high-strength aluminium alloy. A round blank is required with a diameter of ,10 111111. First you bore a 14 111111 diameter hole down the full length of the material. The 16 mm rear bearing seat is machined to give a tight fit. A ball-bearing with an external diameter of 16 mm can he used as a gauge. The forward recess should have a diameter of 20 mm. First machine it to a length of only 2.3 mm. Do nut determine the exact length of the recess until you come to Rt the whole rotor assembly together, including compressor guide system and intake nozzle. The 18 mm bore hole is constructed to take the slide hush ("i) as well as the pre-load spring (6). The slide bush (5) should fit easily into the bore hole. A radial bore hole is made in the flange to push through the capillaries (·-i2). The exact diameter is dependent on the external diameter or the capillaries. It is necessary for luhricanr ro be able to escape from between the slide hush and the shaft tunnel. For this reason a channel approx. U.5 mm high and I mm wide is milled in the 18 mm diameter bore. This is easy to do with a shm1 l 111111 milling bit and a mini-drill. 111e six 2.6 mm diameter fixing holes are created together with the corresponding threaded bore holes in the guide vane holder ( 15). The pre-load spring (6) can he wound from steel wire with a diameter of l.4 to l.5 mm. It does not necessarily have 10 he spring sreel wire. 111e spring is construcced so that it is slightly conical in shape. This requires a conical mandrel with a 5° chamfer. It is wound to a diameter of between U and 13.5 mm. Due to variation in the hardness of steel wire. you need to experiment to find the precise diameter. When Rnishecl, the pre-load spring should fit smoothly into the bore of the shaft tunnel. The pre-loading when assembled should be 2U to 30 N.

Turbine guide system The guide vanes 0 Round, 20 0

Tool .steel

Round, 20

Tool steel lnconcl 7 13 High-1emp .steel AIZnMi-,.Cu Spring steel Tool,deel Al 99.9 AIMg Al 99.9 AIMg AIZnMgCu

Round. l'i 0

Drawing Sheet No.

Notes

Finished part No. 2038 KKK Type 0608/602 839. GR\l( "

2 18 2 2 2

Type D6U8/hU2 839. GR\X" Heady-made, see .suppliers list

Hound. 25 0 Sheet 1.5 thick Hound, 60 0 Sheet 1.5 thick Round, ll"i 0 Round. 120 0

18 2 3 3 3 l'i l'i

Alternative to 3.1-3.:3 Ready-made. see .supplier:; list

16 i

CrNi 18/ 111

Sheet 0.'l thick

Ready-made, see suppliers list

CrNi 18/ 10

Sheet 0.3 thick

Ready-made. see "uppliers list

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73

Home Built Model Turbines

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Home Built Model Turhines

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Constructing individual components Rotor (1.1 to 1.9) To make the comrressor nut ( 1.1) you need a highstrength aluminium alloy. In order to avoid balancing difficulties. it is best to machine the final share when ii is assembled with the compressor wheel ( 1.2 ). spacer ring C (I ..:3l and the bearing C ( 1.4 l. The compressor wheel ( 1.2) is a replacement pan for the KKK turbo charger. It is supplied ready to use and does not require any reworking until the fine-balancing stage. l11e shah ( 1.5) is 111ade from non-alloy or low-alloy tool steel. Perfect cnncentricity between the components is essential. Otherwise. when :1ssemhled. you will have to battle with dynamic imbalances. This is achieved by using a precision lathe. You first need to pre-turn the shaft and carry oul the precision work between u:ntres. The bearing seats are turned down ro an oversize of :1pprox. 0.01 mm. before being lapped to size. You can use 608 size standard hearings as a calibration gauge. To make things easier, the sp:1eer ring C ( 1.3) and .-spacer ring T 0.7) are madc from the same material as the shaft. lmponant: the faces must be ex:1ctly coplanar to each other. The turbine wheel (l.8) is :n·ailahlc as a finishcd co111ronent or as a Glsting. The casting needs to he bored. machined to the correct diameter or better ground down and finally, balanced. To drill thi:- hole you need high-grade tool steel (TiN coated) or a c.1rbide drill-bit. It can he drilled to a role,~ince of 0.01 mm and an exact fit achieved hy grinding down the shaft. The ideal fit is a light press-fit. A low speed lathe and a carbide toul are uscd to machine rhe turbine wheel to the correct external di:11neter. Screw the turbine wheel to the shaft, the spacer ring T and an old ISO 608 hearing. To screw the components together. chuck the shali in the lathe and turn the turbine mu ( 1.9) tight using a T-handled :socket wrench. This will prevent the shaft from bending when tightening. The external diameter of the rurhine wheel should he 0.3 m111 less than the internal diameter of the outer guide vane holder t 10.2). For this reason, only perform this operation once the corresponding components arc finished (see section ' fin:11 asse111bly' in the TK-th components together using 15 spot welds. placed between the slots of the inner v;111e holder. Next the outer vane holder (10 ..2) is pressed 01110 the centring device. You press the vanes from the outside through the slots of the outer vane holder until th e y snap into the slot~ of the inner vane holder. Once the vanes havl" been shortened on the outside so that they protrude only approx. 0.5 mm, weld the vanes 10 the vane holders as hesl as possible. Next, turn the outer welding seams, remove the centring device ;md complete the welding hetween the vanes and the inner v:me hokier. TI1e next step is to centre the unit on the lathe. so as to machine the vane hokier to an internal diameter of li6.4 mm. A variance of between 0.1 and 0.2 mm from the nominal size of 66.4 111111 is nut a problem. If the diameter is already 6(>.4 m111 before it is machined, the diameter of the turbine wheel cm he made around 0. I 10 0 ..2 mm bigger. With the assembly chucked in the same position, machine the 2(> mm diameter of the centre ring (10. •) to press At the shaft tunnel (2.1).

See the respective section in the description or how tu construct the TK-50.

Housing (11)

Combustion chamber for the KJ-66 made by .Jfichael Rang and Heiko Naupold. part (5.5) by means of hard-soldering. spot or TIG welding. The glow plug Atting (6.2) is either hard-soldered or welded to the outsiue. You need to pull out the coil of the glow plug (6.1) with ;1 pair of Ane tweezers, so that it protrndes hy ;1pprox . 1 nun . If you do not want glow plug ignition, you will need to weld two fixing struts (5.-.) to the facing positions on the front part (5.ll. Six ribbon-wires (5.9) complete the combustion chamber assembly.

Fuel supply system (7.1 to 7.4) An alternative option is to machine the fuel connection (7.1) from an M4 screw. Th8 mm using a •i5° mandrel. This r. Bend the oil supply line (9.2) so that the oil connection ( 9.1) is roughly in the correct position respective to the corresponding drilled hole in the housing. Oil the inside of the housing at the front. Insert the complete unit, guide syste1TI1shafl tunnel. centrally in the housing, but do not press it into its final position yet. Push the oil connection through the drilled hole in the housing. Adjust the unit so that the M3 threaded holes on the guide system are aligned axially with the corresponding drilled holes in the housing. Press the unit into the housing to its final con(·entric position. If the position of the drilled holes in the housing does not correspond to the threaded holes in the comprL·ssor guide system (4). you will have to remove the unit again. This is easy to do hy using a plastic rod with a diameter of 21 mm as a driving mandrel, pushing it into the shaft tunnel and forcing the unit out with soft caps of a hammer.

Once the drilled holes are in the right place, screw together the housing and guide system. Screw tight the oil pressure connection. Complete the adjustment and balancing of the turbine wheel as described for the TK-')0_ The most effective gap size between the vane tips of the turbine wheel and the guide vane hokier ( 10.2) is 0.15 mm. For the purposes of adjustment the unit can be pushed from behind into the shaf1 tunnel, in the samt> way as you would do for balancing. Make sure that the lid (3.3) sits tightly on the housing. Ir should require a medium force (hand pressure) to fit it. Should it not fit tightly. rework it with the spinning tool. lnser1 the spring (2.2) and hush (2.3) in the shaft tunnel. Fit the shaft with ball bearing ( 1.6), spacer ring T (1.7) and the turhine wheel in the shalt tunnel. Fit the spacer ring C (1.3) to the shaft. Lightly oil the seating for the compressor wheel. Use hot air to heat the compressor wheel to 50°C and fit ii to the shaft. Pull the compressor nut tight. This involves holding the shaft and using a T-handled socket wrench on the turbine nut. Check that the rotor turns freely. Put on the lid and screw it to the guide system. Check the gap between lid and compressor vanes. Take off the lid again and glue it to the intake nozzle (3.1) and the ring (3.2). Once the adhesive has hardened and you have smoothed over the seam, you are ready to assemble the front part. Hefore running the engine for the first time the seating between the lid and the housing should he sealed using a silicon compound. To do this, loosen the front screws approx. l mm and push the lid forward the same distance. Apply a thin head of the silicon Staling compound round the edge of the lidr housing. Screw the lid down again tightly and wipe off any excess silicon. Once the sealing compound has hardened, the engine will be operational and you can give it a trial run. This will not require the exhaust gas nozzle to he fitted.

94

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Chapter 6

Important Safety Instructions have compiled the following list of risks involved in working with model jet turbines. However I provide no guarantee as to its completeness. Such is the versatility uf the human imagination that it will never be possible to foresee all potential occurrences. This is particularly true of the myriad of possible mistakes. Whilst one person might blow pure oxygen onto his jet turbine instead of compressed air, someone else will wipe away drops of oil from the suction area with a cloth whilst the turbine is running or use his finger to check the suction effect - needless to say, all these experiment