Designing and Building a Miniature Aero-Engine E-Book

Chris Turner

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 262

Veröffentlichungsjahr: 2014

Beschreibung

Designing and building a miniature aero-engine is an exciting and rewarding task. Whether a professional engineer or an amateur looking to build an engine to fly your model aeroplane, this book will safely guide you through all the stages of designing and constructing an aero-engine in your workshop at home. With practical advice and detailed diagrams throughout, the book includes: machine tools, materials and accessories required; designing the engine, including a focus on proportion, valve timing and engine balancing; the manufacture of carburettors, assembly and setting up and, finally, choosing an aircraft for a home-designed miniature engine. Aimed at home metalworkers, engineers, hobbyist aero-engine builders and miniature aeroplane enthusiasts, and packed full of advice and tips, this new book is both instructional and inspirational. Fully illustrated with 163 colour photographs and 65 diagrams.

Leseprobe

First published in 2014 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

© The Crowood Press 2014

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ISBN 978 1 84797 777 9

DisclaimerSafety is of the utmost importance in every aspect of the workshop. The practical procedures and the tools and equipment used in engineering workshops are potentially dangerous. Tools should be used in strict accordance with the manufacturer’s recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

Contents

Acknowledgements

Introduction

  1  Tools and Materials

  2  Machine Tools and Accessories

  3  The Design Process

  4  Designing the Moving Parts

  5  Designing the Static Parts

  6  Carburettors

  7  Making the Parts

  8  Assembly, Bench Running and Setting Up

  9  Choosing an Aircraft for a Miniature Engine

10  Notes on Jigs, Fixtures and Special Tools

Conclusion and Future Work

List of Suppliers

Index

Acknowledgements

I must express my thanks to the following people who gave me their help, support and encouragement in writing this book: my wife and family for their forbearance, in particular our daughters Louise and Alice for caring for their mum and allowing me time to ‘do my thing’, and our eldest son Paul for helping me at the computer when things were beyond my ken. Also our son-in-law Mark Alexander, who puts things right (over the phone) when my computer has a bug.

Thanks also to my friends in the modelling community who show polite interest in my engines, tolerate my many experiments, and listen to my nerdy discourses. I must thank the members of my home club, the White Horse Model Club, for their support, in particular the chairman Adrian Hiley who has always given helpful advice. Also Eifon Herbert for the wonderful flying shots of my 1923 ‘Pander’ monoplane, and the White Horse Model Club for permission to use photographs taken at the flying site and subsequently used on the club website.

Very special thanks are due to Ian Melville for his painstaking work proof reading the manuscript, and reworking my somewhat pedestrian calculations into a more succinct version. Lastly, thanks are due to my modelling friends who offered their help and allowed their models to be photographed. Not all were included for reasons of room, context and other factors, but I know they will understand.

Introduction

A friend at the model aeroplane club where I fly my models once said to me: ‘You are a very rare animal, Chris!’ I laughed, and asked him to explain. He then said: ‘There are people who make and fly model aeroplanes, and there are people who design and make model engines, but there are not many who design and make model engines and fly them in their model aircraft!’

It is with this thought in mind that I felt I could help those who may have had aspirations, but who wondered if this goal were too difficult to achieve. I believe that the important factors which govern its successful achievement are as follows:

◆  an interest in miniature aero engines

◆  practical workshop experience

◆  the ability to think creatively

◆  some engineering drawing experience (either working with or making drawings)

◆  having access to basic workshop equipment

This last requirement may vary considerably, and in my own experience influences the design considerably. If the engine is to be constructed with modest means the practical design must be tailored to suit. This does not necessarily limit the design aspirations of the modeller, it only means that a larger dose of creativity is needed!

So what is meant by the term ‘miniature aero engine’? Some incredibly small model motors have been designed and constructed over the years, but the modern trend is for larger engines. Indeed, many motors used in the larger, modern radio-controlled model aircraft would happily power a motor cycle! My own engines range from a 0.55cu in (9cc) flat twin up to a 1.0cu in (16.5cc) flat twin, with several single-cylinder and twin engines in between.

My approach to engine design is to try to make a practical working motor that will fly a model aircraft. It should have the following features:

◆  be reasonably easy to construct

◆  be robust for frequent use on the flying field

◆  be easy to start and operate, with minimum adjustment

◆  sound and look good in a model aircraft

You may ask why this last factor is important, to which I would reply that the sound of the engine (particularly in a scale model) contributes much to the ‘realism’, and also, a lower ‘beat’ is easier on the ear! As for the looks, well, if you have gone to the trouble of installing your creation in an aircraft you surely want it to look the part as much as possible. I have on occasion added bits and pieces such as dummy cylinders (which is easy if the motor is your own design) in order to help the realism of the aircraft modelled, although this is probably going too far for most modellers.

Regarding the sound of the motor, a four-stroke resolves the problem better than a two-stroke because it runs more slowly (not much more than half the speed of a two-stroke) and so has a lower exhaust note. This may be an important factor where noise could be a nuisance, as anyone who has heard the scream of a high-revving two-stroke may appreciate. In scale models this low exhaust beat adds to the realism as it is more like the sound of a ‘full size’ aircraft.

Four-stroke engines are generally more highly prized by modellers. They are more expensive, and often become valued possessions long after their ‘useful life’ in a model. They usually contain far more moving parts than simple two strokes, hence the expense. This seems to add to the interest for modellers who enjoy ‘fiddling’ and making satisfying adjustments. Fellow modellers looking at my engines often remark: ‘Why not make a two-stroke? – Much simpler.’ My reply is to say: ‘If you are going to take the trouble to make your own motor, surely it is worthwhile to make something of value, more of an achievement, and one that will enhance your model aeroplane?’

My experience of designing and making is fairly practical and is a continuing process: ideas – sketches of these ideas – scale drawings to see if parts will work and ‘miss’ each other – more sketches to explore manufacturing methods (can I make it?) – scale ‘general assembly’ drawing – manufacture of parts (plus modifications which often appear necessary) en route – assembly of parts as I go to ensure pieces work together – final assembly and test – much ‘tweaking’ of things such as carburation, timing, even compression ratio – bench running to explore settings and check rpm, fuel use and general ‘usability’ – air test in a reliable model. Finally the evaluation of your realization compared with the aspirations at the onset.

The process does not end there because it continues to the next engine! What have I learned which will produce a better motor in future? There are some things that I find unnecessary in engine construction but which have become ‘traditional’, and there are other things that I do which may appear unconventional, but which have been proven in the air over time. My engines earn their keep: they fly all my models as I no longer use ‘shop-bought’ motors. One big advantage of this is that I don’t need to wait long for a spare (unlike my club mates). Furthermore an intimate knowledge of your own design means that problems are more quickly diagnosed – a strong case for simplicity.

The satisfaction of seeing your own engine take to the air is a most gratifying experience, even if it is being flown by a much trusted club member or friend rather than yourself. Be reassured, most engines are very tough and more than able to withstand the usual club meeting. One of my engines has been giving regular weekend service for many years and has got through three models to date. I have flown mine at club meetings, contests and shows where the sky was full of ‘shop-bought’ engines and some ‘shop-bought’ models too, and they always attract interest, often because I have matched the engine to the aircraft. For example, one of my flat twins is installed in a scale model of a full size aircraft that uses a ‘flat’ motor, as did some ‘golden age’ aircraft.

Whatever type of model aeroplane you choose to fly, you will have the satisfaction of seeing your engine in its proper element.

I have tried in this book to show how many seemingly complex problems may be resolved, and how as craftsman and designer it is possible to find your own way. After all, you have only yourself to please. The lone craftsman working at the lathe need not be concerned about transmitting the information to others, as the manufacture will be carried out by the designer. This enables the work to continue without the need for formal ‘limits’ and ‘tolerances’ because each part may be machined to fit the next and any spares which may be needed are similarly ‘fitted’ to the engine where required. This does not mean that working drawings are unnecessary, only that enough drawing to fulfil the realization, and often rough sketches, may well suffice.

The craftsman/designer may continue through this process using whatever knowledge has been gained through experience in the past, and so the way is open for many different methods, from basic three-view scale drawings and sketches to the use of modern computer-aided design and manufacturing techniques.

1   Tools and Materials

The tools and materials required for the production of miniature aero engines are much the same as those used by most model engineers, though with some special additions. Because of their small size much use is made of Swiss files, fine drills, small taps and dies, and especially ground ‘tool bits’. The author does not wish to set out an exhaustive list of the tools required because many of these are purchased when the need arises, and gradually accumulate over a long period. The emphasis will focus upon initial needs and how the modeller may proceed. Later chapters will mention special tools, ‘selfmade’ tools, and tools only relevant to the job in hand.

Full size aero engines use some rather exotic materials such as titanium and special steel alloys to allow lightness whilst giving adequate strength, and although miniature engines are not generally subjected to such high stresses, they do require special attention to these two factors because flight is, after all, the main goal. Much use is made of aluminium and its various alloys, and some special steels that offer greater strength and also stainless characteristics to combat corrosion. The metal-working processes needed to work these materials sometimes require tools that are especially modified or adapted to make the actual cutting more effective. This often necessitates modification of cutting angles by regrinding tool points, and in many cases, making tools from ‘sticks’ of tool steel (tool bits): these are easily purchased from suppliers (see list of suppliers).

Fig. 1-1: Assorted materials.

Of course this means that a fair degree of skill must be developed in the shaping of tools from very hard alloys by use of the bench grinder. This may seem a daunting task to those who are used to buying the ‘appropriate tool for the job’. There is a positive side to this though, as it is often difficult or impossible to obtain exactly the shape of tool needed to form a radius, make a recess, or bore that special hole. The requirement for different rake angles to suit the material being cut is another factor. Many tools bought ‘off the shelf’ have tungsten carbide inserts, which, although they may be shaped on a green grit wheel, are not as ‘modeller friendly’ as a simple tool-steel ‘tool bit’, which allows the constant regrinding and reshaping often required because it is used for different materials and opera-tions. Such tool bits may be sharpened and reshaped until eventually, like a well worn pencil, there is nothing left but a stub – and even this may sometimes be used in a fly cutter or maybe at the end of a boring bar.

Fig. 1-2: Small tool bit in a boring bar.

The development of the skills needed to sharpen twist drills and other tools on the bench grinder may only be achieved by experience. It is a good thing to enlist the help of an experienced fellow modeller or friend, or join a college class (often full of model engineers!), and so have some very valuable instruction. This is possibly most important from the safety point of view, as grinding wheels can be dangerous if they are not used properly, particularly in their maintenance where there is a requirement for dressing and ‘truing’ with a diamond. Technical colleges often arrange safety certificate courses which cover the methods of dressing and setting of tool rests, and general maintenance to ensure your ‘wheel’ is safe.

Special tools normally made by the modeller are also necessary to cater for the task of holding parts that will not fit in a conventional chuck, or which need accurate location relative to one another, or need a guide to ensure accurate alignment. These may include drill jigs to ensure the repeatability of holes relative to each other, milling and turning fixtures to hold difficult shapes, and pilot mandrels to ensure the alignment of parts being pressed together – for example, the assembly of bushes in connecting rods and suchlike.

Drawings are available for many useful tools designed for home construction for use in the lathe and milling machine. ‘Hemingway Kits’ (see list) offer a comprehensive catalogue of kits for making many tools which are either not available ‘off the shelf’ or are very expensive. A typical example is their precision boring head, which is most useful in the manufacture of accurate holes for the engine builder – for example, holes in a crankcase for the location of cylinders. Other tools include a ‘clamp-type’ knurling tool which, unlike the usual tool obtained from suppliers, exerts little or no side force on the lathe tool post as it clamps the work being knurled between two wheels.

Some tools require the use of castings, and these may be supplied as part of the kit along with all the other materials necessary, saving the modeller the problem of ordering small, uneconomical quantities of special materials such as phosphor bronze, which may be expensive in small quantities.

The manufacture of these special tools is really part of the design process for the miniature engine maker, because the designer must resolve the problem of how the parts are to be made, which includes all the engineering processes and tools required. The more complicated the engine, the greater the need for tools specially made for individual purposes. This part of the design process is often quite a challenge and every bit as interesting and rewarding as making the engine itself.

HAND TOOLS

Files and Saws

Files and saws are among some of the most important tools for all engineers. They may appear old fashioned, but a good grounding in their use forms the basis of most engineering processes. An understanding of the need to have datum edges – of making surfaces flat and square to each other – underpins all the more sophisticated operations, whether they are hand or machine, or even computer controlled. To this end it is good practice to take every opportunity to use saws and files, and the checking and measuring instruments, until it becomes second nature to use them, rather than depending upon more complex machining methods which often take time to ‘set up’.

Hacksaws

Hacksaws are needed for the general cutting of metals in the bench vice. These saws are capable of holding a range of blades, the most useful being twenty-four and thirty-two teeth per inch. It is better to purchase good quality blades of an alloy steel capable of cutting the various materials that make up the engine than to compromise with cheaper blades that don’t last. Manufacturers often state the material their blades will cut.

Fig. 1-3: Saws.

Junior hacksaws are needed for cutting small thicknesses and where the large hacksaw may be unwieldy. This saw leaves a narrower ‘kerf’, has smaller teeth, and exerts less pressure on the work. Fig. 1-3 shows a full size hacksaw capable of taking 10in and 12in blades at the top, a junior hacksaw in the middle, and a piercing saw at the bottom.

Files

There is a multitude of different types and sizes of file from which to choose. Most useful for small work are 6in, 8in and 10in flat, round, and square section, smooth and second-cut files. For the finest work, Swiss or needle files are required, and for cutting very hard materials, diamond files. These may be bought in a handy wallet of assorted shapes which normally suffice for most work (seeFig. 1-4).

Other shapes and sizes may be added as and when needed. Many model makers keep a set of new, or nearly new files especially for use on brass, because brass is a material which demands a sharp-toothed file. This material can be quite hard, and there is often a tendency for the file to ‘skate’ over the surface without cutting. File handles for all files are normally supplied separately.

Fig. 1-4: Example of diamond files.

Hammers

Hammers are very useful tools in the right hands; the model engine maker will use them most frequently to apply judicious blows to pin punches, centre punches, and parts reluctant to slide by hand. Much skill may be exercised in directing a measured tap in the right place, and in the lathe the use of a hide mallet in order to ‘true up’ a piece of raw material prior to final tightening of the chuck can become a very useful skill.

Most useful are ball pein hammers, particularly small ones for delivering a sharp but accurate tap exactly where it is needed and the ‘ball pein’ part is essential if riveting parts together. Hide mallets or mallets with a nylon or soft metal facing find their uses too, for delivering measured blows to ensure parts are in contact with the parallel strips in the vice when setting up for milling, because parts sometimes tend to ‘ride up’ when being tightened in some vices. Despite the violent nature envisaged by some, hammers are essential when used correctly.

Slip Stones

Slip stones (India stones) are another useful hand tool, particularly for dressing seatings for ball races because they remove very small amounts of metal, leaving the surface smooth and polished. They may also be used for dressing tool points between regrinds.

Slip stones are available in various types of grit and grade for different materials, and they are normally flat, square, triangular and round. Plastic wallets are sometimes available containing assorted shapes and grits for a range of materials and purposes.

HOLDING AND CLAMPING TOOLS

There are numerous ways to hold the work, and clamps may be ‘G’-type clamps, calliper-type clamps which extend to hold large pieces of work, or toolmaker’s clamps (parallel clamps). They each have their own advantages depending upon the task in hand, and this very quickly becomes clear in use.

Clamping work on drilling and milling machines is one of the most important uses for clamps, yet for many years machinists made there own by milling tee nuts and bolts to fit the slots of their individual machines. Eventually manufacturers realized that with a continuing emphasis on safety and holding work securely, something easier was needed, and now it is possible to purchase very efficient, easy and quick systems for clamping all manner of work. Many modellers still prefer to make their own, however, feeling perhaps that they can ‘tailor make’ the tools to better suit their needs.

Model maker’s universal vice: This very useful tool can be set at any angle desired owing to the ball joint on which it is mounted. There is usually a ‘table clamp’ forming the base, which allows it to be fixed temporarily on to most surfaces for use in filing delicate pieces which need close attention. (SeeChapter 2Fig. 2-13.)

Fig. 1-5: Modeller’s hand vice.

Hand vice: This useful tool may be employed when a component can be held in the hand and filed, but where just a little bit more clamping pressure provides the steadiness required for more accurate work. (SeeFig. 1-5.)

TOOLS FOR CHECKING, MEASURING AND ‘MARKING OUT’

The basic tools for marking out include scriber, steel rule, engineer’s square, ‘odd legs’ or ‘Jenny’ callipers, dividers, centre punch and some form of datum surface (seeFig. 1-6). The best solution here is a small surface plate used in conjunction with a scribing block or height gauge. It is quite possible to manage with very basic tools, because much of the work for miniature engines is turned on the lathe, making marking out somewhat specialized.

Fig. 1-6: Basic ‘markingout’ tools.

Typical marking out often involves the placement of holes on a turned part such as those for the cylinder in a turned or milled crankcase, or the placement of bushes for receiving tappets, or simply marking where fixing studs may be required. Much of this may be achieved with the odd legs callipers, if only as a guide for further indexing with the slides after picking up a datum edge.

Having said all that, it must be admitted that a typical part which does require complete marking are the rocker arms for the valves because they will be made out of ‘gauge plate’ (HCS); but even here there is a lot of leeway because the important features will be machined to suit adjacent parts, and so the marking out process becomes mainly a guide. The lines are used for ‘roughing out’ and a close watch kept on sizes as you go by means of micrometer measurements, or by use of a modern digital calliper (seeFig. 1-7).

Fig. 1-7: A digital calliper.

‘Vee’ blocks are useful for dealing with work which is round, as they allow secure holding with the clamp and may be drilled as well as held for marking out.

The main reason for ‘marking out’ is to place an accurate representation of the part to be made on the surface of the chosen material. To this end a layer of copper sulphate or ‘marking blue’ was traditionally applied to the surface in order that the subsequent scribed shape would easily show up. One modern method of achieving this is to use a bold permanent marking pen where the lines are required, and the scribed lines are then drawn into the pen marks. Where holes are to be drilled, crossed lines indicate the centre, which is then punched with a centre punch and a light hammer or with an automatic centre punch to give the twist drill a slight indentation in which to sit. Twist drills are notorious for ‘wandering’, which is often the most common cause of misalignment of holes. The use of a centre drill to start the hole, and then a pilot drill to clear a path for the web, especially with large drills, helps to avoid this.

Large holes may be ‘boxed’ by scribing lines round a scribed circle drawn with the dividers, so putting the desired hole inside a ‘box’. When drilling commences it is then possible to watch the circle made by the drill point as it enlarges to meet the sides of the ‘box’, and offers the opportunity for a slight correction before the hole is fully formed. (SeeFig. 1-8.)

Fig. 1-8: Example of marking out for a ‘boxed hole’.

TWIST DRILLS

Twist drills are commonly available in either plain carbon steel or high speed steel (HSS). It is always better to purchase HSS twist drills because the steel alloy from which they are made is far stronger and more durable than plain carbon steel (see the following section on materials). The accompanying diagram of a typical twist drill shows all the important angles governing the efficient operation of the drill; it also shows the names of the various parts, because grinding the drill requires an understanding of these features in relation to the metal removal (seeFig 1-9).

Fig. 1-9: Features of a twist drill.

Twist drills are commonly available in two types: straight shank, and tapered shank. Most small work is done with straight shank drills because they fit into a variety of different chucks both on the lathe and in the drilling machine. Tapered shank drills are more often used for larger holes – for example, 10mm and upwards – where the rigidity of a shank that fits directly into the machine spindle offers maximum support.

A good range of twist drills is necessary to bore the sizes of hole required in making a miniature engine. Large holes will be needed to allow passage of the boring tool, for example when opening up the chamber of the crankcase, whilst very small drills will be needed for drilling the fine passages in the carburettor. A set of metric drills from 1mm diameter up to 10mm diameter may do for a start, but as progress is made there will be a requirement to increase this range at both ends.

A set of imperial sizes will also prove useful because many holes are an ‘odd’ size, and to have the flexibility of either metric or imperial is invaluable. Fine drills are available in tenths of a millimetre, and also ‘number’ drills for almost any size of hole from 0.0135in (No. 80) to 0.182in (No. 14), and so it is worthwhile to increase your range of drills until most sizes of hole come within your capability. A visit to model engineers’ exhibitions and shows during the year is worthwhile, as there is always an abundance of tools on offer (some secondhand bargains), and many stands sell small wallets of fine drills in 0.1mm increments.

Before drilling holes which are required to be precisely positioned, it is often wise to use a centre drill. This drill has a thick shank and is double ended, with a tapered part leading to a small twist drill point, which is designed to help the accurate location of the start of the drilling operation. The thick shank aids rigidity and helps to prevent the drill ‘wandering’ at the start of the hole. The use of this drill is essential on the lathe because the tapered part is designed to make a hole for the location of the lathe ‘centre’; but it is good practice to use it in drilling on the bench drill also, if accuracy is to be maintained. (See Fig. 1-10.)

Fig. 1-10: Centre drills.

Twist Drills for Different Materials and Purposes

In order to cut with maximum efficiency the various features of a twist drill may be changed according to the material being cut. Looking at the table showing drill points for different materials, it will be seen that the included angle of the point, the helix angle, the web thickness and the land all (ideally) differ to suit the characteristics of the material being cut. In mass production work this matters most because the rate of production and hence the profit is directly linked to the efficiency of the metal cutting. For the modeller, however, such a range of different drills to cater for each material would be far too expensive and something of an ‘overkill’. (See table Fig. 1-11.)

Fig. 1-11: Table of drill point angles in degrees for various materials.

The inclusion of the table showing the ideal requirements is intended to demonstrate that attention to these features is important, and that small modifications to these areas by some judicious shaping on the bench grinder (see appropriate table) may help considerably when cutting various materials. The situation is further complicated by the thickness of the metal being cut. Normally it does not matter whether a hole is shallow or deep in a particular material, but when cutting thin sheet metals some special precautions are necessary. Thin sheet metal is notorious for causing twist drills to ‘catch’ as the drill ‘breaks through’. The cause for this is that the ‘lips’ of the drill catch in the material, which then tries to ‘ride up’ the flutes which have become a very effective ‘screw’. The figure shows the modifications to the twist drill which may help to avoid ‘snatching’.

There are various precautions which can also be taken to overcome this, including the use of a solid ‘backing’ for the drill to ‘bite’ into (timber is good), increasing the included angle of the point, altering the helix angle, and also by simply clamping the work securely. Most of the foregoing will no doubt be ‘second nature’to the experienced model engineer, and it is often the case that twist drills, having been sharpened for a specific purpose, are kept ‘as they are’ until the situation arises again. The cupboards of such a modeller will contain many such tools, very often old ones which have been modified as new tools are purchased. In this manner many useful ‘special’ tools gradually accrue for use in the bench drill and in the lathe. (SeeFig 1-12.)

Lastly an important possession for the modeller who intends to cut a lot of screw threads is an engineer’s pocket book or book of tables, which lists the tapping drill sizes for the screw threads needed. These books also include a lot of useful information, such as wire gauges, letter and number drill sizes. One such book is the ‘Zeus’ book (seelist of suppliers).

Fig. 1-12: Grinding modifications to drill point angles.

REAMERS

Reamers are necessary for finishing holes to size accurately, and are classified as hand reamers or machine reamers. Both are designed to give the closest size to the finished requirement, but hand reamers are of greatest value to the engine builder because they have a very slight taper from the business end to about one third up their flute length. This slight taper allows some adjustment in hole size, as it is possible (within this taper) to make ‘tight’ holes – for example, about 0.002in (0.05mm) below size. Hand reamers have a square on the shank for use with a wrench, but they can also be used in the drill by rotating the chuck by hand, or at a slow speed with a judiciously careful feed. Machine reamers are parallel all the way up and do not allow this flexibility of operation.

It is necessary for both types of reamer that holes are drilled first with a drill a little below the finished size in order that the reamer can remove a little metal – approximately 0.010–0.020in, or 0.2–0.5mm, depending upon the size of the hole. The amount of metal which is left for the reamer to remove should ideally be kept to a minimum as the tool is not designed to remove large amounts. In practice much depends upon the sizes of drill available (another case for a good range of drills).