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Design and build of model aeroplane engines
updated 26/11/2013
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I started in January 2012 to make my first model engine. My first information was the PDF file for the
Boll Aero 1.8cc that can be downloaded from the model engine website run by Ron Chernich This website is a superb source for all things to do with model engines and well worth a visit.
This will be my first complete engine although I have made a lot of bits for old engines over the years. I have had the original drawing for a while but I decided to do a 3D model (SolidWorks) to get a feel for the task. I nearly always do this for anything I plan to make. Of course I couldn't resist tinkering. I wanted to work in real metric for a start so I have been going throught the design converting it to metric and while I was at it I made a few "improvements":- Here is an exploded view of the engine and links to the PDF drawings I used to make the 2 shown below.
NOTE: these drawings should be error free but please be aware that I did them for my own use and not as a definitive drawing set. Any errors may be reported to me.
exploded engine
PDF drawing list

I have completed 2 engines and both have proved to be easy starting smooth running engines. I will be posting a set of drawings as soon as I have finished checking them. I have called the engine a "Nelson 2cc" as it has little in common with the Boll that inspired me to start the process. Only the basic concept remains common and that is also shared with a number of other makers designs for example the the ML Midge.
finished engines finished engines


Some building notes

Crankcase

case 1
Tapping the holes
case 2
Boring the case
case 3
Turning the chamfer
case 4
The finished case
case 5
The drawing
The crankcase was machined from some scrap bar. The holes were tapped in the milling machine - I have a small flat ground on the shank of my taps so that I can rotate them with a tapwrench whilst they are lightly held vertical by the chuck. I machine undersize holes for the front housing, the back plate and the cylinder using a slot drill in the milling machine then I can dial off of these holes to set the block up in the lathe for final turning.
Conrod turning and end radiusing jigs

I have been looking at the article on the Model engine website about using an endmill to machine the radius on the ends of a conrod. For the engines I am making I needed a couple of conrods (actually I wound up making 5) and I thought that I would follow that method but could make life easier with a couple of jigs. >
heading left The first is to hold the end of the flat rod blank in the lathe for turning the shank. I didn't fancy fiddling about with a 4 jaw chuck so I made a simple jig that acted as a back centre and also drove the rotation. This has worked very well and I did all the turning of the 5 rods in less than 30mins. heading left
heading left To machine the end rads I first removed some of the extra material in the milling machine then I had a bit of a think on how to do the milling of the radius. I like the method described by Ron Chernich to make the replacement rod for the Taipan 66 but I thought that I could make it even easier. I decided to use my trusty ML7 and work with the rod horizontal and I though that it would be easier to see what was happening. I already had a set of N0. 2 Morse taper milling cutter holders from when the lathe was my only milling tool and I thought that it would be even easier to control the rod if it was flat on a plate and I could look from above. I also found that by using a pin and spacer at the holding end I could ensure that the rod stayed parallel to the plate and it was very easy to control. Another thing I found was that, if I set the horizontal centreline of the rod end about 1mm off center so that the cutter was mainly cutting with the down moving teeth, the rod stayed down and wasn't lifted up by the cutting forces. A secondary benefit was that the cutting could be easily seen where it blended into the radius of the shank. Of course this was possible only if the rod was turned over to do the second side. This has all worked well and I improvised a stop which made the cutting repeatable - I will update the jig to add this to the design. Different thicknesses of rod will need different spacers but these are easy to make.

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