Part 8

After a too long period finally the parts about the transfer ducts. I've got lots of E-email about when I would continue the article.
Lots of reasons and excuses but I didn't have the time too write about it. Now the model flying season is over I have made some time to finish it.

The shape of the transfer ducts will not be described (it is not a secret) because this would make the article boring. The idea behind the transfer ducts and how to get them in your negative mould will be the main issue.

Lets start with the front transfer duct. Axially the side transfer ducts are different of shape but the idea is the same.

The blue line in this picture shows the shape of the front transfer duct. The depth of the transfer duct is 1.7 mm. The surface area increases 10%, this to accelerate the gases.This means that the transfer duct will be less wide on the most upper part.

This picture shows 3 drawings. First about the top view. The green lines most close to the cylinder shows the transfer duct on the highest points. The outer green lines is an offset of 2 mm (you need a certain wall thickness). The blue lines most close to the cylinder shows the transfer duct at the lowest point.
The front view shows the shape of the transfer duct and here it is clear to see that the duct gets less wide. The 2 holes in the middle of the duct are to screw (M2 bolts) the ducts on sticks.

Take a good look at the pictures below and you will understand what I mean with sticks. The 3 transfer ducts are screwed onto these sticks and slide into a middle part.

This middle part has a diameter of 16 mm. The cylinder will have a diameter of 18 mm so there is not much to turn away later. Realize that every solid part in the negative mould will be an empty space in the wax crankcase.
The side view shows the front transfer duct (green line) and the so-called 'middle part.'

The duct in the middle is the front transfer duct (see also the drawing above). The other 2 are the side transfer ducts. These are also screwed on sticks and fit in the 'middle part.'
See picture on the right for the 'middle part'

The 3 ducts fitted on sticks.

The middle part where the sticks will slide in.
Below a drawing where part C is the middle part

How to make the three ducts: This is easier as I could imagine myself. The cylinder boring in the crankcase is 18 mm diameter . The middle part has a diameter of 16 mm. This means that when the front duct is 1.7 mm deep (measured from the outside of the cylinder) the duct should have an outer diameter of 19.7 mm (which is 18+1.7) The inner diameter should be 16 (the same as the outer diameter of the middle part. So what you make is in fact a tube or pipe. You know how your front transfer duct should look like (see drawing above). The side transfer ducts are a little different. Because the side transfer duct is 3.3 mm deep at its lowest point and 1.7 mm at its highest point the tube will will be a conical. Calculate the angle! The inside of the tube will be 16 mm diameter. Mill the ducts out off these tubes but make the holes with M2 thread first. Make sure that you work from a certain reference point.

The drawing on the left shows the side ducts. As you can clearly see is that the ducts starts deep (3.3 mm). They are there only 5.5 mm wide.
Remember the article (How I make my engines) where I wrote about the ducts.

I wrote:
First, the side porting is different. At the lowest point the port is approx. 3.4 mm deep and 5.5 mm. wide. At the highest point it is only 1.7 mm. deep and as wide as the port in the cylinder. The square surface area decreases by 10 % so assuring an acceleration of the gasses.

The idea behind the deep side ports is following: take one of your engines and take the cylinder and piston out. Take a good look inside at the side ports. They are nicely formed. Put the backplate in.
Conclusion: the backplate closes a part of the side ports so the effectiveness of these ports will be less.

I still believe in these kind of ducts but can't prove that it is better.

Well, the ducts are one thing but a crankcase needs sometimes a certain wall thickness. So ones you have decided how the ducts should look like make an offset of 2 mm all around the duct. Make good drawings, decide what kind of milling tool you will use and ask your wife if you didn't make a thinking mistake. Mill the duct outsides (=duct inside + wall thickness) out of the mould halves.

Above a drawing with 3 parts (A, B, C) and an overview with crankcase (negative in mould). Part A fits into part B and part C fits in Part B and holds also part A in place. Above part C you see the top view of the middle part (part C) and 3 slots where the 3 sticks fit in. Later when the wax crankcase comes out off the mould the first action is to pull the middle thing out (Part C). The ducts with sticks will stay in. Then the next action is to take the front duct out and finally the 2 side ducts. Make the sticks so long that they are looking out off the mould.
Next action is to pull part A out off the crankcase and than part B. The inside of the crankcase is ready except for some turning afterwards to bring it to the right dimensions.The parts A and B are made because you don't want to have a massive inside. Part A has a diameter of 10 mm there where the 12 mm crankshaft will be later. Part A is 14 mm where the 16*8*4 mm ball bearing will be later. Part B has a diameter of 19 where the back ball bearing (21*12*5) will be later. These parts are made so that you don't have to turn to much away later.