Help with design of a Flat Twin .18 c.i Model Airplane Engine

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MickeyDD

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Hi all,
This is my first post since I got my Sherline lathe and mill about 12 months ago.
I have just completed my first two single cylinder .049 model airplane engines (one is the ML Midge diesel and the other one is the EZE glow).
I would like to build a flat twin boxer (both pistons firing at the same time) model airplane engine and I have a few questions. I will be using pistons, cylinders, heads and connecting rods from two K&B Torpedo .09 green head engines. I will fabricate the crankcase, crankshaft, exhaust, backplate, front plate, needle valve assembly etc. I am not sure about how to do the intake for this type of engine. I have been unable to locate any plans for a twin engine that would show the machining required for the intake.

a) What crankshaft timing setup is required? Single rotary opening in the crankshaft for both cylinders with the venturi in the middle? Rear intake with rotary disc? Reed?
b) Should the crankshaft have to be much bigger than the stock one from an .09 engine?
c) Are there any other considerations when building a twin that I should be aware of?
Any drawings or sketches would be greatly appreciated so that I can get started.

Thank you in advance. I would like to start making some chips.
Michael
 

WOB

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The Fig Tree Pocket Twin found here http://adriansmodelaeroengines.com/catalog/product.php?cat_id=40&pid=253 on page 78 is similar to what you want. It is a diesel, but could easily be built as a glow-plug engine. It uses reed valve induction which is much easier to build than any other design. I have just finished building it and will be attempting a first run soon. The only part that is a little tricky to make is the one piece crankshaft, but a simple jig held in the lathe chuck makes it fairly easy.

WOB
 

MickeyDD

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Hi WOB,
Thank you for the quick reply. I will check out the Fig Tree Pocket as you suggest and will post my progress with this project.
Michael
 

Brian Brown

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Hi Michael
You mentioned that two pistols fire simultaneously, that sounds like one bank followed by the opposite. If you are building a two stroke which I assume is the case as you are using K&B Torpedo components, then crankcase compression could be an issue. Two strikes use the up stroke of the piston to create a partial vacuum to draw the fuel mixture into the crankcase, and then compress that mixture so as to charge the cylinder via port(s) opened by the descending piston. The ascending piston compresses the mixture which is then ignited by the glow plug. Simultaneously the rising piston draws fuel into the crankcase this restarting the whole cycle. Now, with one bank ascending and the other descending, then there will not be any crankcase compression, which is vital.
Hope that this makes sense.
 

Brian Brown

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Sorry Michael,
i misread your post, I somehow assumed that you were making a flat four.
apologies,
Brian
 

Arild

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Hi Michael
You mentioned that two pistols fire simultaneously, that sounds like one bank followed by the opposite. If you are building a two stroke which I assume is the case as you are using K&B Torpedo components, then crankcase compression could be an issue. Two strikes use the up stroke of the piston to create a partial vacuum to draw the fuel mixture into the crankcase, and then compress that mixture so as to charge the cylinder via port(s) opened by the descending piston. The ascending piston compresses the mixture which is then ignited by the glow plug. Simultaneously the rising piston draws fuel into the crankcase this restarting the whole cycle. Now, with one bank ascending and the other descending, then there will not be any crankcase compression, which is vital.
Hope that this makes sense.
The pistons are moving the opposite direction. This mean you will get double the underpressure and double pressure compared to a single sylinder...= no problem at all.
 

Arild

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Sorry Michael,
i misread your post, I somehow assumed that you were making a flat four.
apologies,
Brian
... And I didn't read this before answer to your first post! (and you absolutely right, for a four sylinder boxer, he will struggle a bit) :)
 

777engman

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View media item 1245Actually flat four two strokes are very simple to make, set up as two pairs firing 180 deg from each other on separate sealed crank case cavities.View media item 1246this engine is center rotoary valve intake, while the V8 is twin center rotay valve intake
regards
Dean
 

MickeyDD

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Dean,
I tried to pull up the link to your V8 engine but got an error message. Could you please send another link. It could be my next project.
Thanks,
Michael
 

Crawdad

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I'm getting error messages when I try to access either album, Dean. It appears that I don't have permission to access those albums. Is this a setting that the owner would be able to change?

Bill
 

Cogsy

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I'm getting error messages when I try to access either album, Dean. It appears that I don't have permission to access those albums. Is this a setting that the owner would be able to change?

Bill
Where are you trying to access this 'album'? The links given above are both just links to the 2 images shown in the thread, hosted in the media section of the forum, so no plans, etc.
 

stackerjack

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Where are you trying to access this 'album'? The links given above are both just links to the 2 images shown in the thread, hosted in the media section of the forum, so no plans, etc.

You need to be logged on to see the links.
Jack
 

Crawdad

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Where are you trying to access this 'album'? The links given above are both just links to the 2 images shown in the thread, hosted in the media section of the forum, so no plans, etc.
Cogsy - when I click on either of the links in messages #8 or #10 in this thread, I get "Home Model Engine Machinist" page with the message "You do not have permission to view media within this album.". I am logged in - evidenced by my being able to respond to messages such as this one. My terminology is derived from that used in the error message.
 

Cogsy

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Cogsy - when I click on either of the links in messages #8 or #10 in this thread, I get "Home Model Engine Machinist" page with the message "You do not have permission to view media within this album.". I am logged in - evidenced by my being able to respond to messages such as this one. My terminology is derived from that used in the error message.

Hmm, can you see the pictures in the posts themselves? These pics are the media that the links point to and are just individual pictures rather than albums, plans, build logs, etc. I'm not sure why you can't access the media itself though.
 

Crawdad

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Hmm, can you see the pictures in the posts themselves? These pics are the media that the links point to and are just individual pictures rather than albums, plans, build logs, etc. I'm not sure why you can't access the media itself though.
Nope, there's just a hyperlink, no pictures visible. From the error message, I'm guessing that there's something to do with user privilege that is inhibiting some of us from seeing the pictures.

Bill
 

lohring

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Any of the intake types will work. The easiest will be a Cox style reed valve. See the below links for the basic design:
http://www.exmodelengines.com/product.php?productid=17896&cat=268&page=1
http://www.exmodelengines.com/product.php?productid=17903&cat=268&page=2
You can also make a slightly better version:
Reed Valve.jpg

A crankcase rotary valve will give better performance. It can be in the crankshaft or a separate valve as shown. It should open around 120 degrees before TDC and close about 50 degrees after TDC.
Russian Drum Valve.jpg

The best performance will come from a disk valve or bell valve:
Disk Valve.jpg HB Bell Valve.JPG
They can start with similar timings but can run much higher timings, especially with a tuned pipe.

Lohring Miller
 

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