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Discussion in 'A Work In Progress' started by edholly, Oct 12, 2016.
And the penny drops !
Thank you diesel pilot. A quick glance at the drawing you've linked to made me realise my school boy error.
I can now do the math using the cosine rule. :thumbup:
I have always been of the opinion that most home machined engines like the Holly Buddy do look a bit squared edged which detracts a bit from their aesthetics and in fact I designed the HB this way and encouraged those making one to customize their engines by softening this aspect of them.
Just prior to the HB came a small 0.5cc conventional engine and in fact a picture of it can be found on page 1 - it is the 2nd photo in this thread. That engine inspired the internals of the HB and it gives as good a power as any of the late 50's early 60s commercial .5cc motors. It has slightly softer lines but still has that square edged look. One way around that is to do a casting, but then the cost associated with that defeats the objective in my opinion. I have spoken to Steve Jenkinson who did the HB plans about doing CAD plans for the .5cc one, but somehow we never got past the talking about it stage ..
I have been playing around with an idea of inserting a tube in a tube and have done a mock up, albeit on a larger scale than this engine will have. But of course not not sure if it will work - only one way to find out I guess. A photo of this mock up is here alongside the HB and the .5cc little fella. This item is pressed together with Loctite and pinned with 4 x 1mm music wire as can be seen in the photo below. I am of the opinion that this will work in a tiny motor like this where the firing pressure forces are relatively small, an hydraulic lock on starting probably the worse it will experience.
For the next project I would like to design and make a 020 size diesel. To make it an easy handler - a longish stroke and a piston port induction is envisioned, a Mills .75 uses this type of porting for instance.
I want it to be a practical engine that can be used to power a small free flighter, or lightweight RC model. I love really small diesels.
I have started down the road of putting pencil to paper. The engine from necessity with the cylinder in a cylinder design will need quite some thread machining, which shouldn't be too difficult as I think it can be done in the primary machining process before removing the work from the lathe in the first instance.
Some rough measurements are 9/32" bore (.2813") and a 0.32 stroke. With tiny diesels a longer stroke and piston length I think will help with gas seal on the piston / cylinder walls. This gives a capacity of 0.0199 CI.
The Cox 020 is called a Pee Wee so what should we call this ? ... How about the Wee Dee ... for little diesel ?
I will start a new thread on this if there is any interest shown ...
Good luck with it, any small engine demands precision fits and low friction to run at all, this is why DC abandoned the Bambi as it was too expensive to make consistently well, I guess Cox had very sophisticated machine tools so could overcome the difficulty of achieving tight tolerances.
I am working up to larger sizes, now have a 7.5cc diesel which flies a 7foot span plane well.
Thanks Bollaero - I will start another thread as I have started on the initial design phase. Yes fits will be very important and never considered friction - an easy build will minimize friction errors so will try to achieve that. Like your suggestion of Teeny D - but might stick with Wee Dee ... cheers
Pictures or videos?
The current AeroModeller January 2019 magazine has a 4 page feature on the Holly Buddy written by Maris Dislers.
I've been using Autocad for about 5 years now and I find I can do a drawing in about the same time as it takes to draw one with a pencil. The advantages are: Mistakes don't show and I can copy it as many times as I want and print it exact size. I learned how to use it by borrowing 3 books from our local Library and going through them during one Winter. There are also tutorials on You-Tube.
In the UK, the Model Engineering Workshop Magazine is currently running an article on ALIBRE, another drawing tool. You can also download a FREE TRIAL of the sofwtare through the magazine.
I have been thinking for a long time about building a flat twin engine - but always got hung up on the need to use either spit big-ends or use a built up crankshaft. Well I have finally bit the bullet and decided to make a twin using a "sort of" built up crankshaft. It might not work, but I think it will and only one way to find out. The twin is based on the Holly Buddy and if you are interested the thread for it is at https://www.homemodelenginemachinist.com/threads/a-flat-twin-version-of-the-holly-buddy.31463/
regards to all ... Ed
Split big ends are a feasible option, see my flat twin at https://www.homemodelenginemachinist.com/threads/longest-project-ever.27251/ Only you may need very thin bolts to hold them together. In my flat twin I used m2 Allen bolts.
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