Holly Buddy (oz13674)

About this Plan

Holly Buddy 25. Model aircraft diesel engine.

Design by Ed Holly. CAD drawings by Steve Jenkinson. 9 pages of A4 drawings.

Planfile includes build notes.

Quote: "The main aim of designing and making this engine was to try to inspire others to think about designing their own engine. There are a set of parameters that for a sport engine are pretty standard within a fairly small range. These parameters are based around the timing of the 3 main variables if a front rotary engine such as this, or just 2 parameters if a piston port (or sideport) engine. Once these parameters are decided then it is just a matter of supplying the clothing to support them and there is an infinite number of ways that this can be done.

For a first design you want something that will give reasonable power and be relatively easy to start, so go a bit conservative with the parameters. Look at the various engines of the 50s and 60s and see what timing they were running and how they - A) arrange the inlet and inlet timing, B) arrange the transfer porting and its timing and C) arrange the exhaust port and its timing. You don’t want to go too big on any of these to start with, but of course you can change them with development and new components. Probably the most critical item of all is the cylinder which incorporates the transfer and exhaust ports, and having made one for the initial build then you can change the whole character of the engine by redesigning it with more overlap, bigger ports etc but retaining the rest of the components. This way you can experiment without having to build a whole new engine.

So what about the Holly Buddy. Well it was designed around a transfer period of 107° and an exhaust period of 122° with the rotary intake at the venturi of 140°. These I believe are quite conservative, but when taken into account that the transfer passages are quite small and finish not that far below the exhaust opening I think they complement one another. It would be quite an interesting engine to make 3 transfer / exhaust passages instead of 2 thereby increasing the flow (but not the speed) of gas through these ports, this is the kind of thing that manufacturers would have done to settle on a design, and it is a pretty simple manner to do this ourselves, it is just time.

The CAD plans that follow are as per the engine built, but you are encouraged to redesign the various components to make the engine truly unique to you. Some of the simple things would be to extend the nose a bit, change the shape of it, reduce the thickness of the flange where it is bolted to the crankcase square, even mill into the crankcase here to recess it a bit – there are many things you can do with this simple piece. Then there is the muff, so long as it holds the cylinder down and has a reasonable surface area on the fins, especially at the top, why not make it tapered or a ball or an elliptical outer shape..."

Direct submission to Outerzone.

Quote: "Hi, I have just finished my 22nd model aircraft diesel engine - most of which I have designed over the years. There are a few of us that build model engines - all mine are diesels - around the world and there is no central repository for plans of them. They are out there but not easy to access. Therefore my query is would it be possible for OZ to incorporate engine plans as a section within your fantastic plans collection. I would certainly help you with quite a lot - and I am sure others would jump in and add as well. One of my engines is the Holly Buddy - which was tested and written up by Maris Dislers and published in Aeromodeller magazine some time ago... Kind regards, Ed Holly (Sydney Australia)"

Supplementary file notes

Review, from Aeromodeller, Jan 2019.

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