A 5cc version of the 1.3cc Mills Diesel.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

In my youth,many years ago,as an avid Aeromodeller,my favourite engine was the Mills 1.3cc Diesel. At one time I had four of them. They were popular due to their reliability and easy starting from hot or cold.The design was clearly about right for the style of modelling at the time,being free flight and control line in reasonably modest airframes.      I have often wondered why the design  was not taken further and a larger version produced beyond the 2.4cc that did appear.   Recently visiting the excellent site hosted by Adrian Duncan (AdriansModelelAeroEngines.com) for historical informationof “Classic” model engines, I found a reference to a prototype 5cc Mills that they did not continue with. Its external design was not exactly attractive, trying to get the weight down etc, but I then formed the idea of making a 5cc version as my next project. Several yeras ago I made the 1.3 cc version, from Ron Chernichs “Motor Boys” drawings, so a straight enlargement from theses was decided upon.

Most of the engines I have made have been from bar stock,with a few from castings. A rough sketch of the size of the block of aluminium for the crankcase made me decide it would make too much swarf, so decided to make a pattern and have a casting made in L25 Ali.   The design of the cylinder porting and timing has been copied exactlyfrom the 1.3cc design.  The only changes I have made to the internals is to add a ball race to the rear of crankshaft, alter the prop driver to collet fixing rather than taper the shaft and drill the crankshaft for lubrication of the front bronze bearing.  With the crankshaft, I decided not to machine this from solid, but to build it from separate parts. I have tried this once before on another engine a few years ago and it failed due to me having a thread to a shoulder for fixing the disc to the shaft, then silver soldering. It failed on that shoulder after a backfire.  This time I have kept the shaft parallel and the hole tthrough the disc a sliding fit.The front side of the drilling was very slightly chamfered, to act as a reservoir for the brazing material.  It all went together  very well and I have clean brass witness rings each side.

The aluminium casting did not machine well and has a lot of micro porosity that can be seen on the machined surfaces. This has prevented me having the crankcase bead blasted, as I normally do. So. I decided to have the crankcase finished Black. After testing several high temperature paints for resistance to Ether/Castor oil, I gave up and arranged to have it Powder Coated with a local company,using a Polyester based powder coat that proved impervious to Diesel fuel.   The first powder coating was not a success visually, as gas expansion  from the porosity  when heated to 200 C, blew bubbles in the paint, which then collapsed, leaving craters on the surface finish.  Afterrubbing down, filliing thecraters with JB Weld, followed byanother careful rub down the second attempt at coating was much better, but still left a coulple of marks.  Now the engine is finished and assembled, I am quite pleased with overall appearance and I think it resembles the Mills smaller brothers.   An accurate measurement of the finished engine gives a swepped volume of 5.07cc.

On the test bed, I had no trouble in finding the point of ignition. Some really vicious backfires once or twice that nearly knocked the “chicken stick” out of my hand, but then the magic sound of it bursting into life for the first time.   So far I have had a total run time of 30 minutes, in a series of around 4 minute rich runs, with the occasional burst to full 2 stroking. With cool down between each run. Fitted with a 12″x 7″ APC propeller, using Model Technics 1000 fuel. Max RPM is 6200 to date.   It has all the typical Mills characteristics of superb starting. Hot or cold, it starts first flick after a prime.  A very satisfying project.

[PropPeteParticipant @proppete]

Very impressive, Keith! Are you going to put it on your test rig to see what thrust you can get from it?

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Hi Pete,

Will do that after I have given it some more running in. Watch this space

Keith.

[JasonBModerator @jasonb]

That has turned out well, shame about the casting though I quite like the black which is a bit different to the usual bare alloy.

[half whitParticipant @half-whit]

Well done,

More excellent craftsmanship and as ever beautifully presented.

Could I ask why you use the 1000 fuel, I understand 1000 is the easiest startin fuel is that right.

Also do have any specific methods for determining the point of ignition.

Reason I ask is I  still have to run my boll aero 18. I  must get some fuel and make a start

Geoff

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Hi Geoff,  Thanks for comments.  So far as fuel is concerned, when we had the choice, I could not see much differance in the 1000 or 2000 fuel ,so far as easier starting is concerned.  I purchased  5 liters of 1000 when I had a chance, so that is what I use.   My new engine starting method starts at final assembly of the engine. I push the contra piston level with the top edge of the cylinder and do not screw it down past that point.   With the engine on the test stand, initially I do not put any fuel in the tank.  With a port prime only, I then flick 6 times, If no bang, I screw down the contra 1/8 turn and repeat port prime and 6 flicks. This is repeated until I get a bang or two. At this point I put fuel in the tank, open NV about 3 turns and usually get some sort of continuous running. I let it run rich for a few minutes, hopefully not getting too much black oil. Make a note of your settings and you should be able to repeat an easy start.   Let us know when you get the Boll-Aero going.

Keith.

[Ramon WilsonParticipant @ramonwilson3]

That’s come out well Keith despite those issues with the casting porosity. It certainly looks the part and good to hear it starts easily and runs well. Just one comment – personally (and certainly for bench running), I would consider a 7″ pitch prop far too coarse. High pitch props on a static engine can place a fair old load especially when first running. I prefer to use 4-5″ pitch wooden props to take that load off and let the engine rev – but not get ‘hot’ – usually that black exhaust is a good indicator of that. Just been running in a new PAW 19 TBR on a 10 x 3.5 tornado destined for a slightly scaled up Tigress T/R

What’s next?

Tug

[half whitParticipant @half-whit]

Hi Keith

Thanks for your reply, most helpful regarding your start procedures for a new engine.

Fully intend to run the boll aero soon, although finding a fuel supplier has not been easy. Leeds model shop stock d1000. I’m over in Yorkshire next so will call in.

Will keep in touch with the progress on my thread

Geoff

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Tug,

Good to here from you again. I seem to have quite a few props.but limited in the 12 inch range,so it will have to do for the moment and I do appreciate your comment. Good luck with the PAW.

Keith.

[Ramon WilsonParticipant @ramonwilson3]

True old saying Keith – ‘Owts better than nowt int crisis’😉. Leeds Model Shop had some nice wooden props a while back – might be worth a look.

PAW is bedding in nicely – oily MT D1000 to start then onto SMC ‘Sport Mix’ and final running on their ‘Hi-performance’. Aagh – LOVE it

[half whitParticipant @half-whit]

Hi Ramon,

It will be just the fuel I need from Leeds model shop. Finding a local north west supplier has been futile. Tried steve webb models in Frodsham only to find the shop is now permanently closed. Another one bites the dust!

Speaking of wooden props my dad was a lifelong aero modeller who all his props for many years. I still have one which I would guess is around 60 years old

[Ramon WilsonParticipant @ramonwilson3]

Hi Geoff – I guess that’s more a fan or impeller than a prop but what a fine piece of craftmanship.

Many years back we had some full size aviation overnight at the airfield where we flew. Lot’s of comings and goings of people and one came over to our group to ask if we ever used wooden props. Turned out it was the son of the person who made the original ‘STANT’ wood props of the 50-60’s. He told us his dad used to get him to stamp the blue logo on the blades – I sent him the only one I had

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