BollAero .49 Glow (oz14694)

About this Plan

BollAero .49 Glow Engine.

Direct submission to Outerzone.

Quote: "March 2015. Building the Boll Aero 49 Glow.

This engine follows on from my two previous diesels of 1.8 and 4.4cc displacement, which have been made by people in various parts of the world, giving their makers and myself much satisfaction in getting their engines running, in most cases at the first attempt to make an aero engine.

One of the problems encountered in some countries is the difficulty in buying model diesel fuel, or the ether needed to brew it yourself, so together with a larger displacement I decided to do a glow ignition engine, with the possibility to fit spark ignition as well.

I have kept to my priorities of very simple rugged construction above all else, with high performance and minimum weight not being so important, but the end result should be a very docile and usable engine which is meant to be flown.

The decision to use rear disc induction was made for various reasons, I wanted to avoid possible weakness in the crankshaft due to a large inlet hole, bearing in mind that in most cases the shaft will be made from any type of steel of unknown specification which comes to hand, and the diameter and consequent bearing friction needs to be kept reasonably small to be practical.

Rear induction also leaves more space for a home made variable carburettor, leaving the front end free to fit a spark ignition system if so desired. For those who may crash the engine, it is preferable to hide the delicate carb at the back of the engine, and safer for needle twiddling fingers to be further from the prop.

The induction disc is also used to help balance the engine, as the cut away part assists the crank web balancing in reducing the vibration inevitable with a large cast iron piston.

This engine has nearly the same stroke as my 4.4cc diesel, which has allowed me to carry over some of the dimensions with minor changes, but the much larger bore suits the different characteristics of a glow engine, which needs to run at much higher revs than a diesel to have a useful power output.

The piston fit is not so critical as for a diesel, in some of the early engines in the 1940s it was common to supply a separate glow head with a diesel engine, the idea being that when the engine was too clapped out to run as a diesel, it could still be used as a glow engine, although how well this worked in practice I do not know.

Small glow engines can be tricky for the home builder, as the lower torque generated makes it difficult to get them running when first assembled and a bit stiff, whereas a diesel has the torque to overcome the friction and allow the engine to run in and free off.

Glow engines also require more open porting, to achieve the required higher revs compared to a diesel,but I have still kept to a simple configuration which is easy to machine.

My usual principle of using standard alloy extrusion sizes has been continued, so the crankcase lower part is laid out to use 1.5 inch square bar, although for my prototype in fact I used a slightly larger bar which happened to be lying around, but using the correct size saves machining, and the surface finish is perfectly ok without further work.

For an engine of this size, to avoid starting with a very large block of metal, which would be difficult to source, and very expensive, it is necessary to make the crankcase in two parts, but each one is very simple, and easily bolted together to make the complete case. It is a good idea to use two simple components rather than one complex one as it avoids scrapping a lot of work if you do slip up on the machining, although with reasonable care there should be very little risk of making an irrevocable mistake.

The parts are designed with a view to making them easy to hold in the chuck, which also makes things easier than starting with castings which have an irregular shape which may require making fixtures to hold them before machining can start.

Machining model aero engines from solid bar has become common practise in recent years, with top quality engines such as the Laser four strokes being machined from the solid instead of using castings, so my approach is not in any way old fashioned or primitive, even if my designs are unlikely to be made on CNC machining centres.

The porting in the cylinder liner is fairly simple, the transfer ports are angled upwards to reduce the distance between the exhaust and transfer opening. This was done using a milling cutter to cut a series of holes, and the resulting holes were then filed to blend together and smoothed with a Dremel grinder to produce the finished shape. The porting is still a bit restricted compared to a modern engine, but it is sensible to limit the stress on home made components which might cause problems, and at normal sports flying revs the performance difference is not significant.

If you want a state of the art high performance engine, I suggest that unless you are very highly skilled you go out and buy one, but you will not gain the satisfaction of having built it yourself! My aim was to achieve sufficient power to fly a 6 to 7 foot span vintage style model, which would originally probably have been powered by a lOcc spark ignition engine. I was very pleased to meet a modeller recently who had done very little metalwork since his school days, but had successfully completed my 4.4cc diesel and got it running without any major problems.

This is the essence of what my designs are about, there is no point in publishing plans for complex engines which are beyond the scope of people like myself who do not have elaborate equipment or advanced machining skills. In most cases if the drawings look too difficult the normal aeromodeller will not even start to make the engine.

This is in no way to criticise the maestros who do successfully create scale models of multi cylinder aero engines, but their skills and and the patience needed are way beyond my ability. However with any new activity one does learn by experience and skill levels gradually do improve , which is rewarding, as my own plane building expertise , which started over 45 years ago, probably plateaued many years ago!

Screw threads. As I have BA and BSF taps and dies, these are what I used for the tapped holes, but of course you may use equivalent metric or unified threads if you prefer.

The drawings show M3 bolts, these are readily available with various head types, and a range of lengths, crucially nyloc nut are also easily available, and were used for the head bolts and the disc spindle bolt. Similarly, metric sizes of silver steel can be substituted for the inch sizes shown on the drawings.

Crankcase: I like to start with the crankcase, as most parts are fixed to it. The machining is straightforward, a reasonable finish is needed on the large bore so that the end flanges can be turned to a close fit to give a seal, I did not use any gaskets in the engine, although if needed they could be used, or a gasket sealing compound. The top face of the crankcase is also a sealing face... "

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