FLYING STEAM ENGINES

Compressed air engines were, with the use of Rubber motors the pioneer source of power for model aircraft until the advent of model Internal Combustion engines in the mid 1920's. Less common was Flash Steam no doubt because it was, and indeed remains a very demanding way of getting a model aloft. I speak from personal experience!

At that time there were combined contests where Rubber and CA. (compressed air) competed on equal terms and of course it improved the breed of both. Eventually the contests were separated and the last contests incorporating CA. as a class was I think in UK about 1928/30. I have for many years had an interest in performing a few experiments with CA using modern materials particularly for the pressure tanks which often represented half the weight of the rest of the machine. In those days brass tanks made of thin shim material 0.004-0.006" (0.1-0.15 mm) were the normal method used for making the tanks. Brass was preferred probably because it was easy to join using soft solder.

My mind turned towards Kevlar and Carbon laminates, however the contruction of such a tank is not straightforward unless weight is not a major consideration, which rather defeats the object! A tank 2" diameter to take 100 psi could be as thin as 0.006 to 0.010" (1.5-2.5 mm) at which thickness it would be almost as delicate and flexible as a paper tube of similar thickness, unless of course it were pressureised. For the same reasons it is predictable that making and fitting equally flexible the end pressure domes would present problems too. I toyed with the idea of moulding the laminate in one application over an expanded polystyrene core but that too required special consideration. Getting the core out completely and avoiding excess absorbtion of the resin by the polystyrene was the end of the idea as the thought of dissolving it out with thinners was too much of a fire risk and a possible chemical risk to freshly cured Epoxy resin.

Recently (May 2008) a small sub-group on the yahoo FFML (free flight mailing list) talked of using beer cans and other low weight freely available alluminium low pressure containers adapted to serve the purpose. This is where I am now, trying to use beer cans twice! Starting with a humble beer can you have to use several of them to make a tank of any real size and the obvious way of doing that is to join them end to end. So I made a very simple tool of alluminium with two diameters formed on the overall diameter, one being of such size to allow insertion (just) into the can then a conical section to the outside diameter of the identical can. There is only about 0.008-12" difference in the two diameters, is arguably a waste of time bothering with a tool but it does make a fair job of it and the result is more consistant. In fact I did not form my first tank of Beer cans, instead I used cans of a "EURO" form of Red Bull the Stimulation drink; after all I am after more performance! These are only 2" diameter (52 mm). As soon as I had made the tank as far as the last can I turned an alluminium adaptor to fit through the shallow domed end fixed it with Epoxy and then stuck it in position at the business end of the last can. As soon as it was cured I was in the workshop where I connected the compressor and at about 80 psi I was rewarded with this little event!

I know I should not have just filled it up willy nilly! Next time I will hydro test, I have the kit here and it is daft not to use it. I thought the dome was a bit shallow and the next stage was to spread the longitudenal stress into the body of the tank and prevent the blow out in future. The whole idea is to make a much lighter tank of some intrinsic strength for handling the model unpressurised, attachment of wings, wheels and the tail. However I also want eventually to produce a simple to make structure; such that it will withstand much higher pressures than is possible with pop (soda) bottles or the old brass tanks.

With this in view I quickly realised that I had cut away most of the radial strength of the Red Bull type tank in that I had removed BOTH ends of all the intermediate cans in the four cans I had used at that point in time. To get over the problem of the blow out I removed the end of that can, recovered the original alluminium adaptor and added another complete can and the same adaptor. I think in future I will cut away much of the intermediate can's bases but leave a ring as shown by the beer can (left). This will provide a series of really hard areas along the cylinder to relieve stress and allow attachment of wings etc. As a matter of weight saving do not think that the ends weigh so little so, "why not leave them in place". In fact if you do the experiment you will find that removing both ends of any beer can halves its weight!

This is how the Red Bull type can looked before I began fitting the balsa end plugs, these were made of three plies, grain at 120 degrees of medium/soft fixed into position using epoxy filled with micro ball filler such that it would still reasonably easily squeeze out of the joint. I fitted the plug carefully making a former that fitted the female dome accurately. I trod on the balsa dome without jumping! There was about 0.01" (0.25 mm) of crush measured but I am sure it will do the job when the Kevlar covering has been added.

I am a little bit disappointed by the weight which ended up at 70 grams including the white plastic adapter to fit direct to the AH motor. The cans alone epoxied together with the alluminium air feed stub firmly fixed in place with a threaded collar sealed with epoxy weighed 40 grams. The balsa plugs added 3 grams = 43 grams. The Aramid fibre and epoxy added another 18 grams and the white plastic AH adapter 9 grams = 70 grams altogether, however I have retained the virtue of an easily fixed motor which can be used on several different models with a minimum of fuss.

Laying up the fibre took less time than I had imagined with 40 Mins pot life I need not have concerned myself at all. Very few bubbles appeared and caused no trouble in that they were removable with the course bristle brush, most of the bubbles appeared at the joints between the cans. The outer layer at the joints is prominent, this is to be expected and I am glad I thought to fill these tiny 'steps' and smooth them over before laying up began. I will follow the work done so far by building a test tank of the same materials, made just two cans long and hydro-testing to distruction, I fondly hope that the single layer of Aramid will remain intact up to at least 200psi (14 Atm). I intend to use the real tank up to half the rupture pressure. As far as I can see there is little doubt that this very simple contruction could produce pressure vessels of very high performance indeed just by adding more layers of Aramid Fibre. Below are pictures of the complete cylinder, the plastic adaptor between tank and the standard AH motor is fairly easy to spot. The modification to the crankshaft allows me to attach any propeller I want as long as it can be drilled out to 2.5 mm; (the diameter of the crank} which I threaded ISO standard 2.5 mm course pitch. This was not too easy to do if you want details just mail me.