FLYING STEAM ENGINES

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Fuels and Burners .

So far I have used meths (Wood Alcohol) and propane for steam engines designed for model aircraft use. I have yet to use petrol or mixtures of kerosene and petrol. The meths burner in Comet Too is a simple half round tank with wick tubes extending into the furnace volume. The furnace is fed with air from a forward facing air scoop under the burner casing. The scoop as two functions, it feeds air to the flame of course but it also controls to some extent the rate of evaporation as the cool air hits the flat bottom of the fuel tank before it enters the furnace volume. This is not my design it is David Parkers' and for simplicity and light weight it is very good. Such simple burners are not so nice if the aircraft noses over on take off spilling boiling alchohol around . It sounds disastrous but it is not! In these circumstances the plane is close by and an alcohol fire is very easily killed by a little water. I always have a two litre plastic pop (soda) bottle full of water near to hand with its lid perforated with a dozen or two 1.5m.t14. (1/16'') diameter holes. This has worked every time (three times in 100 or so take offs) with only minor blackening of the airframe and no damage to the engine, boiler, or radio gear. This was with doped covering,. If had left the covering undoped it would have burned far less rapidly. The worst fire only took a evening or so to fix. The air frame now bears a few scars with honour and they are all good talking points. I have never had a fire in flight.
The burner for the Groves engine is propane and I may well opt to change this fuel for either butane or petrol in future. I initially chose propane because it has a significantly higher (30%) calorific value weight for weight than butane. It also has a much higher vapour pressure which demands a much stronger tank which is correspondingly heavier. Net gain Negative! My ten gram fuel tank weighed in at 52 grams empty-far too heavy. I have read various recommendations regarding test pressures for LPG tanks, I test butane tanks at 120 psi and propane at 500 psi. These pressures are a little higher than those required by the UK Model Power Boat Association. I have an uncomfortably bloated looking propane tank to remind me of what could happen if I got really careless!
Self pressurised petrol (gasoline) burners are I now think the best way forward for light weight simple and reliable operation in model aircraft. This is not new, it is ancient proven technology being used in every Colman petrol camping stove and lantern that I have seen. As the name implies it uses no pump to raise pressure but relies upon the vapour pressure of the fuel to drive fuel through the nozzle. As the flame begins to heat the tank through radiation and conduction through the tube that feeds the burner from the tank so the pressure increases and the flame becomes more fierce. Clearly this process cannot be allowed to go on and on and it is controlled by a fuel control valve that restricts the flow to a point where the heat getting back to the tank is just sufficient to keep that pressure up and thus maintain a safe equilibrium. The weight saving is considerable as the tank need only be of the actual size required as no free airspace is needed as a reserve of pressurised air above the fuel. If you doubt the pressure will be enough put a thimble full of alcohol or petrol into a pop (Soda) Bottle put the cap back on and leave it for a few minutes and the bottle will be noticably pressurised. This pressure will increase if you put the bottle in the sun or any safe (NO flames please) warmed space. Boyles Law, Charles Law and the General Gas Equation covers this area of simple physics. If you really want to understand how steam engines work a grasp of these principles are the source of all you observe and use in your car engine, it is THE heat to mechanical energy conversion technique for steam and diesel locomotives, jet aircraft and nuclear powered boilers in power stations and ships. The fuel used in Colman stoves is what I knew as SBP no 4, special boiling point number 4 or white petrol and it will I think become my fuel in the future.

Skylark's burner is a yellow flame oil burner known normally as a pot burner, it has been very reliable but it can be a smoky flame which requires a significant space between the end of the flame and the cooler tube of the Monotube to ensure that heavy soot deposits do not become a problem. I am considering a change of format and building a blue flame burner which will produce a much shorter flame and no soot at the obvious cost of more maintenance, not this season though!