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Fuel | Designs for aircraft | Hydroplanes | Practical |
Skylark | I.C. Engines | Compressed Air |
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.
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!
Fuels and Burners .
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.