If we work on the principle that 4 stroke is more economical than 2, gas (petrol) has a higher calorific value than methanol and use recirculating lubrication rather an mixing the oil in the fuel then the specific fuel consumption (gals/hpHour) is greatly improved.
All this is well understood but why on earth would you need to bother about it for a model plane?
Going back to 1960 there was a 10 lb limit for RC model planes in the UK. The endurance record for such a plane had just been set, if I remember correctly, at 15 hours using a fairly conventional high wing plane with fuel held in two external sprung loaded 'bladder' tanks under the wing and using 2 stroke glow with petrol added to the fuel.
My Dad was a serious model engineer with an interest in small internal combustion engines after reading the report in the Aero Modeller suggested an engineering approach would produce a much better result.
As a keen 14 year old aero modeller we discussed what would be the likely minimum power that would be required to fly a 10 lb plane and how much of that could be fuel. It seemed possible that a 'good' 5 cc (0.3 cu in) would fly an efficient glider type plane and structurally 50% fuel weight should be feasible.
Much of course would depend on the engine used and given that he had longed to make a small IC engine he designed and built a 5 cc petrol 4 stroke aero engine to find out.
It took nearly 2 years by which time the 10 lb plane limit had been scrapped and the FAI had abandoned the model plane 'endurance' category!
On its display stand for the 1962 Model Engineering Exhibition now with its 'performance' carburettor. The initial 'get it going' one is underneath.
With a ringed piston and a hemi head with 45 degree inclined valves it matched a 'cooking' 5cc 2 stoke for power with only a small weight penalty.
It also used a dry sump with an ingenious recirculating oil system so it ran on straight gas (petrol) from the garage.
At the time I doubt there was an equivalent any where in the world.
He also prepared this drawing.
It uses the underside of the piston as a pump with one way valves in the back plate so the crankcase is alternatively below atmospheric pressure so sucking oil in and feeding it to the big end and then above atmospheric to blow it via a separate pipe back into the oil tank.
At full power it had virtually half the fuel consumption of a 2 stroke but even more important its part throttle consumption was whole orders better. An important characteristic for a plane that would need ever reducing power as the fuel load burned away.
My Dad's exhaustive testing indicated the engine would be able to run on a declining 5 lb fuel load for well over 24 hours. Of course it never did or even fly come to that.
I still have the engine and did consider putting it in plane but it is now firmly retired particularly as I do not posses the engineering skills to repair it in anyway.
