Engines for Model Aircraft - Chris Murphy

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There is a vast and bewildering range of sizes and types of power plant available for powering model aircraft: 2-stroke, 4-stroke, rotary piston, single or multi cylinder, reaction jet, turbine jet and solid fuel rocket-to cover the major types. Not all of these are universally available or suitable for all aircraft types. As well there are compressed air, CO2 and steam types-though all of these latter by strict terminology are motors and not engines, since they do not burn any type of fuel within the engine.

Historically model aero engine date back to the early 1900s and in a few cases a little earlier-the first successful flight by a steam engine powered model took place in France around 1870, and in 1896 Professor Samuel Pierpont Langley’s large steam powered ‘aerodrome’ successfully flew over the Potomac river, predating the Wright brothers by a number of years. In Britain several model engineers produced internal combustion engines for models-these were generally large, heavy, and of low power-and coupled with the rudimentary aircraft then being built, not very successful-though a single cylinder 25cc model aero engine was commercially available as early as 1910 from Gamages in London, and David Stanger was manufacturing a V-4 4-stroke model aero engine as early as 1907. In the US, the Baby Engine, manufactured in Connecticut, first appeared in 1911.

Steam and ic engines were being developed and used successfully in model boats around the turn of the century-but weight was not an issue in this area, so progress was far more rapid. The true mass produced air cooled 2-stroke model aircraft engine did not make its commercial appearance until 1934 when following its devastating competition success of the prototype engine in the hands of Maxwell Bassett, (including setting a world endurance record of 13 minutes 53 seconds on September 13 1932) competing against rubber powered models over the period 1931-33, the Brown Junior engine was put into commercial large scale production. This was a lightweight spindly looking 10cc spark ignition engine rated at 1/5 HP at 4500rpm on a 14 inch propeller, and spawned a host of imitations and developments. Commercially it was very successful-over 60,000 being produced and sold before WW2.

The Brown Junior 10cc ignition engine, first large scale production commercial model aero engine.

The light weight of the engine was largely offset by the need for the engine to carry an ignition coil and on board batteries, plus a timer to cut the ignition circuit –a payload of around 5-6ozs on top of the engine weight. The outcome was models were large and light-7-8ft wingspan being typical. The small model had to await the development of smaller engines-and even when these did appear-with the Atom 09 (1.6cc) of 1940-the models were still handicapped by the need to carry an ignition coil and batteries-the weight of which did not reduce markedly with the reduction in engine size and weight. [despite many improvements this is still the case-even the smallest spark ignition engines still require a coil and battery to energise it-and this-even with the use of NiCd or NiMH batteries-may weigh as much if not more than the engine!] Even today-the smallest spark ignition engines are around 1.5cc displacement-and generally either completely homebuilt or conversions of modern-usually glowplug- engines. Very small spark ignition engines remain rare-and largely impractical curiosities.    Modern spark ignition engines however are in wide use in large R/C models-either converted from small industrial engines-often used in chainsaws, leaf blowers and weed trimmers of around 25-60cc, as well as larger multicylinder engines which straddle the boundary between large models and UAV’s, and are generally equipped with magneto or electronic ignition systems. The traditional spark coil, points and battery system is only used on vintage engines of the early era by some vintage aficionados.

The Arden 099, shown mounted with all the ignition equipment (less batteries) required for operation was the smallest commercial practical size of ignition engine

The 30s and early 40s were a period of rapid development of the US model aircraft 2-stroke engine, which remained largely a lightweight, low power spark ignition unit. In other parts of the world progress was slower-engines were less refined, heavier, fewer in both number and design, and did not have the vast resources of the USA’s light engineering mass production capacity behind them.  In Europe, at war since 1939, a revolutionary new type of model aero engine-the compression ignition ‘diesel’ appeared on the market almost unnoticed in 1940 in Switzerland. And it was truly revolutionary-doing away with the unwieldy coil and batteries, the problems of oiled contact points and fouled spark plugs by means of a high compression cylinder set up, adjustable ignition timing through the use of a variable contra piston, and the use of a self igniting fuel mix. However WW2 meant that news of the model diesel did not reach the wider world aeromodelling community until after the end of the war-the diesels only starting to proliferate in Europe around 1946-47-and in the USA yet another revolution was waiting in the wings-the glowplug engine-which first appeared on the scene in 1947.

The 2cc Swiss Dyno of 1940 was the first commercial model aero diesel engine, and set a design trend that lasted nearly 10 years

[This is going to take a while, Chris has provided hundreds of great photos but I need to work out which is which and where they fit best into the above article - Mark]

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