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All volunteers are asked questions by the visitors to MOTAT and frequently ADH volunteers are questioned as to how the radial engine works as it is so different from the inline engines. SO the first question is why did it evolve? This one is easy, a problem that was already emerging was that inline engines were getting heavier and longer thus reducing the power to weight ratio and also putting the pilot further back in the aircraft. As airframes were improving so the need for more horse power was called for and the radial was the perceived practical answer…….However….

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Before the Radial engine was successful there was a fully rotating Rotary engine which was fixed directly to the propeller. This, as you may imagine, meant the enormous spinning engine and propeller, mass rotating together as a single unit caused great flying difficulties. It was used in some very successful and famous aircraft on both sides in the first world war but was surpassed by the radial engine which was fixed to the airframe with only the crank shaft spinning the propeller, not the whole engine. And so it was that the radial overcame the mass rotation problem of the rotary.

The first person to develop a radial engine Was Jacob Ellehammer (1871-1946) way back during 1903–1904 when he used his experience in constructing motorcycles to build the world's first air-cooled radial engine. This was a three-cylinder engine which he used as the basis for a more powerful five-cylinder model in 1907. Aircraft manufacturers were slow to adopt them preferring instead to use the more trusted inline four or Vee design.

Jacob Ellehammer was a Danish born watchmaker whose genius for invention deserves a separate recognition at a later date as he paralleled our own Richard Pearse for diversity and ingenuity of creative engineering.

In a radial engine the pistons are connected to the crank shaft by a master rod and an articulating rod assembly. One piston, generally the uppermost one, has a master rod which is directly connected to the crankshaft with a weighty counterbalance opposite it. The remaining pistons connect their rod attachments to a separate ring around the edge of the master rod. Through this it makes all rods connect to the crankshaft indirectly or directly in the case of the master rod.

Four-stroke radials always have an odd number of cylinders for each bank, this allows a consistent every-other-piston firing order to give smooth operation. For example, on a five-cylinder engine the firing order is: one, three, five, two, four, and restarting the cycle with cylinder one.

This firing sequence always leaves a one-piston gap between the piston on its combustion stroke and the next piston on compression about to fire. The active stroke directly helps to compress the next cylinder to fire reducing vibration and making the motion more uniform. If an even number of cylinders were used, an equally timed firing cycle would not be feasible.

There was a prototype two-stroke radial Zoche aero-diesel which had an even number of cylinders - sometimes four or even eight - but this was not problematic, because they were two-stroke engines with twice the number of firing strokes as a four-stroke engine for each crankshaft rotation. Diesel driven aircraft operated commercially by a German airline briefly in the 1930s flying from Germany to South America.

The radial engine normally uses fewer cam lobes for valve lifting than other engine types. As with most four-strokes, the crankshaft takes two revolutions to complete the four strokes of each individual piston (induction, compression, combustion, exhaust). The camshaft ring is geared to spin slower and in the opposite direction to the crankshaft. The cam lobes are placed in two rows for the intake and exhaust. For example, four cam lobes serve all five cylinders, whereas ten would be required for a typical inline engine with the same number of cylinders and valves.

Most radial engines use overhead poppet valves driven by pushrods and lifter on a cam plate which is concentric with the crankshaft just like your car does. A few engines use sleeve valves such as the 14-cylinder Bristol Hercules as used on our Solent aeroplane and also the powerful 18-cylinder Bristol Centaurus. These are quieter and smoother running but require much tighter manufacturing tolerances. They offered greater power but were designed and built toward the end of the life of radial engines which were by then being rapidly superseded by the more efficient jet engine which offered a much better power to weight ratio and a longer service interval ratio giving us further and faster flights but also lowering travel costs for the passenger.

It is sad to see the old piston engines disappear but they were too complicated, too heavy and too thirsty for modern economies of flying - but oh-boy did they sound great! Now please excuse me while I re-charge my hearing aids.

-By Henry Swan

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