Past Talks : NOVEMBER 2023

“Early Aero Engines 1900 – 1920” by Graham Mottram

You can tell that Graham Mottram is a man of influence and knows his way around the FAAM. On entering the auditorium the centre of attention, on which we could feast our eyes, was an authentic Clerget rotary engine in all its metallic-majesty. Perhaps I am just weird, but they do look good and the energy potential is just bursting to come to life.

To Graham, a keen interest in early aero engines has been long standing. The feed across from automobile engines to aero engines was apparent from the outset.

In 1861, a German engineer, Nicolaus Otto developed the first successful internal combustion engine that led to the liquid fuelled engines we are now familiar with.

Ten years later, in 1871 another German engineer, Carl Benz, with the financial help of his wife Bertha, developed his own two-stroke engine incorporating many features we now take for granted, including speed regulation system, battery-powered ignition, spark plug, clutch, etc.. In due course he fulfilled his ambition to add the engine to his very novel ‘horseless carriage’ which was the first of its kind to be commercially available.

Meanwhile, in the USA, Professor of Aerodynamics Samual Pierpont Langley was contracted by the US Government to build a piloted, heavier-than-air craft. His models were successful, but a full scale flight eluded him. Frustratingly for Langley the Wright brothers made a controlled flight in 1903, using a 12hp engine. By teaming up with engineer Charles Manly who oversaw the manufacture of a 50hp 5-cylinder radial engine, Langley eventually built the ‘Aerodrome’ aircraft which also failed to fly. However, it is recognised that the project did contribute a better engine to aviation.

Shortly afterwards, in 1909, Louis Bleriot famously made the first flight across the English channel, using a radial 3 cylinder Anzani 25-30hp engine.

During this cross-channel flight, it became apparent that the aircraft speed was not enough to keep the engine cool. Good water cooled engines were being built, including the French 50hp V8 Antoinette and the British Wolseley 60 hp, but the water cooling added a heavy weight penalty, making them doubtful for aviation.

As a result, the idea emerged that better cooling could be achieved by increasing the flow of air over the cylinder heads by fixing the crankshaft and rotating the cylinders themselves.

The French Gnome engineering company introduced the 7-cylinder rotary Monosoupape (meaning single valve) engine around 1910-1911 and Graham told us that a Farman aircraft fitted with the Monosoupape won the cup for every race it entered at the Reims Air Races.

At this point Graham showed a clip of film with a gentleman in the USA opening the rear bonnet of his 1905 era Adams-Farwell car and starting the 5-cylinder rotary engine – “spins like a top” he said.

The time had come to answer the questions of why the whole engine rotated and how did it actually work? Conventional in-line engines set-up a harmonic vibration and to overcome this a fly-wheel is required, adding weight. By rotating the crankcase and cylinder heads the flywheel effect was achieved, with less weight and so was the cooling problem reduced by the speed of the rotating engine. The engine rotated around the centre of the crankcase while the hub for the piston crankshafts was slightly offset, allowing the pistons to complete their stroke as the engine rotated. Cylinders were always odd numbers, such as 5, 7 or 9 cylinders and the engine fired on every other cylinder to ensure smooth running.

On the Clerget engine in front of the speaker, each of the 9 cylinders was machined from a single billet of metal, along with the numerous, essential cooling fins. It is very impressive to see. The HT voltage spark was generated via a rotating Paxolin disc brushing against a fixed magneto. Similarly the carburettor is fixed at the end of the crankshaft (which is fixed to the aircraft) and fuel flowed through the hollow crankshaft to each cylinder. Engine lubrication used a castor oil total loss system, which easily consumed at least 2 gallons of oil in an hour and guaranteed the pilot would not suffer constipation, having inhaled the laxative effect of the oil vapour! It is interesting to consider the gyroscopic effect of ½ ton of engine spinning at the front of the aircraft – some flywheel. Precision engineering was also required throughout the manufacture to ensure the engine was finely balanced.

Around the beginning of WW1, the engineer W.O. Bentley turned his attention to aero engines. In his travels in 1913 he had come across the use of aluminium alloy to manufacture a piston, which not only made it very lightweight, it ran cooler and improved the power output. Bentley bought a set of these alloy pistons for his hill-climb event car and they promptly allowed him to win every event. Bentley then made himself available to be commissioned in the RN and became the liaison between the RN and engine manufacturers. However, his improvements were not always well received by manufacturers so with assistance from the RN, Bentley redesigned a Clerget engine and created the more powerful, 9-cylinder, 230hp Bentley BR2 engine which was fitted to several aircraft types including the Sopwith Snipe and was still in service well after the end of WW1. After some investigation our speaker, Graham Mottram, firmly believes that the Sopwith Baby aircraft in the FAAM is fitted with an early ‘hybrid’ proof-of-concept Bentley engine.

Bentley was also asked to build under licence the Renault 70-80hp V8 in-line engine as used by the Royal Aircraft Factory. However, Bentley was not impressed by the engine and answered ‘No’ and instead designed his own engine. In 1912, Peugeot built an engine with overhead valves and overhead camshafts, which worked very well. Mercedes subsequently built six racing cars using their own version of an overhead valve and camshaft engine, and went on to win just about everything at the following French Grand Prix. One of the winning cars was sent to Mercedes in London, where Bentley got to hear that it was languishing in a basement. He had it taken out, sent it to Rolls Royce, ostensibly as a basis for an airship engine, but with some redesign it ended up as the RR Silver Ghost engine. Bentley also improved several engine designs from various other manufacturers via introduction of aluminium cylinder heads.

At this time engine manufacturers took every opportunity to capitalise on any new developments and improvements and so names such as Beardmore, Hispano-Suiza, Siddeley, Sunbeam, Wolseley, all appear with their versions of engines containing the best elements from other designs.

Meanwhile, across the Atlantic, in Graham’s words, the USA found itself incapable of successfully building foreign engines, so they created the 400hp, 45º V12 Liberty engine which was used in several aircraft, including the DH9 – it was not a great engine according to Graham.

An audience question asked which is best, in-line or radial engine? To which Graham responded that there is no simple answer. The USA favoured radial engines in WW2 and the resistance to battle damage is impressive, some returning from operations with a whole cylinder missing, but the engine still running, etc.. However, it was easier to develop more power on in-line engines designs, whereas radial engines could only increase complexity by adding more and more rows of cylinders. I have read that the Convair B36 bomber had six, 4-row 9-cylinder engines, requiring well over 300 spark plugs to change. In parallel the UK built its own power-house of an engine in the mighty Bristol Centaurus – described by some as a 2000hp Swiss watch.

Talking of engines inevitably involves talking detail. I have omitted a fair amount of detail as you probably noticed. Even so engines are a fascinating subject, so a big thank you to Graham Mottram for a well illustrated and absorbing talk on a complex subject.