Old Rhinebeck Aerodrome:
Technical Corner

ROTARY ENGINES AND WHY THEY SPIN

The reason for a spinning rotary engine seems at first so illogical that it begs for an explanation. Fortunately when the facts and reasons for its purpose are known and understood it is inevitable to come to the conclusion that only the rotary engine format, and no other, could have accomplished what it did in fueling the terrific growth and performance of early aircraft.

The brothers Louis, Laurent, and Marc Seguin owned and operated an engineering manufacturing firm of high integrity and long standing in Paris. Their grandfather had built the first French locomotive in 1829 and pioneered suspension bridge construction, among many other innovations. Interestingly, the family descended from the very first aviators, Joseph and Etienne Montgolfier who designed and built the first hot air balloon. By 1906 they began focusing on the then relatively new internal combustion engine. Being engineers of top caliber they were up to date on the very latest advances in metallurgy and it came to their attention that a new alloy of steel, namely chrome nickel steel, had wonderful properties of strength, corrosion resistance and machinability that could well be used in a gasoline engine. They decided to build an engine out of it. So the simple explanation for why the rotary engine came to exist could be summed up by saying that it existed because chrome nickel steel required it! But a little more explanation is in order.

After working with the new material, the Seguins hoped to build a lightweight aircooled engine for airplanes entirely out of this wonderful new steel which was very strong and stable in heat coefficient, that is to say, it expanded and contracted with heat less than other metals. They did not intend to incorporate aluminum, which at that time was a poor material, not well alloyed or reliably manufactured. Aluminum from the 1915 era onward is much like modern aluminum, but the aluminum of 1908 is brittle and eventually crumbles to the bauxite dust it came from. They wanted to build an engine that would be reliable and strong. Chrome Nickel Steel was the key. They soon realized after running the prototype that not enough air for cooling could pass over the hot cylinders at the very slow 30-40 mph speeds of the early airplanes unless aluminum heads were used. The cylinder head and exhaust valves of an engine, though not so much the cylinder itself, becomes extremely hot at high power settings. Their brilliant solution then was to retain a cylinder made entirely of steel and to let the cylinders spin around the crankshaft, making their own high speed breeze. In fact regardless of forward speed, the cylinder head of a rotary engine at full power of 1200 rpm is moving through the air at 150 mph. The rotary engine incorporates the only all steel cylinder ever used in an air cooled engine. All others use either an aluminum or heavy cast iron cylinder head.

The terrific strength of Chrome Nickel Steel allows all the major parts of the engine to be very thin and also very light. Light weight was considered the key to successful airplane design in 1908 and for many years after. Consequently an engine of comparatively large swept volume could be made while still retaining a light weight. The "Power to Weight Ratio" became a benchmark of aircraft engine design in 1908 and still is today. The rotary engine led the competition by a mile. Realizing they had a winner the Seguins named the first engine the "Gnome" (an industrious little worker). And it was. The advantages and reliability of the Gnome were as obvious and desirable to aeronautical engineers of the day as the turbine engine was 50 years later.

The first Gnome Omega 7 cylinder rotary engine of 50 HP. was purchased by Louis Paulhan and mounted on a Voisin biplane. The big biplane astounded the crowd at the 1909 Reims Meet, not for the swift performance of the lumbering boxkite itself, but for the consistent and unchanging droning as it circled the field hour after hour without stopping. It was clear to all present that true reliability had at last been created and the engine itself became the sensation of the show. No longer relegated to short flights around the home field, a true cross country aeroplane was born. Four thousand rotary engines were built before the Great War broke out in summer 1914. (But not all were sold. An enterprising dealer must have found a warehouse full of them in France for as late as the 1930's, new 1909 engines were imported, advertised and offered for sale in the US.)

Improvements and imitators grew in the next few years. During the war very high powered Gnomes and similar Le Rhones of 100 to 160 HP were the chosen engine of many fighter airplanes. (The zenith of the design was the British Bentley rotary which achieved 230 HP. in the Sopwith Snipe.) The high power to weight ratio kept them in strong favor throughout the war, notably Sopwith, Nieuport and Fokker making the most use of their wonderful characteristics. Tens of thousands were built.

But as swiftly as they dominated the aviation scene for 10 years, they disappeared completely at the end of the war in 1918. Why? The answer is partly in reality and partly in perception.

The prewar rotaries were expensive engines. Worth it, but expensive. Much of the flying of those days was exhibition flying, aviators charging huge amounts to large crowds that would see their first aeroplane fly. The incomes of many pilots were astronomical as a result. So too the expenses, but it could always be made up at the next exhibition. If the rotary used a gallon of expensive castor oil which it flung out needlessly every hour, what matter? If the fuel consumption was a bit high there was always room for a bigger gas tank. During the Great War, governments payed the bills, and the stream of expensive parts, gas, oil, and labor were thrown away along with an entire generation of young men in Flanders Fields. But when the war ended, and individuals had to pay their own bills to fly, a simple and inexpensive machine that sipped fuel and used no oil was the priority instead of the large, heavy and powerful airplanes that had come before. The rotary, for all its advantages, was considered an extravagant device. And that was not the way the peacetime flying world was headed in the lean years of 1919 and beyond.

In the intervening 90 years many myths have been written around the rotary engine, some intentional and some old repetitions. In the 1930's the pulp magazines told of daring tales of dogfights, and being written by men who never flew behind a rotary they got the details wrong. Stories were dramatically embellished, the real operational characteristics lost. Today many pilots have the sense that a rotary is an unthrottleable full speed engine with a terrible torque effect on the airplane its attached to, and the castor oil it throws will make your bowels move.

Fortunately, none of this is true. It does throw a castor oil mist, leaving that wonderful smell many know from days of model airplane engines, but not enough to effect the pilot. Why castor? Not, as you might have heard, because it is insoluble in the gasoline mixture fed into the crankcase. It so happens that oil made from the pressing of the castor bean maintains its lubricity under high heat and pressure (such as the cylinder wall of a rotary engine) better than any other lubricant ever made, including the finest petroleum lubricant blends made today. Castor is quite simply a superb oil.

The issue of torque and gyroscopic precession is hardly felt on the Le Rhone 80 HP. engine, even in very small airframes such as the Nieuport 11. The larger Le Rhone 120 HP. and Gnome 160 HP. engine have a noticeable effect, but only mild control pressures are needed to counter them, far less than would be required during a crosswind or flying a modern airplane on a gusty day. And whoever started the rumor that rotary engines are unthrottleable should be throttled! In fact, all rotaries have two piece air/fuel throttles that act very much like their modern single piece counterparts once they are understood. They allow single lever normal throttle response from idle to full speed and anywhere in between. The exception to this is the Gnome Monosoupape, which has a clever pilot controlled cockpit switch allowing it to run on 1, 3, 6, or all 9 cylinders giving a fine variation in power output, just like a throttle. So if that is true, what was the purpose of the blip switch on the control stick that would short out the ignition? Interestingly, factory manuals often discouraged cutting the ignition dead by use of the blip switch and emphasized the use of the throttle instead. Unfortunately today we don’t fly from the large square fields of WW1 where we can always land in any direction right into the wind. So there are some circumstances, such as landing an airplane with no brakes and a tailskid on a narrow runway, where instant full power may be required to put a blast of air over the rudder to help with steering to a stop. The blip switch is very effective for this, and gives power more reliably in a hurry than the throttle might. So today when we land a valuable WW1 fighter we make our approach and landing "on the blip" as an extra margin of safety and controllability. The fuel/air throttle is already set in the right place for full power. Release the blip and we don’t have to fiddle with the mixture lever in any way during a critical phase of flight.

The major manufacturers of the rotary were as follows; Gnome and Le Rhone in France (actually the same firm); Oberursal, originally a German licensee of Gnome/Rhone that built a direct copy; Clerget, also of France, arguably the best FrenchWW1 design; Siemens and Halske, the great German engineering firm who took the concept in an interesting direction by gearing the engine 2 to 1 for even more torque; and Bentley of England who adopted linered cylinders, aluminum pistons and many other detail improvements to produce the highest powered version of them all. There are other, lesser manufacturers who briefly came and went.

Decades ago, Cole Palen and Paul Richards called the great engine and car designer W.O. Bentley in England to inform him that they were running a B.R.2 engine at the Aerodrome. Mr. Bentley’s initial reaction, duplicated by Paul in a broad British accent was, "Good heavens! What are you boys doing with my engine?" The Bentley powered Sopwith Snipe was Cole’s most prized possession, the one item of the initial 6 plane collection at the Roosevelt School that he placed a separate bid for in case he wasn’t the high bidder on the whole lot.

Rotary engines are smooth running and powerful, turning large diameter propellers up to 9 ft. long at very slow and efficient rpms that allow tremendous thrust. They are perfectly suited to the high drag airplanes of their day and when carefully understood and maintained are quite reliable. They are fun to watch and fun to fly. As of course they must have been, to be the great success story they created for themselves in the history of aviation.

Chad Wille
Director of Maintenance, Old Rhinebeck Aerodrome.