Rotary Engines

Engines are obviously a key component of a successful airplane design. Aircraft engines these days are so reliable and powerful that pilots sometime forget how central they are to successful flight.

Early version of the Wright brothers glider in flight

Early version of the Wright brothers glider in flight

When the Wright brothers first flew a man carrying powered airplane in 1903, numerous other aircraft designers had already fashioned successful glider aircraft. The thing that allowed the Wright brothers to be the first to fly a powered aircraft was the design and use of a lightweight and powerful engine.

The 1903 Wright Flyer’s engine was exceptional in several respects. Chief among them was the first use of lightweight aluminum for the engine crankcase.  This four cylinder engine produced 12 horsepower which was more than adequate as the Wright’s specified the need for a mere eight horsepower to achieve successful flight.  They were correct on this calculation with their historic flight on Dec 17, 1903.

Front view of rotary engine showing copper fuel-castor oil intake tubes

Front view of rotary engine showing copper fuel-castor oil intake tubes

The challenge remained, however, to produce a light weight and powerful engine for the more advanced and larger aircraft on the drawing boards. For a variety of technical reasons the rotary engine came into favor for these early aircraft designs.

The rotary engine is unique in that the crankshaft is fixed to the aircraft’s fuselage and the cylinders rotate around the crankshaft. The propeller is bolted onto the cylinder assembly.  Note that this is the complete opposite of the widely used radial or in-line engine, whereby the cylinders are fixed and the crankshaft rotates.

In these early days of flight the rotary engine offered several advantages. First was smooth running due to the lack of reciprocating parts combined with the fact the rotating crankcase and cylinders acted as a flywheel.

Clear view of the spinning rotary engine cylinders on a Fokker Dr.I triplane

Clear view of the spinning rotary engine cylinders on a Fokker Dr.I triplane

The second advantage was that a rotary engine did not require a separate and heavy flywheel, for reasons stated above.

Finally, a rotary engine provides its own cooling as the cylinders rotate in the airstream. Thus, no radiator and associated weight is required to maintain normal operating temperatures.

A rotary engine lacks a true sump; lubricating oil needed to be mixed with the gasoline. Castor oil was the lubricant of choice as it mixed well with gasoline.  The gum forming characteristics of castor oil were not relevant as the oil was burned away during engine operation.

As a result of the layout of a rotary engine it was difficult to have an easy throttling system. Various valve designs allowed for some use of a throttle.  Many pilots employed a “blipping” scheme whereby the spark plugs could be intermittently turned off, resulting in a semblance of thrust control for landings.  However for most of the time the engines were run at full speed.  Again, I cannot imagine how the pilots of that era managed to maintain formation with this lack of precise throttle control when operating in larger multi-ship flight formations.

Perhaps the main issue limiting the advancement of rotary engines, that there was no getting around, is the fact the spinning cylinders acted as a large and powerful gyroscope. During level flight this was not really noticeable to the pilot.  But in a turn, there was no doubt as to the effect of gyroscopic precession on the aircraft.

Due to the direction of rotation of the engine, turns to the left were slow, took a lot of effort and the nose would likely rise. On the other hand turns to the right were almost instantaneous with the nose tending to drop.  This was an important item for new flight students to understand, as failure to account for this unexpected effect could result in the aircraft departing controlled flight.

While the rotary engine filled a key need at the time for lightweight fighter aircraft power, the design had built-in deficiencies that ultimately relegated these power plants to the museums of the world.

Towards the end of the war manufacturing technologies with lightweight materials, combined with improvements regarding ignition systems and valve timing allowed in-line engines to achieve higher revolutions per minute (rpm) of up to 2,000 rpms and supplant the rotary engines in use.

Original rotary engine manufactured during World War I

Original rotary engine manufactured during World War I

Rotary engines typically were limited to around 1,200 rpms. As the engine speed was raised even slightly the large aerodynamic drag increase of the rotating cylinders cancelled any true advantage.

The last practical use of rotary engines was in 1918 at the conclusion of World War I. It is still amazing to view these complex designs in operation, with the further knowledge that each of these precision workpieces was hand made.  See the videos below for additional details of the long past era of rotary engines.

Rotary engine in front of Sopwith Camel fighter aircraft

Rotary engine in front of Sopwith Camel fighter aircraft

Fighter aircraft that operated from 1914 to 1918 make for fascinating radio control models. As the aircraft of that day was essentially underpowered, the planes had to fly “on the wing.”  This is design characteristic is conducive to a well performing RC model.

There are a wide range of aircraft designs and paint schemes to emulate with your version of one of these early flyers. The rotary engine is closely identified with these aircraft and offers a unique visual effect that can be replicated on a model.

Electric powered models that make use of dummy rotating cylinders are much easier to construct due to the lack of vibration from a gas engine. Many modelers achieve a convincing rotary engine presentation by using a free-wheeling approach whereby the cylinders as the airflow to induce rotation.

 

Interesting view of a 1909 Gnome rotary engine operated on a test stand

Well done animation of a Clerget rotary engine

Fascinating view of some “detective work” on an improperly assembled original Gnome rotary engine