Rotax Aircraft Engines, I
Rotax four stroke aircraft engines have proven themselves as reliable engines. They can be bought as experimental (UL) or certified (A and F) engine and there are three main models to date.
The original engine is the 80 Hp 912, from which the 100 Hp 912S was developed and they also have a 115 Hp turbocharged 914 engine.
Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the french market) and the 914 has red covers and a turbo. These are the most obvious differences.
Technical data on the Rotax engines can be found in these pdf files: Rotax 912 UL/A/F, Rotax 912 ULS/ULSFR/S and the Rotax 914 UL/F.
Rotax Aircraft Engines are popular in the experimental aviation community especially in two seat Ultralight (ULM) and Light Sport Aircraft (LSA) models.
One manufacturer uses them even in a four seater: Dyn Aero. They are even building a twin Rotax engined powered aircraft: Tecnam and Dyn Aero use the 912S model for that.
Rotax four stroke engine design
The three four stroke models all share the basic design you will find on most aero engines: four cylinders in boxer / horizontally opposed configuration.
The Rotax has two altitude compensating Bing carburettors (no mixture control), dual electronic ignition, gearbox with 1 : 2.43 ratio (the 912 and early 914 have 1 : 2.27 ratio's), liquid cooled cylinder heads with a radiator, air cooled cylinders, external oil sump with radiator, mechanical fuel pump near the gearbox (the 914 has two electrical fuel pumps).
A number of options can be installed on the engine: vacuum pump, alternator, controllable propeller (some options can not be installed at the same time).
Complex engine?
When looking at a Rotax engine installation the first thing that everyone notices is the amount of wires, coolant and oil lines, its compact size (small, compared with an O-235) and lightweight. To hear a Rotax run for the first time has the same effect, it is not what you are used to and when one flies over the sound is quite different too, you will notice the sound of the gearbox.
Other aero engines roar when they take off at full throttle, these Rotax engines whisper! Which helps with the noise statement.
Liquid cooled
Rotax was one of the first engine manufacturers who designed their engines with a liquid cooling system. Some say its too complex, but in my experience there hasn't been much trouble with it. The system consists of a coolant bottle with pressure cap and a cam shaft driven water pump to cool the cylinder heads.
A radiator sits usually below the propeller in the wind stream. In winter time you might need to block some of the airflow through the radiator. By using a thermostat in the coolant pipes together with a thermostat (82-90°C) in the oil system the engine will remain on temperature perfectly.
Non direct drive
Where the Lycosaurus engines are direct drive and run the propeller at the same RPM as the engine. The Rotax engine is normally run (cruise) between 4900 and 5100 RPM (engine RPM, divide that by the gear ratio to get propeller RPM) where they have their maximum torque. Maximum take-off RPM is 5800 for 5 minutes. Maximum continuous is 5500 RPM and idle is around 1400 RPM.
Rotax FLYdat Indicator
There are a number of engine RPM indicators available on the market today which have the right color/rpm bands for a Rotax four stroke. These are very handy and it is a lot better than having to remember all the numbers like you need to do with a FLYdat. One look is just enough with one of these, see the picture.
