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 (click to see their images) 80 Hp 912, the 100 Hp 912S and the 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 for a four seater, DynAero. Some are even building a twin Rotax powered aircraft, Tecnam and DynAero.
Rotax four stroke engine design
The three four stroke models are basically the same: four cylinders in boxer / horizontally opposed configuration, two altitude compensating (no mixture control!) Bing carburettors, 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, controlable propeller (some options can not be installed at the same time).
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.
Rotax FLYdat Indicator
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. 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.
Operating a Rotax
Before starting this engine it is recommended to open the oil sump, leave the cap off and rotate the propeller by hand (ignition off, of course) until you hear a murmling sound coming from the sump. This way you know for sure all oil is in the sump and the engine is loose (especially helpful in wintertime). Now is also the time to check the oil level. You can do that after the flight too, but be very careful then. Hot oil can burn very nasty.
Cold start
Cold starting a Rotax is not that difficult: use the fuel pump (if any) for about 5 seconds, activate the choke fully, place the throttle in idle position (or else the choke will not work on these Bing carbs) and activate the starter. A ten seconds maximum will save it from overheating, you will need to wait two minutes to cool the starter before you take another attempt to start the engine.
When it starts, close the choke slowly and take over with the throttle to keep RPM constant. If it does not start it is probably flooded: close the choke, open the throttle and start again. Close the throttle when the engine fires. If it still does not start you might have used not enough choke.
After starting
Rotax CHT Temperature Chart
When the engine starts, keep it running at or below 2000 RPM. Avoid gassing it like a car. Warm the engine for the first two minutes up at 2000 RPM, this way the oil slowly warms up and is distributed through the engine. After two minutes increase RPM to 2500 and continue warm up. The engine is ready for taxi and run up checks when the oil reaches a minimum of 40 - 50°C. Oil operating temperature is between 75°C and 110°C. Make sure that the oil reaches this temperature to boil off any condensation that might have accumulated, at least once every flying day. This keeps the rust out of the engine.
Run-up
The ignition test is done at 4000 RPM with a maximum drop of 300 RPM and no more than 120 RPM difference on both ignition circuits. If you have a hydraulic constant speed propeller then this would be a good time to cycle the propeller lever three times to circulate the cold oil in the propeller back to the engine.
Keep the RPM at 4000 and check if the voltage from the regulator is at least 13.8V so that it supplies power to the electrical system; check amps if equipped with an ammeter (positive charge). If your aircraft is fitted with a vacuum pump for the gyro's make sure the suction is in the green.
All of this is done while keeping an eye on oil temperatures and pressures. If all checks are done reduce RPM to stable idle (at least 1400 RPM) then return to 2500 - 2700 RPM.
Inflight checks
During flight oil temperature should be between 75°C and 110°C, cylinder head temperature should also be around 80-90°C and must not exceed 135°C depending on the type of coolant used, for details: move your mouse over the image to the right. Oil pressure is in the range of 2 - 5 bar (29 - 73 PSI) and on average around 4 bar (59 PSI).
Postflight
After landing and taxi back make sure to cool the engine for a couple of minutes before shutting down. Else the risk of boiling cylinder heads will be high. Cooling a 914 is even more important, reason: the turbo. The turbo can be red hot while operating and runs at a maximum of 160000 RPM !! It needs a minimum of five minutes cooling time before the engine can be shutdown. Turbo chargers are expensive machinery so make sure to cool it properly and that the engine is within operating temperatures before take-off.
