Peter Kl. wrote:
The drawing is misleading. Text and drawing spec that the max difference shouldn’t be more than 400mm but the oil level is shown well below.
My personal take on the Rotax instructions;
The "drawing" is an illustration - ie it should not be taken too literally.
The Rotax 9 oil distribution system is a compromise between weight & efficiency. It needs all the help it can get.
The delivery system (pump) is only just up to the job - which is okay as long as the set up (tank location) works in its favour.
The oil level in the external tank should always be above/close too the same level as the oil pump (the pump should be primed at all times) so there is no additional effort to "lift" oil and pressurise the engine system.
The return system is dependent on crankcase pressure - lift the external tank (oil level) too high and you will increase crankcase pressure which may put additional stress on seals, reducing their service life - make the pump work harder, making lubrication less efficient.
So Rotax have determined that the oil /level tank has an optimum range, in relation to the engine centre line, within which the system will work well.
Prestart 'Burping";
Demonstrates the tank to engine height relationship is within acceptable limits. No burp; tank too low and all the oil has drained from crankcase to tank - Very slow to (no) burp; tank too high, hand cranking unable to raise crank case pressure sufficiently to drive oil up to tank.
Removes excess oil from the crankcase allowing for less cranking energy & pressure on seals
Allows the pilot to assess the condition of the engine by feeling cylinder pressurisation, smooth turning & unusual noises.
Imparts a degree of pre start lubricating, reducing wear.
Gives the pilot the opportunity to assess sufficient oil in system.
Makes the pilot feel good.
