Hello all
The dry sump and location of the oil tank are relevant. On the small pump, everything except the injected engines that have a larger volume version, the burp is more important. We all know why, moving any residual oil from the crankcase back to the sump tank. The reason it may accumulate is mostly the pump design and oil filter location relative to the oil tank position.
The pump design has a camshaft that drives the pump via an engagement pin on the legacy smaller pump. The lubrication of that cam bearing area (cam directly on the cam bore casting of the crankcase) gets it lube from the crankcase oil vapor that is under pressure from piston blowdown. The drive pin in the oil pump is lubed by the oil within the oil pump. The shaft that holds the pin is a simple shaft that passes via a bore in the aluminium casting of the mail housing of the pump. This is the issue. When you shut off the engine there will always be a slight pressure from the oil tank to the oil pump. The small gap from the shaft to housing and the gap from the cam to the crankcase cam bore can allow oil, over time, to migrate into the crankcase. The higher the tank the faster the transfer. As noted this takes time so the once in the morning is normally just fine. If the aircraft sat for the week it may have more oil in the crankcase built up and it takes a few more turns.
When we consider the engines with an oil tank below the oil pump inlet, as we see in a number of gyrocopter installations like the Magni from Italy, there can be other issues. In this case the oil filter may not be perfectly sealed. This then can see oil migrate from the filter back to the oil tank. If you have one of these it will show up as low or no oil pressure for a few seconds while the oil filter fills itself. Burb the system and problem solved, the filter will fill up with the prop rotation and no worry about the lack of oil pressure.
The injected engines have a new design in that they move about 20% more volume. To help with that oil migration issue the shaft that drives the oil pump from the cam has a physical seal in the housing. You will note that injected engines are much easier to burp than the legacy pump models. There can sill be residual oils that drain from upper galleries within the engine and the injected also has a much larger area below the crank within the crankcase, this can hide a lot of oil that might give you incorrect readings to what the actual oil level is. For this few reasons burp the injected also, it only takes a few mins to know your oil level is correct.
Cheers
