Jeff,

The supply lines to my redrives are AN4.  The oil flow rate is restricted internally by the construction of the redrive.  Without a vent, the air initially inside the unit is entrained in the oil primarily by the action of the gears, the air is eliminated through the drain, and the unit is quickly completely filled with oil. This happens regardless of the size of the drain. At this point, the oil out flow rate must equal the oil in flow rate.  The size of the drain limits the internal pressure of the unit.  The larger size drain allows the oil to be removed by gravity alone and prevents the oil from coming out of the added vent.  With the vent connected to the engine crankcase above the oil level, the pressure inside the redrive is now the same as the pressure inside the crankcase.

Steve Boese

> On Mar 5, 2021, at 5:51 AM, Jeff Whaley jwhaley@datacast.com <flyrotary@lancaironline.net> wrote:
>
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>
> Hi Steve B., with respect to your response about drain hole diameter ...
> What is the size of your oil inlet hose?
> Did you experiment with oil inlet size - if oil won't all come OUT perhaps we are putting too much IN?
> Just a thought.
> Jeff
>
> Finn,
>
> When I installed a vent in the top of my RD-1C drive it acted as an additional drain.  The drive operated completely filled with oil until the size of the original rear drain was increased to an ID of ~5/8" (1/2" NPT pipe).
>
> Steve Boese
>
>
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