enginehistory.org

[mabbott]

interested to know more about the impact of Oil viscosity on aircraft engine output.

The context is; what limits are imposed on the engines output (rpm) due to engine temp (is it a physical limitation due to the oil or is there more to it)?

[jschauer]

Most U.S. designed radial engines are limited to 1,200 rpm until 40C degree oil temp is reached.

[mabbott]

Thanks jschauer, I should clarify further - were there physical limitations ie. the engine can't be operated over say 1,200 rpm because it can't or could you push the engine over the 1,200 rpm mark but risk engine damage?

[rwahlgren]

It is not that the engine does not have the power to overcome the viscus friction from the thicker lubricant. Its is more that the lubricant does not flow as well when cold and thick as it does when at operating temperature. Remember oil in the average "Large" radial engine is equivalent to SAE 60 and yes it is not a multiviscosity oil, like your 5W-30 you may use in your car, this is very thick oil especialy at temperatures below 40 degrees ambient. Not only is it more difficult to pump it through the smaller passages when cold it also does not so easily fly off the rotating and reciprocating parts either, nor does it like to be tossed up inside the crankcase to lubricate wrist pins, nor scrape off the cylinder walls very fast or find its way back to the scavange pumps, let alone sift through the suction screens to get there. So there is lots more than first thought as to the why it should be restricted in speed till the lube oil warms up.

[kmccutcheon]

I have always heard to keep the rpms low until the oil is warm to protect the propeller thrust bearing. It is located in the nose case and is serviced by small oil passages. If run loaded at high speed before lubrication becomes reliable it can be damaged.
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Kimble D. McCutcheon