Instrumentation for Your Propeller Speed Reduction Unit

Welcome Everyone to another installment of Kit Aircraft Mods.

Today I want to talk about the instrumentation needed to properly monitor the PSRU attached to your engine. This critical component of your airplane’s propulsion system needs to be monitored closely and I want to discuss the parameters that will be monitored and the placement of the sensors so that the proper information is getting conveyed to you the pilot in the cockpit.

To start off, I will identify the two variables that we want to monitor. First is the gearbox temperature and second is the pressure of the lubricating oil flowing through the bearings.

The first important question one needs to ask is: “The temperature of what?” The PSRU is made up of differing temperature zones. The placement of the temperature sensor will have a big impact on the type of information you will receive at the gauge.

The PSRU in my airplane has a lower reservoir and the gear gallery as well as the upper main bearing area. Each of these areas will exhibit differing temperatures when the engine is running. The lower location for the thermocouple which is the suggested location from the manufacturer measures the temperature of the oil after it returns to the oil reservoir and before it is siphoned back to the pressure pump.

The oil in the system is pressurized by the lubrication pump and is then filtered and distributed around the PSRU to be sprayed into the bearings that support the shafts in the gearbox. After lubricating and cooling the bearings and gears it falls back to the reservoir. To put a temperature sensor in the mid case area would not work very well as the thermocouple would be extended into air that has oil falling through it. Air is an insulator and would not return the same reading as one immersed in the oil.

There is the output shaft and bearings that are housed in the top most portion of the PSRU that are heat sensitive and important to keep cool. This area is much like the mid section in that oil is being sprayed into the bearings and is allowed to drip back to the pan. In my opinion this area is the most critical from a temperature standpoint and it is the area with no temperature sensor attached to it.

If you work through a failure analysis where there is a leak in the lube system that dumps most or all of the lubricating oil overboard you will see the thermocouple mounted in the lower sump stop doing its job. At the very least it just stays at the same temperature. As the oil drains out of the sump and isn’t replaced by falling oil from the bearings the thermocouple will come out of the oil into the air. This insulates it from the rising temperatures in other parts of the gearbox.

When the oil is all out of the system the pressure pump will lose its suction and the oil will stop flowing to the gears and bearings. It is at this point that the main bearings on the propeller output shaft start to heat up. It is at this precise moment that you need to see the temperature indicator climbing as well. An alarm at this point will give you a much better chance of getting back on the ground safely and will go a long way towards keeping your gearbox from catastrophic failure.

The second parameter you need to monitor is the oil pressure. This sensor should be mounted in the same manifold that is used to distribute the pressurized oil to the various lubrication ports.

I have these two sensors wired directly to a dedicated gauge on my panel and this has worked well thus far. I am thinking though, that I might add another sensor to my gearbox to understand both the oil temperature and the main output bearing temperature. One will help me understand how well the oil is being cooled and the other will help me understand whether there is trouble a brewing in the main bearings.

I hope this discussion helps with your project and that you enjoyed reading it. I invite all of you out there reading my posts to comment

I have been working on a new eBook about the my “Alternative Engine Experience”. It chronicles the decision making process and things you should think about when considering an alternative engine package like my LS1. I should have it on the site shortly.

I Hope it’s Clear Above and Your Visibility is Unlimited!

Dave

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