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Steve,

Your cooling system is a closed system with two heat exchangers, one being the engine adding heat and the other being the radiator removing heat. The delta T of the coolant across the engine and the delta T across the radiator must be the same. Since there isn't much you can do to change the characteristics of the engine as a heat exchanger, there isn't much you can do to change the observed delta T.

As you change the characteristics of the radiator side of the system, the inlet and outlet temperatures will change together until a steady state is reached and the radiator can remove the amount of heat produced by the engine. With increasing power, both the inlet and outlet temperatures will rise by the same amount until the mass air flow through the radiator together with the OAT results in the required amount of heat removed from the system. The goal is to keep the average of the inlet and outlet temperatures within reason. Delta T by itself isn't a good indicator of the suitability of the radiator side of the overall system.

This all assumes that a thermostat isn't being used. With a thermostat, the delta T across the radiator will be variable until the bypass is completely closed, at which point the above behavior will be seen.

The attached plot shows the effect of changing the air inlet and outlet sizes but the delta T is essentially constant.

Steve Boese

RV6A, 1986 13B NA, RD1C, EC2

From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net>
Sent: Sunday, November 29, 2020 1:08 AM
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: [FlyRotary] Water Temps

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Hi there people

Has anyone got data of actual delta T from inlet to outlet of their water radiators
And if you do, what is the size of your inlet feeding the radiator, IAS and OAT at those data points.

We have ~30 square inches of air into the water cooling
Water into the engine is these data points was 180F

Delta T KIAS    OAT
10F             153     54
8F             115             54

I’m now fine tuning our water cooling and wanting to compare with other known data.

Thanks

Steve Izett
Glasair SuperIIRG Renesis 4 Port

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Attachment: inlet and outlet area comparison s.jpg

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