I'm not an engineer or anything (just a long time physics student), but that's interesting. I think it's logical that liquid cooling would be difficult in a pusher, because the radiator is in front of the airscrew and thus, it can't benefit from the airflow generated by the propeller.
If this would present problems, i guess it would be in low airspeed scenarios or on the ground, with cruise or high speed flight presenting less of a problem (more airspeed=more airflow through the radiator)
Of course, there were many liquid cooled aircraft where the rads where not directly behind the propeller, like the wing radiators of the 109 and the near-the-wing-center rad of the spit, but they both had a nose intake right behind the prop as well.
So, it all got me thinking and i've come up with a weird idea.
What came to mind was to use a normal, belly mounted radiator with some sort of airflow augmentation device. A small propeller/fan/turbine in front of or behind the radiator could be used to increase the airflow at the expense of some extra weight. Think of it as a P51 belly radiator with an attached fan.
Of course, if the fan stopped turning it would be worse than not having the fan at all, since the airflow would have to overcome the fan's resistance as well. So, it would either have to be a retractable device to be used during low airspeed segments of the flight (eg, taxi, takeoff and climb) or one that's constantly turning. A retractable one would impose space and weight penalties though.
So, it could be that the fan turned constantly, but its speed could be regulated by the pilot. Along things like carb heat and cowl flaps found in most piston engine systems, there could be an extra lever or spring loaded switch that would control the fan's speed. With today's electronics, it could be possible to have a servo governor mechanism or something like that, so that you would control the fan's RPM through a spring loaded switch that modulated its pitch for example.
When the oil temp went up, you would increase the fan's speed to generate more airflow. This would cool the oil as the heat would be absorbed by the radiator, increasing the temperature in the radiator as a result and lowering its cooling potential. So, when the radiator temp went up you would open the radiator flap to help that warm air escape.
So, why have a flap in the first place and not leave it wide open? You might be flying in a cold day and as you are climbing at high power and low airspeed the engine is getting warm, but if you leave the flap wide open it would cool the air way too much because of the low outside temp (oil needs to be somewhat warm, otherwise it won't flow well enough).
Finally, i don't know if it would work at all, but there's also two more issues about such an installation. We don't know the possible effect on the propeller. A small fan in the radiator assembly is still a mini-prop in front of the main prop, it could create problems with the airflow that goes through the prop.
The second is the placement of the airflow augmentation fan. Would it be best to have it in front of the radiator so that it pushes the air in, or have it behind the radiator so that it sucks air into it like a big vacuum cleaner? I think that placing the fan in front of the radiator would look too much like a turbine assembly and in those, the resulting air compression generates increased heat. Good for a turbine engine where you want to burn the air with some fuel, but not so good for a radiator that's supposed to be receiving cool air. I don't know if it would in fact work like a turbine and i'm not sure if placing the fan behind the radiator would negate such effects, but someone with a better knowledge of aeronautics could shed some light on this.
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