Air Cooling or Water Cooling

(Note: There are regular readers of this blog who know more about engines than I ever will and who probably think it is presumptuous of me to write about engine matters. As if that has ever stopped me.)

One of the perennial arguments between gearheads in aviation is whether it is better to cool piston aircraft engines with air or with liquid.

Air cooling has its problems. You have to arrange for a good airflow over all of the cooling surfaces on the engine (the cooling fins on the cylinders and the oil cooler) without any hot spots. If you rapidly pull the power back on some large engines in a descent, you can cool them to the point that they will "snarf and barf" if you add power quickly and some people have believed for years that you could thermally stress the engines. All that cooling airflow means that you have what is known as "cooling drag" as you push all of those parts through the air. For large engines, it means you have to use radial engines to make certain that all of the cylinders are equally cooled, which means a lot of drag. It can be a major headache for very large opposed engines to cool them evenly, something that was a problem with the eight-cylinder O-720s in the 1960s.

You can solve a lot of that with a good liquid cooled engine. You can arrange the cylinders in a compact line or a V-configuration to ensure the smallest frontal area. When the engineering matured, it became possible to direct the airflow through the radiator in a way that not only was there no cooling drag, but that the heat added to the airflow resulted in a net thrust gain. This was done in the P-51. Liquid cooling also allows for high horsepower, as the cooling is very efficient. Lycoming had a prototype liquid-cooled piston engine under development that could produce 5,000 horsepower and was targeted for 7,000 horsepower, which was more powerful than the largest steam locomotives. It wasn't until the introduction of the very large "corn-cob" radials, such as the R-3350 and the R-4360, that radial-engine powered fighters could keep up with the aircraft such as the P-51.

If you look at the use of piston aircraft engines, you will note that liquid-cooled engines, at least after the OX-5s went out of service in the early 1930s, were largely confined to the Army Air Corps. In the mid-1920s, concerns about maintenance and durability led the Navy to stop using liquid-cooled engines. You have to remember that carrier landings are pretty violent affairs, so the more stuff there is hanging in the engine, the more there is to break. A broken coolant line on an aircraft engine can be a real problem over land, but over water, it likely would get the aircrew killed.

Liquid-cooled engines also proved to be more susceptible to battle damage. It was not unusual for low-flying liquid-cooled fighters to be brought down because a bullet had pierced a radiator. The radial air-cooled engines were just tougher; airplanes can and did come back with battle damage to the engines that would have brought down an air-cooled engine.

There was also arguably a weight penalty for liquid-cooled engines, as the coolant and radiators have to be hauled aloft. As some wags put it, liquid cooling an aircraft engine made as much sense as air cooling a submarine's engine. For the airlines, between that and the added maintenance costs of the coolant system, there was little interest in liquid-cooled aircraft. Oh, some were made, such as the Avro York and the Lancastrian, both derivatives of the Lancaster bomber, but they were not popular.

Interest in liquid-cooling is resurrected from time to time. Mooney teamed with Porsche to produce the Mooney PFM, which was a flop. Continental produced a liquid-cooled engine for the rear engine of the Voyager, but never marketed that size engine to anyone. RAM Aircraft uses liquid-cooled engines for some of its modifications.

But they are niche applications. Air cooling rules.