Flying and climbing faster on the same power - reducing cooling drag

The title of this section says it all. There are two ways we can fly and climb faster, either develop more power or reduce the airframe drag. Today's air-cooled engines require a substantial amount of air pressure in order to move enough volume of air over and through a large and complex engine structure. This creates a lot of drag and that drag adds to the overall parasitic drag of the airframe. This type of drag is called cooling drag. In addition it is very difficult to get the airflow over the engine to pass evenly by each of the cylinders. The result is uneven cooling of the cylinders that frequently results in cylinder to cylinder temperatures varying as much as 50F from each other. This substantial variation of cylinder temperatures makes fuel efficiency a much more difficult problem since the hottest cylinder will typically limit how lean the other cooler cylinders can be run. Using an air-cooled engine means that there are few options on how to pickup cooling air and where the heated waste air can be dumped.

Cooling air pressure = Drag

We all want to minimize the drag on our aircraft to achieve maximum performance and their are a myriad of modifications on the market to help reduce drag. However, the one major source of drag that nobody can really do much about is cooling drag. Generally, cooling air pressure drop is expressed in terms of Inches of Water ("H2O) - a much more sensitive scale than the inches of mercury ("Hg) scale we use to express atmospheric pressure. The typical 4 or 6 cylinder air cooled aircraft engine typically requires a minimum of 6-8"H2O of air pressure in order to force sufficient air through the engine to keep it cool. The typical GA aircraft engine installation generates anywhere from 8-12"H2O of pressure - well more than needed in level flight.

In short - high cooling air pressure translates directly to cooling drag. Wind tunnel testing has shown that as much as 10% of the mechanical horsepower generated by an air cooled aircraft engine is lost to cooling drag - just to keep the engine cool!  What makes this worse is that in addition to the parasitic cooling drag that is the result of the cooling air pressure drop across the engine, there is also additional airframe drag that is created by the turbulence in front of the engine cooling inlets that are typically located near the top of the cowling. This turbulence creates airflow separation over the airframe - especially over the top and sides of the cowling and airframe which are low pressure areas and that adds even more parasitic drag over the entire length of the airframe.

Lower cooling drag =
better performance =
better fuel economy =
better range.

It isn't so much about the volume of air required to cool the engine but the pressure needed to force the airstream through the heat exchanging surface.