Cooling drag has been identified as something that has to be addressed if you are going to increase speed without boosting power or changing the prop. It is fairly obvious that you have to minimize the air flowing through the cooling system if you are going to minimize cooling drag. Under "Traditional Engines" a lot of good information has been exchanged on the cooling drag question. I have cleaned up the flow downstream of the cylinders and reduced the volume in this zone of the cowl before the outlet. The net result was a 4 kt increase in speed. Now, there is still room for cleanup in the post cylinder fin flow path but the dominant effect is a greater mass of air is allowed to flow through the system and the cooling drag is actually increased and the CHTs continue to go down. It has been pointed out that the air mass flowing through the system is now much more than is needed to cool the engine and that and further internal flow cleanups without controlling the air mass in the system will probably slow the plane down. The results of personal experiments with my RV-6A support this view. It has been said by several people that at this point you have to control the the air mass flowing through the cooling system by reducing the inlet and/or the outlet if you want to increase the speed. OK, that sounds right and I have thought of some approaches I can take to do that but it seems to me that if you could fine tune the opening in flight based on CHT observations you could optimize the flow for the current configuration and continue the internal flow cleanup without having to "cut 'n' try" the inlet/outlet to re-optimize for the new configuration. This would certainly be more motivational for making internal cleanups and ultimately get the cooling drag minimized. Cowl flaps reflect this kind of approach but it seems to me that that approach is a compromize suitable for a fixed system configutation. If you want to develop the best configuration and you are not worried about the demands of product liability etc. you would want to be able to be able to reduce the port(s) beyond some arbitrary nominal position and deal with it like a trim control.
The question is, what is a good configuration to achieve the desired control? One posibility might be a cone in orifice.
Bob Axsom
The question is, what is a good configuration to achieve the desired control? One posibility might be a cone in orifice.
Bob Axsom