For now, I am going with the 15.5 x 12 APC prop. I did the engine break in using an 18 x 12 2 blade APC.

I too have seen it suggested that the outlet should be 2 to 3 times the inlet. I guess the best answer for the size of the inlet is, big enough to get the cooling done. The ratio of inlet to outlet is to help insure good airflow. Inlet size will matter, but it depends on things such as what size engine, what type of flying, how lean it's run, type of fuel, so fort and so on. Regardless of the size of the inlet, the ratio should still be about 2:1 to 3:1. After reading as many threads I can find on engine cooling, it seems that 1 to 2 inches of inlet is what I find being described for a plane and engine in this size range. I'm going to try and start with the minimum inlet possible to avoid cutting more of the cowl than necessary. I don't think that the spinner gap would provide much airflow without the fan. I would think that with the fan, the effect of pulling air through would decrease the overall size needed for the inlet and outlet. If it works out with just the spinner gap, I'll be very very happy. The other area that I will be looking into are baffles. I have made a couple of baffles, but may make further adjustments. From what I've read, as well as what I've seen on full scale air cooled engines, it's best to keep the airflow as close to the cylinders as possible. I will probably work over my baffles again before testing. I'll be sure to take some photos.

With the chin scoop of the mustang, I think cooling would be a bit easier. One thought I had about the duct from the front to the scale radiator exhaust, are you going to have to make the duct removable in order to access the radio bay? I'll be very interested in your solution as my next project will be a TF P-51B. If the RCV works out well in the spit, I may use another in the P-51. The other route I am considering would be electric. I really do have a desire to turn 4 blades. Electric or the RCV are the only reasonable options I've seen.

Scott