I want to put togeather a large stunter with a round cowl (corsair, fw190. GeeBee, etc.) how much prop effectiveness do I loose over the use of an inline cowl design. The cowl I was thinking to use is about eight inches in dia. to cover a saito 72, wing apprx 740inchs square?

Real airplanes with ring cowls allowed most of the air to enter the front and exit through a big cooling exit behind the engine, thus, they didn't add as much drag as it looks.

These airplanes also had pilot-controlled cowl flaps.  If you needed to make the engine run hotter, you could close the cowl flaps. and you would take a noticable hit on airspeed.

If you built the model with the cowl exits wide open, it probably won't add too much drag.

Although a prop works by "sucking" the plane foreward, you still have he deal with the drag behind the prop.

 Study the sea fury and FW 190 full size  ducting .

There are a couple of considerations about a large round front end on a stunt ship rather than the "in-line"  appearance of most stunt designs.  As was mentioned above, Al Rabe has already shown us that his semi-scale designs of his Bearcats (several sizes and versions), and his Sea Fury and his later version of the Sea Fury in the Critical Mass can be flown competitively with large diameter front ends.

One may ask, how can this be?

One thing is the frontal area of a stunt ship is not increased significantly by putting on a large diameter nose.  Look at the frontal area of our stunt ships.  Most of it is in the wing.   The additional frontal area of a large diameter nose over a conventional "in-line" type design is almost negligible.  (Some of that increased nose area is really in front of the wing so that part is not an increase in total design frontal area.)  Now, there might be some additional drag for a slight increase in surface area of the fuselage, but this is almost negligible as well.  And when you start to understand that probably most of the drag of our CLPA model comes from the lines themselves, any slight increase in drag due to increased frontal area and fuselage surface area is almost insignificant.  Even more insignificant when it is realized that our CLPA models are a fry cry from being optimized for minimum drag anyway.

Now, as far as the propeller and its efficiency is concerned, unless the propeller diameter is taken to some ridiculously small diameter compared to the nose diameter, there is little lost in how well the propeller will work.  I do not have the exact figures readily available, but a significant portion of the thrust generated by the propeller comes from  the 30 to 70% portion of the propeller disc radius.  This is not to say that tip and hub shapes of the propellers are not important.  Those sections have some influence on how well the 30 to 70% section of the propeller really works.

So, a large diameter front end does not really hinder the performance of a stunt model because of "increased drag" or a decrease in propeller efficiency.

It would be a good idea to exhaust as much air as possible from the engine compartment if for no other reason than to facilitate cooling.  Also, reducing pressure in the engine compartment certainly will not be a negative action.

Keith