For John Schroeder:

John:    No, I  havent determined if the 4 inch duct is "really" necessary.

I went to the 4 inch duct because I am trying to dump the heat from my A/C condenser which is located under the floor of the baggage compartment  only a smidgen above the retracted gear leg..  (Actually I was a little surprised to find that the 4 inch blower I am using  only increases the cu-in flow rate, over the 3 inch blower, a small amount.  SOooo, maybe going to a 4 inch duct was overkill ). From my experience with marine refrigeration and airconditioning, I know that its critical to dump the heat from the condenser if you want to relieably cool anything.  My system is designed to operate primarily on the ground when the plane is not pressurized  because the turbo air that is entering the plane is always slightly above ambient.  I figured that the 4 inch air supply might give me the reliability I needed.

When I get back flying, I will do some ground testing with and without the duct air and then with partial duct air.  Maybe I can go back to the 3 inch duct without any problem.  Anyway, thanks for the thoughts.  Best regards, Bill Hogarty