N91CZ Cooling Drag
Using the cooling system data that has been collected (ref. “Cooling System Evaluation”) it is not too difficult to calculate the actual cooling drag for the aircraft. As time permits I will prepare a report on the steps required to do this. (See also, “The Development for Reciprocating Engine Installation Data for General Aviation Aircraft”, Frank Motts) In the mean time, I thought it worth while to discuss the results.
The horse power lost to cooling is essentially the net amount of energy extracted from the air passing through the cooling system. It enters the system with a certain amount of energy. It losses energy due to flow losses and gains some back from heating. The net effect is typically a loss and usually more rather than less. How much energy is lost while still keeping your engine cool is what defines the efficiency of the cooling system. In the case of N91CZ the total cooling drag was reduced by more than half. The results are plotted as a function of altitude below. Further reductions could be made by reducing the exit area. This would restrict the mass flow through the system but would also cause temperatures to rise. Wouldn’t cowl flaps be nice!?!