Convergence, Divergence and Other Project Rambelings

I think I am getting the picture as to why most airplane builders don't attempt to optimize cooling drag. Its a pain...

So the idea is to take the air in, slow it down and thereby increase its pressure, make it flow through whatever you want to cool and then try and return its local free stream velocity. Sounds simple enough until we realize that all of those ducts take a lot of space, especially when you are trying to do it to a 40+ year old design that is very space limited to start with. There is just no room left under my cowl. The only option at this point is to make more space.

As I mentioned, I removed the Mocal 13 row cooler and picked up a Positech 5 row for an O-300. It has about the same area of cooling fins but is narrower allowing less hacking of the lower cowl. After consulting with Dick Keyt and Joe Coraggio (whose blog can be found here: Garaggio.wordpress.com  ), I was going to attempt using a smile inlet under the spinner to supply air to the cooler. The problem I ran into was that a smile inlet big enough to take in the air I need looks pretty strange below a 9 7/8" spinner. Even if I could have taken the air in at that location, it would have to make a very sharp turn right after it entered the inlet.

I am pretty sure I will end up with a small rectangular duct that measures about 1.75" by about 4". This should allow me to construct a duct of proper proportions to slow the air down. The exit side convergent duct is going to be easier as I have a little more space to work with.

I think I can get the cooler up a little further into the existing collage of cowling pieces but the more I move the cooler up the more difficult the oil lines become.

Anyway, I spent the better part of the day trying to figure out how to balance all of this. The picture shows the current location and the spline depicts the rough shape of the new lower cowl. At this point, I really wish I would have just started from scratch on the entire cowl...

Whats up next:

• Permanently mount the cooler and get it plumbed
• Revise the lower cowl shape
• Make the inlet, the divergent duct and the exit duct for the cooler
• Suppress excessive use of profanity
• Build the right side cooling baffle and cowl inlet
• Lots of heat shielding
• Cowl exit duct
• Cut new pushrods and set valve train geometry
• Tidy up the baffles
• Body work and primer on cowls

Hoping to have all of this buttoned up and have it airworthy by the end of the month (which means it will take 2 months ; )