|
For 1 year, I ignored the paint bubbles and water blisters and impossibly difficult cold weather starting of its O-200-G (ground power unit) engine. After a magneto failure and then a broken throttle cable I decided on a full winter overhaul at home.
Two and a half years later, after lots of advice and encouragement from my Long Eze mentor John Moss, I flew again. The work done included sanding off the heavy RAF micro and re-profiling with West system and replacing the O-200-G engine with a rebuilt O-235-J2A. The updraft cooling was retained for one more year until the sight of broken baffling and slightly cooked No.4 cylinder (the result of dog fighting with Bob Englert ) prompted some serious thinking. Also, despite the more powerful engine, and 175kts top speed, Bob and Ken Miller were still way faster.
The NACA scoop would have been a good speed mod but did not address the updraft cooling problem, plus it makes the bottom end look ugly. Armpit scoops looked better but still cooled the exhaust pipes before cooling the cylinder heads. I also wanted to reduce drag by putting those top cowling bumps to some use.
Inspiration came from a number of sources. I had spent Oshkosh 93 looking at Pushy Galore and talking to Gary Hunter about that tiny side intake. There was an article in Sport Aviation about Nemesis and a picture of a lovely plenum. There was also a photo of a military LongEze with top-side intakes in the CSA newsletter.
An initial balance sheet of pros and cons came out in favor of two top-side intakes feeding separate plenums. Now that the aesthetics and design prejudice had been decided, it was time to look for scientific justification for my choices. I made some working assumptions after reading all the NACA technical reports I could find on the subject of air cooled engines. (http://techreports.larc.nasa.gov/cgi-bin/NTRS) Select NACA full text reports, and search for "air cooled cylinders". The number of hits is astounding and makes my efforts seem more like archaeology than I want to admit. All the relevant reports are pre-war. Try NACA report 719 " The problem of cooling an air-cooled cylinder on an aircraft engine" and report 555 "Airflow around finned cylinders."
I learned that cylinder head temperature was related to the mass of fuel/air mixture being burned, and that cylinder temperature is related to mass of cooling air flow which in turn depends on pressure difference across the cylinder.
I also learned that with our fixed pitch propellers, engine power is limited at low speeds and can only reach a maximum at top speed at sea level (with a max CHT mixture if you dare to lean). Our large speed range and fixed pitch propellers were actually an advantage.
 
A simple idea, but not so easy to implement!
Top
It seemed that nature had conspired to give canards an Eze solution to cooling efficiently with plenums.
My reading also showed that submerged NACA inlets are only slightly better at pressure recovery at lower mach numbers. In any case, my intakes were meant to hide the cowling bulges and so traditional ram air intakes were used.
As a first step, Ken Miller helped me install an Allegro engine monitor and fuel flow gauge. I would need to measure CHTs and EGTs to validate what I was doing. I also borrowed an ASI from Bob Englert to measure the pressure difference across the cylinders.
I started by making and installing RTV/Bid baffles on the cylinder heads and bases. I used three layers of bid and the wraps are substantial. Maybe two layers would have been sufficient. These were made flat on a work surface with plastic wrap on each side, and before the RTV set, I trimmed them to rough shape and wrapped them around an old cylinder to dry. Duct tape kept the wraps in place until final trim and glue to the fins. This resulted in a neat installation. Small patches were RTV'ed into the gap between the cylinders.
 
Underside of #4 cylinder View of #1 cylinder
Top | 
