Improved Engine Cooling Baffles

When I built my Bede BD-4C, I decided to install an air plenum over the top of the engine. I built the plenum out of aluminum and was not entirely thrilled with the results. It was a lot of work and I figured that it was likely to crack. It did. But before the plenum cracked, it got the airplane and me through phase 1 flight test, to and from AirVenture, and through about 80 hours of flight.

My first repair idea was to build a new plenum out of fiberglass, since it would be easier to mold to the complex shape required. I started that process by carefully measuring the clearance underneath the cowling. I realized that the sides of my existing engine baffles were tall enough that I could seal them to the cowling top with flexible baffle material, instead of making a separate plenum. As a bonus to this approach, the interface between the air intake openings on the front of the cowling and the baffles on the engine would be much simpler.

I took the airplane “down” for a couple of weeks of maintenance so that I could do three tasks:

• Fix the cooling baffles
• Replace the cracked front windows
• Fix the wiring for the navigation radio

With help from Jake and Candy, I got all three done. Here is a photo of the baffling that I ended up with. Yes, I know that it’s ugly. I have three styles of flexible baffle material in two colors and way too many small pieces. When I have nothing better to do, I will replace all of the small pieces with a new nice neat set.

I made a test flight yesterday and was happy to find that the engine stays cooler than it did before. The old plenum did not seal well to the air intakes in the cowling. A lot of the incoming air did not make it into the plenum and so it did not help cool the engine. Now, pretty much all of the air does its job and the proof is in the cylinder head temperatures at low airspeed, during climb.

Previously, I could do an initial climb at 95 knots but the hottest cylinder would quickly get up to, and exceed 435 degrees Fahrenheit, which is the beginning of the yellow zone for my Lycoming IO-360 engine. (Maximum allowed temperature is 475 degrees Fahrenheit.) I had to increase airspeed to 105 knots, which reduced the climb rate, to keep that cylinder head cool.

During my test flight, I climbed for five minutes at 82 knots or less. This is much slower than I would normally use during climb for more than a few seconds, certainly not for several minutes except when I am testing how well the engine cools.

The hottest cylinder flirted with 435 degrees a couple of times but never stayed that hot. Here are the graphs. (Click to view them larger.)

There are gaps in the aluminum baffles and between the aluminum baffles and the engine block. I still need to seal those with oil resistant RTV. That will further improve the cooling efficiency and bring the temperatures down even more.