Initially, before any additions, the pressures were 2.0 inches H2O and 3.0 inches H2O. It shows 4.6 inches of H2O across the cylinders and 7.2 inches of static pressure in the plenum above the cylinders. The example shown is from late in the experiments and at high velocity. The manometers themselves were mounted on a board in the cockpit so that a copilot could photograph them for later data reduction. The other end was mounted just below the cylinders under the engine and was plugged and drilled. One end was mounted in the upper plenum next to the tube from the first manometer. The second manometer measured the difference in pressure across the cylinders labeled 1 and 2 in the photo.
The end of that tube was blocked off and holes were drilled around the periphery of the last inch of the tube so that it was clearly sensing static pressure. The other end was secured to the fuel injection spider in the plenum above the cylinders. One end was plumbed directly into the static system in the airplane. The first manometer measured the pressure difference between the static pressure and the plenum above the cylinders labeled 3 and 4 in the photo. The other measured the difference in pressure across cylinders 1 and 2. One measured the pressure difference between the static pressure and the plenum above cylinders 3 and 4. These were made of long lengths of clear tubing and some water with red dye and a drop of dish soap in it (Figure 1).įigure 1: Two manometers were used for testing. To measure this, and to better understand the pressures created by the NACA ducts and the changes, two manometers were used. Lycoming specifications state that there should be a drop of 6 inches H 2O for adequate cooling. One check to learn more about the airflow through the engine was to measure the pressure drop across the cylinders. To give away the ending, I knocked 55 F off the CHTs with a simple fix. I will document what I tried and also detail the engineering solution. I call these the “hacking” phase and the “engineering” phase, respectively. Traces showed that air was indeed flowing into the ducts, but this gave no indication of how much.Įarly efforts to make the engine run cooler were based on suggestions from flying colleagues latter efforts were based on studying literature to find a good engineering solution. First, I tried to see how much air was going through the NACA ducts by putting smoke oil on the top and flying around the pattern. This led me to study different methods to get more air through the engine. On climb-out and in cruise, my engine was overheating with cylinder head temperatures (CHTs) above 425˚ F. The combination of no external scoops and a smooth paint surface made the NACA ducts very ineffective. When I painted the airplane, after three years in primer (a color I called “blotch white”), I took the scoops off. I added external scoops on the rear edge of the NACA ducts and that helped, but looked crude and not very elegant as the NACA ducts were supposed to be low-drag, internal scoops.
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