We have been working our way through our flight test program. However, high CHTs kept us from really making progress until we made a few changes.
Continental’s guidance for the O-200A limits CHTs to 525 degrees max with recommended max during cruise of 420 degrees. We were seeing peak temps during climb-out of 505 degrees, within the max, but still hotter than we wanted. Plus, the temp wasn’t really falling in cruise.
We are using Mike Busch’s free Savvy Analysis software to painlessly analyze the data. I strongly recommend it if you are using a Dynon or any other engine monitoring system that stores data that you check out this software.
Here is a graph of one of our early flights. It shows a peak CHT on cylinder #3 of 505 degrees and a cruise CHT of 480 (note the red line is set at 480 degrees).
We are not running an oil cooler. Oil temp was high but we were below the max allowable of 225 degrees.
What to do?
We focused our attention on the cowl. In order to draw more air out, we added a bigger lip to the cowling. For now, we just duct taped in place.
This helped but we still needed more airflow, especially on cylinder #3. Here you can see the inlets. Cylinder #3 is on the left in the photo and #4 is on the right. Note how much more air is hitting cylinder #4.
Here is a close up of the opening in front of cylinder #3. We were concerned that the air coming in was skipping past this cylinder.
Here is the inlet in front of cylinder #4. Note how much more of the cylinder is exposed to the airflow.
We decided to open up the inlet in front of cylinder #3 by trimming back the fiberglass and replacing it with an aluminum baffle. The top green line is where the baffling used to hit the cylinder and the lower green line is after our modification.
Since we decided to go with a new lip, I cut off the old lip which enlarged the outlet as well. Green line is my cut line.
Here is the removed material.
After each of these changes, we pulled the data and reviewed the impact. If it helped, it stayed, if it did not, we tried something else.
During all this testing, the outside air temperature has dropped 10-20 degrees depending on the day. Of course, this has helped quite a bit. That said, our cowling changes have made a big difference and CHTs are no longer a concern. In this graph, you can see that on take off the highest the CHT is under 450 degrees and in cruise at 2500 rpm we are sub 400 degrees.
We will have to wait and see how we do next summer when the weather warms up. For now, we are happy with the numbers and have really started to put some test time on the aircraft. All other aspects of the testing are going as planned.
Until next time…