Oil Cooler Inlet Reduction

 
 
The plans call for an inlet to be cut into the cowling to feed the oil cooler. I used the specified size for the inlet of a 13 x 4 inch opening. It now appears that I may have made the opening too large. If the inlet is allowing too much air into the oil cooler baffle, that air will eventually make its way into the lower cowl, and might even pressurize it. If the lower cowl pressure rises too much, the cooling air from the top cowl doesn't migrate through the cooling fins properly. Essentially, the air stalls and temps rise. The solution is to either make the air exit larger, and endure increased cooling drag, or reduce the oil cooler inlet area to the minimum dimensions that still provides acceptable oil cooler temperature.

See the following message threads (be sure to read the whole thread) on the AeroVee Yahoo Group:

Waiex builder Bill Hebestreit posted these photos of his modifications. He used aluminum tape to close off his opening unti he determined through trial and error the optimal opening size. In his case, he selected 5.6 sq inches.
He then constructed a cover that attached on the outside of the cowling to set the inlet area.
I followed his lead and used blue painters tape to close off the inlet a bit at a time. I collected test flight data for various opening sizes and used it to determine the optimal size. It appears that about 5 square inches is perfect, although I haven't gotten test data for really hot days yet.
Looking at the flight data is very interesting. Trying to make sense of it and pull out underlying correlations is much more difficult! However, there are several trends that are apparent.

First, Oil Temperature, Carb Temperature, and Oil Cooler Inlet Area are directly related to each other. This makes sense logically, as well. Cooling air enters through the cowl inlet and into the oil cooler baffle. The air then absorbs heat from the oil cooler, and spills into the lower cowling, feeding the AeroCarb. Because the oil cooler feeds the carb (according to the standard installation practice), oil temps and carb temps are directly related: As Oil Temp goes up, Carb Temp goes up as well.

The next correlation is the size of the oil cooler inlet area on oil temp. This also makes sense logically. Ingesting more cooling air into the oil cooler results in lower oil temperature. The data shows quite a bit of scatter. This is due largely to other variables changing, such as Outside Air Temp (OAT), Aircraft Weight, and Density Altitude, among others. You can see the trend lines for cold, medium, and warm OAT. They vary somewhat, but still correlate well. Looking at the graph, the trend lines cross at around 5 sq. inches and 175 degrees. For hotter or colder OAT, the oil temp should range from 150 to 200 degrees, but still within limits.
Looking further, the next correlation relates Oil Cooler Inlet Area to Carb Temp. This is similar to the previous correlation because oil temp and carb temp are directly proportional. The graph shows the effect of Inlet Area on Carb Temp, adjusted for OAT. In this case, I'm looking at the Delta (difference) between Carb Temp and OAT. There is a fairly strong correlation: as Oil Cooler Inlet Area is reduced, Carb Temp is increased. Again, this makes sense because as oil temp goes up, so does the air spilling off the cooler and thus into the carb.
Next, I looked at overall cooling efficiency to determine the resulting trend of changing the Oil Cooler Inlet Area. This correlation is not as distinct, and I'll continue to evaluate and/or refine it as I collect additional data. The theory suggested that ingesting too much cooling air into the lower cowling will reduce the pressure differential between the top and bottom of the plenum, and reduce cooling to the cylinders and heads. Charting average CHTs vs. Inlet Area suggests this may be true. I also included Average CHTs (condensed into a single data point for each inlet area) adjusted for OAT (this is why the yellow curve is lower). I think this shows the correlation a bit better. The CHT trendline bows to a minimum at an inlet area of about 4-6 sq. inches. I think any smaller inlet area and the effect of higher oil temp and carb temp both conspire to increase CHTs.
Looking again at the efficiency of the baffles, the baffle temperature is the increase in temperature (Heat Rise) over OAT. If I express that Heat Rise as a fraction of CHT (in an attempt to normalize the data), it appears that I'm getting max Heat Rise (and thus cooling efficiency) at around 6 sq. inches. The data is skewed a little by the data collected during extremely cold OAT (16 deg), corresponding to the 7 sq. inches data point. Again, this seems to confirm the optimal opening of around 5 sq. inches.
In summary, it appears that the best compromise of the oil cooler inlet area for the standard carb induction setup is approximately 5 sq. inches. However, should the carb draw unheated outside air, the carb temperature will no longer be coupled to the oil temperature. This means that I should be able to run a slightly smaller Oil Cooler Inlet, thereby increasing pressure differential and increasing cooling overall. Plus, as an added benefit, the engine feeds on denser air and makes more power. Next project: Cold Air Induction!

 

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Updated: 31 Jan 08