Anyone figure a good way to cool cylinder #3 ?

[pappas]

Actually, upon further consideration, stated a little differently, Area 51 might be right.

I am considering placing a small NACA duct in the top of the cowl with the output tube pointing directly onto the #3 cylinder. Of course, that would also direct the airflow at the #3 probe as well. So...the question is...am I cooling the cylinder or the probe? I'm thinking likely both. I'll let you know!

[Scott Todd]

I don't know Lou. In all seriousness, the air in the upper plenum/cowl area is very turbulent. The whole thing works because we slow the air down as it passes into the cowl thus raising the pressure. Its the pressure that forces it thru the cylinder heads. I think adding another scoop will add to the total mass flow but again, its so turbulent in there, I'm not sure how much of it will go directly thru #3. If there were also an inner scoop terminating real close to the head without blocking the regular flow, it may help.

When I visited, I don't remember if you had filed all the flashing from between the fins. I think this can help. Also, is there air coming into the lower cowl for the carb? This can help equalize the pressure from the top, which is bad. This is why you always hear people recommend opening the bottom cowl exit. The whole point is to create the largest pressure differential from the top to the bottom to force the air by the heads.

You can also block a little of the coolest front cyclinder. Again, the total mass flow will be the same but more air will go to the other three.

I know Lou is a sharp dude and probably knows all this. But for the other readers, the trick is to raise the pressure in the upper cowl, and lower it in the lower cowl. People are always hesitant to enlarge the lower exit but it really helps most of the time. The air is so turbulent inside and outside the cowl down there, you are just not adding enough drag to noticeably slow the airplane down. Its WAY more important the engine get proper cooling. Also make sure your baffle seals are good. Any little leak is bad. The air just rushes out instead of going by the heads.

I haven't stayed at a Holiday Inn lately but I'm actually an Aerospace Engineer, semi retired.

Scott

[Area 51%]

Just for the record, my comment about airflow to the probe was supposed to be tongue-in-cheek. There ARE threads on this forum that talk about cooling the oil temp probe to get the desired reading. I do not buy into that method.

My real point apparently got lost in translation. That is, the Aerovee Assembly Manual states their recommended CHT location gives the same temperature reading (or close to it) as a probe under the plug. I discounted that theory by having one probe under the plug, and one in their suggested location.

[daleandee]

Do you trust where your CHT probes are? I only have two located at the rear cylinders. I installed the probes under the spark plugs and was running high temperatures (400 plus) in climb during the first 25hrs as expected. No relief came at 30hrs, so I relocated a probe to the recommended spot about an inch away from the plug. Abbra-Kadabra....that probe has never topped 400deg no matter how hard I try. The other (still under the plug) sits on 420+ during a prolonged climb while the other is cooling it's heels at 375. Across the board, I get a 50deg difference between the two.

Many moons ago Charlie Radford, myself, and a couple of others were members of the "Hot Headed Club" and Charlie had done some testing very much like you have done and came to the same conclusion. There was a builder at that time that claimed he could climb all day in the heat of the day and never exceed 280ºF. I suspect his CHT sensors were under the valve cover bolts.

I fly a different engine now and have the opposite concern i.e. CHTs in the low 200's in the winter and mid to upper 200's in the summer (reading under the rear plugs). So I'm no longer a member of the Hot Headed Club as now ... cooler heads prevail . 8~)

Dale

[pappas]

Area 51,
I certainly recognized the humor in your post. Don't think I was ripping on you, I was just moving the gag along. ;-)

[peter anson]

You could run a "blast tube" to force outside air onto the #3 CHT probe to cool it, like others have done to bring their oil temperature down.

OR..........

Do you trust where your CHT probes are? I only have two located at the rear cylinders. I installed the probes under the spark plugs and was running high temperatures (400 plus) in climb during the first 25hrs as expected. No relief came at 30hrs, so I relocated a probe to the recommended spot about an inch away from the plug. Abbra-Kadabra....that probe has never topped 400deg no matter how hard I try. The other (still under the plug) sits on 420+ during a prolonged climb while the other is cooling it's heels at 375. Across the board, I get a 50deg difference between the two.

If the damn probes weren't $65 each, I'd relocate the other one.

I recently changed the probe position on my 3300 engine and it made a significant difference. I had to do a tear down after a prop strike and noticed that the front two cylinders looked like they had been running hot even though they had indicated cool. The new probe position indicated exactly that. My new probes cost about $2 each. I bought a 20 metre coil of k type thermocouple wire on eBay and made my own - really easy. I have tested these and they are very accurate. See http://www.ansoneng.com/sample-page/other-stuff-of-interest/economical-temperature-sensors/
Durability remains to be seen.
Peter

[radfordc]

Many moons ago Charlie Radford, myself, and a couple of others were members of the "Hot Headed Club" and Charlie had done some testing very much like you have done and came to the same conclusion.

I can't recall the exact numbers I got (and the old Sonex forum posts are long gone) but it was a significant difference between under the plug and in the factory recommended location. When I did move my probes to the recommended location I tried to keep the CHTs under 385 at all times. 385 corresponded close to the 425 redline temp. Human nature being what it is, seeing 385 somehow didn't bother me as much as seeing 425! I guess the engine didn't care either since it didn't let me down.

[Scott Todd]

Reading directly under the plug can be influenced by combustion. That temperature is thousands of degrees. Imagine if you moved the thermocouple 1/4" down thru the aluminum where it was near the interior surface of the combustion chamber. The gradient changes pretty fast from there. The factory recommended position take this into account to get a representative, overall CHT. Moving it further out, or blasting air on it, will give bad readings.

[pappas]

The dilemma then becomes that if you increase airflow to the cylinder, you are by default increasing airflow over the probe. You can't do one without the other. The factory configuration does the same thing. Of course, the intent is to cool the cylinder, but the probe will feel the effect as well.

[Scott Todd]

But the probe is hooked to a BIG thermal mass. We were tongue and cheek about it a few posts back but I don't think increasing airflow over the probe will do much. I haven't built a thermal model in MANY years but I did a lot of Heat Transfer analysis on Jet and Rocket engines. The large mass of the head compared to the mass of the probe will essentially keep feeding it.

Fin effectiveness in taking heat away from a source is mostly about the area and flow around them. There are a lot more variables in to consider but you get the jest. The area of that tiny probe is negligible compared to the area of the surrounding fins. They don't call it Rocket Science for nothing ;)