cowl air exit hole size advise seeked

[bvolcko38]

Graeme,
I would have preferred a cable, but its a big hassle to take the cowl off when cables are involved. Much easier to just unplug an electrical connection.

[peter anson]

spoke with nick at jabiru usa / arion aircraft, and in his opinion the more air I can let OUT of the cowling, the better.

The Jabiru installation manual recommends an outlet about 3 times as big as the inlet. They are talking about installation in a Jabiru which is not quite as fast as a Sonex but still a fairly slick aircraft. I seem to recall working out the Sonex recommended outlet area and it was only about 1.5 times the inlet. I followed the Jabiru recommendations and my early model engine with the older thick finned heads has always run cool. In fact, on long descents it runs a bit too cool so Bill Volcko's motorized outlet would probably be a suitable improvement.

Peter

[GordonTurner]

Hi Peter

What is your engineering assessment of the B model side outlets? To me, it sounds good, moving the outlet from a likely relatively high pressure area to a more neutral pressure area.

Gordon

[peter anson]

Hi Gordon,
I think you might be overestimating my engineering abilities. I guess that on climb exhausting the warm air on the sides should be an improvement. Can you think of any other light aircraft that use side outlets? I suppose most radial engine aircraft effectively do.

Peter

[sonexsteve]

I have a Wittman Tailwind driven by an O-200 which was built in the 1980s with side openings on the cowls, only small holes on the bottom for the exhaust and fuel/battery/crankcase drains.

I had expected that this setup would be more efficient than the traditional bottom rear of the cowl air exit, mainly based on Sonex going to the side openings for the B models, but sadly not so - summer temperatures in the 20s and 30s (degrees C) result in the oil temp running very close to the limit, CHTs are good though.

A previous owner appears to have tried to improve the situation with a small area airscoop bottom centre of the cowl, but without knowledge of what the situation was before that installation I can’t really say whether it helps or hinders.

The cowling construction does not lend itself to opening out the exit holes, expert advice said just fly with it, so I’ve not experimented with trying to get more air through the cowl.

Steve Moody
Leamington Spa, England
Sonex A s/n 1383, will be G-SNXA when finished.

[n502pd]

In N Texas it can get pretty hot, and if well near 100 deg, I may not fly. However, I, in foresight to high temps in nature here in Tx, enlarged the exit area, by 25 %, making the width and length of the openibg bigger by dimentionally 25%. Area wise probably a bit bigger. In the two and half years of flying (35.5 hrs)...I know, I am slow!...I have not seen chts anywhere near close to 400, in climb out or cruise. I will admit that I probably am runninbg a tad rich, but I am more than pleased with how mine runs. I also increased the size of the seaplane lip the same percentage.

And now, I suppose, since I have been caught bragging, the old s**t can will be full really soon!!

[gyroron]

Well I will update and say that I am stupid... I put my cowling back on yesterday ( its been off for the last few weeks chasing a electrical problem ) , and low and behold, on the left side of the cowling, there is plenty of room as it is, for me to get a shop light up into the cowling. I had tried a few weeks ago to stuff a light in there through the right side opening and it wasn't even close to fitting... But that is probably due to the venting for the crankcase and the battery tender lead that hangs out of that side along with the right bank of exhaust. On the left side there is just the exhaust and no problem getting a light up inside.

I still wonder, due to how much I can see the firewall being pushed in, if the exit holes should be bigger. I literally have no more than a little over a hour of time flying this plane so far and its been two flights on relatively cool days. I may leave it alone till I have more time in it, and in hotter weather to see how cool the engine is running and then decide if making the holes bigger will help or hurt