cowl air exit hole size advise seeked
Posted: Sun Dec 08, 2019 8:58 am
by gyroron
On my Waiex, powered by a 3300 Jabiru, with dual 3 into 1 straight exhaust pipes… There are fairly large rectangle openings in the bottom of the fiberglass cowl for the air to exit. The builder of the plane added steel metal ( I think its aluminum ) inserts with lips at the front, to fill up those large openings, and now the cowl only has much smaller openings in the metal, big enough for the exhaust pipes to exit and maybe 2-3 inches of space around the exhaust pipes.
I am thinking more of a opening would be better. I can see the firewall being pushed in during flight from all the air pressure in the cowling. I also want to have a opening large enough to be able to put a drop light with a 100 watt bulb in it, up into the cowling to leave in there between flights over the winter so my engine has some heat in it. Right now I can’t hardly put my hand into cowling around the exhaust pipes because the opening are so restricted.
Couldn’t find much info via the search function.
Re: cowl air exit hole size advise seeked
Posted: Sun Dec 08, 2019 11:49 am
by rick9mjn
here is my .02 cents of info…
here is a link to jabiru SB. “see 4th an 6 borm the bottom of lins ,link =
http://jabiru.net.au/service/service-bulletins/
…also see link =EngineCooling
…also general info link = viewtopic.php?f=8&t=238&start=40
…also see manual list link =https://jabiru.net.au/service/manuals/
…also general info link = https://jabiru.net.au/community/engines/
/…/ upon using early morning / non coffee- brain cells , i think i remember, of reading (may be out of Tony Bingelis books) that the “”“exit””" of the cowling , should be 20% bigger than the air inlet…
//…//. also i saw on other forums that you should “”“”“never”“”“” run a Jabiru engine “at high power” with the cowling off…
like i said just my .02 cents , good day /rick
Re: cowl air exit hole size advise seeked
Posted: Sun Dec 08, 2019 11:53 am
by sonex1374
gyroron,
The exhaust tunnels are added to the holes cut into the fiberglass in order to make the cooling airflow exit the cowling properly. If you simply cut rectangular holes in the cowling skin the cooling airflow would not work very effectively. The tunnels are designed to turn the exiting cooling airflow more parallel to the airflow of the surrounding outside air. The deflector lips assist in this process, as well as create a mild low pressure zone at the mouth of the outlet to help “suck” cooling airflow through the inside baffles and draw it out.
The size of the exit in the cowling determines how much air passes thru the baffles (e.g. the exit “throttles” the cooling). The first thing a builder can do to increase the mass of air flowing through the cowling and baffles is increase the exit openings. However, cutting them too large can do two potentially bad things - 1) drawing too much cooling air thru the cowling and overcooling the engine, and increasing the total drag on the airplane from all this extra cooling, and 2) changing the geometry of the air exit and rendering the exit less effective.
In practice, overcooling is not that likely in a Sonex, and flight testing has shown the extra drag to be minimal. However, ruining the geometry is a real possibility depending on how you cut the exit. Visualize the Sonex in a nose-high attitude where the surrounding free-stream airflow is striking the underside of the cowling. If you’re not careful you can cut an exit such that the free-stream air tries to flow into the exit, rather than being directed smoothly around it. If the free-stream air tries to flow in, the cooling air can’t get out, and the engine runs hotter despite the larger exit size.
This is one reason the side exits of the B-Models are potentially an attractive solution - no amount of nose-high attitude will ruin their effectiveness. Extreme side-slips may be another story though…
Now what does this mean for a Jabiru exit? In practice you should aim for approx 50-70 square inches of cooling air exit area to ensure proper cooling of the engine. If you start with the basic tunnels that Sonex provides and trim the edge of the aluminum tunnel “floor” you can easily get to the proper exit area. If you remove too much material (going aft) eventually you’ll create geometry problems and cooling efficiency will suffer (again, picture the airflow trying to enter the exit in a nose-high attitude).
You’ll have to measure your current openings to see how much exit area is there, and examine your in-flight CHTs to see if the engine is running cool enough. If CHTs are too high, increasing the exit area could help. However, depending on the size of the heater you want to squeeze up into the cowl, you may not be able to cut them big enough to fit the heater before you start to negatively affect your cooling flow and increase the CHTs (obviously heaters vary greatly in size, so there is no firm answer here).
My advice is to size the air exit to do the job of cooling the engine properly, then shop for a heater that will fit whatever size opening you result with. If this still won’t work, perhaps the solution is to cut an access door in the side of the cowling that you can open to insert the heater.
Jeff
Re: cowl air exit hole size advise seeked
Posted: Sun Dec 08, 2019 4:33 pm
by fastj22
Initial flight testing with the B-model side exits are very good in regard to cooling. And they are big enough you could put a trouble light through them if you can find an incandescent bulb. I always thought the A-model bottom exits were undersized.
Re: cowl air exit hole size advise seeked
Posted: Sun Dec 08, 2019 6:42 pm
by GraemeSmith
Sonex A - I’ve increased the size of the bottom negative pressure slot and extended the negative lip all around it. That lip is there to prevent air flowing back into the slot at a high angle of climb and encourage negative pressure in level flight. You will find it on the bottom of nosebowls of most Cessnas. The bigger slot has helped but not perfect yet for the back two cylinders in a Vx climb. CHT’s still get up there (I’m being conservative and want to stay low end of recommended ranges).
An unintended consequence has been the lower oil cooler is now working excessively well and I’m running low oil temps. Had to close off the “smile” input slot at the front!
For next summer I’m seriously thinking of adding an extra opening cowl flap - like the C182 300hp and Piper Arrow complex type operation I fly. Opened for take off and hard climbing and can then be closed and streamline one you level off.
Re: cowl air exit hole size advise seeked
Posted: Mon Dec 09, 2019 9:30 am
by gyroron
It is easier for me to post pictures onto facebook, so I will go there to the sonex builders page and upload some pics of the cowl. If any of you also go there, please take a look at my pics and tell me if you think just looking at the pics if you think the openings are too small. Thanks
Re: cowl air exit hole size advise seeked
Posted: Mon Dec 09, 2019 9:51 am
by bvolcko38
I have a moveable cowl flap. I close it for soaring and open it for times when engine is running. I used the plan dimensions for the fixed flap but attached a linear actuator to move the flap. Seems to work well
Re: cowl air exit hole size advise seeked
Posted: Mon Dec 09, 2019 1:34 pm
by GraemeSmith
Bill,
Nice! Kinda where I was headed - but with a simple Bowden cable! 
Re: cowl air exit hole size advise seeked
Posted: Mon Dec 09, 2019 3:07 pm
by WesRagle
Hi Guys,
Since gliding isn’t a normal flight regime for a Sonex, it’s probably best to design a Sonex cowl flap can’t fail full closed.
Wes
Re: cowl air exit hole size advise seeked
Posted: Tue Dec 10, 2019 7:54 am
by gyroron
spoke with nick at jabiru usa / arion aircraft, and in his opinion the more air I can let OUT of the cowling, the better.
Re: cowl air exit hole size advise seeked
Posted: Tue Dec 10, 2019 10:03 am
by 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.
Re: cowl air exit hole size advise seeked
Posted: Tue Dec 10, 2019 9:25 pm
by peter anson
gyroron wrote: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
Re: cowl air exit hole size advise seeked
Posted: Tue Dec 10, 2019 10:21 pm
by 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
Re: cowl air exit hole size advise seeked
Posted: Wed Dec 11, 2019 1:39 am
by 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
Re: cowl air exit hole size advise seeked
Posted: Wed Dec 11, 2019 12:54 pm
by 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.
Re: cowl air exit hole size advise seeked
Posted: Mon Dec 16, 2019 11:11 pm
by 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!!
Re: cowl air exit hole size advise seeked
Posted: Wed Dec 18, 2019 9:32 am
by 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