cowl air exit hole size advise seeked

[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.

[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 =http://www.jabiru430.com/EngineCooling.html
....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

[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

[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.

[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.

[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

[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

[GraemeSmith]

Bill,

Nice! Kinda where I was headed - but with a simple Bowden cable! :-)

[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

[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.