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Has anyone checked the pressure drop across their engine in flight using a U-tube manometer? I was having CHT problems with my Jab 3300 Sonex and checked the pressure drop across the engine in flight and found it to be too low and the pressure under the engine to be too high. I opened up the cowling outlet and dropped the CHTs considerably. I just acquired a Onex with a Jab 3300 and noticed the cowl outlet area is smaller than the inlet area in spite of the CHTs being OK. Has anyone out there checked the inlet/outlet areas ratio on their Onex? How about CHTs? Is anyone having high CHT problems? If so, have you checked the inlet/outlet areas ratio?

Vince Homer

I hope you have the increased outlet size engineering change in the bottom cowl. I'm at 160 hours with an Aerovee and my #1 cylinder, left rear, has always run hotter than the rest, both on EGT and CHT. The other three stay in the mid to high 200's. I suspect the cause is due more to the intake manifold uneven feeding than anything else. The rear cylinders both run hotter than the front two. That cylinder gets too hot on climb out but richening the mixture keeps it from going above 390. At cruise and 3000 rpm it cools down but still remains 30 degrees hotter than the rest. The plugs show no difference, front to back, top to bottom. The oil cooler on top may have something to do with it because it's bleeding pressure on top straight out the back of the baffle. I haven't done the manometer test because the problem? is manageable.

I'm new to the Onex. What does the engineering change set the cowl outlet area to?

Vince Homer
Onex 016

I have never heard it expressed in area. I believe the original opening for the tricycle gear was triangular shaped. The change made it straight sided with a large radius in front with an aluminum lip underneath around the front. The nose gear and exhaust do eat up part of the area but the change was a large improvement in cooling air flow. The change was issued as a .pdf drawing. If you print it as a poster, it will come out full size.

to add, onto post #4 via/onex107....here is some info i found via researching the web.>>>>>>>

5.9.14 ONX-P02 NC
New Detail -11 Cooling Lip
Sonex has been experimenting with new cooling exit for Onex Cowling. We were able to achieve a significant improvement in cooling.
Added detail and cutout for new lower cooling lip ONX-P02-11. Increases exit cooling area and creates more low pressure for engine cooling. Download:

http://www.sonexaircraft.com/documents/ ... lation.pdf
.
vhhomer...... note to.>>>>>>>
and also the looking into "" pressure drop across the engine in flight"" could/should be the key to fixing the problem....
...also another thing to look into , is to look at ""a photo", "side view" of the new ((( rans s7 LSA ))) airplane's cowling ,
there you should see a 'big lip' on the lower / exit of the cowling.......

imho-imbw,,,,,,i my humble opinion,i may be wrong
......good luck ,good day / rick

I was having CHT problems with my Jab 3300 Sonex and checked the pressure drop across the engine in flight and found it to be too low and the pressure under the engine to be too high. I opened up the cowling outlet and dropped the CHTs considerably.

How many times has this come up before? My recollection of calculating the outlet/inlet ratio from the Sonex drawings was that it is about 1.5 while the Jabiru installation manual recommends a ratio of at least 3. I haven't done any measurements with a manometer. I just made the outlets bigger than what Sonex specifies and closer to what Jabiru specifies and it works. My CHTs at cruise are usually between 120 and 130 C.
Peter
Sonex 894
304 hours

Sonex/Aerovee: I'm seeing number 1 temps climb through 400F during extended climb testing and wondered if there have been any successful cowl exit (or inlet, for that matter) mods to help #1 stay cooler. During climb it runs 50F hotter than two others, 100F hotter than the coolest CHT. Thanks.

I just flew with instrumentation and got the following:

2580 rpm, 114 mph ind, delta P across engine was 4.2 in H2O, press. under engine was 1.6 in. H2O, oil in to cooler 91 C, oil out 78 C
2740 rpm, 122 mph ind., DP across engine was 4.5 in H2O, press, under engine was 1.7, oil in 91 C. oil out 77 C
3100 rpm, 155 mph ind., DP across engine was 7.0 in. H2O, press. under engine was 2.5 in H2O, oil in 93 C, oil out 77 C

Tests at 3400 feet, ambient temp 65 F, CHTs and EGT were all within limit'

It is obvious that on this day I didn't have an oil temp problem. I believe this is for two reasons; The oil level was at the upper limit on the dip dipstick and I pulled enough out to bring it to mid-way in the range. perhaps the oil was foaming a bit when I had the high oil temp problems. Secondly, the ambient temperature today was 30 F lower than the day I had the problem. The oil cooler seems to lower the oil temp about 16 C today and probably much less on a 95 F day.

Next I'm going for an easy fix, improving the airflow rate into the oil cooler and that just might give me better hot day capabilities.

If there's a way to do it and I can figure it out I'll post some photos of the tests.

Vince Homer

Hi vince,
What cooling baffles are installed? The alum. Box ones that sonex sells? We modified ours inside in the back by adding a curved alum flat from the top to midpoint on the back. Trying to get the airflow smoother , also added a small lip inside rt bank of cyl, middle cyl to help direct some additional air down thru the mid cyl. Also yes, use a 1 to 3 ratio for exit cooling. We dont have any prob. W cooling. Running our jab 3300 w rotec carb.
By the way you own two of merle's planes? We didnt see him at rio linda this year. Is he still flying and doing well?

WaiexN143NM
Michael

Michael,

I apologize for taking so long to get back to you. We were on a cruise for the last month, just got back. Merle is fine, still flying and looking for another Onex project, this time he's thinking about a UL engine. Yes, I do currently own two of his planes, but the Sonex is going into Barnstormers shortly. The hangar is getting crowded and it's hard to justify keeping the Sonex. Obviously, I haven't flown in a month and have to get over the cruise crude. Maybe later this week.

I will measure the cowl outlet that Merle put in and compare the area to the Onex drawing. I suspect it wouldn't hurt to increase the area. I did that on the Sonex and got a nice drop in CHTs.

Vince Homer