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Old Koreelah

Updraught cooling

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I presume Jab's new allow barrels will cool down much faster than the old steel ones.
Bit of a joke really,as the engine they based the Jabiru on,the KFM had Alloy cylinders ,actually the cyl and head were one piece too.And all these Jabiru high temps probably would never had occurred.I guess the steel cyls were added due to economies.
colin

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I just revisited that:

Aluminium has somewhere between 4 and 6 times the thermal conductivity of steel (depending on steel type).

 

(And pure silver has twice the thermal conductivity of aluminium, so if born with silver spoon in mouth, be sure you didn't stir your tea with it first!)

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 You make a good point with the fact the engine cools off quickly. hence the need to apply carb heat FULLY in cruise before you descend, and IF the conditions warrant it descend with power on. This requires you plan the descent  to begin a lot earlier than normal and keep ahead of the show. Aircraft that theoretically DON'T NEED carb heat still have no heat source worth considering when the engine cools UNLESS they have a thermostat controlled liquid cooled system and even then the heat may not be sufficient at times.. I've used electric systems,  which may cover the situation. (I'm not sure) and a temp sensor of the carb body would take some of the guesswork out of it.  (as OAT doesn't tell the whole story about icing likelihood).  It doesn't happen that often but when it does it might be unexpected and ruin your day.. Low cloud bases indicate high Relative humidity. ( A good clue)  Nev

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 Most "individual cylindered "  aero engines  have steel (and finned)  air cooled cylinders. If you are going to use alloy castings, to get reliability you need some pretty exotic Vacuum die casting  set up to avoid porosity and inconsistent quality. That is really expensive and not suited to low production runs. There are very few alloy cast cylindered aero engines. The Franklin is the only one I can think of  with a cast iron liner pressed in, but with industrial ( large lawn mower etc) it's very common.( Large numbers made.)

  Arguably, the NEW Jabiru engine cylinder and head should have been one piece, which most aero engines effectively are, to eliminate the weakness of having a join in a critical area.

    The  common steel cyl /alloy head is screwed and shrunk on at the same time effectively for all intents and purposes a permanent  fixture. It's almost universally mounted at the base being a load carrying part.. it's still a known failure point to fail at the "shrink band" in the alloy part.  Steel cylinders are quite thin for lightness and cooling and  can distort in use. (Many things can) They are often nitrided for wear resistance ALL steel cylinders  can rust if not used properly.. They need good baffling and directed airflow to cool evenly. Jabiru concentrated on cooling  the (separate) heads and pretty much neglected the barrels... Nev

Edited by facthunter

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2 hours ago, facthunter said:

...Jabiru concentrated on cooling  the (separate) heads and pretty much neglected the barrels.

I agree, Nev. The original Jab cowl ducts cover barely half the barrels. Given how poorly steel conducts heat, that must mean the lower half gets bugger all cooling. Even my third and latest version of these ducts perpetuate this defect, which is one reason I'd like to build new ducts under the engine's hot bits.

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 OK. while heated (and therefore expanded) air rises it's not a large effect especially in a small distance.  Even an open fire with a good chimney will occasionally allow a big puff of smoke out  when a wind gust happens outside and that's a really small pressure change..

   Re the cylinders it's the uneven nature of undirected cooling airflow where the material's conductivity counts most. The AJS Porcupine hot parts were made of silver borrowed from the Country's mint. Rudge M/Cycles 4 valve and Gypsy Major 1 c's were bronze heads. Very Heavy. The heads and particularly the exhaust port areas are the highest temperature areas and 4 valves  common in many aero (particularly British)  engines which caused problems when air cooled mainly in the area between the two exhaust valves, tend to crack. Nev

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50 minutes ago, Old Koreelah said:

Yes Nev, that's one reason it might be better to get the cooling air to the hottest area first.

Not necessarily; if the resultant is the hot area is then cooler but a larger area becomes hotter it’s back to the drawing board. I can remember driving along beside a test truck cab and seeing the wool tufts pointing forward rather than back.

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When I built my aircraft I did a fair bit of research in to air cooled aero engines. The original Jabirus had an awfully designed air flow with a very small air exhaust. Eventually they came up with an extraction kit with a big downward facing lip to generate suction.

NASA produced a very comprehensive 151 page report in 1981 called  "Experimental Investigation of the Aerodynamics and Cooling of a horizontally opposed, air cooled aircraft engine installation". This is a very thorough document and you can understand the reason why the exhaust has to be a minimum of 3 times the volume of the intake. It is much easier to suck the air through than to try and blow it through. Good sealing around the intake and plenum chamber(s) is pretty important. My engine runs cool even when the temperatures are in the mid to high 30s. You can read the report or download it at https://ntrs.nasa.gov/search.jsp?R=19810013485

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the cowling we see is only a small part of the cooling control. It is getting the cool air to the top of the engine, but what we need is cool air passing between the fins. That means we have to have ducting practically touching the tops of the fins and following their contour to keep the airflow between the fins. The basic Jabiru ducting only gets the air to the top of the engine and from there it can get away from the fins, resulting in hotter operating temperatures.

My RV4 has to have a power reduction not long into the climb, to keep the temps below 400 degrees, but pushing the nose down and reducing power a touch works well.

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The jab plenums force the air to go down between the fins. There is nowhere else for it to go. I have small baffles between each cylinder in the plenum at the top which diverts air downward between the cylinder heads. Having over a metre wide air exhaust with a lip creating suction which pulls the air through is the major contributor to the low temps. A separate NACA duct completely separate from the rest of the engine for the 7 row Positech oil cooler also helps. EGTs are around 900 deg F in cruise & the CHT is almost always at the bottom of the green. I usually climb out at 70 knots at about 1500 fpm then increase speed to about 80 knots and about 1000 fpm. It runs cooler than any other jab engine installation I've seen. My oil stays quite clean and I have never had to top up between changes which I do at 25 hours. The numbers were higher with my old wooden prop but the bolly has what they call a taurus (I always thought that was a bull) which is a twist in the very last inboard inch or so of the trailing edge of the prop which is designed to aid airflow into the engine.

 

20151111_154644.thumb.jpg.6616b60c1df9788c496846edf71a8f8b.jpgAir exhaust is the full width of the airframe with a suction generating lip. Oli cooler on the right

 

20151111_154731.thumb.jpg.3298aaea37223575aaf42052c73bbd96.jpgSeparate NACA duct feeding cool air direct to the oil cooler

Edited by kgwilson
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I watched it and it did. I guess the guys at Bolly can't spell.

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