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jeromedelamare(at)free.fr
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 Posted: Tue Sep 11, 2007 11:58 am Post subject: RE : Cylinder cooling issues - Guidance from Jabiru |
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Greater than 6 inch of water ! Sure ?
In my manual :" The required pressure drop across the cylinders at 1.3 Vs in take off configuration is 4.3 cm (1.7") water gauge, minimum."
To obtain a dynamic pressure of 6 inch of water you need an Indicated Air Speed of 180 km/h (100 kts).
Jérôme
-----Message d'origine-----
De : owner-jabiruengine-list-server(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] De la part de Andy Silvester
Envoyé : mardi 11 septembre 2007 16:02
À : jabiruengine-list(at)matronics.com
Objet : Cylinder cooling issues - Guidance from Jabiru
--> JabiruEngine-List message posted by: "Andy Silvester" <info(at)suncoastjabiru.com>
The following information was received by Dealers from Jabiru, and is posted in advance of being incorporated into manuals and warranty policy:
To avoid overheating problems of the Jabiru engines in non Jabiru installations, it is vitally important that all installations provide a greater that 6 inch of water, air pressure differential to provide enough cooling for the Jabiru engine. Attached is a diagram of the tube placements to measure this and photos. We are in the process of having this included in the engine manual as a condition of warranty.
Attached is a diagram and photos that demonstrate how to perform this test.
Andy Silvester
Suncoast Sportplanes Inc.
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www.suncoastjabiru.com
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Andy Silvester
Joined: 10 Jan 2006
Posts: 18
Location: Marica, Rio de Janeiro, Brazil
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| Posted: Tue Sep 11, 2007 12:39 pm Post subject: Re: RE : Cylinder cooling issues - Guidance from Jabiru |
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Jerome,
Three things to point out:
1. The latest test has different reference points than before, when the pressure was compared with ambient and at an airspeed of only 1.3 Vs.
2. The higher pressure differential reflects both pressure and suction, i.e. inside the cooling duct is compared to a point adjacent to the cowling outlet.
3. The 6" differential will be obtained at normal cruise speeds, which is easier to monitor in the air rather than (in my view) trying to maintain a speed of 1.3Vs and looking at the manometer.
I hope this clears-up the confusion. My advice is to use the later information rather than that published in existing manuals - they will be updated soon.
Andy Silvester
Suncoast Sportplanes Inc.
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jimlc
Joined: 19 Mar 2007
Posts: 21
Location: Loveland, CO
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| Posted: Tue Sep 11, 2007 8:45 pm Post subject: RE : Cylinder cooling issues - Guidance from Jabiru |
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Andy Silvester
Suncoast Sportplanes Inc. writes:
"To avoid overheating problems of the Jabiru engines in non Jabiru installations, it is vitally important that all installations provide a greater that 6 inch of water, air pressure differential to provide enough cooling for the Jabiru engine. Attached is a diagram of the tube placements to measure this and photos. We are in the process of having this included in the engine manual as a condition of warranty.
Attached is a diagram and photos that demonstrate how to perform this test."
On slow moving aircraft this may be very difficult to achieve. I will do my best on my Kitfox 5, but for those of us who have already purchased our engines and paid for them, this is an unfair position for Jabiru to take, and a warranty amended after purchase is likely un-enforceable. The warranty at the time of purchase is a part of our contract and cannot be amended unilaterally. Let new buyers beware!! I want to be happy and will do my best to achieve the 6 inch of water differential, but this does not feel good.
Jim Crowder
[quote] --
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jimlc
Joined: 19 Mar 2007
Posts: 21
Location: Loveland, CO
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| Posted: Tue Sep 11, 2007 9:05 pm Post subject: RE : Cylinder cooling issues - Guidance from Jabiru |
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Andy,
You say: "3. The 6" differential will be obtained at normal cruise speeds,
which is easier to monitor in the air rather than (in my view) trying to
maintain a speed of 1.3Vs and looking at the manometer."
It now sounds like I may have jumped the gun with my previous posting, but
Jabiru's idea of warranty is suspect as in the case of the coils. My
previous engine was water cooled which presented its own problems. I will
see how this all plays out. I do appreciate your getting out in front of
these issues. As I previously stated, I will do my best to be happy.
Jim Crowder
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matronics(at)rtist.nl
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| Posted: Wed Sep 12, 2007 12:12 am Post subject: RE : Cylinder cooling issues - Guidance from Jabiru |
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6" pressure differential.. Hmmm, I'm starting to understand why Jabiru included a vacuum pump pad in the design..
Rob
[quote] ---
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Andy Silvester
Joined: 10 Jan 2006
Posts: 18
Location: Marica, Rio de Janeiro, Brazil
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| Posted: Wed Sep 12, 2007 5:36 am Post subject: Re: RE : Cylinder cooling issues - Guidance from Jabiru |
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I'm not sure if further explanation is needed on this latest cooling advice but here goes anyway:
1. Jabiru engines get installed and operated in a huge variety of aircraft. The only installations Jabiru can report and advise confidently on are (of course) in their own aircraft. The efficiency of the cooling for the heads is as much about the cowling design (extract, etc) as it is about the ram-air ducts provided. What they've now done, through factory testing of different installations, is come up with a set of measures for cowled-in installations which can be replicated in the field and objective comparisons made with the advice. The cylinder head cooling WILL be correct if the pressure differentials they specify are met. This gets us past all the problems with different CHT measuring methods, calibration of sensors, hot/cold junctions, etc. etc. If the pressure differential is right, so will be the head temps.
2. The guidance is intended for those who are trying to reduce CHTs, either to be within limits, or to bring the temps. down further from a normal-to-high situation. My experience has shown that CHTs on all cylinders should stay below 300F, 150C in cruise, for good head life (to 1000 hours). Brief excursions above these temps are OK when in the climb. Yes, the published upper cruise limit is higher, but we know that heads (and compressions) last longer with the lower CHTs. Temperatures of 220F, 105C should be achievable with the 6" pressure differential mentioned. Cowling design, oil cooler location and relative airflow, propeller choice, ambient air temps, type of fuel used, fuel / air ratio and speed of aircraft all affect CHTs. As a dealer I can only advise based on experience; Jabiru publish what they will and make the 'official' rules.
3. Warranty won't, and can't be retrospectively changed! The guidance is published now for advice, but Jabiru have said that IN FUTURE the cooling requirements and specification will be included as part of warranty. Quite honestly, how we dealers are to ultimately prove or disprove this in warranty claims is beyond me, but we'll see.....
4. So why is head temperature so connected to life of parts? We get feedback that "Lycoming and Continental' have higher limits and don't have problems (yeah, right). What you have in a Jabiru engine is a fine balance between power and weight. To get the low weight there is obviously less material in heads (etc.) and so heat build-up can get to the structure quicker than in heads weighing twice as much or more (and they do). Also remember that CHTs are measured only at one point on the head, and some parts of the head are much hotter (around the exhaust port). When a Jabiru head is exposed to high temperatures which soak through the entire structure, the aluminum can distort, both losing the bolt-torque and hence head-to-cylinder seal, and also valve seats can start to move, be drawn into the head material or simply twist around and lose valve seal. Let's be clear - if the head temperature is kept within limits, and the lower the better, it will last the course.
Andy Silvester
Suncoast Sportplanes Inc
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