Its been quite a while but many years ago our gliding club bought a thermal shock alert from Ian for our Super Cub tug. I did a fair bit of reading at the time (as I was mainly the poor soul stuck in the tug) on avoiding engine damage through to rapid a cooling rate.
Of course this was more applicable to the Lycoming and might not apply to the Limbach but the literature was strongly supportive of damage primarily resulting from the initial and subsequent power reductions from the highest head temps through the first half of the cooling phase. However as Rob said, we are very conservative with our cooling procedures and maintain gradual power reductions until CHT is around the 100deg. We have often done a lot of high cycle work for Scouts etc. Another minute or twoof cooling before shutting down means little compared with head replacement. Regards Laurie Hoffman 46531233 ________________________________ From: Rob Thompson <[email protected]> To: [email protected] Sent: Mon, 2 August, 2010 8:53:38 AM Subject: Re: [DOG mailing list] cracked heads I completely agree with Ian but I think there still needs to be lots of care taken through the lower temp range cooling. I generally try to cool very slowly though the whole range and try and get the minimum temp as low as possible until it won't go any lower in idle. Haven't cracked a head in 20 years ..touch wood. Rob Thompson PO Box 129, Lawson, NSW, 2783. mobile 0429 493828 ............................................. ________________________________ From: Ian Mc Phee <[email protected]> To: DOG LIST <[email protected]> Sent: Sun, 1 August, 2010 7:59:02 PM Subject: [DOG mailing list] cracked heads I have heard of a few cases recently of cracked heads in Limbachs usually from plug to exhaust valve. In talking to Nigel Baker he correctly pointed out to me it is caused by shock cooling of engine. I can say air cooled GA aviation engines should not cool at rate of more than 40degF/min (22degC/min) but ideally you should aim for lower than this like 30-35degF/min BUT DAMAGE IS ONLY DONE FROM HIGH TEMP TO MEDIUM TEMP and not from a medium temps to a low temp. Thus if you can get that maximum temperature really low then you will have a lot less problems infact I believe head problems should be totally a thing of the past in aircooled motorgliders as they are with Pawnee Tugs these days (I can remember in 1976 when we started using C180s for towing and then 18months all the cracks started and $4000 to $6000 repair bills) Limbach tech bull 53 (or is 54??) point 4 clearly states CHT should not go above 180degC in climb or words to that effect. Those temps in Limbach manuals of 230degC or what ever is, is total CRAP. Nigel is correct but I say get the CHT <170deg C AND cool down slowly from 170deg c to 150deg c and the heads WILL LAST the life of the engine no iffs or buts. It is very difficult to cool from say 195degC to 170degC without doing serious damage. Another way to check is a Lamba meter OR some means of measuring the CO (needs to be about 6-7%) I think I am right but I am open to thoughts on this one. It is not like a car as we are using high power settings and huge changes in density altitude. Ian M
