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Old January 6th 18, 08:33 PM posted to rec.aviation.soaring
Dave Springford
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Posts: 250
Default Debunking the Shock Cooling Myth

I have to disagree with the premise of the OP as well. My club running a 180 Citabria, a 235 Pawnee and a 260 Pawnee had a plethora of cracked jugs over the years. We had a couple of guys, who designed and built a monitor that measured (amongst other things) CHT of all 4-6 cylinders, RPM, airspeed, altitude.

They installed it in a towplane and flew 100's of tows in each type to measure what was happening using many let down techniques. What they found is that a good let down procedure really depends on the aircraft type. For example, the Citabria with it's tight cowling was very sensitive to airspeed changes combined with power reductions after glider release and the 45-50 F cooling rate/min could easily be exceeded at this point in the let down if power was reduced to rapidly and airspeed increased too rapidly. The Pawnee with its huge cowling and lots of cooling air was not sensitive to this rapid cooling at the top of the descent, but if aggressive cooling did not occur after glider release, then upon pulling back power on final to land the cooling rate spiked above the recommended value.

Having recently flown a Pawnee with a JPI EDM700 installed that displays the cooling rate of the most rapidly cooled cylinder, I saw exactly this. The cooling rate spiked right about touchdown, but could be easily reduced by adding power after landing (1000-1200 rpm seemed to work).

The monitoring system was also installed in an L-19 at another club and as I recall it was also found that the L-19 is quite sensitive to rapid cooling right after glider release.

I don't claim to have the best let down around as we are still cracking jugs at times and many other operators don't crack any, but to say that shock cooling is a myth is just wrong based on my experience and the measured data.

If you have a procedure that is working for you and preventing cracked jugs, stick with it!
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