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Can a piston helicopter experience shock cooling in flight?

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Can a piston helicopter experience shock cooling in flight?

Old 11th Jul 2021, 17:03
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Can a piston helicopter experience shock cooling in flight?

Let's say I cruise close to MCP, temps are within limits. I fly solo in the R44 with half the fuel. Destination comes in sight, but I fail to anticipate and I have to completely lower the collective to stay on the glide path.
So no power, but the big fan at the rear still forcing cool air at 102%...
Could it be a problem in, say, the Canadian winter?
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Old 11th Jul 2021, 17:55
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I very much doubt it. I think the absolute ambient temperature is irrelevant - it’s the rate of change of temperature that matters.

some info here: https://www.lycoming.com/content/how...ng-your-engine
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Old 11th Jul 2021, 18:37
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I think your biggest concern will be rotor overspeed. I never manage to fully lower the lever when light but never been told about temperature issues except carb heat if not injected
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Old 12th Jul 2021, 00:11
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Considering it takes more than a minute to cool down after landing at 70% engine rpm even on a very cold day with no load at all, I seriously doubt it would be a problem in flight.
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Old 12th Jul 2021, 01:58
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I think it’s a very real possibility if flat pitch. Once I was flying in the H269 coming off a mountain range at about 8000ft AGL and decided to give my student a simulated engine failure. It was very cold and on the way down I looked at the CHT to find it had plummeted. It was a long time ago since I’ve flown pistons but it had dropped from high green to 30 deg very quickly.
I very gingerly reintroduced power to slowly increase the temperature and was concerned about cracking the cylinder head.
After that when performing the same exercise which wasn’t often I’d watch the CHT and reintroduce partial power to reduce the rate of temperature drop before removing again and also to reduce the increase in temp at the bottom when you rapidly applied full power.

If I recall correctly it was also a danger in fixed wings with the same lycoming engines and rolling to idle from high power settings.

That exercise was for a practise Auto and shouldn’t be used for a normal approach. If say you have some power say 3 inches Manifold pressure above idle I think it would not be a factor at all. In the auto the fuel is only a trickle to keep the engine turning over whereas in flight even at low power settings you are burning quite a lot to drive to rotor system so are introducing enough heat that thermal shock should not be an issue.
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Old 12th Jul 2021, 03:38
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The only shock cooling I've ever experienced in the 22 was when crossing a micro climate barrier with the doors off, and the temperature suddenly goes from 90° down to 50°!

,...although that's much more pleasant than in the other direction.
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Old 12th Jul 2021, 12:16
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SLFMS
You sure about that ? as the CYL head temp on a 269 runs around 350 Fahrenheit to get to 30 degrees Fahrenheit is pretty impressive
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Old 12th Jul 2021, 12:53
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Originally Posted by Hughes500 View Post
SLFMS
You sure about that ? as the CYL head temp on a 269 runs around 350 Fahrenheit to get to 30 degrees Fahrenheit is pretty impressive

Hughes, no not sure on the exact numbers it’s been about 15 years since I’ve flown a 269 so they might not be bang on perhaps it was 50. I do very clearly remember that it had dropped right off the gauge from been in the upper green. At the time I’d flown fixed wings so was a wary of cracking the cylinder head. It was during an auto with engine at 2000 rpm probably close to 0deg C or a little below..
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Old 13th Jul 2021, 18:29
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Thank everyone for your input, much appreciated. Of course it's the rate of cooling which matters, no more than 50 ºF/min according to the Lycoming link. In airplanes we are taught a winter procedure for simulated engine failures: keep some power and lower some flaps to get an equivalent performance. Never heard that in helicopters, but we learn in an R22 with two POB, so simulated engine failures don't even last a minute, should be OK I guess...
However an R44 at 700 lbs below MTOW loves to glide! Better watch my CHT gauge next time. Even better, learn to anticipate arrivals before next winter
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