That was my point, later I will do the math if I can figure it out. Edit: Ha never mind figuring out the math for that but I would assume quite a bit as I have never seen it and I typically do a max performance descent biweekly in our aircraft.
Shock Cooling
- Thread starter Douglas
- Start date
That was my point, later I will do the math if I can figure it out. Edit: Ha never mind figuring out the math for that but I would assume quite a bit as I have never seen it and I typically do a max performance descent biweekly in our aircraft.
USMCmech
Well-Known Member
Since the "shock cooling" phenomon wasn't an issue when radials were king, I'll bet that it's a case of "that's what my CFI/A&P told me to do in my Bonanza, so I'll do the same thing here". Since some radials run 40"+ MAP, you'd have to start your power reductions a half hour from the runway.
A constant cruise airspeed/500 FPM descent isn't ever going to harm your engine, no matter what you do.
Don't let your CHTs get hot in the first place. If you are seeing CHTs over 400 in cruise something is wrong with your engine.
Hacker15e
Who am I? Where are my pants?
Well....not so much. That comes from a guy named Randy Sohn, who was a USAF B-25 instructor back in the 1950s, and then had a long airline career flying radials. If you Google him, you'll find that he has written many essays on different aspects of how to properly operate big pistons -- lessons that were "common knowledge" back when everyone flew with big radials all the time, but have been lost to time with very few people flying them. Very interesting stuff to read. It's required reading for anyone who wants to fly warbirds today.
There are actually some other issues that are directly correlated with this, such as the reciprocating load phenomenon, and the 1" per minute ROT is part of an entire package of how to treat a big radial correctly so it not only doesn't kill you, but so that it also has a long and healthy life between overhauls.
Besides....40" is takeoff power. You're not cruising around at those MAPs.
1"/min was SOP at Airnet in the barons. It took some planning to work out descents/approaches, especially when Chicago wants you 170 to RUNTS and you're trying to figure out how to toss out gear and flaps.
In the end, it was their plane and they were paying me to do something that wasn't unsafe, so I played the game.
Real world...you'd have to rip the cowling off, toss the power to idle and dip the cylinders in ice water to drop the temps 50 degrees/minute. Slight
I wouldn't worry about it.
-mini
In the end, it was their plane and they were paying me to do something that wasn't unsafe, so I played the game.
Real world...you'd have to rip the cowling off, toss the power to idle and dip the cylinders in ice water to drop the temps 50 degrees/minute. Slight
I wouldn't worry about it.
-mini
BrewMaster
Well-Known Member
I have 50 hours multi time and was ALWAYS taught 1-2"/1-2mins. When you have turbo-charged, high-performance engines, you want to make sure you don't do 2 cooling actions at once (descent and power reduction) My ex-MEI/buddy who let me build about 30 hours or so in his 340 just found a 2 inch long crack in the block just behind the alternator. They really don't know how it happened, but that's just one more reason to really take care of the engine.
I know yours were normally aspirated, but we were always told it was more of a high-performance issue with high MP. I guess the words "shock cooling" never came up although they did say don't do 2 cooling actions at the same time.
I know yours were normally aspirated, but we were always told it was more of a high-performance issue with high MP. I guess the words "shock cooling" never came up although they did say don't do 2 cooling actions at the same time.
USMCmech
Well-Known Member
How does changing altitude seriously affect cooling?
It dosen't, airflow through the cowling (either more airspeed or opening cowl flaps) does. If you keep those two variables the same, the power reduction will only cause a modest rate of cooling, nowhere near 50deg/min.
I stand corrected, I've read some of his stuff, but haven't seen anything about cooling yet.
In a "proper" descent where you just power back and airspeed remains the same how can you apply 2 cooling actions at once? Unless the CFI's descent procedure was to throttle back and dive bomb the deck, then 1-2 inches might be worth thinking about.
Here from an article:
"(A Lycoming Flyer article once stated: "Engineering tests have demonstrated that valves will stick when a large amount of very cold air is directed over an engine which has been quickly throttled back after operating at normal running temperatures." See 101 Ways to Extend the Life of Your Engine, page 96.) But it's a big jump to go from that to saying you can make a cylinder head crack just by pulling the throttle back too quickly."
This article: http://www.avweb.com/news/maint/182883-1.html
It all falls back to know the limits of degree change per minute on your CHT and stay within those limits, if you do that the procedure you use to do it is completely irrelevant. But like USM stated repeatedly, a 500 FPM descent (a reduction of about 5MP or 500 RPM or any combination of the 2) will not put your aircraft in any situation where shock cooling is an issue. Shock cooling is a factor primarily on cold days with high speed descents and the power at idle, my advice, stop worrying about it.
[o/t]
Hacker, are you on the "farm" forum? I've been wondering what's up with Randy? I don't think he's posted there since January. I let my membership lapse and any word about him is probably in a forum that I can't read, so I'd appreciate an update if you have one.
BrewMaster
Well-Known Member
Hey man, I'm not saying I agree, just saying that's how I was taught and that's what Douglas was told too. Obviously it's not all that unheard of. I would rather take my chances on being conservative with a couple inches per minute than cracking a block or whatever else could happen.
Also, where is 50 degrees a hard number? Who says at that exact point it will make a difference? I never said anything about shock cooling (in fact I made a point of it), I was just taught that it was good practice on engines that can produce a lot of MP.
R
Roger, Roger
Guest
A cracked crankcase is more likely to be either a bolt that was not properly torgued, allowing the case to flex and fatigue, or a case that was just plain worn out. The shock cooling myth doesn't even mention crankcases, just cylinders.
USMCmech
Well-Known Member
Thosands of pilots were taught to fear the "downwind turn". Famous authors talked about flying "on the step". Virtually every puppy mill (including the one I went to) teaches that you shouldn't ever run your engine "oversquare". All of these myths are 100% wrong.
"Shock cooling" is just another myth that is based on flimsy evidence. While hard science shows the opposite is actually true.
BrewMaster
Well-Known Member
I'm not arguing, I actually prefer your style, it's a lot easier, but I doubt "the opposite" is 100% true. I'll ask him about it next time I see him.
See my post #12, that 50 degrees is from the lycoming manufacturing document I linked in that post. I wasn't trying to say that to argue with you I actually thought you were agreeing to begin with.
JJ429PA
Well-Known Member
If "shock cooling" could do so much damage, then what about the reverse? It seems that if drastic temp changes could do significant damage, it would happen from firing up that plane that's been sitting on the ramp all day chilling. Watch how fast those CHT's rise.
It seems like rapid temperature changes by themselves are not going to destroy an otherwise healthy engine, but could very well contribute to the demise of an engine with issues already.
It seems like rapid temperature changes by themselves are not going to destroy an otherwise healthy engine, but could very well contribute to the demise of an engine with issues already.
Test this theory out on a piece of glass at your house. Heat it up over a flame on the stove then run it under cold water (careful it may explode instead of just shatter). The point is, rapid cooling tends to have a more detramental effect on materials than rapid heating.
However starting on a cold day presents an entirely different problem. The oil sitting in the sump while the metal cylinders grind up and down the unlubricated piston sleeves at 1800 RPM because someone didn't start the engine properly (keeping it at/around 1000 RPM or below after start till warm up). This same person is usually the same guy taxiing around at 1500+ ridding the breaks right down to the run up area, but what ya gonna do.
JJ429PA
Well-Known Member
Yeah I was thinking about that after I posted it...a hot object rapidly contracting would be worse than a cool object rapidly expanding. However I stick by my idea that it would do the most harm by exacerbating an existing problem rather than causing one on it's own. I agree with a majority of the responses that it has become somewhat of a "boogy-man". Yes, it's valid in theory, but it's pretty far down the list of things to lose sleep over.
Safety first, always. If you have the time to 'slow-cool' it, why not? I'd opt for that to be on the safe side, but there aren't a lot of things I'd give it priority over. But if you're afraid to shoot an approach or something because of it? C'mon....
Douglas
Old School KSUX
G.B. Red. :clap:
jdlilfan
Well-Known Member
Kind of catch 22 with carb engines in the winter. Many POH's call for the power to be set to 1500 for the first minute to allow proper fuel vaporization while the engine warms. I agree with this as well because I have experienced many of 152 engines die soon after starting when the student starts monkeying with the throttle in below freezing temps.
Hacker15e
Who am I? Where are my pants?
Well, again, there are certainly truths behind what eventually become WOMs, and this is one of them. It is certainly not an absolute, but it remains a good ROT.
If you read Randy Sohn's "Warbird Notes" entry about reciprocating loads, it will show that there IS an important link between MP and RPM and the overall health of your piston (radial, at least) engine -- and that the 1" per 1,000 RPM is a pretty solid ROT.
http://www.douglasdc3.com/sohn/3.htm
ppragman
FLIPY FLAPS!
I dunno, I do about 1" per nautical mile in the continentals it works great to fly a normal pattern and be not too hard. These lycomings I've been flying seem to be pretty much bullet proof. I don't like the idea of going 20" to 15" or some combination therein in rapid succession if I don't absolutely have to.