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Debunking the Shock Cooling Myth



 
 
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  #21  
Old January 9th 18, 12:46 PM posted to rec.aviation.soaring
Tango Eight
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Default Debunking the Shock Cooling Myth

On Monday, January 8, 2018 at 7:07:06 PM UTC-5, wrote:
Yep Charlies, that was I was explaining in my earlier post, hard slip= slow speed, large rate of decent and disrupted airflow into cowl hence keeps the cht up. Works liike a champ but half the guys out there flying have ever done a hard slip and even less use it on a regular basis.
Dan


The L-19 will slip hard enough to dump a lot of fuel overboard if the tanks are full. We don't consider this an operational advantage.

Still, in terms of angle off the direction of motion, it isn't that big an angle (guessing 20 degrees, rudder on the floor). Intuitively, I doubt that it is possible to significantly disrupt the intake airflow with a steady state slip. Someone bring the data, prove me wrong.

Evan Ludeman / T8


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  #22  
Old January 9th 18, 05:25 PM posted to rec.aviation.soaring
[email protected]
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Default Debunking the Shock Cooling Myth

Evan, the data was found refering to a 6 cylinder cht that we had in our pawnee. Also reconfirmed on a 200hp Stinson also with 6 position cht. Doubt away, the technique does work, been using it for 30 years, have never cracked a cylinder. And regarding your bird dog (glorified 170) venting fuel, I hope your not towing with topped off tanks, that extra weight costs you time and money in the long haul unless where you operate doesn't have handy fuel available. We always operated with 3/4 tanks but note we were flying in extreme density altitude conditions.
  #23  
Old January 9th 18, 05:50 PM posted to rec.aviation.soaring
Charlie M. (UH & 002 owner/pilot)
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Default Debunking the Shock Cooling Myth

I will agree the slip angle likely makes minimal difference in engine cooling.
I WILL say a high slip angle makes the tug come down pretty fast with not a lot of airspeed, adding some "G load" at the same time helps as well.
Airspeed (straight on or angled) is still flow through the cylinder fins.
So.......lower speed and slightly reduced airflow can only help, the slip adds to sink rate. That IS part of the goal....correct?
Turnaround time, lack of adding to maintenance..........right?

  #24  
Old January 9th 18, 06:32 PM posted to rec.aviation.soaring
K m
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Default Debunking the Shock Cooling Myth

On Saturday, January 6, 2018 at 6:40:13 AM UTC-7, wrote:
from SSA Clubs and Chapters:

Over the years I've come in contact with various club towplane operating procedures that made some attempt to address "shock cooling". These techniques often have the effect of substantially reducing the tow capability of a towplane during a busy day by increasing cycle time. Being the kind of person who always wants to know "why" when these kinds of facts are presented, I learned that there was no sound thermodynamic or metallurgical reasons for the practice. When I became our clubs Chief Tow Pilot and maintenance go-to guy I changed our procedures and worked with our towpilots to understand how our operational procedures might make our engines last longest yet yield the best tow service. Our procedure once the glider releases is to smoothly pull power to whatever setting is desired to get back to the pattern soonest and reduce mixture aggressively. If you belong to a club that has a prescribed "cool-down" procedure for towplane descents, you may be spending more on tow services than need be. The following from a more expert authority might be useful.
-----

Shock Cooling: Time To Kill The Myth

RICK DURDEN

Some years ago, I had one of those “what in the world are they thinking?” conversations with a pilot who was towing gliders as a volunteer for the Civil Air Patrol. While he thought it was important to volunteer for a good group, he was ready to quit because of a screwy power reduction procedure imposed on the pilots by someone high up in the organization.. The procedure was ostensibly to prevent cylinder cracking due to shock cooling during descent after the glider released. However, the procedure he described took so long that, even if the glider did several minutes of soaring during its flight, it was on the ground well before the tow plane. As a longtime tow pilot, this struck me as ludicrous.

The anti-shock-cooling exercise required a series of small reductions in manifold pressure, each followed by flying around for a period of time before making the next, while the airplane descended slowly, burning lots of fuel. If shock cooling actually existed and caused cylinder cracking, it would probably be cheaper for the operation to have bought a bevy of cylinders and kept them on hand for replacement than pay for the fuel they were going through to avoid a phantasm.

I used to be astonished at how aviation myths, particularly when it came to engine operation, have such incredible staying power. Now, when I hear one spouted, I just shake my head in admiration of the influence of ignorance and belief over data. With some folks, the laws of physics, aerodynamics, metallurgy and thermodynamics are trumped by unwavering faith in their particular superstitions.

Nevertheless, when aviation superstitions get in the way of safe, efficient engine operation and addressing real risks of damage to engines, they need to be exposed for the nonsense they are, particularly when they are adversely affecting others—such as the glider operation that could only get off a few flights an hour. Such practices, especially when they are taught as fact to new pilots, only perpetuate the foolishness.

The widely respected Daniel Patrick Moynihan put it eloquently: “Everyone is entitled to his own opinion, but not his own facts.”

There is absolutely no hard evidence that making a large power reduction will cause cracking of the cylinders of a horizontally opposed piston aircraft engine. Because people like examples, we’ll start with a few: Bob Hoover regularly shut down and feathered the engines on his Aero Commander Shrike during airshows—going from max power to none—and never cracked a cylinder. That’s consistent with what skydiving and glider tow operators have known for decades—their engines hit TBO without much in the way of cylinder problems, even though they descend rapidly at low power settings. Flight schools, with their repeated touch and goes, don’t go through cylinders at a disproportionate rate.

Let’s look at the numbers involved in engine cooling, starting with the small role that the cylinder fins play. Only about 12 percent of the heat generated by combustion departs from the engine via the cooling fins. The biggest proportion, 44 percent, goes out the tailpipe. Eight percent, almost as much as is handled by the cooling fins, is dissipated through the oil. Most of the rest is dissipated via the big, metal prop bolted to the crankshaft.

The engine manufacturer that has published data on the potential for shock-cooling damage—Lycoming—said to avoid the risk of damage, pilots should limit CHT reduction in flight to 50 degrees F per minute. The good news is that, even assuming such a rate of cooling will damage an engine—Lycoming said that damage potential existed only if done "consistently"—it’s nearly impossible to cool an engine that fast in flight even by shutting it down. In an article written by Kas Thomas more than 20 years ago and reprinted in AVweb, he went through the published test data—which showed that cutting engine power by half only reduces CHT by 10 percent or so. That kind of CHT drop isn’t capable of trashing cylinders—and isn’t anywhere close to the CHT change that occurs in the opposite direction on takeoff—shock heating, so to speak. And there’s never been any data to indicate that the massive shock heating during takeoff harms the cylinders.

Thomas also pointed out that flying through rain reduces CHTs by nearly as much as a 50 percent power reduction. There’s no history of airplanes regularly flown through rain having to constantly replace cylinders.

In fact, the real shock cooling comes at the end of the flight when you pull the mixture to idle cutoff and the CHTs drop at more than 100 degrees per minute right away—yet every engine goes through that sort of shock cooling and manages to survive it.

In the last 20 years, graphic engine monitors have become common in general aviation—and the data they provide further support conclusions reached before they were around regarding the minor effect of big power changes. Many monitors are set to alarm if the CHTs show a drop at a rate of more than 60 degrees per minute. Pilots are discovering that it’s nearly impossible to hit that rate without slamming the throttle shut and diving—which isn’t comfortable for anyone in the airplane. Mike Busch, A&P and principal of Savvy Aircraft Maintenance Management, told me during a conversation at an AOPA Fly-In that he’s tracked how fast CHTs will drop with various power reductions in his Cessna T310R. His observations were that it unusual to have CHTs drop at a rate of even 30 degrees per minute even with aggressive power reductions when ATC gives a slam-dunk approach.

In one of AVweb columnist John Deakin’s excellent articles on engine operation, he noted that when he waited 18 seconds to restart the engine of his Bonanza after running a tank dry, the CHTs only dropped 10 degrees..

In my opinion, It’s time to put the shock cooling myth to bed, so that pilots can worry about things that really are a risk to their safety and wallets—such as runway loss of control accidents. After all, with more than 25 percent of accidents that cause damage to the airplane and engine arising from loss of control on landing rollout it seems to me that rather than designing complex power reduction strategies to avoid a mythical risk of damaging an engine, we should be practicing crosswind landings to protect a real risk that actually does damage engines—and the airframes wrapped around them.

Rick Durden holds a CFII and ATP with type ratings in the Douglas DC-3 and Cessna Citation and is the author of The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing it, Vols. 1 & 2.


To the OP, Initiating discussion on engine management is good, but as others have pointed out, the accompanying article is pretty much pointless. To the authors defense, I am sure his editor pushes for sensationalism and hype for the sake of web traffic or magazine subscriptions as the case may be. This type of "National Enquirer" style journalism has caused many (Including myself) to quit reading aviation publications and blogs (And for that matter, Facebook).
So keep reading and posting this stuff but understand that the vast majority of Aviation Journalism is garbage.
  #25  
Old January 9th 18, 07:23 PM posted to rec.aviation.soaring
danlj
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Default Debunking the Shock Cooling Myth

Yes, Rick's research harmonizes with mine.

Ed Kollin, who ran the Exxon engine-testing lab before retiring early to market Camguard, says "There is no such thing as shock cooling, but shock heating is damaging. Going to full power on an aircraft engine that is not yet at operating temperature stresses metals that have not expanded to fit properly."

It's also true, as I've learned from experience, that CHTs near manufacturer CHT redlines are safe only briefly at best. Aluminum begins to soften above 400°F, and rings begin to lose temper. A single overtemp incident by a mechanic working on my aircraft's engine, not following book mixture-adjustment procedures, caused 4 broken rings, gradually discovered during later inspections.

Ed recommends CHTs optimally at or below 380°F, and has noted that severely LOP mixtures with low CHTs lead to lead fouling of exhaust-valve guides, leading to exhaust-valve sticking.

I see guys taking off with barely-warm engines frequently.
  #26  
Old January 10th 18, 12:30 AM posted to rec.aviation.soaring
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Default Debunking the Shock Cooling Myth

On Tuesday, January 9, 2018 at 1:32:52 PM UTC-5, K m wrote:
To the OP, Initiating discussion on engine management is good, but as others have pointed out, the accompanying article is pretty much pointless.

So I was the "Original Poster", and fair enough on what you said. I didn't sign my name to it because I only posted the info, I did not originate either what seems to come from the SSA Clubs and Chapter group or the article author.

I did want to start a discussion, and glad I did - I've learned a lot and this is not an area of expertise at all for me, so I'm in listening mode.

I do notice that the article AND a lot of the posting by people defending their well honed procedures that there is very little scientific method here.
There appears to be a lot of correlation mixed up with causation. A lot of anecdotal stories and little statistical data based on controlled conditions.
This is understandable of course as it would take a large institutional type research budget to set up statistically significant control groups and study one factor at a time.

Someone lobby for NASA to do a study, otherwise we can go with crossing our fingers as we cut back the power and clutch tight on the rabbit's foot.


Chris
  #27  
Old January 11th 18, 12:28 PM posted to rec.aviation.soaring
Tango Eight
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Default Debunking the Shock Cooling Myth

On Tuesday, January 9, 2018 at 7:30:15 PM UTC-5, wrote:
On Tuesday, January 9, 2018 at 1:32:52 PM UTC-5, K m wrote:
To the OP, Initiating discussion on engine management is good, but as others have pointed out, the accompanying article is pretty much pointless.

So I was the "Original Poster", and fair enough on what you said. I didn't sign my name to it because I only posted the info, I did not originate either what seems to come from the SSA Clubs and Chapter group or the article author.

I did want to start a discussion, and glad I did - I've learned a lot and this is not an area of expertise at all for me, so I'm in listening mode.

I do notice that the article AND a lot of the posting by people defending their well honed procedures that there is very little scientific method here.
There appears to be a lot of correlation mixed up with causation. A lot of anecdotal stories and little statistical data based on controlled conditions.
This is understandable of course as it would take a large institutional type research budget to set up statistically significant control groups and study one factor at a time.

Someone lobby for NASA to do a study, otherwise we can go with crossing our fingers as we cut back the power and clutch tight on the rabbit's foot.


Chris


See Soaring mag, Dec 1992, pg 40 & April 1993, pg 22. Good study for those particular aircraft (Citabria and Pawnee). Too bad about the poorly legible figures.

Rapid cooling -- in excess of manufacturers' recommendations -- is no myth. Whether it causes damage over the long or short term is another issue altogether.

best,
Evan Ludeman / T8
  #28  
Old January 11th 18, 08:35 PM posted to rec.aviation.soaring
Brian[_1_]
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Default Debunking the Shock Cooling Myth

There is a line of thinking that most cylinders failures are caused more by Manufacturing issues than anything else.
One way to collect data for this especially when aircraft production was higher was to look at trade-a-plane adds for aircraft that still had factory engines in them. As I recall for a while it was pretty easy to see that certain years of C-210's rarely went over 500hrs without a top overhaul. I believe you can still see this with Franklin engines in Stinsons.

As the fleet ages and aircraft are on the 2nd, 3rd or more engines it is harder to see patterns like this, as it is hard to collect data on who made the cylinders and when they were made.

On the other hand I love engine monitors and prefer to treat the engines nicely when practical to do so.

  #29  
Old January 11th 18, 09:40 PM posted to rec.aviation.soaring
[email protected]
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Default Debunking the Shock Cooling Myth

On Thursday, January 11, 2018 at 3:35:55 PM UTC-5, Brian wrote:
There is a line of thinking that most cylinders failures are caused more by Manufacturing issues than anything else.
One way to collect data for this especially when aircraft production was higher was to look at trade-a-plane adds for aircraft that still had factory engines in them. As I recall for a while it was pretty easy to see that certain years of C-210's rarely went over 500hrs without a top overhaul. I believe you can still see this with Franklin engines in Stinsons.

As the fleet ages and aircraft are on the 2nd, 3rd or more engines it is harder to see patterns like this, as it is hard to collect data on who made the cylinders and when they were made.

On the other hand I love engine monitors and prefer to treat the engines nicely when practical to do so.


We had multiple failures during the ECI debacle. We overhauled with ECI and had 3 failures, all in the air. Heads came off. They replaced free of charge. After the 3rd, our overhaul shop told them we wanted our money back. They refunded all our money plus some bucks toward install of replacement Lycoming cylinders.No questions asked. Not long after that the AD came out.
FWIW
UH
 




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