Thrust and Power

[mhcasey]

Apart from their different dimensional content, what's the difference? Is there a time in flight that I can have excess power, but not excess thrust, and vice versa?

TGrayson, this one's for you especially since my question comes from the "core concepts of flight." By the way, thanks for the response on Spins. It helped out quite a bit.

 

[tgrayson]

Apart from their different dimensional content, what's the difference? Is there a time in flight that I can have excess power, but not excess thrust, and vice versa?

Congratulations on asking one of the most difficult concepts to explain in all of aviation. Most engineer types throw up their hands when trying to explain this in an intuitive fashion, because it's an inherently mathematical concept, and the math isn't very satisfying to most pilots. I've been trying to refine my discussion of it over the years, so let's see how this works:

Of the two, thrust is the more fundamental concept, because it's what moves the aircraft foward. However, this thrust will result in some velocity for the airplane. It just so happens that some performance characteristics correspond more closely to the imaginary value we get when we multiply excess thrust times the velocity it achieved. We call the result of that calculation "Power".


Imagine two airplanes departing a runway at a climb angle of, say, 5 degrees with respect to the horizon. One has a velocity of 100 knots and the other has a velocity of 200 knots. If both airplanes weigh the same, then their excess thrust must be identical, because the climb angle is given by

Sin(climb angle) = Excess Thrust/Weight​

What that formula means is that the aircraft will angle upwards until the excess thrust is "neutralized" by a portion of the aircraft's weight along the flight path. If you could develop thrust equal to the weight of the airplane, you could fly straight up.

With two airplanes flying the identical flight path, you should be able to see that the faster airplane must be climbing at a higher rate because its velocity is higher; at the same angle; it must get to a given altitude sooner. The quantity of excess thrust can't predict this, but the quantity of excess power can.

Is there a time in flight that I can have excess power, but not excess thrust

No. Ignoring the math, the answer is buried in the definition of "excess". By definition, we're talking about power or thrust in relation to whatever is required for level flight. If we have "excess" of either one of these things, it means we're not in level flight, so there must be *some* climb angle (or descent angle) and some climb rate (or descent rate).

By the way, thanks for the response on Spins. It helped out quite a bit.

I'm glad, thank you. It's a very complex topic.

 

[B767Driver]

I liked TG's response. BTW, very few common texts will even approach the topic of thrust and power in any detail.

FWIW, a couple of generic, non-mathematical concepts to add

1: Rate of climb depends upon horsepower. Power by definition is the rate at which work is done. A time value involved with both, power and climb rate.

2: Thrust is a force. A push if you will. The angle at which you can climb depends upon the magnitude of the push you receive. No time value involved...but an angle dependant upon the force of push or pull you receive.

 

[mhcasey]

Thanks fellas. One question: The 100 knot and 200 knot aircraft are using the same thrust to climb at 5 degrees, but in all likelihood the 200 knot aircraft, all other things equal, still has more thrust available? It seems that the 200 knot aircraft could climb at a steeper angle (all other things equal...by the way, isn't there a latin short term for "all other things equal?") at which the 100 knot aircraft would not generate sufficient thrust to balance or exceed drag. Or am I going the wrong direction here?

 

[tgrayson]

The 100 knot and 200 knot aircraft are using the same thrust to climb at 5 degrees, but in all likelihood the 200 knot aircraft, all other things equal, still has more thrust available? likelihood the 200 knot aircraft, all other things equal, still has more thrust available? It seems that the 200 knot aircraft could climb at a steeper angle (all other things equal

They may or may not being using the same thrust. All we know is that excess thrust is the same. For any given amount of drag, there are two airspeeds where it occurs (except for min drag), one slower than ldmax and one faster.

Yes, the 200 knot airplane could climb at a steeper angle, but only at a lower airspeed, as long as the thrust output of the engines remains the same. If the pilot increases his AOA, then his drag would go down, and his excess thrust would go up (assuming thrust available doesn't vary with airspeed), and he would climb at a steeper angle, all with the same throttle setting.

...by the way, isn't there a latin short term for "all other things equal?")

"Ceteris Paribus" according to Google.

 

[mhcasey]

Sorry I should have been a little clearer. The 200 knot airplane will be able to attain a higher climb angle at a lower airspeed, and from Sin(climb angle) = Excess Thrust/Weight it follows that at that higher climb angle and constant weight, it will be producing more excess thrust at the same (call it max) power output.

"Ceteris Paribus" man it's been too long since high school physics!

Alright I'm going to divert this thread back to the avweb article (http://www.avweb.com/news/pelican/182081-1.html) you posted in my thread in CFI Corner. I'm a bit perplexed here. Near the bottom of the article he states,

"All else being equal, any engine will be more efficient if operated at full throttle. If you don't want all that power (or fuel flow), use a lower RPM, a leaner mixture, or both. Of course, full throttle makes it tough to get slowed down in the traffic pattern, and it can be tough on the brakes while taxiing, so the throttle can come in handy now and then. But it's best to avoid using it during climb and cruise, and I'll be talking more about this in another column."

Am I wrong here, or is he advocating just leaving the throttle full open in climb and cruise? Assuming I level off at say 3,000 feet, I'll have to pull my RPM back quite a distance to reduce my power properly, which would seem to put significant stress on the engine as well, including causing the engine to tend toward a vacuum in the same way it would with the throttle closed (as a result of the slow cylinder movement limiting airflow rather than the closed throttle). Was the idea in the article more oriented toward a decent climb heights where the MP with full throttle open will be significantly less?

I think the issue here is with the application of "efficiency." Obviously in the performance charts, at a higher MP there is more fuel flow, and therefore more g/h consumed. Full throttle will probably not result in a more efficient trip due to the aircraft's drag characteristics, but the engine itself, if it were to be removed from the aircraft (or the aircraft placed in a vacuum or some clever device to maintain constant TAS) would be running more efficiently.

Another "efficiency issue," which aircraft will break down sooner? I'm guessing the full throttle always engine since it will encounter more "events" per unit of time flown than the engine operated at lower MP, thus legitimizing my CFI (who also happens to own the flight school and aircraft) demanding that I constantly reduce MP to an obnoxiously low level in flight (though she is just as quick to ensure that I don't close it too much in a descent).

A fellow CFI student of mine told me "Just make sure the RPM is always higher than the MP," as in 23"MP, use 2300<RPM. Is there any truth to this claim?

OK I'm out of breath. I'll read more avweb and check back in a bit.

 

[tgrayson]

Sorry I should have been a little clearer. The 200 knot airplane will be able to attain a higher climb angle at a lower airspeed, and from Sin(climb angle) = Excess Thrust/Weight it follows that at that higher climb angle and constant weight, it will be producing more excess thrust at the same (call it max) power output.

Yes. BTW, I modified my post from what you may have first read. I decided that you weren't making the interpretation I thought.

is he advocating just leaving the throttle full open in climb and cruise?

In climb, yes. He argues strongly against throttle reductions after takeoff, for good reasons. In cruise, I believe he is arguing that you should fly at altitudes that permit full open throttle, because it's not efficient to artificially obstruct the airflow, which is what the throttle does.

Assuming I level off at say 3,000 feet, I'll have to pull my RPM back quite a distance to reduce my power properly, which would seem to put significant stress on the engine as well

I believe he is advocating flying higher than that, so that it's not necessary to pull the RPM way back.

I think the issue here is with the application of "efficiency." Obviously in the performance charts, at a higher MP there is more fuel flow, and therefore more g/h consumed. Full throttle will probably not result in a more efficient trip due to the aircraft's drag characteristics, but the engine itself, if it were to be removed from the aircraft (or the aircraft placed in a vacuum or some clever device to maintain constant TAS) would be running more efficiently.

Again, I think his point is for a given power setting, you're better off with a full throttle and a lower RPM setting, as long as the low RPM/MP combination doesn't violate any restrictions from the aircraft or engine manufacturer.

Another "efficiency issue," which aircraft will break down sooner? I'm guessing the full throttle always engine since it will encounter more "events" per unit of time flown than the engine operated at lower MP, thus

There's no real data on that. Some advocate that low RPM, high MP is better, because there's less vibration. Others speak of cylinder pressures and others temperatures.

Deakin has access to an awful lot of state-of-the art engine test data, so his opinions are well-informed, although I would never take what anyone says as gospel.

legitimizing my CFI (who also happens to own the flight school and aircraft) demanding that I constantly reduce MP to an obnoxiously low level in flight (though she is just as quick to ensure that I don't close it too much in a descent).

Let's draw a distinction between low power levels and the particular method you choose to get there. Lower power settings than 75% will probably prolong the life of the engine, but you can get there through a variety of different combinations of MP, RPM, and mixture. I've never seen any data that shows that one combination is better than another. Lots of arguments, but no data. Deakin is just making an argument.

A fellow CFI student of mine told me "Just make sure the RPM is always higher than the MP," as in 23"MP, use 2300<RPM. Is there any truth to this claim?

No, that's a myth. You will see quite a few "oversquare" power settings in most POH's. Take a look.

 

[mhcasey]

A few more along our line of discussion:

Is idling a fixed-pitch prop for a descent, etc. just as harsh on the fixed-pitch engine as a variable-pitch? I guess particularly a quick descent would be since you're closed throttle, but the RPM can get up there.

Can a non turbo charged engine achieve manifold pressure greater than 29.92 (standard conditions at sea level)? If so, how (other than flying really fast so there is some ram air?)

What exactly is the difference between "exploding" and "burning," and what about running an engine at low RPM and high MP causes detonation?

 

[MoMatt]

I'll join in.

A fellow CFI student of mine told me "Just make sure the RPM is always higher than the MP," as in 23"MP, use 2300<RPM. Is there any truth to this claim?

If you haven't learned already, be very aware of the "a friend of mine said," even if the friend is a space shuttle pilot (ok, maybe I'd give them some credibility).

Instead, go out and find the answer on your own, so when somebody asks you the question, you can tell them where you found the answer.

Is idling a fixed-pitch prop for a descent, etc. just as harsh on the fixed-pitch engine as a variable-pitch? I guess particularly a quick descent would be since you're closed throttle, but the RPM can get up there.

Not quite clear on the question. A fixed pitch prop will overspeed (above RPM redline) if allowed, so you must adjust throttle as necessary during descent.

A constant speed prop will maintain a constant RPM (as set by the pilot with the Propeller lever) as long as the propeller governor is able to keep up. If you have a high RPM setting, and then close the throttle, the propeller governor (all the one's I've seen, anyways) will not be able to maintain the high RPM setting, so RPM decreases.

In other words, just after the throttle is closed, the propeller governor decreases the pitch of the prop blades in attempt to keep the high RPM setting. The prop blades soon reach their limit of travel (against a "stop") and the blade pitch stops decreasing, so RPM decreases.

Is this more "rough" on an engine connected to a fixed pitch prop than one connected to a constant-speed prop? I don't think so. Might be rough on the crankshaft if you get in the habit of yanking the throttle closed instead of moving it smoothly.

Can a non turbo charged engine achieve manifold pressure greater than 29.92 (standard conditions at sea level)? If so, how (other than flying really fast so there is some ram air?)

In short, no. Nothing's free; I don't see how a normally aspirated engine could achieve a higher MAP than the ambient air pressure. Anyone else?

Edit: Should've read the question first. Disregard long explanation of turbo charging below.

I believe this is technically "turbocharging," where the alternative design, turbo-normalizing, maintains 29.92"MAP up to a critical altitude.

Look at the design of an engine's turbocharging system for the answer. It has nothing to do with aircraft airspeed, and everything to do with how well the turbo works. Most turbochargers use engine exhaust to turn a turbine blade, which in turn rotates an impeller that compresses intake air (on it's way into the intake manifold and into the cylinders). The compressed intake air on a turbocharged engine would be compressed to above 29.92" at MSL, while the compressed intake air on a turbonormalized engine will be ~29.92" all the way up to the critical altitude.

Find a picture, it'll make more sense. As aircraft airspeed is increased, more "ram-air" enters the turbo's intake, and any excess gets dumped overboard.

I'll leave the question about exploding and burning to someone else.

 

[MoMatt]

By the way, I always find these websites very helpful with flying:
http://www.whittsflying.com/web/index.htm Looks like it's updated; cool.

http://www.av8n.com/

 

[tgrayson]

I apologize for the delay. I’m really not an engine guy, so I wanted to skim through my reference books before posting.

Is idling a fixed-pitch prop for a descent, etc. just as harsh on the fixed-pitch engine as a variable-pitch? I guess particularly a quick descent would be since you're closed throttle, but the RPM can get up there.

You seem to know that the prop driving the engine is considered a problem. As I understand it, the damage is called by “ring flutter”. The rings can become unseated because the cylinder pressure that normally exists helps keep them in their grooves. Low RPM helps by increasing cylinder pressure and reduces the violence of rapid direction changes as the pistons move up and down.When you pull a the throttle of a CS prop to idle, the props will go flat in an attempt to maintain its RPM. A fixed pitch prop won’t do that and would tend to have a lower RPM. By the logic in the previous paragraph, the fixed pitch prop would be less likely to damage the engine. Of course, you can pull the RPM back on the CS prop during descent.

Can a non turbo charged engine achieve manifold pressure greater than 29.92 (standard conditions at sea level)? If so, how(other than flying really fast so there is some ram air?)

Seems unlikely; your intake losses appear to always amount to a few inches.

"exploding" and "burning,"

Normally, when the spark plugs spark, there are two flame fronts that progress out from the source of the spark. This is a slow (relatively!) steady burn. When the cylinders are unusually hot, the fuel/air mix ahead of the flame front can ignite before the flame front reaches it, causing an unusually high temperature and pressure. This is regarded as an explosion, or “knocking.”

and what about running an engine at low RPM and high MP causes detonation?

The idea appears to be that the worst time to have peak pressure is when the cylinder is TDC, or Top Dead Center. This causes the highest temperature and highest pressure to occur when the volume inside the cylinder is at its minimum, causing an increase in temperature and pressure over normal. Normally, peak pressure happens after the cylinder is already moving away. The books don’t say this explicitly but I suspect the expanding combustion chamber is a key aspect of keeping the cylinders cool. Thermodynamic theory makes assumptions of the ideal engine having a constant temperature and pressure in the cylinders, something that can only happen if the volume increases as the fuel burn. An expanding volume lowers temperature and pressure. Anyway, low rpm alters the timing of the engine, moving the peak pressure towards TDC, which, as I mentioned at the start of the paragraph, is the worst time to have it.

Note, though, the RPM is got to be pretty low. On a random Lycoming, you’ve got an allowable spread of about 5” “oversquare”. The only Continental I looked at is less. Any setting in your POH will be fine with probably quite a bit of margin.

 

[mhcasey]

This thread will never die.

Situation: Have to make a quick descent in a constant speed prop single...Is it:

1) More effective to go high RPM, or low RPM in at attempt to increase drag?
2) Worse for the engine to go high or low RPM?

I'm looking here for normal operations, as in the controlling facility kept you way high and you need to get down quick without ruining the engine.

 

[tgrayson]

This thread will never die.

Situation: Have to make a quick descent in a constant speed prop single...Is it:

1) More effective to go high RPM, or low RPM in at attempt to increase drag?
2) Worse for the engine to go high or low RPM?

I'm looking here for normal operations, as in the controlling facility kept you way high and you need to get down quick without ruining the engine.

High RPM isn't going to produce more drag until you start generating reverse thrust, such as when you have very low power settings, high speed. Not great for the engine due to

1) Excessive engine cooling rates and
2) Ring flutter

If you're developing positive thrust, a higher RPM will just generate *more* power.

Operationally, if I understand your meaning, your best bet may be to reduce excess power by as much as possible, while keeping your engine warm. You could do this by pulling your prop way back, while keeping a reasonable MP. For instance, looking at my Lycoming engine manual for the IO-360, an MP of 22 with an RPM of 1800 is ok and generates only 80 HP.

Now, I can't find my Arrow Manual, so let's assume that this C182 Information Manual I have sitting here is really for an Arrow. If so, level flight at 8,000 ft at 132 knots requires 163 HP. Doing what I did with the prop only provides me with 80 HP, so I have a power deficit of 163 - 80 = 83 HP. This HP is equivalent to a descent rate of 83 HP * 33,000 (ft-lb/min/HP) /2600 lbs = 1053 ft/min. Not bad!

 

[mhcasey]

OK great that's kind of what I was thinking...excess power = rate of climb, power deficit = rate of descent. I was just thinking that possibly with a high rpm setting, the prop could be windmilling significantly and generate a good bit of drag (which if I understand correctly is the "reverse thrust" of which you spoke). Of course, like you also point out, that would cause significant ring flutter.

 

[tgrayson]

I was just thinking that possibly with a high rpm setting, the prop could be windmilling significantly and generate a good bit of drag (which if I understand correctly is the "reverse thrust" of which you spoke).

Yes, reverse thrust is the origin of the propeller drag. What we think of as a negative AOA on the prop that is providing the torque to keep the propeller spinning. (The profile drag of the metal blades is small; stopping the prop is the next best thing to feathering.)

With idle thrust, I'm not sure that a high RPM setting is draggy in *all airplanes.* In the Arrow, you can improve your glide range by pulling the prop back during engine-out practice, but I haven't seen much difference in a C182 or the Mooney that I fly. I'm sure that would change if the engine were truly stopped.

 

[USMCmech]

You seem to know that the prop driving the engine is considered a problem. As I understand it, the damage is called by “ring flutter”.

Not true, engine braking is does not cause "ring flutter" which does not even exist. If the rings were that loosely installed they would not be able withstand the pressures of normal combustion. Furthermore the rings are designed to take pressure in both directions on the compression stroke, and the power stroke.

When you are descend with very low MP, and high RPM you are doing the exact same thing as a Freightliner coasting down a hill. They do this all the time causeing zero damage.

The one exception (and the source of this myth) is a radial engine. Due to the peculier desgin of the crankshaft in a radial. If a pilot lets the prop drive the engine, it will cause dmage to the main crank bearing (not the rings).

Since almost all of us fly flat engines with "normal" crankshafts we can disregard this. Go ahead and "downshift" and let the engine add some drag.



Now let us address the myth of "shock cooling".

Somehow this transformed from a freak event useually brought on by gross abuse of an engine, into a bogeyman that would cause cylinders to shatter like glass.

The two largest varibles in engine cooling are airflow, and power, with airflow being the far larger influence. Therfore if you reduce power by 3-4" creating a reasonable constant airspeed descent you will see a very gradual cooling of the engine. "constant airspeed" = constant airflow = constant cooling rate.

I used to fly skydivers where I would climb at full throttle for 25 minutes and almost redline the CHTs, then with a mear 2 minutes to cool down begin a Vne descent. This routine is repeated 5-10 times per day every weekend at hundreds of DZs across the country. If that dosen't create "shock cooling", then what does?

If anything would cause a cylinder to "shock cool" then dumping cold water on it would, right? What do you thingk happens when you fly through a rain shower?


Be kind to your engine, but these two myths are not something to worry about.

 

[tgrayson]

Not true, engine braking is does not cause "ring flutter" which does not even exist.

Lycoming acknowledges that it exists, as does every other engine authority.

Now let us address the myth of "shock cooling".

"Myth" in your opinion.

Therfore if you reduce power by 3-4" creating a reasonable constant airspeed descent you will see a very gradual cooling of the engine. "constant airspeed" = constant airflow = constant cooling rate.

That wasn't the context of this discussion.

Here's a copy of a Lycoming publication:

Avoid Sudden Cooling of Your Engine​

Sudden cooling is detrimental to the good health of the piston aircraft engine. Textron Lycoming Service Instruction 1094D recommends a maximum temperature change of50 deg F per minute to avoid shock cooling of the cylinders.

Operations that tend to induce rapid engine cool down are often associated with a fast let down and return to the field after dropping parachutists or a glider tow. There are occasions when Air Traffic Control also calls for fast descents that may lead to sudden cooling.

The engine problems that may be expected when pilots consistently make fast let downs with little or no power include:

1. Excessively worn ring grooves accompanied by broken rings.
2. Cracked cylinder heads.
3. Warped exhaust valves.
4. Bent push rods.
5. Spark plug fouling.

Generally speaking, pilots hold the key to dodging these problems. They must avoid fast let downs with very low power (high cruise RPM and low manifold pressure), along with rich mixtures that contribute to sudden cooling. It is recommended that pilots maintain at least 15" MP or higher and set the RPM at the lowest cruise position. This should prevent ring flutter and the problems associated with it.

Let down speed should not exceed high cruise speed or approximately 1000 feet per minute of descent. Keeping descent and airspeed within these limits will help to prevent the sudden cooling that may result in cracked cylinder heads, warped exhaust valves, and bent push rods.

The mixture setting also has an effect on engine cooling. To reduce spark plug fouling and keep the cylinder cooling within the recommended 50 deg per minute limit, the mixture should be left at the lean setting used for cruise and then richened gradually during descent from altitude. The lean mixture, maintaining some power, and using a sensible airspeed should achieve the most efficient engine temperatures possible.

The operating techniques recommended in this article are worth consideration as they will be a positive step toward saving dollars that might be spent on maintenance. Whatever the circumstances, pilots must plan their flight operations so that the potential damage caused by sudden engine cooling can be avoided.

 

[USMCmech]

Lycoming acknowledges that it exists, as does every other engine authority.

I apologize, I worded my response poorly.

Ring flutter does exist, but it isn't caused by downshifting.

If ring flutter is caused by engine braking, then why doesn't it show up in automobile engines? Bear in mind a VW, Porsch, Harley Davidson, engine is identical in design to an Lycoming.

I have never seen in ANY engine manual discourage downshifting because it might cause "ring flutter". A Semi downshifting has the same effect on piston rings as a propeller driving an aircraft engine.

Ring damage is useually the result of poor assembly, not downshifting.

Read the following article regarding the pratice of letting the prop drive the engine.

http://www.avweb.com/news/pelican/186778-1.html

"Myth" in your opinion

Here's a copy of a Lycoming publication:

Sudden cooling is detrimental to the good health of the piston aircraft engine. Textron Lycoming .... recommends a maximum temperature change of50 deg F per minute to avoid shock cooling of the cylinders.

The engine problems that may be expected when pilots consistently make fast let downs with little or no power include:

1. Excessively worn ring grooves accompanied by broken rings.
2. Cracked cylinder heads.
3. Warped exhaust valves.
4. Bent push rods.
5. Spark plug fouling.

Generally speaking, pilots hold the key to dodging these problems. They must avoid fast let downs with very low power (high cruise RPM and low manifold pressure), along with rich mixtures that contribute to sudden cooling. It is recommended that pilots maintain at least 15" MP or higher and set the RPM at the lowest cruise position. This should prevent ring flutter and the problems associated with it.

The operating techniques recommended in this article are worth consideration as they will be a positive step toward saving dollars that might be spent on maintenance. Whatever the circumstances, pilots must plan their flight operations so that the potential damage caused by sudden engine cooling can be avoided.

Let me rephrase myslef

"The myth that "shock cooling" is a problem for the average pilot in normal flying operations." To cause a 50 degree per minute drop in CHTs requires extreeme power and airspeed changes. Not something you would do on a average x-cty flight.

When I flew jumpers I took all the steps recomended, and never saw more than a 100 degree CHT drop over 6 minutes. If any engine had the right to break from abuse it would be a jumpplane, yet they useually run reliably for their full expected life.

However many pilots have adopted draconinan measures to protect themselves against a hazard that was never an issue. Indeed I have seen some concern themselves so much with their engines that they don't manage the rest of the flight. I did a BFR for one pilot who was so concerned that he failed to propperly fly a localizer aproach because he was nervous about retarding the throttle.

Here is my version of "how to avoid shock cooling"

1st - Don't let the engine get hot. If you are crusing with CHTs over 400, something is wrong, get it fixed. 350-380 is a much better range.

2nd - modest constant airspeed descents (500 FPM which you were already doing anyway) will give your engine a nice gentle cool down peroid.

A pilot in a Bonanza who takes off, climbs to 8000, cruises for 2 hours, and then makes a nice genttle descent to a normal landing has nothing to worry about.

 

[mhcasey]

You know, I was a bit confused as to why anyone would ever use engine braking if terrible problems were associated with it...seems much cheaper to replace brakes than crankshafts or rings...

So it seems that even at high pitch, the parasite drag from the prop (in a low MP high rate of descent scenario) will be less than the reverse thrust you'll get from going high rpm, so if you really need to shoot down, that's your best bet. However, avoid prolonged descents and if possible, keep a little mp in to "relieve the suck/reverse crankshaft pounding" or pull your RPM back a bit to essentially do the same thing.

So next question: In a real engine out scenario, would I be better off then to go to a low rpm setting to decrease drag and increase my glide distance (assuming I'm in a single engine with no feathering)?

 

[tgrayson]

Read the following article regarding the pratice of letting the prop drive the engine.

I don't see anything there that dismisses this issue. Deakin says, without criticism:

Ring Flutter​

During most of the time the engine runs, the cylinder rings are forced "down" against their lands, which also pushes them against the cylinder walls. If the RPM is very high, and the MP is very low, there is a large, negative pressure created in the combustion chamber during the intake stroke, due to the closed throttle plate and the piston trying hard to suck air in. This may lift the ring off its land during that stroke. The next stroke is the compression stroke, and while the pressure will be greatly reduced because not much air got in, it's still enough to push the ring back down again. This repetition may well cause the rings to "flutter," beating up and down within the land, and this may well cause damage. ​

Let me rephrase myslef "The myth that "shock cooling" is a problem for the average pilot in normal flying operations."

Agreed. However, the OP was raising the issue of something outside "normal flying operations".

I did a BFR for one pilot who was so concerned that he failed to propperly fly a localizer aproach because he was nervous about retarding the throttle.

Yes, I've seen that too, and it was caused by flight instructors who created an excessive fear of shock cooling. It's difficult to undo that fear.

A pilot in a Bonanza who takes off, climbs to 8000, cruises for 2 hours, and then makes a nice genttle descent to a normal landing has nothing to worry about.

Agreed, but again, that wasn't this topic of conversation.

Pretty much everyone agrees (even Deakin) that it's possible to shock cool an engine, but people differ about under what conditions this can occur. In truth, we simply don't have reliable data to know for sure. We'd have to have accurate data about how a particular engine was operated over a long period of time, its degradation in performance in the same time period, and when it went up for overhaul. We don't have that and aren't likely to ever get it. Best to operate with conservative limits as much as possible, without sacrificing the utility of the aircraft.