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Rapid descents and mixture/power settings

R

RNB

Well Known Member
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On both my factory rep. ride and with my RV10 checkout CFI I recall them emphasizing the ability to make rapid descents in the plane. I had come from a Cessna 172 and really stuck with 500fpm, but I recall 3000 fpm on both the above rides.

Due to wx changing a bit yesterday I cut short a flight. Clouds seemed to be 6000-9000, I was 9500. 20 miles from home there was an opening that I circled down in, but towards the end I was 2500fpm to make sure I was clear of clouds, saw my CHTs get lower than expected. So I have been going down the shock cooling rabbit hole online.

If I were to take what I am learning and critique myself it would be that I likely enriched my mixture too much too early and should have done so incrementally as I descended.

The critiques of others online have me trying to learn and understand things better.

How often do you descend more rapidly than 1000 fpm?
How much do you pull the power back and push in the mixture?
 
RNB said:
On both my factory rep. ride and with my RV10 checkout CFI I recall them emphasizing the ability to make rapid descents in the plane. I had come from a Cessna 172 and really stuck with 500fpm, but I recall 3000 fpm on both the above rides.

Due to wx changing a bit yesterday I cut short a flight. Clouds seemed to be 6000-9000, I was 9500. 20 miles from home there was an opening that I circled down in, but towards the end I was 2500fpm to make sure I was clear of clouds, saw my CHTs get lower than expected. So I have been going down the shock cooling rabbit hole online.

If I were to take what I am learning and critique myself it would be that I likely enriched my mixture too much too early and should have done so incrementally as I descended.

The critiques of others online have me trying to learn and understand things better.

How often do you descend more rapidly than 1000 fpm?
How much do you pull the power back and push in the mixture?
Click to expand...
Personally, for high speed climb, cruise, and descent, I use around 500 fpm. Climbs or descents in excess of 500 fpm can be from mildly uncomfortable to excruciatingly painful to people.

During initial climb, I usually average around 1000 fpm to a safe altitude; for me around 1000 AGL.

A 3000 fpm descent in a -10? Well, it would have to be an emergency...
 
Coming down from altitude, I don’t enrichen the mixture at all, unless I reapply power above 65%.. typically I’ll descend at 18 inches, or 15 inches. I usually pitch the nose over for 500 to 1000 fpm (or whatever rate I’m looking for) then as the speed approaches 180 KTAS, I adjust the power as needed to maintain that speed. I leave the mixture leaned unless I level off and cruise, then I only enrichen to that new power setting. I can usually plan my descent from altitude and never have to go above 55% all the way down to landing, I end up at pattern altitude doing 180 knots and have the power back to 15 inches, then I level the nose and let the speed bleed off. Then power to idle, full flaps and land. My CHT cooling rate alarm is set for 50 degrees/minute. With that technique, I can land without triggering the alarm.
rocketman1988 said:
Personally, for high speed climb, cruise, and descent, I use around 500 fpm. Climbs or descents in excess of 500 fpm can be from mildly uncomfortable to excruciatingly painful to people.

During initial climb, I usually average around 1000 fpm to a safe altitude; for me around 1000 AGL.

A 3000 fpm descent in a -10? Well, it would have to be an emergency...
Click to expand...
 
High power applications. 1" MP per 1000ft or if you really want .5" per 500. Watch your CHTs, while the whole shock cooling topic has been thrashed a number of times and I don't believe too much in it. Your engines health will be happier with gradual reductions. Also you'll stay quicker. In my Turbo Cirrus, I also have to watch TITs because I'm LOP in cruise. So with it I pretty much descend watching TITs and CHTs. Mixture typically full rich at first GUMPS check..
 
This is worth a read - first written around 2000, updated in 2024. The article is a summary of his engine management series, from start-up to shutdown, including descents and shock cooling. Note that Deakin's engine was a turbo. https://www.avweb.com/flight-safety/pelicans-perch-19putting-it-all-together/

Here is his article specifically about descents, which also has an interesting sidebar on descent speeds. https://www.avweb.com/features_old/pelicans-perch-36those-fire-breathing-turbos-part-6-and-final/
 
Deakin San was a wise man.

With our Lycoming engines, that have either an FCU or Carby, you can safely leave the mixture in the LOP position all the way to the hangar door. Even a Go Around at low level/high MAP will result in a perfectly appropriate LOP mixture for 85-90% power. Go try it.

If you fly a lot of different and under powered machines, stay in the full rich habit on final, but from a science perspective there is nothing to it. 2300 hours of never touching mixture from TOC to the hangar door, plus many more hours text flying others planes and never had a stumble. And even if you do, just push it forward.
 
Under normal situations and you are not in winter freezing conditions it will not be a problem. Just avoid decending with the throttle pulled back to idle. If your in a situation you feel you need to get out of, then fly the airplane and don’t worry about anything else until your established back into a comfortable environment.
 
togaflyer said:
Under normal situations and you are not in winter freezing conditions it will not be a problem. Just avoid decending with the throttle pulled back to idle. If your in a situation you feel you need to get out of, then fly the airplane and don’t worry about anything else until your established back into a comfortable environment.
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Hi Rich,

You say avoid descending with the throttle pulled back to idle. But why? I have tested this a few times and compared the data to other flights and in particular climb outs. The rate of change in CHT is very minimal and steady, and less than a climb. So which is harder on the cylinders? High cylinder pressures and rising temperatures, or hardly any pressure and slowly declining temperatures. The data shows the climb is what is hard on them, not the fast descent.

I hope you find that helpful.

Best regards,
 
RV10inOz said:
Hi Rich,

You say avoid descending with the throttle pulled back to idle. But why? I have tested this a few times and compared the data to other flights and in particular climb outs. The rate of change in CHT is very minimal and steady, and less than a climb. So which is harder on the cylinders? High cylinder pressures and rising temperatures, or hardly any pressure and slowly declining temperatures. The data shows the climb is what is hard on them, not the fast descent.

I hope you find that helpful.

Best regards,
Click to expand...
I have probably have done descents of 2500+ fpm and probably at idle a couple of times. I am not a subscriber of the shock cooling camp. Will change mt mind when a metallurgist gives me real world data supporting the theory. Today it is just rehashed data from who knows what unrelated source. Just like the rule that your oil must be 100* before take off, and over square conditions will cause your engine to explode, and lean of peak damages your engine and on and on. I also don’t richen on descents. I go all the way from 14k to pattern without touch the red knob.

What i find most amusing about the skock cooling hysteria is that going quickly from 350 to 300 is somehow bad. Yet on take off, they go from 275 to 375 in an even shorter period of time and that is a ok. This stuff just doesn’t pass the sniff test with me.
 
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No scientific data just the old school info I was taught decades ago. The basic is the throttle open cylinders are getting more fuel thus more thermal energy aka heat. So you don’t have the risk of a cylinder cooling too rapidly. Worked well with my turbo charged airplane and never I had an issue. I know everyone has their own method and if it works for them then that’s great.
 
togaflyer said:
No scientific data just the old school info I was taught decades ago. The basic is the throttle open cylinders are getting more fuel thus more thermal energy aka heat.
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The bulk of the heat is from combustion PRESSURE.....just think your air compressor, no combustion but it gets hot, from the peak pressure. Sadly as my good mate and APS teacher the late John Deakin would say, a lot of what was taught was wrong and we just didn't know any better, until we did.
 
togaflyer said:
No scientific data just the old school info I was taught decades ago. The basic is the throttle open cylinders are getting more fuel thus more thermal energy aka heat. So you don’t have the risk of a cylinder cooling too rapidly. Worked well with my turbo charged airplane and never I had an issue. I know everyone has their own method and if it works for them then that’s great.
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Yes, but this is all based on the "theory" that cooling a cylinder too quickly is a risk. Where is this stated? Who's theory? I have yet to see an authority that I trust, such as Lycoming or a reputable, large scale cyl O/H shop, provide cautions in this area. Lets consider how aluminum is heat treated. They warm it up to around 900* and drop it in a liquid to bring it down to 100* in a second or so. Much more to that process, but rapid cooloing in general is not inherently bad.

Pretty sure that if lycoming saw this as a problem, there would be a warning in there documentation. They are not really bashfull about telling you what to avoid to keep your engine healthy. This does not include the guidance to avoid low map descents during break in, as that is a ring flutter risk, not cooling related.
 
500 fpm on my descents. I don't touch the mixture, just keep it LOP. At altitudes I usually fly I don't do much more than 500 fpm as I want somewhat of a buffer before Vne
 
I don't normally touch mixture on descent and it normally is not an issue. But one time in Florida I had to spend a bit of time down at 1,000ft MSL on a hot day and noticed my airspeed dropping. I had to enrichen the mixture to maintain full power. That's the only time I noticed an issue.
 
I'm a John Deakin advocate and generally don't subscribe to shock cooling theory, at least as an absolute phenomenon. I generally establish the desired vertical decent rate (typically 500FPM from cruise but higher on occasion based upon ATC request-- maybe 1500FPM in a boat anchor approach) and manage airspeed with MP. I'll let it go deep into the Yellow arc if it's smooth, otherwise I'll pull enough inches off to keep the airspeed in check. For me anything over 2000FPM would constitute an emergency decent and if i need a super high rate I'm slowing down, flaps down, pushing the prop in, initiating a steep bank (45* min) to g-up the plane and spiral down without building up airspeed.
 
Auburntsts said:
I'm a John Deakin advocate and generally don't subscribe to shock cooling theory, at least as an absolute phenomenon. I generally establish the desired vertical decent rate (typically 500FPM from cruise but higher on occasion based upon ATC request-- maybe 1500FPM in a boat anchor approach) and manage airspeed with MP. I'll let it go deep into the Yellow arc if it's smooth, otherwise I'll pull enough inches off to keep the airspeed in check. For me anything over 2000FPM would constitute an emergency decent and if i need a super high rate I'm slowing down, flaps down, pushing the prop in, initiating a steep bank (45* min) to g-up the plane and spiral down without building up airspeed.
Click to expand...
Ever tried it flaps up, pushing red line (or even yellow arc) for an emergency descent? Your descent rate will be waaaaaaay higher compared to when you limit it to white arc. At some point you will have to bleed all that speed so starting altitude plays a part in an actual emergency decent.
 
jcarne said:
Ever tried it flaps up, pushing red line (or even yellow arc) for an emergency descent? Your descent rate will be waaaaaaay higher compared to when you limit it to white arc. At some point you will have to bleed all that speed so starting altitude plays a part in an actual emergency decent.
Click to expand...
Putting it into a 45 degree bank and pulling a couple of g's will get you down faster without exceeding Vne.
 
Jereme, Yes the faster you go the drag increases and that helps a heap. If you want efficient, descend at the Vz or Carsons speed, nominally 120-125 knots for most RV's . But if you are like me, I stay high in CTA and cooler smoother air, then descend at just under yellow arc.

Do like the flexibility and the performance you get from an RV10. :-)
 
swjohnsey said:
Putting it into a 45 degree bank and pulling a couple of g's will get you down faster without exceeding Vne.
Click to expand...
RV10inOz said:
Jereme, Yes the faster you go the drag increases and that helps a heap. If you want efficient, descend at the Vz or Carsons speed, nominally 120-125 knots for most RV's . But if you are like me, I stay high in CTA and cooler smoother air, then descend at just under yellow arc.

Do like the flexibility and the performance you get from an RV10. :-)
Click to expand...
Todd,
Yes, when I did testing I did two series:

1. full flaps top of white arc 45 bank steep spiral
2. top of yellow arc if air was smooth, 45 bank steep spiral, no flaps

When I did my commercial rating it was always explained that the 45 bank was to increase drag but mainly to keep PAX under positive Gs. (a lot of people out there REALLY don't like that weightless feeling haha) The descent rate in option two was leaps and bound above option 1. Go give it a try if you haven't. A lot of fun actually.

David, I couldn't agree more. (y)
 
jcarne said:
Todd,
Yes, when I did testing I did two series:

1. full flaps top of white arc 45 bank steep spiral
2. top of yellow arc if air was smooth, 45 bank steep spiral, no flaps

When I did my commercial rating it was always explained that the 45 bank was to increase drag but mainly to keep PAX under positive Gs. (a lot of people out there REALLY don't like that weightless feeling haha) The descent rate in option two was leaps and bound above option 1. Go give it a try if you haven't. A lot of fun actually.

David, I couldn't agree more. (y)
Click to expand...

I think I learned this from John Deakin...."Being lazy and soft" paraphrased of course, although I never flew airlines like he did, his old saying of "doing a whole lota nuthin' " was aimed at simplicity and efficiency. So I am soft, will take a headwind and smoother ride, and I am "efficient" by being lazy :) and it would seem you have formed these same horrible habits also ;-)
 
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