Propeller rpm and effect on engine cooling

FastEddieB

FastEddieB

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Fast Eddie B
I flew with a fellow today in a Mooney M20J. Even with cowl flaps open, the CHT's were getting well above 420° or so in the climb.

Obviously, one can power back a bit and/or shallow out the climb, which he did.

I was under the impression that for a given power setting, engine cooling was better at higher rpm. I think the logic was that cylinder pressures are lower at higher rpm, indirectly leading to lower CHTs.

I've tried Googling a few different ways, and so far can't find a source for that. First, am I right, and second, can anyone point me to a source?

Thanks in advance.
 
FastEddieB said:
I flew with a fellow today in a Mooney M20J. Even with cowl flaps open, the CHT's were getting well above 420° or so in the climb.

Obviously, one can power back a bit and/or shallow out the climb, which he did.

I was under the impression that for a given power setting, engine cooling was better at higher rpm. I think the logic was that cylinder pressures are lower at higher rpm, indirectly leading to lower CHTs.
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I think the fuel flow is the biggest predictor of heat rejection your engine is generating, and at the same MP you burn more at higher RPM. I see you were careful to mention a "given power setting". In my M20E I have to pull back on throttle to get to the same power at a higher RPM. Easily a few inches of mercury difference. I should mention that I have a fuel flow meter, but not an engine monitor, so my CHT figure may not be trustworthy.

As far as cooling air intake is concerned, inlets are too close inboard for the RPM to make a difference. I think only the airspeed matters for it.
 
FastEddieB said:
I flew with a fellow today in a Mooney M20J. Even with cowl flaps open, the CHT's were getting well above 420° or so in the climb.

Obviously, one can power back a bit and/or shallow out the climb, which he did.
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What speed was he climbing initially? I climb Vy (88kts) just off the field, but after pattern altitude transition to a cruise climb of about 120kts. Keeps the temps well under 400.
 
Bill Jennings said:
What speed was he climbing initially? I climb Vy (88kts) just off the field, but after pattern altitude transition to a cruise climb of about 120kts. Keeps the temps well under 400.
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I'd have to guess between 90kts and 100kts. Not sure if 120kts would have resulted in enough climb rate to clear the rising terrain north of Copperhill. After the power reduction, we were barely seeing 500 fpm.

My point was that I thought cooling would be enhanced by leaving the rpm up while just reducing MP, rather than reducing both.

BTW, I've checked PHAK, FTH, Aerodynamics For Naval Aviators and a couple of manufacturers publications, and cannot find a reference to increased rpm aiding in engine cooling. So if it is a factor, it must be a small one.
 
If you're looking for climb-out cooling leave throttle full in and reduce RPM as warranted for noise, etc. Full throttle supplies more fuel. A few hundred RPM has little effect on fuel flow.
 
FastEddieB said:
I was under the impression that for a given power setting, engine cooling was better at higher rpm. I think the logic was that cylinder pressures are lower at higher rpm, indirectly leading to lower CHTs.
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I can't say for certain that this is how the real world works, but peak HP for the IO-360 occurs at 2700rpm. Even at full throttle, less RPM = less HP. And intuitively, less HP should mean less heat.
 
Stewartb said:
If you're looking for climb-out cooling leave throttle full in and reduce RPM as warranted for noise, etc. Full throttle supplies more fuel. A few hundred RPM has little effect on fuel flow.
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Meet my best friend, Detonation.

Okay, okay, detonation isn't likely for a small decrease in rpm but I once had a CFI just about have kittens when I pulled RPM before throttle.
 
asicer said:
I can't say for certain that this is how the real world works, but peak HP for the IO-360 occurs at 2700rpm. Even at full throttle, less RPM = less HP. And intuitively, less HP should mean less heat.
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Sure.

But I was trying to say - making up numbers...

You want to pull back and climb at 75% power - let's say - because your CHT's are too hot. You look at a performance chart and see there are two settings that both result in 75% power:

24" @ 2700 rpm, or,
25" @ 2500 rpm.

Both should give the same performance. My impression was that the former would run cooler, all thing being equal, due to lower combustion chamber pressures which would lead to lower CHT's. But I sure am having a hard time backing that up.
 
Stewartb said:
Full throttle to promote cooling is SOP.
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Okay for a small drop in rpm. With a high compression engine you can get into trouble. Watch fuel flow closely to be sure it is max.
 
Wouldn't less RPM = less HP? Is the case for the old continentals.

This time of year, I use Vx to 500' agl and then drop the nose with cruise climb power and a shallow 500 fpm climb. Cowls open until cruise.

(Oh yea- full rich and full throttle for me. But that could be just how my carb works.)
 
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What airplane do you fly with a high compression engine? How high is your compression? A richer mixture should help your engine stay cooler even more than a low compression one. Full throttle
in an aircraft engine is designed to supply extra fuel for engine cooling. Reduce throttle just a little and you lean your mixture whether you wanted to or not.
 
Stewartb said:
What airplane do you fly with a high compression engine? How high is your compression? A richer mixture should help your engine stay cooler even more than a low compression one.
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Ever fly behind a 210 Franklin?

I'm not going to argue with you about your concepts of fuel flow. I am pointing out the fact that you can get into detonation by running full throttle and reduced rpm. You can deny that if you want. I will point out that SOP for many folks is to pull throttle then rpm with the intent to avoid detonation. HTH and HAND.
 
Some guys will misdirect any comment in order to argue. On climbout you won't reduce RPM below what? 2600? Full throttle and 2600 works great. I do it every time I fly. If I reduce MP even a little? Temps soar. Like I said, SOP is full throttle and RPM as required for noise, etc.

No Franklin time for me. Just CMI and Lyc.
 
Stewartb said:
Some guys will misdirect any comment in order to argue. On climbout you won't reduce RPM below what? 2600? Full throttle and 2600 works great. I do it every time I fly. If I reduce MP even a little? Temps soar. Like I said, SOP is full throttle and RPM as required for noise, etc.

No Franklin time for me. Just CMI and Lyc.
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I have noticed you do like to misdirect comments and fail to see other's points-of-view. HTH and HAND
 
FastEddieB said:
Thanks for that link - I remember that article. Though it doesn't directly answer my question.
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It doesn't directly answer but he does say run it full power for best cooling.

I like his whole series on engine operation and re-read them from time to time.
 
Probably true, depending on your point of view. Full throttle-full fuel flow for takeoff and climbout is an easy topic to find info for. Those who advocate reducing throttle and/or MP to aid cooling are wrong. Anyone can do a little fact checking and come up with that summary. I hope the OP does just that.

Cheers.
 
I think it's more about the power you are asking for and how rich (or lean) the mixture is

In my experience (based on Cirrus (which "auto" controls the prop), 172, and Archer) airspeed and mixture have the biggest help on CHTs, more so than RPM. The 172 and Archer I fly have JPIs... and I've noticed a dramatic difference in CHTs from even just a 10 knot difference, or from going more rich on the mixture. I've tried pulling back from 2600 RPM to 2400 RPM and it doesn't do much for CHTs other than slow me down or slow the climb. But pushing the red knob in seems to give a definite, near immediate, result.. In the Cirrus during the transition training they were very strict on going to a cruise climb pretty much once you're 800' agl

P.S. - Yes we take off at full rich in the Skyhawk and Archer in most places (okay, not Big Bear, etc.), but between 3K to 5K I start to lean "for best RPM" per the POH.. but if the CHTs get high I'll go richer. Can't really climb that well at over 90-100 KIAS in either the Archer or the Skyhawk once you are over 5K in the summer here
 
FastEddieB said:
24" @ 2700 rpm, or,
25" @ 2500 rpm.

Both should give the same performance. My impression was that the former would run cooler, all thing being equal, due to lower combustion chamber pressures which would lead to lower CHT's. But I sure am having a hard time backing that up.
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My feeling is that the former will run cooler per combustion event but then you are running more combustion events per unit time. So unless it is more than 27/25th's cooler, it comes out to be the same or worse. Also, by running faster you are increasing the amount of heat from friction and possibly running the prop (really the whole powerplant/drivetrain combo) in a less efficient region.

But again, that's just my gut feel.
 
Fly your Mooney WOT / 2700 from the beginning of your takeoff roll until after leveling off at your cruise altitude and accelerating. Reducing MP or RPM will reduce power, lower airspeed and reduce climb rate. This leaves you climbing for a longer time, with both less cooling airflow from the reduced speed and less cooling fuel flow.

I climbed in the Saturday heat from 326 msl to 7500, at Vy, WOT/ 2700 / Full Rich. Oil Temp came ear the top of the green; CHT did not. It was less than fifteen minutes from engine start of level cruise, power set and leaned. At 7500, OAT was 68°F, we'll above ISA of 32°F.
 
He got something very wrong. I have 1,000 hours in a J and don't see those temps. Normally #1 and #4 cylinders need metal modification but I think he has some serious rubber baffle issues. On #1 trim down the front deflector baffle 3/4" and #4 bend up the metal baffle to rob some air from the oil cooler. That's worth 35-50f chat right there!


Sent from my iPhone using Tapatalk
 
Aaronk25 said:
He got something very wrong. I have 1,000 hours in a J and don't see those temps. Normally #1 and #4 cylinders need metal modification but I think he has some serious rubber baffle issues. On #1 trim down the front deflector baffle 3/4" and #4 bend up the metal baffle to rob some air from the oil cooler. That's worth 35-50f chat right there!


Sent from my iPhone using Tapatalk
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Check fuel flow first.
If the plane is not setup correctly and is not sufficiently rich of peak, your CHT will be out of control. Backing the power down in this case is realistically the only thing you can do.

Tim

Sent from my LG-H631 using Tapatalk
 
FastEddieB said:
I flew with a fellow today in a Mooney M20J. Even with cowl flaps open, the CHT's were getting well above 420° or so in the climb.

Obviously, one can power back a bit and/or shallow out the climb, which he did.

I was under the impression that for a given power setting, engine cooling was better at higher rpm. I think the logic was that cylinder pressures are lower at higher rpm, indirectly leading to lower CHTs.

I've tried Googling a few different ways, and so far can't find a source for that. First, am I right, and second, can anyone point me to a source?

Thanks in advance.
Click to expand...

To reduce temps you have to reduce power, increase airflow, and/or increase fuel flow.
 
Clip4 said:
To reduce temps you have to reduce power, increase airflow, and/or increase fuel flow.
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It depends on why temps are high. Need more data first, starting with fuel flow.

Tim

Sent from my LG-H631 using Tapatalk
 
Clark1961 said:
Ever fly behind a 210 Franklin?

I'm not going to argue with you about your concepts of fuel flow. I am pointing out the fact that you can get into detonation by running full throttle and reduced rpm. You can deny that if you want.
Click to expand...

Anyone who has owned an older stick shift car knows this one.
 
Thanks for all the responses.

I may see if my friend wants to go play in the next couple of days. We can try different things and I can take along my GoPro to document the effects of different power settings and airspeeds on CHT's in the climb.

After we do that I'll post the results.
 
Clark1961 said:
Meet my best friend, Detonation.

Okay, okay, detonation isn't likely for a small decrease in rpm but I once had a CFI just about have kittens when I pulled RPM before throttle.
Click to expand...

As the only one here (I think) who's actually run these engines instrumented for detonation and doing detonation testing, lower RPM isn't going to hurt you any from a detonation perspective.

To the question of the OP, less manifold pressure will tend to have higher CHTs due to dynamics with fuel flow and also the engine having to make more power internally to suck the vacuum. However sometimes propeller design will promote extra cooling at higher RPMs. Really there are a lot of dynamics in play and I've seen it go both ways.

What happens if he just climbs out at full power? A lot of times the CHTs will be cooler doing that vs. a 25 squared setup. And if you've got obstacles to clear, that makes me ask even stronger, why not full power? You aren't going to hurt that engine firewalling it. I've even done full power climbs to 16k in the 414, which of course is boostificated.
 
FastEddieB said:
Thanks for all the responses.

I may see if my friend wants to go play in the next couple of days. We can try different things and I can take along my GoPro to document the effects of different power settings and airspeeds on CHT's in the climb.

After we do that I'll post the results.
Click to expand...
Maybe your friend should have the baffles inspected.
 
Ted DuPuis said:
As the only one here (I think) who's actually run these engines instrumented for detonation and doing detonation testing, lower RPM isn't going to hurt you any from a detonation perspective.
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Ummm, Ted, I used to drive an old ford pick-up. Had access to "white" gas. It would get into detonation with a high throttle setting and lower rpm. Close the throttle and the detonation went away. Hold throttle steady and allow engine speed to pick up and detonation went away.

No, it wasn't instrumented. Does that mean it didn't happen?
 
Ted DuPuis said:
What happens if he just climbs out at full power? A lot of times the CHTs will be cooler doing that vs. a 25 squared setup. And if you've got obstacles to clear, that makes me ask even stronger, why not full power? You aren't going to hurt that engine firewalling it. I've even done full power climbs to 16k in the 414, which of course is boostificated.
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I agree with full power climb and have always operated the Mooney in that fashion. I run full throttle/2700[1] until I reach cruise altitude and even keep that power for a minute or two once I level off to help accelerate the airframe to cruise speed. CHT's in climb always remain below 400.

[1] That blue knob allows you to have a climb prop in climb and a cruise prop at cruise and I use it that way.
 
Clark1961 said:
Ummm, Ted, I used to drive an old ford pick-up. Had access to "white" gas. It would get into detonation with a high throttle setting and lower rpm. Close the throttle and the detonation went away. Hold throttle steady and allow engine speed to pick up and detonation went away.

No, it wasn't instrumented. Does that mean it didn't happen?
Click to expand...

Too much timing advance at low RPM. Unsure how those "old" cars work, but I've tuned this out of modern cars with an ECU. It's not a usual problem, though.
 
Clark1961 said:
Ummm, Ted, I used to drive an old ford pick-up. Had access to "white" gas. It would get into detonation with a high throttle setting and lower rpm. Close the throttle and the detonation went away. Hold throttle steady and allow engine speed to pick up and detonation went away.

No, it wasn't instrumented. Does that mean it didn't happen?
Click to expand...

Note I said "these engines." Not your Ford's engine.

Yes, low RPM and high manifold pressure is a common detonation point. But on these engines, your detonation propensity goes down with lower RPM because of the loss of power. Ultimately they spin slow at any of their RPMs. It's not like going from 4,000 RPM to 2,000 RPM.
 
Clark1961 said:
Ummm, Ted, I used to drive an old ford pick-up. Had access to "white" gas. It would get into detonation with a high throttle setting and lower rpm. Close the throttle and the detonation went away. Hold throttle steady and allow engine speed to pick up and detonation went away.

No, it wasn't instrumented. Does that mean it didn't happen?
Click to expand...

Thanks internet for the laugh this morning!
 
I seem to recall reading someplace in my Arrow POH that cowling airflow is primarily a function of airspeed, as the prop does not direct a significant amount of air in to the cowling. I would expect that is probably a fairly common condition, as the majority of the thrust in the propeller is developed outside of the root area.

disclaimer: "I seem to recall."
 
I think my thesis depends not on the fact that the propeller provides additional cooling at a higher rpm. It's that the engine generates less combustion chamber pressures at higher rpm, therefore reducing CHT's.
 
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