Question about Max RPM and Max Continuious RPM

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obelix

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obelix
Little confused about the RPM recommendations by Cessna and a 180HP conversion. Trying to understand how they get those numbers and why. Looking at the FAA type certificate for the 172n and the POH, lists the max RPM at 2700. Cessna seems to reference %75 power a lot in the POH, and Cruise Performance chart for both Cessna and the 180 conversion seem to have the same basic numbers. The 180conversion goes as far as to spell out max continuous RPMS as 2540. However in the cruise performance chart they list RPMS at 76% at 6000 and 8000 that are at 2650 and 2700.

My question is, does the max continuous RPM recommendation change with altitude? If so or not, why do they give the recommendation of 2540? What happens exactly if someone flys it at 80 or 90 BHP, up to say 2700 RPM's continuously. Why do they give examples of 75%BHP higher than 2540 at 6000+ PA I just don't see any reference to any of that in the POH, the Airplains paperwork or anything really on the internet. Are these just recommendations or are there reasons for staying at %75 BHP? I attached the Airplains STC pages I'm referring to, but they are the same ones you can see on the web if you google the 180 airplains conversion.
 

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Some airplanes have limits on continuous operation RPMs. Lacking a limitation you can run WFO all day long if you want to.
 
Some motors have max RPM for takeoff,and limited RPM for cruise. Flying at WOT could cause damage according to the manufacturer,also fuel burn goes way up at WOT.
 
frfly172 said:
Some motors have max RPM for takeoff,and limited RPM for cruise. Flying at WOT could cause damage according to the manufacturer,also fuel burn goes way up at WOT.
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Depending on how you lean.
 
Do not confuse WOT with full power. WOT does not equate to full power. Higher altitudes (in non turbo'd engines), leaning, etc... may reduce the power below the maximum even with the black knob pushed all the way in.

Follow the manufacturer's recommendations. The major issue with high power settings is HEAT. Even running full rich isn't guaranteed to fix that. As soon as you've gotten around the issue (usually takeoff/climb over obstacles) that required max power, get down to max continuous (or recommended cruise).
 
I don't know the reason for that 2540 rpm "max continuous" note. My Air Plains supplement (from the late 1990s) has that in there, too. I've been meaning to call them and ask about it.

The newer Air Plains supplements, for the same type airframe, engine and STC, say max continuous 2700 rpm. The one below is from 2012.
 

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Pilawt said:
I don't know the reason for that 2540 rpm "max continuous" note. My Air Plains supplement (from the late 1990s) has that in there, too. I've been meaning to call them and ask about it.

The newer Air Plains supplements, for the same type airframe, engine and STC, say max continuous 2700 rpm. The one below is from 2012.
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Yep, confusing, particularly when they list specific altitudes on the performance page with higher than 2540 RPMs..
 
obelix said:
Yep, confusing, particularly when they list specific altitudes on the performance page with higher than 2540 RPMs..
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I sent them an email asking them to clear it up.. What I believe is that the max continuous static RPMs is 2540, but the in air max continuous is what they have listed on their performance chart, but I really don't know.. Going by the modern doc, it should be 2700 period in the air, with the performance charts being recommendations.. But again, no clue..

Lets see what they say..
 
one of the max numbers could be for sea level which could be why as you go up the charts give you rpm values higher than that.
 
obelix said:
I sent them an email asking them to clear it up.. What I believe is that the max continuous static RPMs is 2540, but the in air max continuous is what they have listed on their performance chart, but I really don't know.. Going by the modern doc, it should be 2700 period in the air, with the performance charts being recommendations.. But again, no clue..

Lets see what they say..
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Yeah, because at 10,000' DA, 2540 rpm would only be about 61% power. 2700 rpm gives 72% power at that altitude.
 
A lot of time, when it is the prop speed (RPM) that is altitude limited, it is because the prop diameter is sized to best absorb the engine power at/near sea level and is bordering sonic tip velocities. As altitude increases, the sonic velocity decreases and the manufacturer puts RPM restrictions in place to keep the prop tips below sonic velocity to reduce noise and maximize both efficiency and prop longevity.

When the engine itself is restricted, the cylinders are sized (cooling) for cruise, but the manufacturer gives a pass for overheating those cylinders as long as the high power period is restricted (usually less than 5 minutes) so as not to weaken the cylinder structure
 
oldShar said:
A lot of time, when it is the prop speed (RPM) that is altitude limited, it is because the prop diameter is sized to best absorb the engine power at/near sea level and is bordering sonic tip velocities. As altitude increases, the sonic velocity decreases and the manufacturer puts RPM restrictions in place to keep the prop tips below sonic velocity to reduce noise and maximize both efficiency and prop longevity.

When the engine itself is restricted, the cylinders are sized (cooling) for cruise, but the manufacturer gives a pass for overheating those cylinders as long as the high power period is restricted (usually less than 5 minutes) so as not to weaken the cylinder structure
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While RPM limitations have been used to meet the more stringent EU noise limits I doubt that this if a factor here. Nor is the cooling ability generally going to come into play as an RPM limit. IME when a particular OEM engine/prop/airplane combination is certified with a lower RPM limit or a timed RPM limit than the engine's the reason has to do with harmonic vibrations of the prop and crankshaft.
 
2540 is at SL, 2550 at 2000 ft. 2600 at 4000 ft etc...
 
Silvaire said:
2540 is at SL, 2550 at 2000 ft. 2600 at 4000 ft etc...
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It would be understandable if they were limiting continuous rpm to that which yields 75% power at given altitudes. But that's not what the old Air Plains supplement said. It just said max continuous 2540 rpm, period, without regard to altitude or % power (see OP).

:dunno:
 
Perhaps the limitation was to reduce HP to the pre-upgrade power levels. Probably would be easier to get the STC.
 
Pilawt said:
It would be understandable if they were limiting continuous rpm to that which yields 75% power at given altitudes. But that's not what the old Air Plains supplement said. It just said max continuous 2540 rpm, period, without regard to altitude or % power (see OP).

:dunno:
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Well that's the way I read it. For one thing the only time you're going to get 180 hp is at sea level so looking at it any other way doesn't make sense.
 
Here is the answer. Do not fear 2700 RPM max continuous RPM in an Airplains 180 conversion of a 172N:

From: Tech [mailto:tech@airplains.com]
Sent: Tuesday, September 15, 2015 5:20 PM
To: Reaper1
Subject: RE: Question about max continuous RPMs, 180HP conversion C172N

Reaper1

The 2540 RPM reference was dropped in the last FM revision.

The static RPM is what the aircraft should be able to turn on the grd and not moving. This is useful in determining the engine health.

2700 is the maximum continuous RPM.

Mike

From: Reaper1
Sent: Friday, September 04, 2015 3:04 PM
To: tech@airplains.com
Subject: Question about max continuous RPMs, 180HP conversion C172N

I have a question about the conversion for NREAPER1 with the A4M engine and the 76EM8 Prop.

The attached airplains doc (page 8) that came with NREAPER1 after the conversion states the max RPMs at 2700, max continuous RPMS at 2540.

The one for download (modern) on your website as the manual (filename 172056-signed.pdf here http://www.airplains.com/index.php/support/drawings-documents/category/23-air-plains-cessna-172-gwi-stc-sa2196ce ) has on page 4 listed 2700RPMs as max continuious RPMs for an A4M engine with an A4M engine and the 76EM8S prop in “Section 1>Descriptive Data>Engine”.

Now under that same online doc from the link above under power plant limitations for that same engine you have “Static RPM Limits: 2250 to 2450”, and again max continuous RPM as 2700.

Then in the cruise performance of the doc attached (page 16) for NREAPER1, it has RPM references greater than 2540 at altitudes 4000ft+.

So my big question is what is the max continuous RPM really for my airplane, I’m sure you can see why I’m a bit confused. My best guess at this point is that the 2540 is max continuous static RPM for the ground only, and the actual max continuous RPMs are the ones listed on page 16 next to each altitude. But I really don’t know for sure which is why I’m sending this to you guys to clear up.

Thanks,
The Reaper 1
 
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