Flying by the numbers - Prop/RPM

[GeeBee]

6 hours ago, Brandt said:

I’m curious as to the mechanics behind the crankshaft counterweight issue. How does that happen?

The counterweight counters the mass of the connecting rod and components attached thereto. In doing so, it creates gyroscopic stability of a two spoke gyroscope. Gyroscopic stability is dependent upon velocity, radius and mass. Since mass and radius is fixed, velocity is the only variable. Once the RPM drops below 2300, the stability of the gyro suffers against a crankshaft producing a lot of power causing flexing as the gyro forces drop out. At low power this is not a problem as the crank is not being stressed, at higher power, the crank begins to flex against the connecting rod forces which can be counter acted by the weights if the RPM is sufficient.

[Brandt]

6 minutes ago, GeeBee said:

The counterweight counters the mass of the connecting rod and components attached thereto. In doing so, it creates gyroscopic stability of a two spoke gyroscope. Gyroscopic stability is dependent upon velocity, radius and mass. Since mass and radius is fixed, velocity is the only variable. Once the RPM drops below 2300, the stability of the gyro suffers against a crankshaft producing a lot of power causing flexing as the gyro forces drop out. At low power this is not a problem as the crank is not being stressed, at higher power, the crank begins to flex against the connecting rod forces which can be counter acted by the weights if the RPM is sufficient.

Thanks

[TGreen]

On 4/22/2025 at 2:25 PM, GeeBee said:

Minimum RPM for an IO-550 is 2300. 
 

IO-550 G5, I Personally I climb at 2500, cruise 2400. @ 65% 50 LOP ((12.3 gph) descend 2400, approach 2500. 

Me too. And at that cruise setting I typically see true airspeed of 165 kts/hr.

[wombat]

I fly a TSIO-520NB in a Rocket.

Take-off:   2,650 RPM @ 38" MP

Climb: 2,500 RPM @ 35" MP

Cruise: Pick a setting from the chart.  Higher headwinds == higher power   Higher tailwinds == lower power   

RPMs are either 2,200, 2,300, or 2,400

MPs are 30, 31, 32, 33, or 40 inches     26" is also an option on the chart, but I never use it..... I didn't buy an airplane to fly slow.

[jlunseth]

I generally fly by the numbers, but have learned to respect the slickness of the Mooney airframe. For example, flying an approach I typically maintain 120 knots to the final approach fix, and going down the glideslope I want to be at 90 knots. 19 inches of MP will usually give me 90 knots. But if I start at 19 inches at the top of the slope I will never get down to 90, the combination of the higher approach speed, the speed going down the slope and the slickness of the airframe will conspire against me. So when I tip the nose over on the slope I will reduce power to 15.5 MP or so, I put in half flaps when speed comes under 100, and then will increase power for the balance of the final approach. There are lots of circumstances where a reduction to allow the airframe time to slow down is needed.

[GeeBee]

13 hours ago, wombat said:

RPMs are either 2,200, 2,300, or 2,400

Not supposed to be below 2300

[wombat]

11 hours ago, GeeBee said:

Not supposed to be below 2300

You are wrong.

30" and 2,200 RPM is an approved power setting

26" and 2,200 RPM is an approved power setting

Also, 25" and 2,200 RPM is listed as minimum descent power

I suggest you reference the documentation

Edited April 26 by wombat

[GeeBee]

10 hours ago, wombat said:

You are wrong.

30" and 2,200 RPM is an approved power setting

26" and 2,200 RPM is an approved power setting

Also, 25" and 2,200 RPM is listed as minimum descent power

I suggest you reference the documentation

I did. Continental CSB09-11A which supersedes documentation issued prior to 11/12/2014. Do you have documentation dated after that date that shows 2200 rpm is approved? This is for cruise power, descent is not affected.

[Fly Boomer]

17 hours ago, wombat said:

I suggest you reference the documentation

Go back to page 1 of this topic.

[gevertex]

On 4/23/2025 at 11:11 AM, M20F said:

M20F I climb 120mph IAS at 2600.  There is negligible difference in FPM but it’s a lot quieter. 

In the J, the climb checklist in the POH instructs us to decrease RPM to 2600 shortly after takeoff. I usually do this upon reaching 1000 ft AGL. Boost pump comes off at the same time.

[wombat]

On 4/26/2025 at 7:05 AM, GeeBee said:

I did. Continental CSB09-11A which supersedes documentation issued prior to 11/12/2014. Do you have documentation dated after that date that shows 2200 rpm is approved? This is for cruise power, descent is not affected.

Interesting.   No, I don't have more recent documentation,  I'd never even heard of this one. I learned something today!

[Ed de C.]

I've got the Ovation 3 with 2,700 rpm STC. I fly using the APC suggestions (the GAMI guys) of everything forward, climb with WOT all the way to cruise level (and then it's usually still WOT), progressively leaning to keep the EGTs in the G1000 white bar range of 1,450 F.  Achieving cruise level, I pull back the rpm as described below and do the "big mixture pull" to set mixture.  My engine climbs with cool cylinders and never threatens the 380 F line.  I make a point to cruise climb at 120 KIAS (or faster in winter) to keep good cooling airflow.  

I normally cruise high enough (7000 feet and above) that I use RPM as a power lever (the throttle is at WOT).  At these altitudes, all we have left is RPM and mixture (when flying LOP).  With my particular prop dynamic balance, I find that 2400 is a bit smoother than 2300, so I don't cruise below 2400.  The higher my cruising level, the higher I'll set rpm.  I use 2400 up to maybe 12k feet, 2500 rpm up to 16k, and 2600 for the rare times I try for higher.  At the high altitudes, I'll go to the effort to fly closer to peak rather than 50 LOP.  I'll also lean to best power when climbing at very high altitudes to eek out as much climb rate up there as possible.  I can keep 500 fpm going all the way up to around 19k feet by pulling back on airspeed from 120 KIAS to 105 KIAS (best rate).  

I find I can get 170 TAS at any altitude over 8,000 feet with reasonable power settings, (FF between 12 and 13.5 depending on altitude) and at mid-high teens I get that 170 knots at the higher rpm but FF is down to 11 gph plus or minus.   I'm finding I really like to cruise my Ovation between 10k and 16k in terms of performance (and how long it takes me to get to altitude).  Weather is another matter of course, but as long as the plane is ice-free, we have a lot of altitude flexibility (for a non-turbo airplane).  

Best,

Ed

[NickG]

1 hour ago, Ed de C. said:

I've got the Ovation 3 with 2,700 rpm STC. I fly using the APC suggestions (the GAMI guys) of everything forward, climb with WOT all the way to cruise level (and then it's usually still WOT), progressively leaning to keep the EGTs in the G1000 white bar range of 1,450 F.  Achieving cruise level, I pull back the rpm as described below and do the "big mixture pull" to set mixture.  My engine climbs with cool cylinders and never threatens the 380 F line.  I make a point to cruise climb at 120 KIAS (or faster in winter) to keep good cooling airflow.  

I normally cruise high enough (7000 feet and above) that I use RPM as a power lever (the throttle is at WOT).  At these altitudes, all we have left is RPM and mixture (when flying LOP).  With my particular prop dynamic balance, I find that 2400 is a bit smoother than 2300, so I don't cruise below 2400.  The higher my cruising level, the higher I'll set rpm.  I use 2400 up to maybe 12k feet, 2500 rpm up to 16k, and 2600 for the rare times I try for higher.  At the high altitudes, I'll go to the effort to fly closer to peak rather than 50 LOP.  I'll also lean to best power when climbing at very high altitudes to eek out as much climb rate up there as possible.  I can keep 500 fpm going all the way up to around 19k feet by pulling back on airspeed from 120 KIAS to 105 KIAS (best rate).  

I find I can get 170 TAS at any altitude over 8,000 feet with reasonable power settings, (FF between 12 and 13.5 depending on altitude) and at mid-high teens I get that 170 knots at the higher rpm but FF is down to 11 gph plus or minus.   I'm finding I really like to cruise my Ovation between 10k and 16k in terms of performance (and how long it takes me to get to altitude).  Weather is another matter of course, but as long as the plane is ice-free, we have a lot of altitude flexibility (for a non-turbo airplane).  

Best,

Ed

Ed, I fly my O3 almost identically to you - I usually cruise in the 15k+ range and like to set 2550 RPM. Also, I follow the Deakin guidance and lean to 1250 on the EGT while climbing WOT everything forward (its what he recommends if taking off from a higher altitude airport - KHND is 2491 MSL)

[A64Pilot]

The statement of lower RPM is “better” is true, everything he stated is true.

However it doesn’t make a whole lot of difference. Go out and do your own testing to see, set a target airspeed, then manipulate throttle and RPM to achieve identical airspeed and write down fuel flow differences. I’d suggest to start with low RPM as if you start with a higher one you may pick an airspeed that isn’t achievable at lower RPM.

As an old A&P /IA/ pilot my opinion is something near the middle part of the green arc is best and I pick an RPM that the engine is smoothest feeling, for me in my little 4 cyl that’s 2300. Of course if the primary driver is speed, max continuous RPM will be the fastest as usually if you want to go fast, you’re at altitude and either at WOT or max allowable continuous MP if boosted. 

From a smoothness and helping make everything last longer as in avionics / airframe and engine I think one of the best things you can do is have your prop balanced to the lowest achievable IPS. You’re really balancing the whole rotating assembly, prop and engine combo. It even slightly makes you faster and burn less fuel, because the energy that used to shake a couple of thousand ponds of airplane is now producing thrust (conservation of energy). But it’s not much different, pretty much in the noise band if you graphed it, other than cost there is no downside to a balanced prop.

Oh, and if engine longevity is a concern there has been a preponderance of evidence for decades that lower power settings significantly increase engine longevity, for instance Lycoming has written in a couple of pubs that for max engine longevity to cruise at 65% power or less. Implication is I believe that even less is better

Edited April 28 by A64Pilot

[wombat]

I made an updated performance quick reference sheet for my plane removing the 2,200 RPM options for cruise.  They are not illegal, but they are recommended against by the engine manufacturer.

I've also included the TIT test procedures and eliminated some things I don't use.

M20K Rocket 305 Power Settings and Performance - Google Sheets.pdf

[kortopates]

I did. Continental CSB09-11A which supersedes documentation issued prior to 11/12/2014. Do you have documentation dated after that date that shows 2200 rpm is approved? This is for cruise power, descent is not affected.

That bulletin can be safely ignored unless you’re flying a Cape Air profile with very high MAP and ROP. Mike has many thousands of hours operating his TSIO-520’s 2100 and 2200 LOP with out any unusual counterweight pin/bushing wear.


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[GeeBee]

11 hours ago, kortopates said:

That bulletin can be safely ignored unless you’re flying a Cape Air profile with very high MAP and ROP. Mike has many thousands of hours operating his TSIO-520’s 2100 and 2200 LOP with out any unusual counterweight pin/bushing wear.


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I am loathe to write this, but I am less than enthusiastic on Mike Busch's (I heard the podcast) take on this bulletin for the following reasons:

1. Continental labeled this bulletin "Critical Service Bulletin" which according to Continental "Compliance Necessary To Maintain Safety". This differs from a regular service bulletin which Continental says, "enhances safety".  Labeling it a Critical Service Bulletin is one whistle stop from an AD.

2. Within the background of the CB, Continental mentions not only TSIO-520 historical difficulties but also IO-520. To my knowledge Cape Air does not operate IO-520 engines, which makes me question if this is really just a "Cape Air" problem.

3. It is easy to finger Cape Air as their operation of the engines is known and defined, but not the thousands of 520 operators. Did Continental see other than Cape Air on the overhaul line?

4. Despite the background which did not include 550s within the bulletin it includes IO and TIO 550 engines which makes me question. "where did that come from?". Is Continental seeing something on the 550 overhaul line that we don't know about?

5. While I respect Mike Bush's operation of his TIO-520s his C-310 is just 2 engines in a fleet of thousands. Thus his data set, two engines is valid, but it is part of a larger data set of which none of us know anything. I would like to know how many "non Cape Air" TIO-520s and IO-520's Continental show pin problems (as well as 550s)

In summary, things are just too opaque to make an intelligent deduction. If there is more data from Continental or Savvy can glean more data from Continental I would be grateful because right now, I just cannot ignore what Continental is saying without more data. They see thousands of these engines a year, and I got to believe something has their attention more than just "Cape Air".

[wombat]

12 hours ago, kortopates said:

That bulletin can be safely ignored unless you’re flying a Cape Air profile with very high MAP and ROP. Mike has many thousands of hours operating his TSIO-520’s 2100 and 2200 LOP with out any unusual counterweight pin/bushing wear.


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I'm assuming you are talking about Mike Busch here.   And while I tend to give a lot of weight to his opinions, until I feel I have enough information to decide that something doesn't increase my risk more than I'm comfortable with I'll usually go with more conservative options.     And I don't have any information on what the actual wear/failure modes were for these four engine stoppages and why Continental issued the SB.   Was this a harmonic issue that causes cracks and sudden failure, or was this a wear issue?     TCM does say " TCM will continue to evaluate these reported counterweight releases in an attempt to establish a root cause" but that was 15 years ago from the original, and over 10 since the update.   I don't think they are actually doing this if they have not done it yet.

There are only 4 stoppages noted on this.  While they are pretty catastrophic, 4 is not a very large number.  But I don't have even an estimate of the fleet size of these engines that run regularly below 2,300 RPM.   So I don't have a way to judge the risk increase.

I don't have much time on my plane so far, but I have not yet used 2,200 RPM in cruise and as such, the cost to me of continued disuse of this power setting is low enough that when combined with my lack of information about the risk increase, I'll probably continue to not use it.    To make life easier on myself I went ahead and made a new in-flight reference sheet that doesn't include them.

@kortopates Can you clarify what you mean by 'Cape Air profile', and what you consider 'very high MAP' (does 30" or 26" count?  What about 35"?)  and assuming that ROP means Rich Of Peak, why that even matters?      I want to be clear that I'm not trying to call you out or say your data or opinions are bad or wrong, I'm trying to say I don't understand this situation and I'm looking to you for help understanding it better.   

 

https://continental.aero/service-bulletins/CSB09-11A.PDF

[wombat]

On 4/28/2025 at 5:12 AM, A64Pilot said:

The statement of lower RPM is “better” is true, everything he stated is true.

However it doesn’t make a whole lot of difference.

I've found this to be the case with a lot of things.   While it's true that A is better than B, (In this case, lower RPM) the improvement margin is not significant and might be outweighed by the benefit of other factors that we hadn't even been considering. 

Some examples:

• 2,300 RPM is more efficient, but the plane is 'smoother' at 2,350 RPM.   Overall, you'll spend less money at 2,350 because there is less wear.
• One hike is 'better' than another, but because it's on the list of best hikes, it's less pleasant than a different trail that has lower traffic.
• A refinance loan might be at a better interest rate, but refinance costs take away the benefit of refinancing.

[kortopates]

I can understand skepticism on ignoring the bulletin but I am not saying to ignore it entirely, just that its safe to ignore if you avoid the very high MAP/Low RPM ROP Cape Air flight profile that led to the issue reported in the CSB. Cape Air flew their TSIO-520 at 2100 rpm and 27" MAP ROP. 

For a little context: 

In June 2007, Cape Air grounded its entire Cessna 402C fleet due to a crankshaft counterweight issue in the Continental Teledyne TSIO-520 engines. The problem, was identified as abnormal wear on the counterweight, which led to three in-flight engine failures in the Cape Air fleet of 49 Cessna 402's. This prompted TCM to first issue, only to Cape Air,  Special Service Instruction 107-5 to replace the 6th counterweights, pins, bushings, plates and snap-rings - the area of accelerated wear ( https://continental.aero/service-bulletins/SSI07-5.PDF)  in order to get them back into the air by. Later despite the problem being limited to Cape Air aircraft Cessna 402 TSIO-550 engines, TCM  issued a much broader Critical Service Bulletin 09-11 for a whole slew of NA and TSIO engines  https://continental.aero/service-bulletins/CSB09-11A.PDF to warn against operating below 2300 rpm. 

So when I came on to work at Savvy years ago I queried Mike about this since he has written a great deal about how safe it is to operate highly oversquare - but I remind you he runs LOP and a lot at WOT.  Below I'll share his response of this:

Paul, I am intimately familiar with Continental CSB09-11A. It's a service bulletin, not an AD, so it is not an operating limitation, just a suggestion. CSB09-11 arose out of accelerated crankshaft counterweight pin/bushing wear that occurred in the Cessna 402C fleet operated by a Part 212 air carrier called Cape Air. The problems occurred in turbocharged TSIO-520 engines that were operated very far oversquare and with worst-case ROP mixtures. The Cape Air operations were very unusual -- extremely high-cycle ops, very short flights, worst-case leaning procedures. 

 

To the best of my knowledge and belief, no similar accelerated counterweight pin/bushing wear has ever been observed in normally aspirated Continental engines, nor in engines operated primarily LOP. The intensity of power pulses (and thus torsional stress on the crankshaft and motion of the counterweights) is greatly reduced during LOP operation. 

 

I have literally thousands of hours flying TSIO-520 engines LOP at RPMs in the 2100-2200 range and no unusual counterweight pin/bushing wear was observed at teardown. Hope this helps. —Mike 

[wombat]

@kortopates Thanks for that additional info!!!   I'll consider my 26" and 30" manifold pressure to be high MAP / Low RPM combo.    I have not been running LOP at all, so that puts me in the danger zone for increased wear. Interesting to note that Continental did say that there were two IO-520's that had this problem as well as the two TIO-520's that I think we can attribute to Cape Air.   So while I'm feeling quite a bit more informed about this and more confident that I am unlikely to just fall out of the sky if I cruise at 2,200 RPM, I'll probably avoid it.  I'll consider it if I've got amazing tailwinds and a flight I need to scrape all the range out of but I'm not going to use it regularly.

 

[GeeBee]

3 hours ago, kortopates said:

I can understand skepticism on ignoring the bulletin but I am not saying to ignore it entirely, just that its safe to ignore if you avoid the very high MAP/Low RPM ROP Cape Air flight profile that led to the issue reported in the CSB. Cape Air flew their TSIO-520 at 2100 rpm and 27" MAP ROP. 

For a little context: 

In June 2007, Cape Air grounded its entire Cessna 402C fleet due to a crankshaft counterweight issue in the Continental Teledyne TSIO-520 engines. The problem, was identified as abnormal wear on the counterweight, which led to three in-flight engine failures in the Cape Air fleet of 49 Cessna 402's. This prompted TCM to first issue, only to Cape Air,  Special Service Instruction 107-5 to replace the 6th counterweights, pins, bushings, plates and snap-rings - the area of accelerated wear ( https://continental.aero/service-bulletins/SSI07-5.PDF)  in order to get them back into the air by. Later despite the problem being limited to Cape Air aircraft Cessna 402 TSIO-550 engines, TCM  issued a much broader Critical Service Bulletin 09-11 for a whole slew of NA and TSIO engines  https://continental.aero/service-bulletins/CSB09-11A.PDF to warn against operating below 2300 rpm. 

So when I came on to work at Savvy years ago I queried Mike about this since he has written a great deal about how safe it is to operate highly oversquare - but I remind you he runs LOP and a lot at WOT.  Below I'll share his response of this:

Paul, I am intimately familiar with Continental CSB09-11A. It's a service bulletin, not an AD, so it is not an operating limitation, just a suggestion. CSB09-11 arose out of accelerated crankshaft counterweight pin/bushing wear that occurred in the Cessna 402C fleet operated by a Part 212 air carrier called Cape Air. The problems occurred in turbocharged TSIO-520 engines that were operated very far oversquare and with worst-case ROP mixtures. The Cape Air operations were very unusual -- extremely high-cycle ops, very short flights, worst-case leaning procedures. 

 

To the best of my knowledge and belief, no similar accelerated counterweight pin/bushing wear has ever been observed in normally aspirated Continental engines, nor in engines operated primarily LOP. The intensity of power pulses (and thus torsional stress on the crankshaft and motion of the counterweights) is greatly reduced during LOP operation. 

 

I have literally thousands of hours flying TSIO-520 engines LOP at RPMs in the 2100-2200 range and no unusual counterweight pin/bushing wear was observed at teardown. Hope this helps. —Mike 

Thanks, that clears up some of my concerns. I wish Continental was more transparent about the issue.