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 Saturday at 04:11 AM 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 Monday at 12:28 PM by A64Pilot