Flying by the numbers - Prop/RPM

[Max Clark]

I went to the Mooney Safety PPP last year (well worth it). One of the points that was made (and is reinforced in their manual) is flying "by the numbers". Aka understanding what configurations will give you what resulting performance. This has stuck with me and has been helpful as I've been working to be a better M20R pilot.

The POH is full forward except for set performance profiles or in descent:

Takeoff (2500 RPM)
Climb 2500 RPM
Cruise 2300/2400/2500 RPM (Section V Performance)
Descent 2400 RPM
Approach 2500 RPM

Something that was taught to me early on (by multiple people) was that it was unnecessary to touch the Prop in flight (I do not have the HP STC and max at 2500 RPM). In my case leaving the Prop full forward results in 2460 indicated in cruise. Dialing back 60 RPM just to push it back in never made sense to me.

I'm curious if you adjust the Prop in flight, when, and why?

[hazek]

Yes, I do. RPM is one component of the power equation: Power (hp) = (Torque × RPM) / 5252

So you can get the same power at either higher RPM or bigger torque. Why lower RPM is advantageous?:

• Better propeller efficiency: Lower RPM optimizes blade angle, improves thrust, reduces drag, saves fuel.
• Reduced engine wear: Fewer cycles, less stress, longer engine life.
• Lower noise: Slower prop reduces tip speed, quieter operation, less fatigue.
• Improved fuel efficiency: Lower RPM reduces frictional losses, better BSFC.
• Smoother operation: Less vibration, more comfort, less airframe fatigue.

[PT20J]

If you have a mechanical tachometer, it's always good to check the accuracy as they can easily be off by 60 rpm.

It's not clear why the prop full forward only turns 2460 in cruise. Does it turn 2500 in climb? The governor should keep it at 2700 in both climb and cruise. 

 

Edited Tuesday at 07:02 PM by PT20J
Corrected climb rpm to 2500

[Ragsf15e]

15 minutes ago, PT20J said:

If you have a mechanical tachometer, it's always good to check the accuracy as they can easily be off by 60 rpm.

It's not clear why the prop full forward only turns 2460 in cruise. Does it turn 2700 in climb? The governor should keep it at 2700 in both climb and cruise. 

 

I think his max is 2500 based on stock ovation setup.

[Max Clark]

32 minutes ago, hazek said:

Yes, I do. RPM is one component of the power equation: Power (hp) = (Torque × RPM) / 5252

So you can get the same power at either higher RPM or bigger torque. Why lower RPM is advantageous?:

• Better propeller efficiency: Lower RPM optimizes blade angle, improves thrust, reduces drag, saves fuel.
• Reduced engine wear: Fewer cycles, less stress, longer engine life.
• Lower noise: Slower prop reduces tip speed, quieter operation, less fatigue.
• Improved fuel efficiency: Lower RPM reduces frictional losses, better BSFC.
• Smoother operation: Less vibration, more comfort, less airframe fatigue.

What MP/RPM settings are you targeting and when? For example, what do you target for cruise and descent?

[PT20J]

1 minute ago, Ragsf15e said:

I think his max is 2500 based on stock ovation setup.

Oops, my bad; I meant 2500 not 2700. I corrected the original. Thanks, good catch. 

[Max Clark]

17 minutes ago, PT20J said:

If you have a mechanical tachometer, it's always good to check the accuracy as they can easily be off by 60 rpm.

It's not clear why the prop full forward only turns 2460 in cruise. Does it turn 2500 in climb? The governor should keep it at 2700 in both climb and cruise. 

 

Stock ovation = 2500 RPM Max

I’m going to get the HP STC at next annual, but pretty sure the seller had this dialed back a touch to “save the engine”

[PT20J]

1 hour ago, Max Clark said:

Stock ovation = 2500 RPM Max

I’m going to get the HP STC at next annual, but pretty sure the seller had this dialed back a touch to “save the engine”

That was my typo. With the prop full high, the governor should adjust the prop to turn 2500 in climb and cruise. 

[hazek]

6 minutes ago, Max Clark said:

What MP/RPM settings are you targeting and when? For example, what do you target for cruise and descent?

Takeoff - Max RPM

Cruise Climb - Cruise climb RPM

Cruise - lowest green range

Descent - descent RPM

[Max Clark]

11 minutes ago, hazek said:

Cruise - lowest green range

What is lowest green range mean?

[Hank]

25 minutes ago, hazek said:

Takeoff - Max RPM

Cruise Climb - Cruise climb RPM

Cruise - lowest green range

Descent - descent RPM

I fly my short-body the easy way:

• Takeoff = 2700
• All climbs = 2700
• Cruise = as set (2300, 2400 or 2500, depending on altitude,  higher for higher)
• All descents = Cruise RPM
• Pattern Entry = prop full forward
• Landing = prop full forward 

[hazek]

29 minutes ago, Max Clark said:

What is lowest green range mean?

Sorry, that was unclear yeah. Lowest RPM setting that is still in the green range is what I meant to say.

[GeeBee]

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. 

[Brandt]

3 hours ago, 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. 

Maybe the IO-550 but the TSIO-550 is 2200

[GeeBee]

28 minutes ago, Brandt said:

Maybe the IO-550 but the TSIO-550 is 2200

Might want to check that

[Brandt]

19 minutes ago, GeeBee said:

Might want to check that

Right back at you.  The TSIO-550-G is not included in the bulletin.  

[exM20K]

Funny, this was just in a “Ask the A&P’s” podcast. The provenance of this SB was Cape Air and their 402’s run at high power and low rpm. According to Mike Busch, there were no other reports of issues, save for a handful of Cape Air reports and maybe one or two others. The SB is based on 4 reports? Good grief, there are thousands of these engines flying, so not on my top 100 list of things to fret.

And, as @brandt points out, the -G isn’t on the list, probably because it didn’t exist them.

-dan
 

[GeeBee]

33 minutes ago, Brandt said:

Right back at you.  The TSIO-550-G is not included in the bulletin.  

You did not say the G just TSIO-550 which is why I said check that

[GeeBee]

8 hours ago, exM20K said:

Funny, this was just in a “Ask the A&P’s” podcast. The provenance of this SB was Cape Air and their 402’s run at high power and low rpm. According to Mike Busch, there were no other reports of issues, save for a handful of Cape Air reports and maybe one or two others. The SB is based on 4 reports? Good grief, there are thousands of these engines flying, so not on my top 100 list of things to fret.

And, as @brandt points out, the -G isn’t on the list, probably because it didn’t exist them.

-dan
 

One Cape Air 402 represents about 24 privately owned aircraft based upon usage hours.

[midlifeflyer]

17 hours ago, Max Clark said:

Something that was taught to me early on (by multiple people) was that it was unnecessary to touch the Prop in flight (I do not have the HP STC and max at 2500 RPM). In my case leaving the Prop full forward results in 2460 indicated in cruise. Dialing back 60 RPM just to push it back in never made sense to me

I’m in the simplification school of thought. When I do constant speed transition training, I teach there are only three prop positions. Full forward, the optional enroute climb (mostly for noise and vibration)! And whatever cruise setting prop position you choose for that flight. 

Sure, you are welcome to choose a full forward prop for cruise, but you might consider other combinations for best power or best economy at your cruise altitude. 

[Brandt]

9 hours ago, GeeBee said:

You did not say the G just TSIO-550 which is why I said check that

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

[Pinecone]

16 hours ago, Hank said:

I fly my short-body the easy way:

• Takeoff = 2700
• All climbs = 2700
• Cruise = as set (2300, 2400 or 2500, depending on altitude,  higher for higher)
• All descents = Cruise RPM
• Pattern Entry = prop full forward
• Landing = prop full forward 

I do the same in my M20K 252/Encore.   Except max RPM is 2600 RPM.  I wait to push in the prop control in the pattern until the RPM drops below my set point to avoid the noise.  So if prop is set at 2300, when I see that it is less than 2300, pushing the prop control in does not change the prop pitch.

Lower RPM in cruise is more fuel efficient as there is more time to extract power from ignition to exhaust valve opening.  

[M20F]

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

[201Mooniac]

9 minutes ago, M20F said:

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

Same in my M20J.  At about 1000' I reduce from 2700 to 2600, definitely quieter with no noticable loss of climb rate.  Also drops fuel flow by about 0.7 gph.

[Hank]

1 hour ago, 201Mooniac said:

Same in my M20J.  At about 1000' I reduce from 2700 to 2600, definitely quieter with no noticable loss of climb rate.  Also drops fuel flow by about 0.7 gph.

I was told at my first MAPA PPP lo these many years ago, that for our 4-cylinder Mooneys, plan on your cruise fuel flow plus one gallon per cylinder for climb. Now if you're going into the teens with a Continental 6, all bets are off!