Question about power settings

[Glen Davis]

I’m new to adjustable pitch props so.....

What is the advantage or disadvantage of setting the prop at 2600 rpm and mp at 21.3 inches OR 2400 rpm and 22.8 mp.  It’s 65 percent power either way.  Only thing I can figure is it’s quieter at the lower rpm.   What else.  Glen
 

 

 

[David Lloyd]

Quieter, less wear and tear, slightly more efficient at the lower RPM setting.  Pick a smooth spot, might be 2450 in your plane, 2375 in another. Really won't matter much.

[Glen Davis]

So which is less wear and tear?  Lower RPM or lower MP?

Glen

[carusoam]

when it comes to prop related questions... I like to invite @Cody Stallings our prop guy...

much of the answer is a combination of engine and prop...

1) Efficiency vs. yellow zones or red arcs...

2) Noise vs. speed...

3) FF vs. power...

The faster the prop spins...

• the louder it gets...
• the more vibration it produces...
• the more revs per hour it goes through...

 

through a wide range of MP...

• 1” of MP is nearly the same bhp as 100rpms... aka the mp/rpm superposition rule...
• FF goes up with either of them...
• When above certain altitudes... you are at WOT... more power is available by increasing rpm...

In the end...

• You will find a cruise power that makes sense to you, based on...
• Power
• vibration
• FF

If you are fortunate... FF doesn’t mean that much to you...

If you own a Mooney...

• you like speed, or
• you like efficiency, or
• you like to choose one or the other depending on the day....

If you like speed the most... you fly using flaming dragon mode... and intend to swap out some cylinders about every 1k hours....  Which is perfectly acceptable in terms of cost and safety....

PP thoughts only, not a personal economist...

Best regards,

-a-

[PT20J]

1 hour ago, Glen Davis said:

I’m new to adjustable pitch props so.....

What is the advantage or disadvantage of setting the prop at 2600 rpm and mp at 21.3 inches OR 2400 rpm and 22.8 mp.  It’s 65 percent power either way.  Only thing I can figure is it’s quieter at the lower rpm.   What else.  Glen
 

 

 

 

25 minutes ago, Glen Davis said:

So which is less wear and tear?  Lower RPM or lower MP?

Glen

Great questions, Glen.

The pressure in the combustion chambers pushes on the pistons so many times per minute creating horsepower (horsepower is the rate of doing work, so if all else is equal, faster equals more power). But, not all of that horsepower drives the airplane. A bunch of items rob power. Let's focus on two you can do something about: frictional loss and propeller efficiency.

Frictional power loss increases with speed, so the engine is more efficient at lower rpms and higher manifold pressures. Mike Busch is a big fan of running at lower rpms and you can probably find a recording of his webinar on this with a little Googling.

A propeller does not operate at 100% efficiency in converting brake horsepower (engine output) into thrust horsepower (what drives the airplane forward). Bob Kromer (former Mooney test pilot) has claimed that the 201 prop was designed for a maximum efficiency at 2500 rpm. However, the efficiency curve (hard to get as propeller manufacturers consider them proprietary) is likely pretty flat around the maximum so 2400 -2600 are likely not significantly different.

So, down low, you will be more efficient choosing a power setting with lower rpm and higher manifold pressure. But, as you climb a normally aspirated engine airplane you will run out of manifold pressure at the rate of around one inch per thousand feet and at some point you will have to increase rpm if you want more power.

As far as wear and tear: manifold pressure doesn't wear out an engine; rpm does. Look at it this way: An IO-360 has a stroke of 4.375 inches. That's 8.75 inches of ring/cylinder travel per revolution. The TBO is 2000 hours or 120,000 minutes. So an extra 100 rpm is 8.75 inches/rev * 100 rev/min * 120000 min =19,886 MILES.

Skip

 

[carusoam]

Which brings up some interesting reading...

Find anything from...

• Mike Busch
• Bob Kromer

Read and enjoy...

These are the guys that are current in airplane technology... not an ancient history technology lesson...

Best regards,

-a-

[Cody Stallings]

From a propeller stand point, it really doesn’t give a Sh!t. It’s just absorbs the power an dampens harmonics.

I personally like to run lower RPM an Higher MP.

I feel like I’m taking advantage of the slower burning fuel an the fixed timing that way.

 

More bang for your buck...:)

[Bob - S50]

I have no proof of this, but I think the most efficient RPM/MP combination will depend on which engine you have, primarily because of differences in static timing.  The rest of this is just the ramblings of an old man.

Two things will affect how much potential energy (stored in the gas) gets converted to speed:  Engine efficiency and prop efficiency.

Skip already covered most of that.  However, from what I've read, the optimum point to reach peak pressure inside the cylinder is about 15 or 16 degrees after top dead center (TDC).  If I'm not mistaken, most A3B6 engines are timed at 20 degrees BTDC while the A3B6D engine is timed at 25 degrees BTDC.  That's a big difference if you are trying to hit peak at the same 15 ATDC.  It's my belief the the 25 degree timing was optimized to produce maximum power at full power so Mooney could hit the magic 201 mph on 200 HP.  I'm guessing at 20 BTDC engines make just slightly less power at full power.

If you reduce RPM, peak pressure will move closer to TDC.  As you increase MP, peak pressure also moves closer to TDC because the flame front spreads more quickly.  If you are operating at the optimum point, moving peak pressure closer to TDC reduces power and increases stress on all the moving parts and the bearings.  If I have an engine that is timed at 25 BTDC, and I reduce RPM but keep the MP high, I think it moves peak pressure earlier than optimum.  However, if I have an engine timed at 20 BTDC, which is likely operating later than optimum at full power, running lower RPM and higher MP will move peak pressure closer to optimum, or at least, not as close to TDC.

So bottom line, if I have an IO360-A3B6 engine, I'd tend to choose 2400 RPM.  But since my plane has an IO360-A3B6D engine, I operate my engine at 2600 RPM in cruise.  I have also noticed that doing that reduces my CHT's and blow-by which means I end up with less oil on the belly of the plane (still lots, but not as much).  Of course, once you are operating at full throttle, the only way to increase power at a given mixture is to increase RPM.  So once you are full throttle, 2600 RPM will move you through the air faster than 2400 RPM.

Rambling done.

[jetdriven]

You can set the A3B6 at 25 degrees by undoing the service letter. The extra 5 Degrees of timing really wakes it up, and is a large difference on the lean side of peak.  

[carusoam]

7 minutes ago, Bob - S50 said:

I have no proof of this, but I think the most efficient RPM/MP combination will depend on which engine you have, primarily because of differences in static timing.  The rest of this is just the ramblings of an old man.

 

The author of the 310hp STC for the long bodies recommends full power and 2550 rpm...

(Fuzzy old memories of a PP)

I invite @StevenL757 because he tends to know the details for this question...

Best regards,

-a-

[Glen Davis]

Wow!  That’s a wealth of information.  But I’m going to have to read it several times to digest it.  Thanks all.   Glen. 

[PT20J]

11 hours ago, Bob - S50 said:

If I'm not mistaken, most A3B6 engines are timed at 20 degrees BTDC while the A3B6D engine is timed at 25 degrees BTDC.  That's a big difference if you are trying to hit peak at the same 15 ATDC.  It's my belief the the 25 degree timing was optimized to produce maximum power at full power so Mooney could hit the magic 201 mph on 200 HP.  I'm guessing at 20 BTDC engines make just slightly less power at full power.

I believe you are correct regarding the timing. Lycoming changed all the IO-360s to 20 deg except the dual mag versions because the dual mag wasn’t available with a reduced lag angle for starting. The timing that gets you the max power is called the maximum brake torque (MBT) timing. Keep in mind that at full power, the engine will be at full rich mixture and the MBT doesn’t vary a lot. That’s how Lycoming could make the change and still meet the certification spec. The tradeoff for slightly less power is cooler CHTs which I understand was the objective. As Byron noted, it’s a different story LOP.

The key idea behind the Surefly magneto variable timing is to attempt to keep the timing near the MBT optimum over a range of cruise power settings. This improves efficiency and LOP operation.

Skip

[201er]

I found that despite what the POH says, it will develop more speed for equal % power the higher the RPM. Mike Busch doesn’t believe me. But, I ran a bunch of small tests and it’s what I’ve anecdotally noticed in cruise over the years. Particularly LOP. 
 

The higher RPM brings out a few more knots but it also increases vibration and wear and tear. The oil burn seems to go up and the quality of the oil seems to go down after running higher RPMs for that batch of oil. I’ve done some long cross country’s cruising primarily higher or primarily lower RPMs that required an oil change when completed and that was what I observed. I’m almost always cruising 60-65% power in any case.

This is all anecdotal, but @jetdriven observed the same about the speed vs RPM.

[Cruiser]

I did some quick comparisons on a one hour trip at 4000' which allowed changes to both MP and RPM. 

In all combinations the FF was lower when the MP was high and the RPM was low. 

[Ragsf15e]

15 hours ago, Glen Davis said:

Wow!  That’s a wealth of information.  But I’m going to have to read it several times to digest it.  Thanks all.   Glen. 

To give you a more basic answer, there are lots of options and none is “correct”.  They all depend on what you’re trying to accomplish.  The variation between settings will be small, possibly undetectable (ie “wear and tear”).  You will however be able to see slight CHT differences, a few knots difference and/or a little fuel flow difference if you have a good engine monitor.  

It’s not “wrong” to run at 2700 rpm or 2400 or even less.  I would say though that most people cruise 2400-2600.  If you’re sightseeing and minimizing fuel burn is the goal, try lower rpm /mp.  Climb should be at 2700rpm, although some people may reduce to 2600 for noise abatement.  The engine is certified to run at 2700 though, so use that for the best climb.
 

As noted, above about 5000’, you can leave the throttle full open and just adjust the rpm (within poh guidelines).  You will see speed, chts, and oil temp (delayed) change slightly.  If you cruise above 10,000’, try 2600 to gain back a few knots.

If you do cruise low enough to adjust both mp and rpm, decide what your objective is (speed, mpg, wear and tear, etc) and play around until you find out what works for you.

[skydvrboy]

I was just playing around with power settings last week to see what kind of mpg I could get.  My first leg (48 minutes) was at 2100 rpm, 16" mp, LOP, during which I burned right at 3 gallons including takeoff (105 mph).  I didn't really like running it there because there seemed to be more vibration than at higher RPM settings.  Also, the RPM was VERY sensitive to control inputs and a quarter turn on the prop knob would send RPM's much lower.

During my second leg (1.8 hrs), I ran 2400 rpm, 14" mp, LOP, during which I burned 7 gallons (105 mph).  Fuel measurements were by my self calibrated stick, so give or take a half gallon.  Still, I was impressed.  Given my 64 gallon tanks, that is a LOT of range and a VERY long time aloft!

[larrynimmo]

8 hours ago, Cruiser said:

I did some quick comparisons on a one hour trip at 4000' which allowed changes to both MP and RPM. 

In all combinations the FF was lower when the MP was high and the RPM was low. 

When I ran at 2200 rpm, 25”MP I could get a true 20MPG (Statue miles) on my plane, but my fuselage rivets we’re failing.  Much less vibration at 2500 RPM

[EricJ]

8 hours ago, 201er said:

I found that despite what the POH says, it will develop more speed for equal % power the higher the RPM. Mike Busch doesn’t believe me. But, I ran a bunch of small tests and it’s what I’ve anecdotally noticed in cruise over the years. Particularly LOP. 
 

The higher RPM brings out a few more knots but it also increases vibration and wear and tear. The oil burn seems to go up and the quality of the oil seems to go down after running higher RPMs for that batch of oil. I’ve done some long cross country’s cruising primarily higher or primarily lower RPMs that required an oil change when completed and that was what I observed. I’m almost always cruising 60-65% power in any case.

This is all anecdotal, but @jetdriven observed the same about the speed vs RPM.

Since hp = (torque*rpm)/(scaling constant), it is usually expected that hp will increase with RPM.   Since the propeller can vary the load at different rpms, it may not be constant so torque may vary a bit, but generally one still expects higher rpm => higher output power.   So it seems credible to me.   I think the % power calculations are estimates that may not fully take everything into account.   Fuel flow is a pretty good proxy for power, too.

 

Edited November 9, 2020 by EricJ

[jaylw314]

2 hours ago, skydvrboy said:

I was just playing around with power settings last week to see what kind of mpg I could get.  My first leg (48 minutes) was at 2100 rpm, 16" mp, LOP, during which I burned right at 3 gallons including takeoff (105 mph).  I didn't really like running it there because there seemed to be more vibration than at higher RPM settings.  Also, the RPM was VERY sensitive to control inputs and a quarter turn on the prop knob would send RPM's much lower.

During my second leg (1.8 hrs), I ran 2400 rpm, 14" mp, LOP, during which I burned 7 gallons (105 mph).  Fuel measurements were by my self calibrated stick, so give or take a half gallon.  Still, I was impressed.  Given my 64 gallon tanks, that is a LOT of range and a VERY long time aloft!

Yeah, if you're willing to fly around at 115 KIAS, that can take 6-7 gph.  Can you imagine sitting in the plane without a bladder break for 10 hours? 

Edited November 9, 2020 by jaylw314

[jetdriven]

22 hours ago, Cruiser said:

I did some quick comparisons on a one hour trip at 4000' which allowed changes to both MP and RPM. 

In all combinations the FF was lower when the MP was high and the RPM was low. 

That is true, but the aircraft slows down, and your fuel at destination is also the same even though it takes more time.  

[201er]

4 hours ago, jaylw314 said:

Yeah, if you're willing to fly around at 115 KIAS, that can take 6-7 gph.  Can you imagine sitting in the plane without a bladder break for 10 hours? 

Yes. It was actually 11 hours cause of the headwind.

[MikeOH]

5 hours ago, jaylw314 said:

Yeah, if you're willing to fly around at 115 KIAS, that can take 6-7 gph.  Can you imagine sitting in the plane without a bladder break for 10 hours? 

Wide mouth Gatorade bottle...just sayin'

[Cruiser]

18 hours ago, jetdriven said:

That is true, but the aircraft slows down, and your fuel at destination is also the same even though it takes more time.  

Here is the data from my JPI830 for the different settings. I was wrong above, the altitude was 3500' autopilot was holding heading and altitude 

MP - RPM - MPG

LOP settings

25 - 2500 - 12.6

25 - 2400 - 13.2

25 - 2300 - 13.8

25 - 2200 - 14.3

24 - 2400 - 12.3

24 - 2300 - 13.6

24 - 2200 - 14.5

23 - 2400 - 12.3

23 - 2300 - 13.1

23 - 2200 - 13.8

ROP settings

25 - 2500 - 9.2

25 - 2400 - 8.8

24 - 2500 - 9.5

24 - 2400 - 10.0

24 - 2300 - 10.4

24 - 2200 - 11.1

there are a couple of reading that don't follow the trend and these might have been due to changing ground speeds but the trend is clear, higher MP and lower RPM gives you better fuel efficiency. 

[bonal]

For our old C with a dog house it always comes down to control of temperature. In warmer weather and in cruise lower rpm always brings temps down vs higher rpm. As much as I like the higher speed running 2500 I will always run 2400 which brings my oil and cylinder temperature back down to acceptable levels on hot days. Not sure if this directly relates to wear on the engine but it seems logical.

[squeaky.stow]

On 11/8/2020 at 8:03 PM, Glen Davis said:

Only thing I can figure is it’s quieter at the lower rpm.   What else.  Glen

Glen, I am not an engineer, just a pilot, so take this for what it is worth, which ain’t much. 
 

While not a perfect analogy, I like to think of the propeller governor as being similar to the transmission in your car. Its job is to translate engine torque into thrust. A fixed pitch prop is like a car with only one gear. In the bush plane world you will hear of “climb props” and “cruise props” which is like having a car with only first gear or only 4th gear. With a constant speed prop, we have the equivalent a CVT transmission except that it is not automatically controlled. Takeoff and climb are done in a low gear to allow the engine to get to maximum rpm and thus maximum power. Cruise is all about miles per gallon most of the time, so you want to shift into a high gear and get the engine running at a fuel efficient lower rpm. If you try to shift into 5th while still at 30 mph on the highway on-ramp, it would be pretty hard on your engine. That would be like pulling back the prop rpm while climbing at full throttle. But once you are established at cruise speed, lower rpm should equal more miles per gallon. However there are limits to this. If we could keep shifting to higher and higher gears on the highway, forcing our engine to turn over at slower and slower rpm to maintain 60mph, it would eventually generate too little horsepower to maintain highway speed. Or it might start detonating as we press harder on the gas pedal trying to maintain that power. That’s where the POH comes in. In my airplane, at max continuous cruise power I am restricted to an RPM of no lower than 2400, but at 65% power I can dial it back to 2200. In other words, the engine manufacturer doesn’t want me using 5th or 6th gear at high power settings, but once I dial it back to lower power, I can use whatever gear I want, with 6th gear being the quietest and most mpg. 
Also worth noting that even though my POH is written as if I had a six speed transmission with corresponding MPs to maintain xx% power for each “gear” (2700, 2600, 2500, 2400, 2300 & 2200rpm), in reality my “CVT” will allow me to choose 2437 rpm or whatever I like. My engine monitor will tell me what percentage power that is and if my cylinder temps are happy, then I am happy.
Regards,

Mark