Percent power for a given MP/RPM (Chart discrepancy)

[0TreeLemur]

4 hours ago, PT20J said:

I think maybe we pay too much attention to %power perhaps because we all learned in airplanes with power charts that showed %power and were admonished not to lean above a certain %power.

But I question with modern instrumentation if %power really matters. I have a few MAP/rpm/ FF settings I use and I don’t ever think about %power. 

%pwr is secondary.   First thing I'd like to determine is this: find the optimal cruise rpm for a given MP, given that MP=MP(density altitude) when WOT.  I know higher altitude, lower MP, so more rpm required for the same power.  But is the same power required, or necessary?  Below about 5,000 then we can dial in MP in cruise, but 22-24" is the normal range for me at least.  Secondarily I'd like to keep LOP ops out of the red box, that's where the %pwr estimation comes in handy, particularly down low, where I dial in MP.   I think the way I run the engine is ok, I'd like to know.   What I'd really like is the on airframe data collected by Mooney!  Prolly na ga get it.    I've written a program that duplicates the Lycoming IO-360-A power curves.  It does not include ff as in input.  So I cannot solve my problem.

[MikeOH]

I may be way oversimplifying but if I'm going somewhere (above 5K) I'm WOT, 2550 rpm and set my LOP fuel flow to however fast I want to go!  As long as CHTs are okay I don't worry about all these details of power charts, red-box, and what-not.  Typically this translates to right around 9 gph.  If I'm fighting headwinds then a bit more.

[0TreeLemur]

18 minutes ago, MikeOH said:

I may be way oversimplifying but if I'm going somewhere (above 5K) I'm WOT, 2550 rpm and set my LOP fuel flow to however fast I want to go!  As long as CHTs are okay I don't worry about all these details of power charts, red-box, and what-not.  Typically this translates to right around 9 gph.  If I'm fighting headwinds then a bit more.

That's not dissimilar from what I do.   I'm just looking for a bit more information.   Can you tell me why you choose 2550 rpm?   Do you use 2550 at 11.5K too?   Why not 2600?  

I don't fight headwinds.   Groundspeed is what the wind gods determine it shall be.   If possible, picking a heading can have a much bigger effect on groundspeed can have a lot more effect than engine settings.

[MikeOH]

10 minutes ago, 0TreeLemur said:

That's not dissimilar from what I do.   I'm just looking for a bit more information.   Can you tell me why you choose 2550 rpm?   Do you use 2550 at 11.5K too?   Why not 2600?  

I don't fight headwinds.   Groundspeed is what the wind gods determine it shall be.   If possible, picking a heading can have a much bigger effect on groundspeed can have a lot more effect than engine settings.

Some distant, and probably inaccurate, memory of reading somewhere that was the 'optimum' rpm for prop efficiency.  Nothing wrong with 2600.

[0TreeLemur]

57 minutes ago, MikeOH said:

Some distant, and probably inaccurate, memory of reading somewhere that was the 'optimum' rpm for prop efficiency.  Nothing wrong with 2600.

The efficiency vs. rpm curve for a constant speed prop is really pretty flat between 2300 and 2600.  It starts to drop off pretty quickly above 2600 though.   My question is pretty esoteric- a fancy word for "probably not very important"

[PT20J]

Bob Kromer (former Mooney factory test pilot) has written that the M20J was optimized for 2500 rpm. Roy Lopresti did a lot of flight testing with a heavily instrumented airplane including a torque meter to measure actual engine power. I have no reason to doubt Bob's recollection. And when you say you are trying to pick the optimum rpm, you really have to determine what parameter you are trying to optimize: mpg or TAS. Or, maybe engine longevity? In reality, the slope of the curve (for whatever parameter you are trying to optimize) vs rpm is probably pretty flat in the vicinity of the maximum. In other words, the difference between 2500 and 2550 is probably negligible. A lot of people just go by what seems to produce the least vibration.

The red box is a concept, not a hard rule. As the APS folks have pointed out, CHT is a proxy for cylinder stress. And as Mike Busch has pointed out that if you keep the CHTs under control you won't be in the red box. George Braly told me that he has never gotten any normally aspirated engine to detonate under any condition with a CHT of 400 deg F or below. He has gotten mild detonation at 430 deg F with elevated inlet air temp, 100LL at the low end of the allowable octane rating, sea level maximum MAP and mixture leaned to between peak EGT and 35 deg F ROP. So, you are not going to detonate an IO-360 unless you try really hard. Mike Busch told me that he cruises at fairly low powers (he guessed below 65%, but he doesn't calculate it so he doesn't know for sure). His rationale, and I quote, is, "I'm a longevity guy, not a speed guy," The airlines in the days of the radials cruised at around 55% BHP according to several flight manuals I looked at for the DC-3 and DC-6.

All airplane performance calculations are based on the drag polar. Here is the actual M20J cruise drag polar I got from Lowell Foster when he was an engineer at Mooney many years ago:

C_D = 0.164 + 0.72C_L²

A more fun way to determine the optimum parameters for your airplane though is to just do a bunch of flight testing.

[0TreeLemur]

18 minutes ago, PT20J said:

and when you say you are trying to pick the optimum rpm, you really have to determine what parameter you are trying to optimize: mpg or TAS

That's a good question.   Here's another stab at the question:  When I do long cross country flights, often at 8000 or above, WOT, leaned to approximately peak EGT, does it make any difference to be at 2400, 2500, 2550, or 2600 rpm?  Smooth is good, but what is optimal?   I've played with it a little bit.  Increase rpm and ff goes up.  IAS doesn't seem to change much.  So I don't think I'll be able to determine it experimentally.

 

18 minutes ago, PT20J said:

C_D = 0.164 + 0.72C_L²

Must be 0.0164 + 0.072 C_L²

[PT20J]

21 minutes ago, 0TreeLemur said:

That's a good question.   Here's another stab at the question:  When I do long cross country flights, often at 8000 or above, WOT, leaned to approximately peak EGT, does it make any difference to be at 2400, 2500, 2550, or 2600 rpm?  Smooth is good, but what is optimal?   I've played with it a little bit.  Increase rpm and ff goes up.  IAS doesn't seem to change much.  So I don't think I'll be able to determine it experimentally.

Fuel flow went up because the power increased. But, I think your experiment proved that the optimum is pretty broad.

22 minutes ago, 0TreeLemur said:

Must be 0.0164 + 0.072 C_L²

Good catch. Yep, typo. 

[EricJ]

35 minutes ago, 0TreeLemur said:

 Smooth is good, but what is optimal?  

What is your optimization criterion?   Speed?   Fuel consumption?   Fuel efficiency?   Longevity?   There are probably different answers for each.   If you want a blend of more than one, then the TLAR method (That Looks About Right) is often a reasonable approximation.   

[0TreeLemur]

On 1/14/2025 at 11:27 PM, EricJ said:

What is your optimization criterion?   Speed?   Fuel consumption?   Fuel efficiency?   Longevity?   There are probably different answers for each.   If you want a blend of more than one, then the TLAR method (That Looks About Right) is often a reasonable approximation.   

Speed:  groundspeed is determined much more by winds aloft than anything I do with the engine, unless I slow it down for some reason.  And I never slow down unless I'm going to land.

Fuel consumption:  Using MP to determine ff LOP kind of takes this variable out of the mix.  In cruise down low using the throttle to keep MP in the range of 22-24"  I'll run the richest cyl. about 15-20 degrees LOP.  Up higher WOT I'll run at peak EGT.  Using this method, my cruise CHT's max out at 300-340 this time of year, 340-370 in the summer.  Down low I get 8.5 < ff < 9.2, when 22" < MP < 24".  When MP< 22 WOT, I see ff in the 8.3 to 7.8 range from about 7k to 11.5k cruising altitude.

Fuel efficiency: see speed.

Longevity: yes, that is a concern.  I'd like to be as gentle as possible on the engine while cruising at a decent IAS for a given altitude.

So- given my means of setting the engine: ff=f(MP), what is the "best" rpm to use?   My hunch is that it is the higher one flies, more rpms are optimal, particularly after you go to WOT and no longer control the MP.  Various articles I've read mention that it  is better to use higher prop speed flying higher because of reduced air density.  But how many rpms?  2550?  2600?   Higher engine speed means higher power and higher pumping losses, which are probably minor, and maybe a bit higher engine wear.

Realizing that I'm talking about sensible operation and a second order effect in a portion of the performance curves where the response surface is pretty flat- maybe it just doesn't matter as @PT20J wrote that Bob Kromer suggests.  Maybe I just optimize for smoothest ops, which tends to be about 2500 rpm.  I guess we can consider my question answered unless someone else can offer some reason to run at a different prop speed above 7000 ft.

[PT20J]

Generally a propeller will have maximum efficiency at a particular advance ratio J=V/nD, where V = TAS, n = rpm and D = prop diameter. So, faster true airspeed requires higher rpm (and vice versa). But if you need to fly high (perhaps for wind, weather or terrain) in a normally aspirated airplane, you may need to run higher rpm than optimum just to generate the power required.

[Fly Boomer]

15 hours ago, 0TreeLemur said:

And I never slow down unless I'm going to land.

Made me laugh.  

[Pinecone]

On 1/14/2025 at 9:10 PM, 0TreeLemur said:

I'd just about bet that the JPI uses that relationship when it knows the engine is LOP.

It does.