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

[mkrakoff]

Hi all! I've been flying around a new to me M20J (and loving it!) and am delving into perf charts in the POH and directly from Lycoming. I'm finding a discrepancy b/w their calculations and am wondering if it's me, more accurate manufacturer calculations, or some factor (wrong chart?) I'm missing that is driving the differences. For example, the POH shows 22"/2200 at 4000' and Std temp to = 55% pwr. the Lycoming chart is below and looks like the same MP/RPM/ALT combo yields 118 HP at 4000 and Std Temp. More like 59% Thoughts? Thanks!

 

Edited October 29, 2020 by mkrakoff

[carusoam]

MK,

That chart clearly lists the engines that it applies to...

Is your engine on the list?

If not, why?

Beyond that... the engine manufacturers generate the charts before they get installed in the plane...

Once installed, the intake and exhaust have strong effects on their output...

The POH often covers these differences... but even POHs can get stale over the decades...

 

So... what are you trying to get out of this chart with respect to your flying?

Start with your POH...  get to know it really well... then build on that experience...

To build on that experience... get a copy of the most recent version of your POH... they got updated over time...

PP thoughts only, not a mechanic or CFI...

Best regards,

-a-

[mkrakoff]

Whelp, beyond just academics, looking to accurately operate at 65% for a LOP and GAMI test. Although the GAMI test is going to be at WOT and 65% or less so that is a bit easier (fly high). 

IO-360 A36BD, so I believe this is the correct chart. 

Operationally, it would be nice to have a chart that works for every possible MP/RPM/Temp/ALT combo. The manufactures chart would allow that, but I'd like to know that my numbers are accurate. 

At the end of the day though, operating LOP, I will just be using GPH to calculate my %HP.

[carusoam]

11 minutes ago, mkrakoff said:

Whelp, beyond just academics, looking to accurately operate at 65% for a LOP and GAMI test. Although the GAMI test is going to be at WOT and 65% or less so that is a bit easier (fly high). 

IO-360 A36BD, so I believe this is the correct chart. 

Operationally, it would be nice to have a chart that works for every possible MP/RPM/Temp/ALT combo. The manufactures chart would allow that, but I'd like to know that my numbers are accurate. 

At the end of the day though, operating LOP, I will just be using GPH to calculate my %HP.

There are simplified ways of doing this...

Mapa has a training and uses what they call key numbers... simple and more accurate than anything else...
 

It uses MP and RPM... MP is limited by Alt, and temp has a huge affect on MP and  DA

I’m still looking for the A36BD on chart...?

Check your engine details... I don’t think you really have an A36BD...  

 

There are a lot of details to know... none of them require decimal points to achieve safety, speed, or efficiency...
 

It looks like you may get stymied by the trees... and not see the forest they are hiding... 

So much data and detail... 

Transition training often covers a lot of detail...

MAPA training is a great way to pick up the finer details...

POHs have plenty of fantastic detail...

Keep  asking questions...

Best regards,

-a-

[mkrakoff]

Thanks, I'll definitely be working with MAPA in the future.

Really, it's just an academic pursuit. I know that actual operations are MUCH less precise, but I just like numbers (i'm weird, I know) and it's interesting to review on those cruddy flying days.

Engine-wise, It's an IO-360 A3B6D, so an IO-360 A model. I'm still thinking it's the right chart. 

[takair]

I use an app called “Aircraft Power” where you can enter your engine and parameters to get various power combinations.  Have not fully validated its accuracy, but it seems reasonably correct.

[carusoam]

Liking numbers is helpful in this pursuit...

There are different levels for how much number liking you can pursue...

Some engineers around here are pretty good with numbers...

Some accountants around here are pretty skilled as well...

We even have one guy around here that is into numbers so much... he flys the Pi Plane...
 

Spend some time calculating WnB and T/O distances... know how DA effects these...

Some people have built an excel spreadsheet that may even help with some calculations...

Also, work on the fuel capacity and fuel flow for your plane... it is too easy to run out of fuel...
 

Some MSers have misplaced planes by not getting their numbers quite right... (did I under state that appropriately?)

Best regards,

-a-

[mkrakoff]

1 hour ago, takair said:

I use an app called “Aircraft Power” where you can enter your engine and parameters to get various power combinations.  Have not fully validated its accuracy, but it seems reasonably correct.

I just downloaded it. That app is awesome...Looks like it uses Lycoming's charts, so it matches up with what I came up with, including the POH discrepancy. I'm assuming Lycoming's chart is the "perfect world, perfect engine" scenario. The POH basically low balls the %pwr by about 4-8% for every MP/RPM/ALT/TEMP combo. 

Thanks for the info!

Edited October 29, 2020 by mkrakoff

[carusoam]

mkr,

You should be able to find a couple of books specific to your engine written by Lycoming...

It will have all the procedures that Mooney used to develop the POH...

It may give insight to why we start the engine the way we do... and shut it down the way we do... and operate it too...

Some procedures have been highly edited (or over edited) before they got into the POH...

You may also find a parts manual for your engine, if it is separate... there are some seals and small oil return hoses that are good to swap out every decade or two... if you have an oil leak... this is an excellent resource to understand where it may be coming from...

PP thoughts only, not a mechanic...

Best regards,

-a-

[takair]

36 minutes ago, mkrakoff said:

I just downloaded it. That app is awesome...Looks like it uses Lycoming's charts, so it matches up with what I came up with, including the POH discrepancy. I'm assuming Lycoming's chart is the "perfect world, perfect engine" scenario. The POH basically low balls the %pwr by about 4-8% for every MP/RPM/ALT/TEMP combo. 

Thanks for the info!

I agree, I suspect that the Lycoming numbers are on a Dyno with optimal intake airflow and straight pipes.  Glad it helped.  I find it convenient in flight to figure alternate power settings or verify %HP without that feature on my old analyzer.

[Hank]

7 minutes ago, takair said:

I find it convenient in flight to figure alternate power settings or verify %HP without that feature on my old analyzer.

That's what the MAPA Key Numbers are used for . . . .

[mkrakoff]

2 minutes ago, Hank said:

That's what the MAPA Key Numbers are used for . . . .

So, as a MAPA member, is there an article or link to the key numbers everyone is talking about? I haven't had the opportunity to attend any events yet due to...the current state of the world. 

[PT20J]

Congratulations for figuring out the power charts! BTW, if you are interested in this stuff, I attached an old Pratt and Whitney doc. that delves into detail about how the charts are created.

The Lycoming charts won't be quite the same as the Mooney charts because they are taken for the engine in a test cell with optimized cooling, intake and exhaust systems. They are representative of the best the engine can do. When mounted in an airframe, there are necessary factors in the installation -- bends in the induction and exhaust, air filters, mufflers and the like -- that affect power produced for a given MAP/RPM combination, and the tables or charts supplied by the airframe manufacturer should be used.

On this website, you'll find all sorts of formulas for power LOP and ROP. But, keep in mind that these are all approximations. The only way to know for certain the power produced is to measure the torque (which is what the BMEP gauge did on the radial engines).

Skip

Pratt&WhitneyManual.pdf

 

[carusoam]

29 minutes ago, mkrakoff said:

So, as a MAPA member, is there an article or link to the key numbers everyone is talking about? I haven't had the opportunity to attend any events yet due to...the current state of the world. 

As part of the class... there is a handbook that goes with it...

It has a very well developed procedures section for each Mooney... great for IFR flight.

It also has power settings for each engine.  The key numbers are variations of MP and RPM... each number equates to a specific %BHP...  some people like to use the bench marks for 65% and 75% bhp...

The MAPA training is a big source of the organization’s small income...

Mapa Itself has new people running it...

It would be great if this document could be purchased separately...

 

The actual training is far more valuable than the book...

Many people take the training, and get credit from their insurance company... a small discount.

PP thoughts only...

Best regards,

-a-

 

[Missile=Awesome]

I find the selling of safety information repugnant.  Whatever.

I also dislike when humans speak in riddles...

I just find that annoying.

[PT20J]

2 hours ago, mkrakoff said:

So, as a MAPA member, is there an article or link to the key numbers everyone is talking about? I haven't had the opportunity to attend any events yet due to...the current state of the world. 

The Key numbers are trivial to calculate. You simply look through the power charts and find combinations of rpm in hundreds and MAP in inches Hg that give approximately the same %power and add them together. For instance, in my M20J, any combination that adds up to 49 is about 70% power. It is not exact, but it's close enough to give you an idea where you are operating.

I really don't understand why anyone cares much about %power, though. Maybe it's because the charts in the POH are made that way. Or maybe our first instructor emphasized it. But, what's special about 65% or 70%? Why not use 67%, or 74%? Personally, I just find some combination of MAP, rpm and mixture that keeps the fuel flow and temps where I want them. Up high, I cruise my M20J WOT/2500 rpm and peak EGT and down low I use 24"/2500 rpm and 20 deg LOP.

Skip

[carusoam]

Speaking of key numbers.... 65%bhp is a big one...

We all have an opinion regarding the health and well being of our engines...

Knowing where 65% is can keep us out of the alleged or imaginary red box...

Staying out of the red box is a nice way of getting our engines successfully to TBO...

So...

I can safely operate at 65% BHP and keep my CHTs below 380°F for the most part... I will most likely be able to keep my one engine operating until it reaches TBO... with its original set of cylinders....

The only thing I have to do beyond that... is go fly every week or so.... 

PP thoughts only, not a CFI...

working notes that won’t make it into this post in time...

 

Riddle me this... then... 

Which post are you referring to...?
 

Why is safety so overrated by some people..?

If we are talking about the MAPA manual still....
 

Without selling the information... Trey and Lela Hughes would not have been able to produce it...  

The MAPA manual isn’t as much a safety manual... though it does cover some of that... it is a book of convenience...

It conveniently has gathered in one place, what dozens of Mooney POHs do in hundreds of pages...

 

When it comes to free stuff... is it always the quality you are hoping for?

When it comes to safety... are you looking for the best quality or something that is just OK?

In this case the system known as Capitalism has produced a really convenient book...

It would be really cool...

• if we could get more credit from our insurance company for being MAPA trained...
• Get a tax write-off for being MAPA trained...
• Have a system that pays for MAPA training... 

Wait a minute...

I think I found the system... that pays for MAPA training... it’s called capitalism!   
 

Safety is incredibly important...

Our first contact with Mooney specific training comes from Transition Training... another thing that shouldn't be sold... But somebody has to pay for all that is involved... Or it just gets lost...

Trying to figure all this stuff out by oneself... may take longer than one has...
 

Good thing we don’t have to pay by the electron used... these long posts would cost an extra fifty cents!

 

PP thoughts only, I never directly got paid for anything related to safety...

 

 

 

 

[PT20J]

5 hours ago, takair said:

I use an app called “Aircraft Power” where you can enter your engine and parameters to get various power combinations.  Have not fully validated its accuracy, but it seems reasonably correct.

Thanks for that, I didn't know about that one. It's kind of fun to play with.

 

[carusoam]

Back on topic...

Why % power, MP, and RPM, and FF don’t match up very well...

The formulas required are so complex, we don’t have the instrumentation on board to cover it....

 

Where things get simple... we fly LOP... there is a simple formula for that... but it requires direct measurement of FF...

The most common mode of flight is ROP... to use the simple formula we use while LOP... would require an air flow sensor...

Oddly... automotive electronic ignition systems use an airflow sensor... but certified aviation hasn’t adopted that quire yet...

So... we use another calculation of measuring airflow through the known geometry to estimate The actual air flow... that is our MP sensor at work... 

When the engine is being developed at the engine manufacturer... they collect tons of airflow, and fuel flow and MP and RPM data... with the engine connected to a dyno....

 

Why using MP and FF is so hard while ROP... we never quite know how much fuel is delivering power... and how much is being used for cooling...

It wouldn’t be that hard to write down all the details.... then build a reference book...

The real surprise comes from engine monitors that estimate the BHP% while ROP... this is a magical calculation that may not match what your POH says... it has taken years to get them to where they are now... and they probably don’t share how they calculate this number...  

Best regards,

-a-

[PT20J]

I knew someone would bring up the dreaded red box 

What we need is a cockpit warning light

[Speed Merchant]

Mooney developed an installed power chart as part of the graphs list in the original post. To develop this chart the test plane is instrumented with a torque meter that installs between the engine crankshaft flange and prop. From the in flight torque reading and RPM you get horsepower. Flying the plane with the induction system and exhaust installed at different altitudes yields much truer results than testing an engine static on the ground (at one altitude) without the actual prop and exhaust installed. Installed charts are much different from the ground based charts. Most of the methodology used to develop the charts was spear headed by a guy named Eddie Allen at Boeing during early WWII. He was the father of performance flight testing. 

[Greg_D]

18 hours ago, takair said:

I use an app called “Aircraft Power” where you can enter your engine and parameters to get various power combinations.  Have not fully validated its accuracy, but it seems reasonably correct.

Where does someone obtain this app?

[Glen Davis]

I can't find it in the Apple APP Store.  Maybe it's only for android?

[PT20J]

4 minutes ago, Glen Davis said:

I can't find it in the Apple APP Store.  Maybe it's only for android?

I downloaded it from the Apple store -- it's only for iPad.

[Bolter]

For anyone looking at the OP's original chart (Lycoming 12700-A) and thinking it does not apply to IO-360-A3B6D for the J, the caption in the Lycoming manual has more detail:

"Figure 3-21. Sea Level and Altitude Performance IO-360-A, -C, -D, -J, -K; AIO-360 Series" which is more helpful than AEIO-360-A which sounds more like something said along with "Old MacDonald had a plane..."

For those interested in the Lycoming manual aLycoming 360 Oper Manual 60297-12.pdfs the source: Lycoming 360 Oper Manual 60297-12.pdf  I will try to upload it.