Percent power/performance chart question

[Philip S]

Engine savants,

I have a 76 C model with an O-360 A1D. I have noticed the numbers I get in flight differ from what you might expect from the performance charts in the POH. I typically fly about 9-11,000. On a recent flight at 10,000 my power settings were WOT, with just over 22" MP and 2350 RPM. The performance chart for 10,000 shows a max MP at 20.2" while the 7,500 chart will get you 22.5" This suggests I'm making about 75% power at 10k, which seems improbable to me, and apparently the guys who wrote the performance charts as well. I can't seem to get the plane to operate LOP despite fiddling with carb heat or tilting the throttle plate a smidge, so I usually am just over 100F ROP. I'd like to be able to operate near max efficiency at times for longer range (only 48 gal usable-which is barely 500 miles or so at 9 gph), and thus am interested in leaning more-say to at peak EGT or roughness, which with a low enough percent power should be safe. I want to stay out of the red box of course while trying this, which means going higher to reduce MP without closing the throttle. I guess my questions are 1. Is 22" MP a reasonable number at 10,000 or is my gauge telling me I'm making more power than I really am? and 2. Since even at 10,000 WOT, 22" and 2350 RPM, leaned just shy of engine roughness only gets me about 365 on the #4 (hottest) CHT, does any of this really matter? and finally 3. Should I give up trying to get this thing to burn less than 9 gph to try and get some more range and fly 100F+ ROP and forget about it?

 

Thanks in advance for any wisdom you all may have.

[0TreeLemur]

I'm skeptical.  If true, 22" MP at 10k feet requires a very non-standard atmosphere.  That means temperature considerably below standard temp (23F), probably less than 0F, and very high barometric pressure (30.4").  If those conditions were true, then maybe.   If not, your MP gauge is off.

[KLRDMD]

1 hour ago, Philip S said:

On a recent flight at 10,000 my power settings were WOT, with just over 22" MP and 2350 RPM.

As was mentioned, you're not actually getting 22" MP at 10k with any normal or reasonable atmospheric conditions. Also, why 2350RPM? I would be at 2500 RPM or higher. There's nothing wrong with 2600 or 2700 even at that altitude.

[jaylw314]

1 hour ago, Philip S said:

Engine savants,

I have a 76 C model with an O-360 A1D. I have noticed the numbers I get in flight differ from what you might expect from the performance charts in the POH. I typically fly about 9-11,000. On a recent flight at 10,000 my power settings were WOT, with just over 22" MP and 2350 RPM. The performance chart for 10,000 shows a max MP at 20.2" while the 7,500 chart will get you 22.5" This suggests I'm making about 75% power at 10k, which seems improbable to me, and apparently the guys who wrote the performance charts as well. I can't seem to get the plane to operate LOP despite fiddling with carb heat or tilting the throttle plate a smidge, so I usually am just over 100F ROP. I'd like to be able to operate near max efficiency at times for longer range (only 48 gal usable-which is barely 500 miles or so at 9 gph), and thus am interested in leaning more-say to at peak EGT or roughness, which with a low enough percent power should be safe. I want to stay out of the red box of course while trying this, which means going higher to reduce MP without closing the throttle. I guess my questions are 1. Is 22" MP a reasonable number at 10,000 or is my gauge telling me I'm making more power than I really am? and 2. Since even at 10,000 WOT, 22" and 2350 RPM, leaned just shy of engine roughness only gets me about 365 on the #4 (hottest) CHT, does any of this really matter? and finally 3. Should I give up trying to get this thing to burn less than 9 gph to try and get some more range and fly 100F+ ROP and forget about it?

 

Thanks in advance for any wisdom you all may have.

 

Wait, how do you know you're 100F ROP if you can't get the engine to run any leaner than that?

[Philip S]

9 minutes ago, jaylw314 said:

 

Wait, how do you know you're 100F ROP if you can't get the engine to run any leaner than that?

Very good point. I should have been more specific. I'be been running 100F rich of the peak EGT I can achieve before significant engine roughness. I usually peak about 1470, a little leaner will sometimes get me 10F beyond peak EGT at 1460, but its rough enough to make the passengers ask questions. Perhaps this isn't the best system. I welcome any other ideas. 

[0TreeLemur]

13 minutes ago, jaylw314 said:

 

Wait, how do you know you're 100F ROP if you can't get the engine to run any leaner than that?

I should have asked- do you have a modern engine monitor?     They are really essential for the kind of engine management you want to do.   We have a JPI EDM900 in our C, and I can dial in the EGT precisely on the leanest cylinder with it.   LOP ops are not recommended for carbureted engines and my engine will not run smooth LOP no matter what I do.  

Using the EDM900 I dial in 80F ROP, which gets me 10.3 gph at 7.5k and 9.2-9.5 gph at 10.5k.  My cruise CHT's with cowl flaps closed are all 355-365F.   CHT's in the 300's are fine according to Mike Busch for Lycoming O-360's.

.

[Philip S]

31 minutes ago, KLRDMD said:

As was mentioned, you're not actually getting 22" MP at 10k with any normal or reasonable atmospheric conditions. Also, why 2350RPM? I would be at 2500 RPM or higher. There's nothing wrong with 2600 or 2700 even at that altitude.

Seems like the gauge is off then. I figured so. Regarding the RPM setting I guess mostly because Mike Busch says so, or for another reason its quieter. I've played with higher RPM, but it doesn't seem to generate much more speed. 

[Philip S]

1 minute ago, 0TreeLemur said:

I should have asked- do you have a modern engine monitor?     They are really essential for the kind of engine management you want to do.   We have a JPI EDM900 in our C, and I can dial in the EGT precisely on the leanest cylinder with it.   LOP ops are not recommended for carbureted engines and my engine will not run smooth LOP no matter what I do.  

Using the EDM900 I dial in 80F ROP, which gets me 10.3 gph at 7.5k and 9.2-9.5 gph at 10.5k.  My cruise CHT's with cowl flaps closed are all 355-365F.   CHT's in the 300's are fine according to Mike Busch for Lycoming O-360's.

.

I have a JPI 700. Four probe CHTs and and four probe EGTs. I'd rather one of the newer ones, but mostly for the ability to easily download the data to my computer from the monitor for graphing trends. I haven't delved into the various adapters and windows programs to get the data off the JPI 700 yet. Debating upgrading to something easier to use with fuel flow. 

[0TreeLemur]

Just now, Philip S said:

Seems like the gauge is off then. I figured so. Regarding the RPM setting I guess mostly because Mike Busch says so, or for another reason its quieter. I've played with higher RPM, but it doesn't seem to generate much more speed. 

I did the same experiment on a recent trip from AL to CO.   Tested the difference between 2400 and 2500 at 8500 ft, when I was fighting a nasty headwind.   Maybe 1 or 2 knots difference for more noise and about 0.4 gph increased fuel flow.   meh.  I went back to 2400.

[0TreeLemur]

2 minutes ago, Philip S said:

I have a JPI 700. Four probe CHTs and and four probe EGTs. I'd rather one of the newer ones, but mostly for the ability to easily download the data to my computer from the monitor for graphing trends. I haven't delved into the various adapters and windows programs to get the data off the JPI 700 yet. Debating upgrading to something easier to use with fuel flow. 

Rather than spend the 0.3 AMU's to OH your MP gauge, for only about 4.7 AMU's more, you can upgrade to an up-to-date engine monitor!  If you are handy and have a cooperative A&P IA you can do most of the install yourself.  That's what we did.  It gave my co-owner the chance to get to know the hardware real well too.  She now has a greater appreciation for how it all works.

Edited May 26, 2021 by 0TreeLemur
underscore the word "only"

[PT20J]

Don’t make this complicated. Replace or get the manifold pressure gauge calibrated. Not a bad idea to check the tachometer too. The EDM 700 is fine for what you want to do. Below 75% you can set the mixture wherever you want. You should be able to run at peak EGT reasonably smoothly. 

[Philip S]

6 minutes ago, PT20J said:

Don’t make this complicated. Replace or get the manifold pressure gauge calibrated. Not a bad idea to check the tachometer too. The EDM 700 is fine for what you want to do. Below 75% you can set the mixture wherever you want. You should be able to run at peak EGT reasonably smoothly. 

Perfect. This answered my main question: if at 10,000 I'm definitely making less than 75% power, I'll lean to peak and make sure those CHTs are happy. Thanks all. MP gauge overhaul in the offing. The tach is separate, a digital Horizon model. It seems spot on, but perhaps I'll have it looked at as well since I'm doing all this fiddling with power settings. 

[0TreeLemur]

57 minutes ago, Philip S said:

Perfect. This answered my main question: if at 10,000 I'm definitely making less than 75% power, I'll lean to peak and make sure those CHTs are happy. Thanks all. MP gauge overhaul in the offing. The tach is separate, a digital Horizon model. It seems spot on, but perhaps I'll have it looked at as well since I'm doing all this fiddling with power settings. 

Agree- digital tachs are typically spot-on if working.  Few measurements are quite as simple as a circuit that displays how many times per second something happens.

[Hank]

@Philip S, my 1970 C generally runs around 20" at 10,000 msl (give or take a little). I lean to peak, then enrichen up to 50°. At 2500, I'm usually indicating 140-145 mph. According to my Manual, this is 70% power and 8.8 gph. If I feel like going slow, I can run smoothly down to about 25° LOP.

To reach this, I first had to let the hinge on the Carb Heat box fail completely and fall open on takeoff . . . Between that, the jammed carb heat cable and the fist-sized hole in the bottom of my muffler where all the pipes came together, I had a pricey annual but she runs 10 mph faster and apparently lost the induction leaks that prevented LOP operations. 

If your O-360 won't do that, then look for and stop your own induction leaks. Happy hunting! Fly safe!!

P.S.--you said 48 gallons at 9 gph will barely get you 500 miles??? Your math is bad wrong. That's 4 hours flying at 140 knots = 560 nm = 644 miles, plus a very generous 12 gallons (1:20 flying time) reserve. According to my book, my 52 gallons at 20"/2500 will take my C at gross weight for 792 miles with a 45-minute reserve. 

P.P.S--since when do 1976 Cs have 48 gallons usable fuel??? My 1970 carries 52 gallons usable fuel. I've flown 4:45 twice, once with three takeoffs, and only added 41 gallons. More than your MP gage appears to be off. Or did Mooney revert to the old, early-60s little tanks between `70 and '75?

[carusoam]

Great questions Phillip...

You came to the right place...

MP gauges can be off for a couple of reasons...  there is a calibrated leak hole in the MP line... that after decades... might not be so calibrated any more...

RPM gauges can be terribly off spec.... but that is easy to check with a smart phone tach app...

If off by 200rpm... that is Similar to 10%power not being there...

 

Check your instruments for accuracy...  before running off the long end of a short runway....

a 10% power reduction turns my T/O roll from 800’ to 1200’...   a 50% surprise difference....

PP thought only, not a CFI...

Best regards,

-a-

[jaylw314]

3 hours ago, Philip S said:

Very good point. I should have been more specific. I'be been running 100F rich of the peak EGT I can achieve before significant engine roughness. I usually peak about 1470, a little leaner will sometimes get me 10F beyond peak EGT at 1460, but its rough enough to make the passengers ask questions. Perhaps this isn't the best system. I welcome any other ideas. 

Aaah, ok, I figured I was missing something! 

As far as I've read, with Lycoming O-360's, detonation is REALLY hard to produce at any cruise powers.  If you're not producing detonation, and your CHT's are below 380-400deg F, how are you going to damage the engine?  Lycoming guidance suggests leaning to whatever you want above 5,000' MSL, IIRC, so just lean as far as the engine runs smoothly and below 380degF--who cares how rich or lean of peak you're actually running?

[Chocks]

Looks like you got some good response here. The only additional thing that I will add. Is as @jaylw314 eluded to, you don’t need to worry about the “red box” at that atmosphere. That helps when you’re limited in performance precision by the carb. 
 

Just tell everyone you have double ram air.

[Hank]

Just now, jaylw314 said:

Aaah, ok, I figured I was missing something! 

As far as I've read, with Lycoming O-360's, detonation is REALLY hard to produce at any cruise powers.  If you're not producing detonation, and your CHT's are below 380-400deg F, how are you going to damage the engine?  Lycoming guidance suggests leaning to whatever you want above 5,000' MSL, IIRC, so just lean as far as the engine runs smoothly and below 380degF--who cares how rich or lean of peak you're actually running?

Actually, my Owners Manual says to run Full Rich above 75% power, and to lean the mixture at 75% and below.

[jaylw314]

4 minutes ago, Hank said:

Actually, my Owners Manual says to run Full Rich above 75% power, and to lean the mixture at 75% and below.

That's why I said "guidance"  There was some technical newsletter that Lycoming put out about the 5000' thing, not the operating manual

[Hank]

7 hours ago, jaylw314 said:

That's why I said "guidance"  There was some technical newsletter that Lycoming put out about the 5000' thing, not the operating manual

I vaguely remember something like that with the flight school's Cessnas . . . . But I don't fly them anymore.  

[Philip S]

8 hours ago, Hank said:

@Philip S, my 1970 C generally runs around 20" at 10,000 msl (give or take a little). I lean to peak, then enrichen up to 50°. At 2500, I'm usually indicating 140-145 mph. According to my Manual, this is 70% power and 8.8 gph. If I feel like going slow, I can run smoothly down to about 25° LOP.

To reach this, I first had to let the hinge on the Carb Heat box fail completely and fall open on takeoff . . . Between that, the jammed carb heat cable and the fist-sized hole in the bottom of my muffler where all the pipes came together, I had a pricey annual but she runs 10 mph faster and apparently lost the induction leaks that prevented LOP operations. 

If your O-360 won't do that, then look for and stop your own induction leaks. Happy hunting! Fly safe!!

P.S.--you said 48 gallons at 9 gph will barely get you 500 miles??? Your math is bad wrong. That's 4 hours flying at 140 knots = 560 nm = 644 miles, plus a very generous 12 gallons (1:20 flying time) reserve. According to my book, my 52 gallons at 20"/2500 will take my C at gross weight for 792 miles with a 45-minute reserve. 

P.P.S--since when do 1976 Cs have 48 gallons usable fuel??? My 1970 carries 52 gallons usable fuel. I've flown 4:45 twice, once with three takeoffs, and only added 41 gallons. More than your MP gage appears to be off. Or did Mooney revert to the old, early-60s little tanks between `70 and '75?

The tanks were resealed which I think is the source for the reduced fuel volume, but I'd have to dig back through the logs. The filler area is placarded "24 gal usable." The way I was running 100F ROP gives 9.2 gph. 

Start/clearance/taxi fuel, waiting for the texan2s to takeoff before me, mild headwinds, etc. Here's a recent flight where I landed with 7 gallons/45 min remaining. It was a clear and beautiful day with 13 knots wind forward of the beam otherwise I'd never have flown this far before a fuel stop. Yes, the tanks were filled to the max prior to departure. 

https://flightaware.com/live/flight/N7048V/history/20210523/1500Z/KNEW/KCDN

Total distance 635 sm. I would have rather landed with at least 1 hrs fuel in the tanks (my general policy, but I'm experimenting, carefully, with trying to get more range). I don't think I'll ever be comfortable flying this far again, especially if there is ANY weather around. This flight was the impetus for my initial inquiry into how I can operate more efficiently to get more range from the plane. I'll try leaning more knowing at 10,000 there's no red box, its just a bad MP gauge, and see how my fuel burn changes. 

REALLY appreciate all the responses from you all. This forum is truly a wonderful place. 

 

[Hank]

Above ~7500msl, I pull the throttle just enough to make the MP needle wiggle. Then I set 2500, lean to peak and advance anywhere from none to 50°.

I suggest flying one tank empty, or at least until it sputters. Switch tanks and fly on. When you land, fill the now empty tank first and confirm its volume. The 24 gallon placards make no sense.

My tanks were stripped and revealed, then filled to the brim with 52.2 gallons. Yours should be the same.

[Tcraft938]

Sounds like you have the reverse issue of me.  Do you have the original or analog (dial) Manifold Pressure gage?  Likely it is in accurate.  Mine has started reading lower than what I believe to be actual, yours appears to read higher.  When on the ground is your MP instrument reading the same or very close to the barometric pressure?  set your altimeter to filed elevation, read the baro, say it's 30.25, your MP without the engine running should read the same.  Unless very cold at 10,000 it is unlikely you are getting every bit of 75% power up there.  Who knows, you Tach might be off also.  

Another thing I've noticed, I have a 63C and the early performance numbers in the POH appear to be for just the cruise phase of flight at best power "150 rich of peak", which my understanding from SAVY is that's flirting or in the red box.  So the 1965 POH at 10,000' says at 2500 rpm 20.25" is 71% power yielding 176mph on 10.3 gph.  Then look at a 1970 POH and will be same altitude, same MP and RPM, same % power but speed is down to 164 mph and fuel burn down to 9gph.  Some pretty big difference that translate to substantial implied range impact.  One is basically 17mpg the other 18.2mpg for a 7% increase in range.  However, reading the charts, the 1965 says best power mixture which is 100 or 150 rich of peak (sorry can't remember), and the 1970 charts is "lean mixture at 75% or below to rough operation and just rich enough to smooth operation".  Also it appear the 1970 POH give range numbers and is including the climb and descent profile, so the gph takes that into account as well.  Perhaps I'm misunderstanding things.  As for range, fortunately, or not, as I age, my bladder and back issues are natural alarms to prevent fuel exhaustion.  LOL  The plane has a 5 hour range, I have a 3.5 to 4

[carusoam]

Double check actual amount of fuel that fits in the tanks for any new 2U airplane...

Most short body Mooneys may get 52gals total.... if their fuel necks have been updated to the 65 or newer style...

The older Mooneys with a door over the fuel cap lost some capacity with that style... aka thermos bottle fuel cap...

 

For long flights with only 52 gallons...  consider getting a FF gauge with a totalizer... or those Ceis floats and a digital display...

Flying gets more comfortable when your math matches the read outs....

 

It only took me a decade to prove that to myself...

Best regards,

-a-

[Urs_Wildermuth]

19 hours ago, Philip S said:

The filler area is placarded "24 gal usable."

That is strange for a 76' model. I would really try to empty a tank totally (e.g. fly it to a minimum or wait until fuel pressure drops before switching), then have the rest drained on the ground and refuel to absolute top. That is how much you can get into these tanks. If it's less than 52 USG, then I'd find out why and what can be done about it.

Mine is a 65 model and it takes 52 USG.

I've been musing about increasing that either via bladders (to 64 USG) or with Monroy tanks (to 88 USG) but the latter are no longer available and as my tanks are perfectly sealed I can't really convince myself to put the bladders just yet. But 12 USG more would be lovely at times.

Clearly, if you do not have a fuel flow indicator yet, better a fuel computer, then get one. Guestimating fuel flow from the POH is never very accurate. Combine that with our rather inaccurate fuel gauges and you give away range.

As for efficiency: The POH is a bit interesting to read in that regard, you really have to start digging into it to figure out the best settings for range. I've done some work on this a couple of years back and it comes out at rather unexpected values.

What I've basically done is to take the max range figures on each power table and feed them into one single table. From there, I recalculated the range via fuel at top of climb and 45'' reserve at the applicable fuel flow. The remaining "cruise fuel" is then divided by the fuel flow to give a range and the distance to climb added.

What I found is that max range is achieved at 10'000 ft with 2300 RPM and 17" MP. This results in a fuel flow of 7.8 GPH and 128 KTAS. Almost the same range is achieved with 18" and 8.2 GPH. Still Air Range calculated like this is a little over 700 NM with fuel remaining of 6 USG. 

I've tried this power setting several times and it works, including the associated TAS (measured with a Shadin FF and the Aspen for TAS). Actually, that is what I aim for to use these days.

For those who prefer high speed cruise, best range again is at 10'000 ft, however I find the TAS values in the POH to be optimistic...  With full throttle and 2500 RPM the POH gives 10.3 GPH fuel flow and 156 KTAS. With 2300 RPM and full throttle it gives 9.4 GPH and 149KTAS. The range calculated the same way as above is somewhere between 630 and 670 NM with fuel remaining of 7.5 GPH.

I have yet to test HSC properly.

The POH gives a maximum long range cruise at 1800 RPM and 37-44% power. Fuel flow is around 6 GPH. Speeds however are absymal, between 97 and 108 KTAS. Range in this regime is about 40 NM more than the long range cruise I quoted in the first paragraph, so imho it's not even worth considering, as first of all the gain is minimal and 2ndly sitting 8 hours only to achieve a probably quite academic range increase doesn't make sense.

I'd actually wonder if someone has ever tried this 1800 RPM cruise. Some feedback would be interesting.

What I'd suggest is to take your POH and start working the same way: Pick out the max range figures per altitude and compile a table which gives you the power settings to achieve that. Then calculate your maximum cruise fuel by deducting your personal final reserve and fuel to climb. Finally add the distance to climb to the cruise distance and you get some pretty usable data.

Needless to say I suppose that the calculated range data are significantly smaller than the ones in the POH. I wonder how many folks have run out of fuel because they believed those figures.

As also TAS is often quite different, it is also very useful to collect real life data and then adapt your tables with them. Quite possibly also the airplane profiles in your Foreflight or whatever you use for flightplanning. That is what the airlines are doing as well (with fuel bias or similar quotas) and it works really well. That is something I've been wanting to do as well but have not found the time. For cruise regimes not at the top of the spectrum, I found the POH to be quite accurate though.