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Power settings for MP < 15"


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Hi Mooniacs

I have a 3 blade hartzell prop in my M20E with IO-360-A1A and a limitation placard saying "Avoid continious operation below 15 inches hg between 1950 to 2350 rpm"

What is interesting is that I never see MP < 21" even on short final when power is almost at idle. When I taxi / idle my MP is around 22" (sea level). This is my first complex airplane so I am not sure if there is something wrong with my MP gauge or it is normal but my question is: what would be the typical scenario when my MP is below 15 inches hg and rpm is in 1950 to 2350 range?

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7 minutes ago, lithium366 said:

What is interesting is that I never see MP < 21" even on short final when power is almost at idle. 

That can't be right.  It's gotta be the gauge.  Otherwise you would have a heck of a time landing or even taxiing.  

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2 minutes ago, ArtVandelay said:

Something is wrong, is MP accurate before startup?


Tom

I will check today. What should I see? All the time I was flying the airplane I was avoiding 1950 to 2350 rpm not looking into a MP gauge on final because it didn't feel right to me

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1 minute ago, lithium366 said:

I will check today. What should I see? All the time I was flying the airplane I was avoiding 1950 to 2350 rpm not looking into a MP gauge on final because it didn't feel right to me

Atmospheric pressure.  ~30" at sea level.

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Just get back from the airport: when engine is off 28.9 inches hg is observed. That corresponds to today altimeter setting minus 1” / 1000 ft airport elevation. All lines and fittings look / feels tight under the cowling but I can’t really access that plastic tubing behind the instrument panel. Will need to bring an airplane to a mechanic. Thank you guys and sorry for dumb question, learning curve :)

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27 minutes ago, lithium366 said:

. . . sorry for dumb question, learning curve :)

Relax, @lithium366, the only dumb question is the one you don't ask. We all start somewhere with our knowledge base, and with MooneySpace, we can learn so much faster than without it!

Welcome aboard, and fly safe. 

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The goofier the question...

The smarter I feel...

Go ahead and as some goofy questions... I feel much better...  

Wait until you answer somebody’s question, and give a goofy answer... it happens.  Try to be pleasant when letting them know...  :)

Things you might get used to...

  • 10” The lowest MP setting you will normally see with the throttle out...
  • 12” Typical MP descending around the pattern, steep-ish descent...
  • 13” Typical MP descending around the pattern, normal descent...
  • 15”  barely staying level at TPA...
  • 16” slowing down to enter the pattern...
  • 18” Cruising at high altitudes WOT...
  • 20” Cruising at 10k’ WOT

If you are seeing 20” MP idling on the ground... that is a sign that air is interning the MP line...

Search for MP leaks, expect to find some loose plumbing between the MP instrument and the nearest cylinder air intake...  sometimes, the calibrated leak that is in The MP line experiences a crack and propagates with vibration... the leak is no longer calibrated and can easily be fixed...

PP thoughts, lots of inaccuracy, because of aged memory and MP depends a lot on weight and conditions... :)

Best regards,

-a-

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Ilya,

At 22” of MP, My Mooney can travel at about 175kts...

 

I was looking to see what sensor you have, where it is mounted...

http://www.chiefaircraft.com/ei-m1.html

This is kind of strange... because digital MP instruments take out a few error prone possibilities...

But, they may have introduced a simple leak with the sensor...

Expect that you have a leak... leaks cause more problems with low power settings more than they do during cruise... just a natural phenomenon with air pressure and air flow...

or your calibration is way off... has anyone calibrated this instrument for you?

 

I think I may know what could have happened...

Wait a minute...

... did somebody hook the existing MP line that was connected to the old mechanical instrument, (with the calibrated leak), to the back of the EI MP instrument..? This would explain the instrument error you are seeing...  the digital instruments don’t need calibrated weep holes to keep mechanical parts clean of the 100LL additives/blue goo... :)

check the installation details, finding a weep hole is not too hard if it is there... did you have this mounted in the plane? Or did you get it that way? If this is the issue... it Looks like an install error...? Have the mechanic cut off the the part of the tube with the hole in it...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

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6 hours ago, carusoam said:

Ilya,

At 22” of MP, My Mooney can travel at about 175kts...

 

I was looking to see what sensor you have, where it is mounted...

http://www.chiefaircraft.com/ei-m1.html

This is kind of strange... because digital MP instruments take out a few error prone possibilities...

But, they may have introduced a simple leak with the sensor...

Expect that you have a leak... leaks cause more problems with low power settings more than they do during cruise... just a natural phenomenon with air pressure and air flow...

or your calibration is way off... has anyone calibrated this instrument for you?

 

I think I may know what could have happened...

Wait a minute...

... did somebody hook the existing MP line that was connected to the old mechanical instrument, (with the calibrated leak), to the back of the EI MP instrument..? This would explain the instrument error you are seeing...  the digital instruments don’t need calibrated weep holes to keep mechanical parts clean of the 100LL additives/blue goo... :)

check the installation details, finding a weep hole is not too hard if it is there... did you have this mounted in the plane? Or did you get it that way? If this is the issue... it Looks like an install error...? Have the mechanic cut off the the part of the tube with the hole in it...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

The instrument was installed somewhere in 2000 by previous-previous-previous owner. I didn’t find a weep hole when I was inspecting the line yesterday but I think it could be also oil in the line. I will ask my mechanic to check that line as the instrument itself shows correct setting with engine off

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Lots of fittings in that line...

The line typically has a 1/8” soft aluminum line behind the panel, crosses the firewall, becomes a larger diameter copper line, then attaches to the intake...

lots of potential contamination from fuel/additives in that line...   probably not too much with the FI’d M20E, IO360.... compared to the carb’d O360... where fuel flows up the intake tubes and evaporates along the way...

less chance of it being oil in that line... oil on the air intake side would lead to a smoke show coming out the exhaust...

Each time the engine shuts down, the vacuum in this tube, tends to draw whatever is in the intake back towards the instrument...

The old instruments were pretty large vacuum cans with lots of volume drawn into the instrument after shut down...

If your mechanic starts at the cylinder end of the line, he can quickly check for both contamination and a line leak.

Potential challenge review...

  • oil contamination... would probably have a smoke show to go with it...
  • fuel additive contamination... possible, but less likely with the IO360...
  • blockage in general... not too likely, because the sensor in the instrument is sensing atmospheric pressure upon shutdown pretty easily...
  • line leakage... any air allowed into this line will keep it from registering the deepest vacuum that occurs during idle.  If you have a JPI or other engine monitor... you might be able to see a slightly higher EGT on this cylinder... realistically it is such a small amount it might get lost in all the noise and variation... got any JPI data you can post?

This would make an interesting JPI experiment to see MP and EGT data...

It must be challenging to set power properly, or at least know how much power the engine is actually generating.  Some people like to lean aggressively below 65% bhp... using MP and RPM as a guide... this would be more of a challenge in the lower altitudes where more MP is available.

great thought experiment... did I miss anything?

PP thoughts only, not a mechanic...

Best regards,

-a-

 

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I have a EI engine monitor and MUX-8A data recorder which I never tried to use. If I figure out how to damp it's data (and more importantly what exactly is being recorded) maybe I can share it then... Looking at https://buy-ei.com/wp-content/uploads/MUX-8A-OIII.pdf if should record both UBG-16 and MP info

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On 7/3/2019 at 1:44 PM, PilotCoyote said:

It just occurred to me that a MP gauge that is reading too low will not be the result of a leaking line; a leak in the line will produce a higher MP than normal... Sorry about that!

The longer threads get...  some additional strangeness becomes incorporated....  :)

So... somebody comes along and attempts a summary...

1) IO360 seems to be running fine as expected...

2) MP indicator never shows any numbers below 15” (Coyote note... the gauge isn’t reading low enough...)

3) labels on the instrument panel suggest staying out of the yellow rpm arc when MP is below 15”..? (Something like this)

4) OP never sees numbers that low on his relatively modern EI MP instrument...

5) Observation 1, The MP is reading much higher at the idle end of the spectrum...

6) Observation 2, The MP is reading close to normal at the WOT end of the spectrum...

7) Logic applied, a small leak in the MP line is a large percentage of the reading, during idle... and only a small percentage at WOT...

8) Experience applied, my M20C’s weep hole cracked and allowed more air to enter the MP line...

  • full range of the gauge with a calibrated weep hole started at 11 - 29”
  • Full range with an enlarged weep hole became 17 - 29”
  • Ilya’s MP line has no weep hole...

9) The instrument still works...

  • but all the MP related power charts need to be recalibrated... to match the instruments ‘new’ range...
  • recalibrate the instrument to the system... its calibration might be wrong?
  • find the leak...
  • find what else may be causing the challenge... 

10) Possible Proof that a leak exists... might appear in the EGT data... more air getting into this intake tube will show a higher EGT than the others, and will peak first during leaning... How good is the Gami spread?

11) Another possibility... There may be another leak on that cylinder’s intake... check the logs to See if the seals have ever been replaced... an inspection may reveal some pretty old cork seals still in there or the modern silicone reddish seals...

12) Getting the data out of the MUX is starting to look like an interesting challenge...

13) measuring a Gami spread typically needs a Fuel Flow instrument... got any FF to go with that? (Peak 1-4 vs. FF)

14) Measuring a pseudo-Gami spread (without FF) takes a little more work... (peak 1-4 vs position of the red knob...) easy to do with a twistable mixture knob... more of a challenge with only a push/pull mixture knob...

15) more Logic... If there is a sizable intake leak, that is going to be the cylinder that reaches peak first...

16) We have an EI guy... the coolest support mechanism that MS has is its members... :)   One MSer is an internal EI guy... so... try to get the data out of the MUX unit... if you need help, let’s ask the EI guy for support...

17) Nothing worse than not having accurate MP and RPM data... we use it on every flight for normal operations...

18) At least with a failed rpm gauge... the smart phone can measure rpm audibly... pretty reliably...

19) without a calibratedMP gauge... it is hard to lean without knowing the engine is below 65%bhp... unless taken above 8k’ altitude... (red box/fin theory)

20) Above 8k’... even at WOT... the max BHP available... is going to be less than 65%. (Outside of the red box)

 

Fun summary... it probably went off the tracks somewhere in the middle... let me know if you see any mistakes...

If Coyote’s note is right... my summary went off the tracks early on! :)

PP thoughts only, not a mechanic...

Best regards,

-a-

 

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Even worse my IE fuel flow instrument seems to be not calibrated correctly. In my recent trip I used 22 gallons (I don’t trust my fuel stick yet so it is how much fuel I topped off since last top off and 2.5 hours flight with 2 landings). My fuel flow instrument shows I used 32 gallons and fuel flow 12g/h at 7500 rich of peak (22.5” at 2400RPM). Again we are not sure in either fuel flow indicator, MP gauge, fuel stick and how accurately I topped off tanks :)

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Btw 22.5” was WOT with ram air open (poor man turbo) at 7500. Don’t quite remember what was the altimeter setting, I think somewhere in 29.8-29.9 range. Given that MP readings should be around 22.5 + ram effect so looks like in cruse I can trust it. At least WOT as you said at high altitudes

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3 hours ago, lithium366 said:

Btw 22.5” was WOT with ram air open (poor man turbo) at 7500. Don’t quite remember what was the altimeter setting, I think somewhere in 29.8-29.9 range. Given that MP readings should be around 22.5 + ram effect so looks like in cruse I can trust it. At least WOT as you said at high altitudes

If there's a leak in the line to the MP sensor, your MP gauge would ONLY be accurate when WOT while the motor is running...

12gph seems awfully high, and I can't think of a way one failure would cause both your MP gauge and FF to be inaccurate.

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