Mooney power charts question?

[Will.iam]

My 252 POH says TIT is more precise than FF so charts are based on peak TIT right? And I’m assuming peak tit is close to or the same as peak egt so i had a thought since my power charts also give a fuel flow setting with RPM and MP so if i set 35% power say on my chart it shows 2200 20” and 6.1 FF so when i set that, i should be at peak TIT but I’m not. In fact when i use my engine analyzer i noticed i was 50d LOP. When i keep adding fuel until i had peak TIT i was at 7.1 or 1 gph higher than power chart. I tried 50% power setting in my chart 2200 25”(24.3 + .7 for isa temp dev) 8.3FF but my TIT did not peak until 9.3FF same 1 gph difference. So i wonder if my “k” value in my FF is off / inaccurate since i need a gph extra than what the charts says to get peak tit. What am i missing here?  Is this a way to verify your FF is accurate or maybe my engine is so worn out that it needs an extra gallon of fuel to make charted peak? Pics for your perusal. Sorry the 50d lop pic is missing some digits as I couldn’t figure out how to slow down the shutter speed on my iphone and the display doesn’t update that fast. I can’t see it flicker but the camera did. 

[Bentonck]

I have found the power charts in my Acclaim manual have little, if anything, to do with reality.  I originally took them as a baseline to work off but now I've just found I'm better off finding the settings myself.  If I used their flow settings ROP it was way too rich for 50d (although I would personally not fly 50d rich, if I was going to fly ROP I'd use at least 100d) ... and LOP their setting was normally either slightly ROP or right at peak....I had to pull another 0.5-1.0g to get it leaned to -50d.....

As @Carusoam says, not a mechanic, just a PP, but I think on a non-new plane your intake, filters, injectors, and everything else come into play and everyone's setup is going to be just a little different, and change over time as things wear.  You know how to lean your plane better than a 30 year old factory manual.

[carusoam]

First...

before using your FF for knowing how accurately you are running your engine...

Compare the FF instrument for its accuracy... aka checking the K factor...

Essentially, check the fuel used, according to the instrument, vs. fuel used at the gas pump...

when filling the plane... it is important to do it the same way each time while verifying the K factor... fill it to the same point on the fuel neck...

Very level, front to back, and side to side... or if doing it the same day... park the plane in the same exact spot while filling the same way...

it is possible to get within a gallon of accuracy over an entire tank full of gas...

it all starts with how accurately this step gets completed...

 

Second, checking TIT gauge for accuracy... for some reason, these instruments get calibrated every now and then...

TIT sensors are in the worst environment possible... they have a tendency to erode over time...

 

Third...

There should be noted on the charts... if they say lean to peak TIT... there will be a procedure to follow to lean to peak TIT the same way as expected...

As Benton eloquently said... the POH can get you exactly where you don’t prefer to be... 50°F ROP... 

fourth...

Peak EGT and Peak TIT should occur closely together...  but there numbers will be very different...

When ROP the mixture may still be burning after the EGT sensor...

 

Fifth...

it is important to lean using the TIT and not EGT...  the health of the turbo vanes depends on TIT...

Too high TIT leads to softening of the blades that allows them to stretch and contact the case walls... turbo vanes work better than turbo nubs...

This is where a calibrated TIT instrument becomes important...

 

NA PP thoughts only, not a TC’d PP...

Best regards,

-a-

[jlunseth]

If you are running at 35% power it does not matter whether you are LOP or ROP or at peak. But assuming you are LOP as you say, your horsepower is determined directly by fuel flow. The formula is 13.7 x FF (in GPH) = HP , then divide that HP by rated HP which is 210, and you know your percent HP. So 6.1 GPH is producing about 40% HP. The formula only works for LOP operation, not for ROP. MP is not related to HP on the LOP side, it only factors in to how many degrees LOP you are operating. More air means more LOP. You can't hurt anything at 35%, according to the GAMI people, when the engine is under 65% HP you can lean it anywhere you want. 9.3 GPH is 61% (60.7 %) so you are still under the 65% threshhold. I have not found the charts in my POH to be very accurate, but then I have an aftermarket intercooler which requires another set of calculations to the charted numbers, and there is probably some inaccuracy in the intercooler charts as well as the POH charts. Carusom is right that the best way to know if your FF is correct is to take a series of flights of about two hours, always fill the tanks yourself and then refill at the end of the flight so you know the tanks were filled to the same place, and take the USD number off you engine monitor, which is based on the fuel flow measurements. Match those up to see if your FF is accurate. When I did it, my fuel flows were accurate to a tenth of a gallon out of fifty gallons total, in other words, the meter told me I had used 50.1 gal. and the total from the ground based fuel pumps for the flights was 50 gal.

PS I should clarify that the 13.7 multiplier is for a TSIO360. The multiplier depends on compression ratio, so it would not be valid for, say, an NA engine.

[carusoam]

 Sixth....

Notes  on %bhp...

we use the limited fuel to determine %bhp...
 

When LOP... gas is the limited fuel going to the engine... so we use FF to determine %bhp directly...

When ROP... air is the limited fuel going to the engine... so we don’t have a direct measurement of air flow... but,  the engine manufacture does... and they defined a formula using MP and rpm as the variables...  the engine guys have put %bhp in the charts...

Note: in this discussion... Fuel = gas + air.   And FF is gas flow.  And gas is short for gasoline...

-a-

[ArtVandelay]

Here’s a relevant discussion, more details and the multiple varies with compression.

https://mooneyspace.com/topic/29739-lop-with-fine-wire-vs-massive-plugs/page/2/

[Will.iam]

Thanks everyone especially bentonck it helps to know the charts are not exact and i figure with my worn engine i probably need more fuel to get the same power. I reference that assumption on the twin jet engines i fly. As i see an engine with high time paired with a new / overhauled engine and you can see the fuel flow is higher with a higher egt on the old engine to get the same N1 fan speed or EPR. But jet engines are very different then piston so wasn’t sure if that holds true especially since continental did that experiment to show on a new engine they on purpose reduced the compression all the way down to 40/80 and still showed that the engine produced max rated power. Just wonder if their fuel use went up to get that same power?

[PT20J]

There are likely two reasons Mooney suggests using TIT rather than fuel flow: First, there is a limit on max TIT that you don't want to exceed for the sake of the turbocharger. Second, the Floscan transducers are not super accurate when installed in airplanes. These transducers are supplied with a K-factor measured at 16 gph and are specified to have a repeatability of 0.5% at that flow rate. http://www.floscan.com/html/blue/aviation.php. However, you will note that engine monitor manuals generally include a method to calibrate the fuel flow by adjusting the corresponding K-factor of the monitor. Since the definition of K-factor is the number of pulses per gallon in thousands, and the measurement system after the rotor simply counts these pulses digitally and displays the result, there would be no need for this adjustment if the measurement were accurate. Sources of inaccuracy can be installation details or perhaps the fact that mechanical fuel pump actually emits pulses that average to the indicated fuel pressure and the non-constant pressure may affect the accuracy. But for whatever reason, accuracy requires calibration and I have found that over 250 hours of careful measurements the accuracy of fuel burn on a tank of gas in my M20J varies depending on whether the tank was consumed over many short flights or one long flight.

What this means is that the fuel flow in the factory chart may differ from your airplane. But once you correlate TIT and fuel flow at specific power settings for your airplane, it should be repeatable.

Since the only means we have to "calibrate" our system is to adjust K-factor based on the amount of fuel consumed (which is also subject to measurement errors) the accuracy of any fuel flow measurement is probably +/- 2% at best. So if you use the original APS LOP rules of thumb of 15 x FF = HP (NA engine) or 13.7 x FF = HP (Turbos), you will be close enough for Government work. For example, according to the attached Lycoming document, above 65% HP, the best power ISFC is 0.400 lb/hp/hr for an 8.7 compression ratio and 0.407 lb/jhp/hr for an 8.5 compression ratio. This is a change of about 1.7% which is below the accuracy of the fuel flow measurement.

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Power_from_fuel_flow.pdf

[jlunseth]

Well, the reason Mooney used TIT is that as the aircraft came from the factory, TIT was the useful number. It is sort of an average of the EGTs, “sort of” being very loose wording. There is some additional combustion going on in the exhaust system, so TIT is generally hotter than any single EGT, but it is still a collective measurement based on the EGTs from all cylinders. EGT was measured by putting one probe on one cylinder at the factory, so that does not tell you much about what the other five cylinders are experiencing. The GAMI people teach that EGT with a good engine monitor is the correct way to determine degrees lean or rich of peak. The idea is to protect all six cylinders (or four if that is what you have) by using the EGT of the cylinder closest to peak EGT. If you are leaning out from the rich side of peak, that is the first cylinder to peak. If you are leaning out from the rich side of peak, across peak and then into the lean side, it will be the last cylinder to peak. That means that all the other cylinders are running further from peak than the one being used to measure degrees from peak. So if, say, cylinder 2 is running hottest and you have that cylinder 20dF lean of peak, the others are more than 20 LOP and are cooler than #2.  I should caution that “cooler” in this context is a relative term. How hot the cylinder is depends only in part on where that cylinder is running relative to peak, it also depends very much on the flow of cooling air over that cylinder, so “coolest” does not mean that will be the cylinder with the lowest CHT, only that the heating from the combustion process will be cooler than in a cylinder closer to peak. 

The reason the GAMI people do not recommend using TIT to determine peak, is that especially if the fuel flows of the respective cylinders are distant from one another, using peak TIT (a collective number remember), you don’t know exactly where each cylinder is, and you can have one or two at peak or even rich of peak while the others are different degrees lean of peak. The collective number does not tell you that you are in reality, beating up on those two cylinders that are just slightly rich of peak, thinking that you are operating lean of peak.

Once you have used your engine monitor enough to know what is going on with each of the cylinders, you can use TIT, or even just use an MP/FF setting that you know from experience is protecting all the cylinders. As I said, with just one EGT probe, the factory did not have a valid EGT reading to go on so TIT is the next best thing, but it does not protect your engine the way the GAMI method does.

You can experiment with this all you want so long as you keep the HP under 65%. 

Things are a little more elaborate in the 231 (my plane) because the fuel flow and MP are interlinked, so changing one changes the other to try to keep the fuel/air ratio constant, and ROP and LOP are not just about pulling the fuel flow further out, they are about changing the fuel to air ratio.

[PT20J]

Every so often someone asks about efficiency gained by running at lower rpm and higher manifold pressure. Generally, it's known that prop efficiency is higher at lower rpm and engine friction is lower at lower rpm, so efficiency should be better (measured in terms of fuel flow per thrust horsepower) at lower rpm. But actual numbers are hard to come by. Looking at the power chart for the IO-360A-powered M20J you'll notice about a 0.5 gph difference between 2400 rpm and 2700 rpm.

The fascinating Lycoming document I posted earlier in this thread gives more data to verify this. For a 360 cu inch engine the ISFC is 0.400 lb/hp/hr at best power mixture. The friction hp at 2700 rpm is 31 hp and at 2400 it's about 24 hp for a difference of 7 hp. 7 x .4 =2.8 lb/hr or 0.47 gal/hr -- about the same as the Mooney chart.

Another interesting subject to ponder is the efficiency loss due to the installation. In the airplane, the exhaust and induction system restrictions rob the engine of some of it's power. I took the Lycoming power curves from the Operator's Manual and created a chart similar to the Mooney chart (note that the Mooney chart reads left to right 75% to 65% and I reversed this on my chart). It's interesting to compare the Mooney chart which tells you what you can expect from the engine as installed with the Lycoming chart which shows what the engine is capable of on the test stand. You can see the loss in efficiency by noting that higher manifold pressure is required at each power setting in the airplane compared to the test stand.

NOTE: While I created this chart by traditional graphical construction several years ago, it is much easier now using the AircraftPower app for the iPad.

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