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

[skykrawler]

So the Aircraft Power app is not installed power.

 

[testwest]

@skykrawler, you are correct. The Aircraft Power app uses the Lycoming charts for its calculations. And the IO-360-A**** are all on the same chart. Here is a screen shot from the app for @mkrakoff 's conditions of standard day, 22"/2200 and 4000 feet pressure altitude:

As stated before, this is not the "as-installed" power in a stock Mooney M20J. This power setting and atmospheric conditions is 55% power in a J. More below!

Mooney was very accurate in its engine power data gathering for the M20J effort. In particular, the airborne engine torque meter, fuel flow instrumentation and air data instrumentation were the best obtainable for a GA airplane in 1975-76, and the flight test engineers, Roy Lopresti, and Fen and Dorothy Taylor, were without peer.

I modeled the M20J IO-360-A3B6D "as-installed" power in Benchmark. Here is a chart under the same conditions as before:

This chart shows 55% in the J, right on the money. And of course, Benchmark allows any combination of every possible MP/RPM/Temp/ALT, just like the OP, @mkrakoff wanted!

So, let's find the lowest altitude where WOT and, say, 2400 RPM gives exactly 65% power, and what the book fuel flow would be at best economy (which in this case is 25 degrees rich of peak EGT, that's how Mooney did it):

So, for @mkrakoff , fly your airplane to 9000 feet density altitude (note for here it is a standard day, so 29.92" altimeter and -3 deg C in the chart is 9000 ft pressure altitude as well as density altitude). Go wide open throttle and 2400 RPM (hopefully your tachometer has been recently calibrated). Slowly lean back to 9.3 to 9.2 gallons per hour. Ideally, you should see 22.5" MP under these conditions, and none of the cylinders has passed peak EGT yet. Manage your cowl flaps to keep the hottest cylinder below 380 deg F. At cruise you "should" not have to worry and you should be able to run cowl flaps closed. Let the engine stabilize, then slowly lean some more. Your first EGT should peak about 25 degrees higher and the fuel flow should be about 8.7 gph. If your first cylinder peaked at 8.8 gph and the last at 8.6 gph you have a really good baseline and your airplane is pretty close to the type design, and you are pretty much exact at 65%! And you could probably run LOP very nicely without GAMIs. Many Lycomings can do this.

On the other hand, under these same conditions, if you get a first peak at "something like" 9 gph and a last at 8.2, you have a high GAMI spread and GAMIs would help. If it's really bad you engine might feel a little rough before the last cylinder peaks. Or, at WOT if you are getting significantly less than 22.5 inches MAP you may have a restriction in the induction system somewhere. A 1" loss under these conditions (21.5" at WOT) equals a 3% power loss, so your peaks would be around 8.3 gph instead of 8.7. I got this answer instantly "flying" the Benchmark power chart just like flying the airplane.

Now then. I would like you to please go fly these numbers and report back your data. And we will go from here.

Hint, we will look at cruise performance next. You should see about 138-139 KCAS, or about 2 knots higher indicated (which computes to ~158 KTAS) in the stated conditions above and around 2600 lb GW. Draggy stuff on the airplane will knock that down some, so the common wisdom is that the J is a 155 knot true airspeed flyer under every day 65% power conditions.

[carusoam]

Norman are you working on a PhD thesis?   

Great pirep regarding the work of Lopresti, Fen, and Taylor...  ( @Speed Merchant may like to see that...)

Thanks for sharing all of this great detail!   They are Slightly more than just the ordinary PP thoughts...

Best regards,

-a-

[testwest]

Bump, hoping for @mkrakoff to reply back here!

[mkrakoff]

On 11/1/2020 at 1:03 AM, testwest said:

@skykrawler, you are correct. The Aircraft Power app uses the Lycoming charts for its calculations. And the IO-360-A**** are all on the same chart. Here is a screen shot from the app for @mkrakoff 's conditions of standard day, 22"/2200 and 4000 feet pressure altitude:

As stated before, this is not the "as-installed" power in a stock Mooney M20J. This power setting and atmospheric conditions is 55% power in a J. More below!

Mooney was very accurate in its engine power data gathering for the M20J effort. In particular, the airborne engine torque meter, fuel flow instrumentation and air data instrumentation were the best obtainable for a GA airplane in 1975-76, and the flight test engineers, Roy Lopresti, and Fen and Dorothy Taylor, were without peer.

I modeled the M20J IO-360-A3B6D "as-installed" power in Benchmark. Here is a chart under the same conditions as before:

This chart shows 55% in the J, right on the money. And of course, Benchmark allows any combination of every possible MP/RPM/Temp/ALT, just like the OP, @mkrakoff wanted!

So, let's find the lowest altitude where WOT and, say, 2400 RPM gives exactly 65% power, and what the book fuel flow would be at best economy (which in this case is 25 degrees rich of peak EGT, that's how Mooney did it):

So, for @mkrakoff , fly your airplane to 9000 feet density altitude (note for here it is a standard day, so 29.92" altimeter and -3 deg C in the chart is 9000 ft pressure altitude as well as density altitude). Go wide open throttle and 2400 RPM (hopefully your tachometer has been recently calibrated). Slowly lean back to 9.3 to 9.2 gallons per hour. Ideally, you should see 22.5" MP under these conditions, and none of the cylinders has passed peak EGT yet. Manage your cowl flaps to keep the hottest cylinder below 380 deg F. At cruise you "should" not have to worry and you should be able to run cowl flaps closed. Let the engine stabilize, then slowly lean some more. Your first EGT should peak about 25 degrees higher and the fuel flow should be about 8.7 gph. If your first cylinder peaked at 8.8 gph and the last at 8.6 gph you have a really good baseline and your airplane is pretty close to the type design, and you are pretty much exact at 65%! And you could probably run LOP very nicely without GAMIs. Many Lycomings can do this.

On the other hand, under these same conditions, if you get a first peak at "something like" 9 gph and a last at 8.2, you have a high GAMI spread and GAMIs would help. If it's really bad you engine might feel a little rough before the last cylinder peaks. Or, at WOT if you are getting significantly less than 22.5 inches MAP you may have a restriction in the induction system somewhere. A 1" loss under these conditions (21.5" at WOT) equals a 3% power loss, so your peaks would be around 8.3 gph instead of 8.7. I got this answer instantly "flying" the Benchmark power chart just like flying the airplane.

Now then. I would like you to please go fly these numbers and report back your data. And we will go from here.

Hint, we will look at cruise performance next. You should see about 138-139 KCAS, or about 2 knots higher indicated (which computes to ~158 KTAS) in the stated conditions above and around 2600 lb GW. Draggy stuff on the airplane will knock that down some, so the common wisdom is that the J is a 155 knot true airspeed flyer under every day 65% power conditions.

Amazing. Thank you so much for the detailed and thoughtful post. I’m headed out to fly tomorrow and will let you know how it goes. 

[mkrakoff]

1 hour ago, testwest said:

Bump, hoping for @mkrakoff to reply back here!

Just out of curiosity, is that benchmark model publicly available or are you working on a Future project with the data right now. 

[mkrakoff]

13 minutes ago, bluehighwayflyer said:

It is available in the Mac App Store.  Here is the developer’s website, which includes a link to it ...

Jim

http://www.seqair.com/benchmark/

Thanks, downloaded!

[testwest]

Anytime! Looking forward to the data. What kind of engine analyzer do you have?

[testwest]

Oh yes, the M20J airplane model, "as installed" engine model and several M20J props are on the Seqair site, so you can download those two and start right in.

[mkrakoff]

17 hours ago, testwest said:

Oh yes, the M20J airplane model, "as installed" engine model and several M20J props are on the Seqair site, so you can download those two and start right in.

Got it thanks. I went flying today but wasn’t able to run the test due to workload. I’ll run a flight dedicated to just this shortly. Also I may wait, as I have an EDM upgrade scheduled next week to a 730/830 from a 700. Right now I don’t have FF connected to the 700 but will after upgrade, so it makes it much more difficult to pull an accurate gami spread. I’ll post as soon as I get good data for you. 

Edited November 4, 2020 by mkrakoff

[testwest]

Excellent, you will like that 730/830 upgrade. Be sure to look at the setup menus and record data at the highest possible rate, I think it is once every two seconds is the highest rate choice.

[testwest]

Bump, how is that upgrade to the 730/830, @mkrakoff?

[mkrakoff]

A great upgrade! I just flew it to a shop for it's big tank reseal so when I fly it back I'll get the flight test data off of it. I tried with a USB I had in my bag, but it wasn't being recognized. I'll bring a fresh formatted USB next time. 

[Tim Jodice]

On 10/28/2020 at 7:22 PM, 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 can't find the app. I searched simply "aircraft power". What am I missing?

[carusoam]

13 minutes ago, Tim Jodice said:

I can't find the app. I searched simply "aircraft power". What am I missing?

Tim,

Not what you are missing... it’s what you have too many of....

looks like you got an extra space in the middle... aircraftpower 

Go MS!

-a-

[0TreeLemur]

Great thread- I just discovered it.   Like many of the folks that posted here, I'm interested in operating the engine correctly LOP.   Probably too interested...  

My J has an EDM 900, and I've noticed that after I set MP and RPM, the %power displayed on the EDM decreases as I lean with little/no change in MP and RPM.  Somehow the JPI uses fuel flow in its %power calculation.  I'm wondering how, and how accurate it is.  The published Lycoming chart for A6B3D is at "best power mixture" only.  Anybody know how it does it?

[Ryan ORL]

10 minutes ago, 0TreeLemur said:

Great thread- I just discovered it.   Like many of the folks that posted here, I'm interested in operating the engine correctly LOP.   Probably too interested...  

My J has an EDM 900, and I've noticed that after I set MP and RPM, the %power displayed on the EDM decreases as I lean with little/no change in MP and RPM.  Somehow the JPI uses fuel flow in its %power calculation.  I'm wondering how, and how accurate it is.  The published Lycoming chart for A6B3D is at "best power mixture" only.  Anybody know how it does it?

I don't know the exact mechanism but the logic goes something like this... (Mike Busch described this calculation in general terms on a podcast once, but it basically just follows logically):

First, the engine monitor has to decide whether the engine is ROP or LOP.  This can probably be pretty easily approximated just from looking at whether the fuel flow is in the plausible range (given those tables) for "best power mixture".  Anything less than a particular fuel flow for a given MP/RPM combo can be assumed to be LOP.

If ROP - Excess fuel, air (MP) is the limiting factor.  % power is given by MP/RPM table lookup.  Fuel flow has little measurable impact on horsepower, probably mostly disregarded.

If LOP - Excess air, fuel flow is the limiting factor.  % power is computed by fuel flow and RPM only, probably using some figures from the BSFC curve.

[EricJ]

59 minutes ago, 0TreeLemur said:

Great thread- I just discovered it.   Like many of the folks that posted here, I'm interested in operating the engine correctly LOP.   Probably too interested...  

My J has an EDM 900, and I've noticed that after I set MP and RPM, the %power displayed on the EDM decreases as I lean with little/no change in MP and RPM.  Somehow the JPI uses fuel flow in its %power calculation.  I'm wondering how, and how accurate it is.  The published Lycoming chart for A6B3D is at "best power mixture" only.  Anybody know how it does it?

Fuel flow tells you the max power than can be produced.  Since the amount of energy in a gallon of gasoline is known, the flow rate tells you how much power can be produced at that flow rate.   If you can somehow compute the efficiency of the burn, that'll tell you how much power is actually being produced.   Subtract losses due to friction, pumping, etc., and you know the power produced.   You can also work backwards assuming the rated power is correct and use the RPM and MP to scale down from that.    So combining the three to make a reasonable estimate of hp, especially from a table of known or expected performance, is a reasonable way to estimate % power.  Depending on how sophisticated the algorithm wants to get, it could also use OAT to improve it a little bit.   My JPI knows OAT, but I don't know whether it uses it for the % power estimate or not.

Cars use Mass Air Flow sensors with intake air temperature sensors and ambient pressure sensors to get a good idea of how much fuel can be added to get an efficient burn, and then look at the amount of O2 in the exhaust for feedback of how close it is to stoichiometric efficiency (or just how lean or rich it is).   Our engine monitors don't have access to those kinds of metrics, so they make do with available inputs like MP, RPM, fuel flow, etc.   Kinda crude in comparison, but I don't think it's intended to be or needed to be an accurate estimate.

[0TreeLemur]

2 hours ago, Ryan ORL said:

I don't know the exact mechanism but the logic goes something like this... (Mike Busch described this calculation in general terms on a podcast once, but it basically just follows logically):

First, the engine monitor has to decide whether the engine is ROP or LOP.  This can probably be pretty easily approximated just from looking at whether the fuel flow is in the plausible range (given those tables) for "best power mixture".  Anything less than a particular fuel flow for a given MP/RPM combo can be assumed to be LOP.

If ROP - Excess fuel, air (MP) is the limiting factor.  % power is given by MP/RPM table lookup.  Fuel flow has little measurable impact on horsepower, probably mostly disregarded.

If LOP - Excess air, fuel flow is the limiting factor.  % power is computed by fuel flow and RPM only, probably using some figures from the BSFC curve.

That makes sense.  I'd like to see the code...

[Will.iam]

1 hour ago, 0TreeLemur said:

That makes sense.  I'd like to see the code...

That is jpi proprietary information. 

[0TreeLemur]

1 hour ago, Will.iam said:

That is jpi proprietary information. 

Sure it is, but Github is full of code that used to be proprietary.   Brave developers release their code so it can be scrutinized and improved upon.  That's my opinion.   It's not like they'd lose customers.  Their product is amazing in my opinion.  I didn't buy it for that feature.  If it didn't have it that number, I'd just use the tables.

[A64Pilot]

Analyzers can’t get % power not really, best they can do is best guess, they just don’t have the data input, sure they can have algorithms to try to get closer, but without a torque meter we just can’t get there, we have to deal with charts.

Garbage in - Garbage out.

Having spent a lot if my flying life doing test flights for data acquisition I can attest that any chart is mostly extrapolated, most test points aren’t flown, your Certifying an Aircraft, survival and success of the company relies on getting it done, so any charts are “close enough” then of course there are variations in everything equipment wise so close enough is often about as close as you can get.

I would throw this out there as a thought experiment.

% power = airspeed IF density altitude and a couple of other variables are the same then airspeed will correlate to % power

Since we can determine % power decently close when LOP, go out and fly say from 50% to as high as you can get at your average cruise altitude in 5% steps.

Graph speed vs % power, then anytime your flying at the same density altitude you should be able to determine % power from the speed you have as long as other variables are close to being the same, weight and CG probably being the biggest ones, not perfect but perhaps close enough

In truth we can only get close determine % power by fuel flow when LOP because the lower the RPM the lower the friction losses of the engine and prop, but it’s likely close enough for getting an idea how hard you are working the engine.

Of course this isn’t necessary, I haven’t done it, but for those that like to “play” it might be worth doing, just as a learning exercise.

What I’m saying is that if 55% power equals 130 kts, then any combination of mixture, RPM and Manifold pressure that gives you 130 kts is 55% power.

Edited January 14 by A64Pilot

[Pinecone]

On 10/28/2020 at 6:42 PM, 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). 

LOP is easy.

14.9 HP per gallon per hour most NA engines.  13.7 for turbo.

[PT20J]

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. 

[0TreeLemur]

8 hours ago, Pinecone said:

14.9 HP per gallon per hour most NA engines.  13.7 for turbo.

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