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M10J Power Settings Issue


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#1 Moonkee

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Posted 25 November 2012 - 08:43 PM

I recently began flying an '87 M20J with the A3B6D engine. Yesterday I decided to make myself a kneeboard reference card for power settings.

In doing this I came up with two questions that a search here didn't answer.

First, why are the Best Power flow numbers included in the Mooney chart? According to Lycoming I can run as high as 75% power with Best Economy flow rates, so why would I want to burn more fuel than that?

Second, the power numbers on the POH chart do not match Lycoming's numbers. For example, the POH says that 25.8" and 2500 RPM will give 150BHP/75% at sea level w. standard temp. while the Lycoming chart says those settings produce 160BHP.

At least one other person has made this discovery: http://www.kilohotel...7829&mode=print

I've attached the power chart and the Lycoming graph. Any wisdom would be appreciated.

Attached File  Mooney_Power_Settings.pdf   41.16KB   11 downloads
Attached File  IO-360_Performance_Curves.jpg   475.88KB   63 downloads

#2 philipneeper

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Posted 25 November 2012 - 09:23 PM

would it be that mooney made its numbers based off weight of aircraft and lycoming did the numbers based on a different airframe?

#3 Moonkee

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Posted 25 November 2012 - 11:21 PM

No. It really has nothing to do with the aircraft. This is simply about how much power the engine makes at a given MP, RPM, and air density. Lycoming says one thing, Mooney says another. The differences are maybe in the 5% +/- range for the numbers I have checked Not huge, but puzzling..

The only thing Mooney engineers should have done is to translate the Lycoming power curves into a table of settings for the pilot, possibly precluding certain combinations due to vibration or other airframe considerations..

#4 DonMuncy

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Posted 25 November 2012 - 11:27 PM

I don't know how they do it, but in the automotive world you can get numbers for power with no accessories or with everything installed. If they are testing on a Prony brake, they may do it without the alternator installed.

Don Muncy


#5 Moonkee

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Posted 25 November 2012 - 11:47 PM

I hadn't thought of that, but it's hard to believe that Mooney would be motivated to fund their own engine dyno testing. But the plot is actually slightly thicker:

The fuel flow numbers in the Mooney charts match the Lycoming charts for the Mooney-indicated horsepower. So, for example, 12.5gph is the Lycoming Best Power rate for 150bhp, but according to Lycoming the 23.5"/2700/SL shown in the table actually produces about 160hp and the Best Power fuel flow for that is about 13.2gph. So, assuming Lycoming knows more about its engines than Mooney, Mooney would have us running the engine about 0.7gph lean of Best Economy. So, even if Mooney was coming up with their own net horsepower figures the fuel flows are wrong.

#6 Cruiser

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Posted 26 November 2012 - 12:38 AM

Information the manufacturer provides on these charts has been gathered from test flights conducted in a new aircraft, under normal operating conditions
I believe the prop has something to do with it. For both load and drag.
aka TomK
A plane is safe in a hangar but that is not what a plane is for.

#7 RobertE

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Posted 26 November 2012 - 06:16 PM

I actually think you're giving Mooney too much credit. I think the numbers vary because Mooney was sloppy. The POH that came with my M20J actually expresses two different VSo speeds. In one section it cites a number that is contradicted in the bank angle section (that is, the zero bank angle figure cited there differs from the VSo figure expressed earlier). Selling just a few hundred aircraft annually doesn't provide a lot of money for careful editing, is my guess.

I say, good catch on your part.

#8 Moonkee

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Posted 26 November 2012 - 08:40 PM

I think the numbers vary because Mooney was sloppy.

Sloppy is certainly a viable theory and that was my going-in assumption. I talked to Lycoming tech support today, got a guy who had been there for seven years. Here's another theory that somewhat fits the facts:

Lycoming tests the engines with induction systems thought to be typical of what the airframe manufacturers are using. If Mooney's induction system is more restrictive than what Lycoming used and the restriction is after the MP gauge tap, then the actual MP that the engine sees will be less than what the MP gauge indicates. Further, that would mean that the fuel flows are correct as shown.

On the numbers I checked, the difference between the Mooney numbers and Lycoming's is smaller at high RPM and it diverges at lower RPM. That's consistent with theory #2 because at lower RPM a higher MP would produce higher instantaneous flow rates, resulting in more pressure drop.

Absent any more information, I'll go with Theory #2, though it would mean that Mooney spent the money do do their own dyno testing -- which seems a bit unlikely.

The POH that came with my M20J actually expresses two different VSo speeds.

Yeah, these things are not perfect. I have a Cessna 182T POH that gives two different sets of best glide numbers that are a couple of knots apart.

#9 Marauder

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Posted 26 November 2012 - 08:49 PM

Sloppy is certainly a viable theory and that was my going-in assumption. I talked to Lycoming tech support today, got a guy who had been there for seven years. Here's another theory that somewhat fits the facts:

Lycoming tests the engines with induction systems thought to be typical of what the airframe manufacturers are using. If Mooney's induction system is more restrictive than what Lycoming used and the restriction is after the MP gauge tap, then the actual MP that the engine sees will be less than what the MP gauge indicates. Further, that would mean that the fuel flows are correct as shown.

On the numbers I checked, the difference between the Mooney numbers and Lycoming's is smaller at high RPM and it diverges at lower RPM. That's consistent with theory #2 because at lower RPM a higher MP would produce higher instantaneous flow rates, resulting in more pressure drop.

Absent any more information, I'll go with Theory #2, though it would mean that Mooney spent the money do do their own dyno testing -- which seems a bit unlikely.

Yeah, these things are not perfect. I have a Cessna 182T POH that gives two different sets of best glide numbers that are a couple of knots apart.


I buy into the Lycoming explanation. Restrictions in the induction system and point of MP measurement would certainly explain the difference.

#10 carusoam

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Posted 27 November 2012 - 01:09 AM

Funny...

The name of this thread begins with M10J....

Thought I might find a fuel injected cadet question.

Then the thread evolved into challenges of POH editing.

It must be hard to build great planes and edit POHs well....

Or did I miss something?

Best regards,

-a-

#11 201er

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Posted 27 November 2012 - 07:22 PM

First, why are the Best Power flow numbers included in the Mooney chart? According to Lycoming I can run as high as 75% power with Best Economy flow rates, so why would I want to burn more fuel than that?


Not sure how it works for your M10J, but on my 201 things are quite different than the book claims. First of all best power can yield 1-5 knots speed increase over economy (peak I'm assuming). But the bigger issue is cooling. Most likely you will NOT be flying 75% power at peak and maintaining CHTs below 380. Thus you will have to choose between spending 13GPH going 100+ROP to get the temps down or 10GPH or less LOP. Oh shucks, here we go again!
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#12 aaronk25

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Posted 27 November 2012 - 07:26 PM

201er.....nope no need for us to "go again" your right!

#13 testwest

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Posted 05 December 2012 - 01:14 AM

Mooney used an in-flight torque meter reading to measure power developed by the engine, as installed, during the initial flight tests of the M20J, back in 1975ish. They did not do any "dyno" testing because it would not have yielded results relevant to certification of the airframe-engine combination. I personally spoke with Curt Lopresti on this and he said that he, "Pop" (as in Roy Lopresti) and the Taylors (Fen and Dorothy) were quite proud of the inflight instrumentation setup for the engine on the prototype M20J.

I believe the horsepower resulting from the Mooney POH data for RPM and MP are pretty close to the mark for the Best Power and Best Economy tables.

Norman and Gretchen Howell

77 M20J, N201JX/Lopresti/Hartzell BA

Frontier Airpark (WN53), WA

Also 77 Aerostar 601P


#14 Moonkee

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Posted 05 December 2012 - 01:31 AM

@testwest, thanks very much. That is exactly the kind of information I was hoping to find with this thread.

As you say, the fuel flow figures in the POH are the same as Lycoming's numbers. So that would say that the engine is producing the HP shown in the POH table (not the higher power implied by Mooney's MP settings) and hence requires the fuel flow that Lycoming's chart shows.

As mentioned, the most likely explanation for the higher MPs then seems to be a more restrictive induction system, which seems odd for a performance oriented airplane. I'm going to try to get a look at an Arrow induction system (the Arrow POH numbers exactly match Lycoming's) and compare it to the M20J's. Maybe the difference will be obvious. Or maybe Piper didn't do as careful a measurement job as Mooney did! :-)

#15 testwest

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Posted 05 December 2012 - 03:25 AM

You are welcome, sir. Always happy to share information. B)

Norman and Gretchen Howell

77 M20J, N201JX/Lopresti/Hartzell BA

Frontier Airpark (WN53), WA

Also 77 Aerostar 601P


#16 jetdriven

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Posted 05 December 2012 - 11:47 AM

There is an excellent article in the August 1976 issue where they taked about how they developed the Mooney 201. One part was where they installed a new low-restriction exhaust and intake air box, and measured all 200 HP through the torquemeter to the propeller. http://books.google....epage&q&f=false
Byron
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#17 bluehighwayflyer

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Posted 05 December 2012 - 12:28 PM

I wonder what happened to the swing out instrument panel referenced in the article? Would have been nice.

Jim
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#18 Piloto

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Posted 05 December 2012 - 02:22 PM

I don't know what is the big deal about the 201 power settings. There is only two power settings to remember full throttle for flight and idle for taxing. Once I apply full throttle for take off it remains full until I start descend.

José

#19 marky_24

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Posted 06 December 2012 - 12:33 AM

There is an app for android called IO-360 that you enter altitude and tempture and what % of power you desire and it gives you all the possible combos for that % of power

#20 Moonkee

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Posted 06 December 2012 - 12:48 AM

There is an app for android called IO-360 ...

The info page says it's based on the Lycoming Operators Manual. If so, the numbers it gives you are not going to match the Mooney POH numbers. Plug in the Mooney numbers and the app will probably give you a HP number that is 5-7% higher than the Mooney chart says. That's the problem that started this thread. (If you have the app, please check the example in the original post and let us know what you learn.)




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