Moonkee

M10J Power Settings Issue

23 posts in this topic

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.com/rv8/article.php?story=20030101212257829&mode=print

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

Mooney_Power_Settings.pdf

post-10984-0-80476800-1353875554_thumb.j

Share this post


Link to post
Share on other sites

*Members that donate $10 or more do not see advertisements*

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

Share this post


Link to post
Share on other sites

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..

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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-

Share this post


Link to post
Share on other sites

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!

1 person likes this

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

@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! :-)

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
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.)

Share this post


Link to post
Share on other sites

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.)

I don't own a 201 yet, still shopping. But I found a POH online and its pretty close to the app.

App says 60% @8000' 2700 rpm 19 mp 8.8 gph

POH says 60%@8000' 2700rpm 18.5 map 8.8 gph

I would guess the .5 difference is induction loss

Share this post


Link to post
Share on other sites

Interesting. I haven't checked numbers in that performance region, but your app exactly matches my POH for that scenario. Try the one I kicked of this thread with: "... 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." and see what your app says.

The bigger difference at the higher power setting does indeed point to intake manifold restriction but given all the Mooney effort to achieve performance it seems kind of improbable that they ended up with something worse than Lycomins' generic manifold and maybe even worse than the Piper Arrow. Maybe it was a tradeoff with the cowling design. Puzzlement.

If you're curious enough, PM me your email and I'll send the POH plus some charts I got from the Lycoming guy including one that is unpublished.

(@Piloto and @JimR, (a) this is partially abstract engineering curiosity and (B) not everyone flies the airplanes the way you choose to do. That's not a criticism, just an observation.)

Share this post


Link to post
Share on other sites
My point was exactly the same as yours, though, that this is a largely abstract discussion ...
Agreed. This whole thing got started when this M20J got added to the list of the half-dozen or so complex airplanes that I fly. The POH numbers were different than the same-engine Arrow and I couldn't understand why, so that got me started digging, got me to this forum, and launched this thread.

But the bottom line is that each of these seas of power numbers really has very little significance in the real world, For a given RPM and % Power, the MP numbers are almost all the same. Plus, I don't really care whether I'm flying at 73% or 77% and the engine doesn't either. So I'm ditching the charts and instead making myself a kneeboard reference for six airplanes -- just six numbers for each airplane: MP and lowest RPM to get roughtly 55%, 65% and 75% power at 4K+ ft. altitude. When full throttle becomes the limiting MP, then it is more or less how much noise you are willing to endure and not really power settings so much. Those six numbers give me an operating range to work within. If I was only flying one airplane all the time I wouldn't even bother with a reference card.

The MAPA training course uses a similar concept with their "magic number" theory that MP+RPM/100 is a constant for a given % power level. Nice and simple too.

Share this post


Link to post
Share on other sites

The MAPA training course uses a similar concept with their "magic number" theory that MP+RPM/100 is a constant for a given % power level. Nice and simple too.

44 - 47 - 50 ~ 55% - 65% - 75% if I recall correctly....it was a long time ago B)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now