Moonkee

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.

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enough, PM me your email and I'll send the POH plus some charts I got from the Lycoming guy including one that is unpublished.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 ( not everyone flies the airplanes the way you choose to do. That's not a criticism, just an observation.)

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.)

@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! :-)

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.

Most electronic failures end up being in connections. If you have a satellite design and go to the reliability guys, what they want to know is how many connections are in the bird and what kind. The fact that this is temperature-sensitive also points to connections. That's why, as Piloto suggested, disengaging and reseating connectors is always hign on the troubleshooting list. When you attempt to transmit, is it that ATC cannot hear you or that you cannot hear yourself on a hand-held in the airplane? If you can hear yourself, then the radio is probably transmitting just fine and it is an antenna connection issue. Deteriorated coax usually just adds attenuation, it does not get intemittent. If you can't hear yourself locally, then it's in the radio as you expect. The fact that the nav flag moves (something almost totally independent of the primary problem) tends to point toward a grounding problem. A weak ground connection, inside or outside the box, can produce very weird behavior. Can your avionics shop leave the box in a fridge overnight and bench test it when it's cold? (I suppose you could do this test if you can get the radio thoroughly chilled and quickly plug it into a warm airplane.) That might provide a clue as to whether the issue is inside or outside the box. If they can duplicate the problem, then it becomes Garmin's problem not yours. If not, you still own it. Troubleshooting this kind of thing can be a b1tc#.

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.

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..

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