oisiaa Posted January 13 Report Posted January 13 I'm only able to achieve about 2640 RPM at full power during takeoff in my M20C. I'm not really worried about this and am pushing it off until my first annual in a few months. Should I be concerned? The prop seems to govern fine and it seems like an adjustment issue.
bonal Posted January 13 Report Posted January 13 First things first, you need to confirm your tachometer is accurate. There are optical sensors that can do this I think they can be found on apps for cell phones. You may have already checked this and are moving on towards correction. Governors are usually not too hard to adjust 1
oisiaa Posted January 13 Author Report Posted January 13 This is on a G3X with a modern Hall effect sensor which I presume to be accurate, but that is a good thing to check first. I don't operate out of short fields so even if there is a marginal decrease in takeoff power it isn't a huge deal and then I'm back to 2600 for climb power which it governs to fine.
Slick Nick Posted January 13 Report Posted January 13 If that’s your actual RPM and not a gauge issue, absolutely that is a big deal. I’d get it fixed right away. Not having full power available is dangerous. Why wait until annual? Could be something as simple as a governor adjustment. If it used to be fine and suddenly changed, that is even more cause for concern. Investigate this further before flying again.
MikeOH Posted January 13 Report Posted January 13 Redline is 2700 rpm. What is the minimum acceptable as I doubt it's the full 2700 rpm!
EricJ Posted January 13 Report Posted January 13 Not a huge deal, imho. Prop governors aren't perfect. Do have it looked at at annual just to make sure nothing is going awry. 1
oisiaa Posted January 13 Author Report Posted January 13 2 minutes ago, MikeOH said: Redline is 2700 rpm. What is the minimum acceptable as I doubt it's the full 2700 rpm! These are my thoughts. I don't know the tolerance, but I'm happier to be below 2700 than above. With that being said, I do want it adjusted up to hold 2700. I go to climb RPM (2550-2600) after gear/flaps up so hitting the full 2700 is more about making good on performance numbers for ground roll and 50' obstacle clearance. Since I never operate on anything remotely close to the limits of the performance charts a few percent off of doesn't seem too risky. Still happy to have the discussion with varying opinions though. 1
MikeOH Posted January 13 Report Posted January 13 @EricJ @PT20J Is there a minimum full-power RPM spec?
Vance Harral Posted January 13 Report Posted January 13 17 minutes ago, Slick Nick said: I’d get it fixed right away. Not having full power available is dangerous. This is hyperbolic. Per the M20C POH performance tables, the difference between 2600 and 2700 RPM at the same manifold pressure is about 2% across a broad range of takeoff altitude and temperature. 2640 RPM is in the middle of that, so call it a 1% degradation in power developed, assuming all gauges are perfectly precise and perfectly accurate. That number is so small as to be completely lost in the noise for actual takeoff performance. Unrelated factors such as density altitude, runway slope, minor cam lobe wear, dirty air filter. etc. are all going to have a much greater effect than 2640 vs. 2700 RPM. Bear in mind also that even if this is "fixable" with a governor adjustment in the field, you must have realistic expectations about what can be achieved. Odds are that tweaking the max RPM adjustment screw in situ is not going to give you exactly 2700 RPM, but rather something like 2680 or 2710 (with a flashing red indication on your G3X), etc. Such is the curse of modern digital instrumentation. Be careful what you try to fix, lest you create a bigger problem than was there in the first place. 2 1
N201MKTurbo Posted January 13 Report Posted January 13 There is a stop screw on the governor that can be backed out about 1/2 turn and you should be able to get 2700. Sometimes the prop cable needs to be rerigged. Is your prop control all the way in? It is best if it is not.
PT20J Posted January 13 Report Posted January 13 15 minutes ago, MikeOH said: @EricJ @PT20J Is there a minimum full-power RPM spec? I've never seen one. But the McCauley manual says within 100 rpm of redline. If the redline can be obtained in flight, then a low static rpm would indicate a lack of power from the engine. For McCauley governors, the high speed stop adjustment is about 25 rpm per turn. 1
oisiaa Posted January 13 Author Report Posted January 13 7 minutes ago, PT20J said: I've never seen one. But the McCauley manual says within 100 rpm of redline. If the redline can be obtained in flight, then a low static rpm would indicate a lack of power from the engine. For McCauley governors, the high speed stop adjustment is about 25 rpm per turn. I've seen 2660 in flight (80-100 knots on climb out prior to pulling back 2550-2600) vs ~2640 at 40-50 knot on the takeoff roll so it seems to be governing properly (at a lower set-point) +/- a small tolerance. My thoughts have always been a simple prop governor adjustment which is why I've never been too concerned. I have an oil change scheduled soon so I'll see about tweaking this the also. I typically operate from fields 2,300' - 4,000' elevation (DA 5,000+). I'm a quick flight away from 500' field elevation. Maybe I'll make that next weekend's mission to see what I can get down low with respect to engine output. 1
bonal Posted January 13 Report Posted January 13 I get 2700 on TO roll but after gear and flaps I turn down to 2500 due to cylinder temps this keeps me just under 400 degrees and I still get about 6 to 7 hundred feet per with two on board and full fuel at 120mph indicated. BUT, taking off is a very controllable action. Getting caught in an unexpected down draft you’re going to want every bit of available climb performance you can get. That one or two percent might be the difference in the outcome. Not really a problem until it really is.
oisiaa Posted January 13 Author Report Posted January 13 5 minutes ago, bonal said: I get 2700 on TO roll but after gear and flaps I turn down to 2500 due to cylinder temps this keeps me just under 400 degrees and I still get about 6 to 7 hundred feet per with two on board and full fuel at 120mph indicated. BUT, taking off is a very controllable action. Getting caught in an unexpected down draft you’re going to want every bit of available climb performance you can get. That one or two percent might be the difference in the outcome. Not really a problem until it really is. Off topic, but does 2500 in climb help with CHT vs 2600? This isn't something I've ever considered before.
47U Posted January 14 Report Posted January 14 3 hours ago, Vance Harral said: Odds are that tweaking the max RPM adjustment screw in situ is not going to give you exactly 2700 RPM, but rather something like 2680 or 2710 After a new prop governor a couple years ago the UPB-16 (P-lead sensor) was around 75 RPM low on takeoff. I tweaked the high RPM screw and now get 2700 with blips to 2710, but only on the first takeoff of the day. Subsequent takeoffs I get around 2670. Not sure why this is. Maybe some increased friction in the cylinders as the engine comes up to operating temperature? I can’t imagine the lower end is responsible.
Ragsf15e Posted January 14 Report Posted January 14 Do C models climb cooler at lower rpm? Generally “new age” GA technique is to climb at full rpm and full mp.
Vance Harral Posted January 14 Report Posted January 14 48 minutes ago, 47U said: Not sure why this is. Anything's possible, but it's worth talking about accuracy vs. precision of the system that's showing you RPM. I don't know what a "UPB-16" tachometer is. Is that a modern system designed this century, or something akin to the old Electronics International R1 that was designed in the early 1990s? To put some nerd numbers on it, for a 4-cylinder engine with the world's greatest P-lead sensor (the P-lead signal is quite noisy) 10 RPM accuracy requires resolving pulse edge differences on the order of 50 microseconds (2700 RPM on a 4-banger is 5400 P-lead pulses per minute, or one every 11,111 microseconds; at 2710 RPM it's every 11,070 microseconds). A CGR-30 or GI-275 EIS with a modern microcontroller and A/D system is almost certainly sampling at much higher than 25 microsecond intervals (need at least 2x oversampling), but it's less likely to be true for something designed decades ago, just based on my general knowledge of hardware available at reasonable costs for consumer applications over the years. A lifetime spent in the digital design industry has shown me several examples of manufacturers deliberately conflating precision with accuracy, and it's left me skeptical that the 10 RPM (or in some absurd cases 1 RPM) indications on digital tachometers are meaningful, especially not with older hardware. Speaking for myself, I start paying attention when max indicated RPM differs from redline by more than 50 RPM or so. I wouldn't personally spend energy chasing down a 30 RPM difference, and definitely not a 10 RPM difference. But to be clear, I don't know what tachometer you actually have, or what digital guts are in it. It's certainly possible your numbers really are accurate enough to be interesting. 1
DXB Posted January 14 Report Posted January 14 5 hours ago, Slick Nick said: If that’s your actual RPM and not a gauge issue, absolutely that is a big deal. I’d get it fixed right away. Not having full power available is dangerous. Why wait until annual? Could be something as simple as a governor adjustment. If it used to be fine and suddenly changed, that is even more cause for concern. Investigate this further before flying again. Meh. I suspect WOT power difference is trivial here. 2600-2650 is about what I get since installing a new PCU-5000 governor a few years back. It behaves identically before and after an engine overhaul last year - so I don't think the engine is underperforming. The tight quarters to adjust the fine pitch stop screw on the governor have made it seem not worth the trouble. Also, if there's real evidence of that a 50-100rpm difference in pitch at WOT makes a more than trivial power difference for my O-360, I'm sure about to be corrected in great detail , and if so I'll fix it at annual...
EricJ Posted January 14 Report Posted January 14 Digital tachs can be pretty accurate, or not, but the governor itself is just a mechanical device that doesn't have the means to be super accurate over all realms of temperature, oil pressure, viscosity, wear, etc., etc. They're generally pretty consistent, but sometimes I think people expect too much of them. 2
bonal Posted January 14 Report Posted January 14 4 hours ago, oisiaa said: Off topic, but does 2500 in climb help with CHT vs 2600? This isn't something I've ever considered before. 2 hours ago, Ragsf15e said: Do C models climb cooler at lower rpm? Generally “new age” GA technique is to climb at full rpm and full mp. I can only speak from my personal experience and am only basing this off my single CHT gauge. On hot weather days if I climb at 2700 I see the indication at just above 400 reducing to 2600 made no visible change to the temperature but reduced to 2500 I can definitely see the needle indicating just below the 400 degree mark. Now I know the original gauge is not calibrated but it’s accurate to itself and the reduced temperature is consistent every time in climb. WOT full rich mixture 120mph IAS and as noted before this gives me around 600fpm with Two and full fuel plus fully loaded with goods from Wallyworld. I know this goes against new age technique but then this is an old age Mooney. 1
47U Posted January 14 Report Posted January 14 2 hours ago, Vance Harral said: Anything's possible, but it's worth talking about accuracy vs. precision of the system that's showing you RPM. I don't know what a "UPB-16" tachometer is. Is that a modern system designed this century, or something akin to the old Electronics International R1 that was designed in the early 1990s? Vance, you never disappoint. Thank you. You deciphered it, it’s an E.I. UBG-16. The module to add RPM came with a several sets of isolators (resistors?) to the P-lead with instructions on how to narrow the combinations down for the best accuracy. I contacted their tech support and told them I have a SureFly, and he gave me the combination for the best result. Upon post install engine run/flight eval, the UBG RPM was generally within 25-50 RPM of the analog, so I am assuming the UBG is more accurate than the analog tach. I guess I should actually verify accuracy of the RPM module, huh? I doubt if I’ll upgrade to a primary engine analyzer… I have to draw the line somewhere and the UBG gives me 95% of what I’m looking for. Since the UBG is not primary, I’ve got all the original analog gauges, too. 1
jeremyc209 Posted January 14 Report Posted January 14 I have the opposite problem, but with an old tach. How accurate are these? Annual is coming up in a couple months and I intended on getting this checked out. This is my tach from take off roll to cruise alt.
PT20J Posted January 14 Report Posted January 14 3 minutes ago, jeremyc209 said: I have the opposite problem, but with an old tach. How accurate are these? Annual is coming up in a couple months and I intended on getting this checked out. This is my tach from take off roll to cruise alt. Just check the rpm with an optical tach and if it is accurate the procedure to adjust the governor is to set rpm to redline in flight with the prop control and then land and adjust the governor stop screw so that it it just touches the arm. I’d remove the safety wire before the flight to make it easier to adjust the screw and then re-safety after a second flight to verify the setting. 1
AndreiC Posted January 15 Report Posted January 15 I had a similar problem, where I could only get 2640 on takeoff. I asked my mechanic to turn the high rpm limiting screw on the governor (PCU5000) enough to make 2700, and ended up with 2730 max rpm, which I did not like (flashing red light on tach). So I had the screw backed off by a turn, and I'm now happy at around 2690. But the job of adjusting that screw is a bear, it is located in the worst possible position. I have a huge admiration for the patience of both mechanics who adjusted this, I would have used quite a few choice words in the process. I also noticed that on first flight of the day I get a slightly higher RPM on takeoff. I attributed this to oil still being cold, and so the governor cannot push it to adjust the prop blades as well as it should.
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