2500 RPM @ cruise in a M20J

[kevinw]

Hi Everyone,

I'm fairly new to MS. I've owned an '82 J for about a year now. I've been flying for 15 years and have about 500 hours. Private with IR. I was always told run the IO-360 at WOT and 2400 rpm in cruise which I have. Take a look at this clip from Bob Kromer and please give me your opinions. For the full article http://www.mooneypilots.com/mapalog/cruisepower.html

 

Bob Kromer
MAPA Executive
Director & Former Engineering Test Pilot

 

Prop RPM at 2500 (2400 in the Eagle Only)
Except for the new M205 Eagle, for the normally aspirated (non-turbo) Mooneys we fly, the best RPM to cruise is 2500. Period. For the engine/prop combination in the Eagle, the best cruise RPM is 2400. You can talk about all kinds of reasons why 2400, 2300, or 2200 might he better, but from a pure technical standpoint, cruising at lower RPM settings makes no sense at all. By selecting cruise RPM lower than 2500 (2400 in the Eagle) you are simply giving away power and better performance for nothing in return. Think the engine will last longer at lower RPM settings? Forget it. TBO is set assuming maximum continuous power (that's max rated RPM). Think the engine will run cooler at lower RPM? Nope. As you increase the spread between MP (high) and RPM (low), internal engine operating pressures go up slightly, making the engine run just about as warm as it would at a higher RPM setting. And I have yet to fly a Mooney that is as smooth at 2200 or 2300 RPM as it is at 2500. So why do it? So try cruising at 2500 RPM. If you have an Eagle,2400. Everything else is giving away performance (our most valuable Mooney asset) for nothing in return.

[The-sky-captain]

My J is definitely smoothest at WOT 2500. I used 2400 for a year or so when I first bought the plane but now cruise strictly 2500 both LOP or ROP. Give it a try.

[kevinw]

My J is definitely smoothest at WOT 2500. I used 2400 for a year or so when I first bought the plane but now cruise strictly 2500 both LOP or ROP. Give it a try.

Thanks, I will. I put a FRM A3B6 in it when I bought it and I'm probably a bit paranoid of doing anything wrong that will damage the engine. I'll give it a try. btw, much of a fuel penalty at 2500?

[jetdriven]

I have noticed that the NMPG at 2500 RPM is virtually the same as 2400 RPM but the 2-3 knots additional airspeed are free. Above 9K or so, 2600 and 2700 is also just as efficient, and faster as well.

 

I think Aaronk25 and 201ER have also noticed this.

[Jeev]

My J is definitely smoothest at WOT 2500. I used 2400 for a year or so when I first bought the plane but now cruise strictly 2500 both LOP or ROP. Give it a try.

 

Exactly the same as me.  Ran it at 2400 for about 100hrs when I first got it and have been cruising at WOT 2500 for the last 600hrs, smooth and a few knots faster.

[ArtVandelay]

From lycoming: Longer engine life may be expected from most engines when the operator is willing to sacrifice maximum performance for conservative cruise operation in the 60% to 65% power range

If you want to make TBO, yes it's designed for max power, you want to go past TBO and save some fuel, drop down to 60-65%

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[ArtVandelay]

I have noticed that the NMPG at 2500 RPM is virtually the same as 2400 RPM but the 2-3 knots additional airspeed are free. Above 9K or so, 2600 and 2700 is also just as efficient, and faster as well.

I think Aaronk25 and 201ER have also noticed this.

You do realize the extra 2-3 knots on the longest of trips is only saving you a few minutes? So really there is no practical reason except to satisfy your obsession for more speed.

:-)

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[tomn]

+1 WOT/2500 always except when needing to slow for turbulence etc. I pull back on MP but leave it at 2500. My plane flies in the yellow arc at cruise so I have to slow from time to time.

[jetdriven]

From lycoming: Longer engine life may be expected from most engines when the operator is willing to sacrifice maximum performance for conservative cruise operation in the 60% to 65% power range

If you want to make TBO, yes it's designed for max power, you want to go past TBO and save some fuel, drop down to 60-65%

Sent from my iPad using Tapatalk

I think the engine may generally show less total wear at 65% power than at 75% power, but most engines fail to make TBO not from wearing out, but from corrosion, other factors caused by disuse, and camshaft / lifter failure.

 

Another thing is that wear isnt RPM dependent, and 2500 RPM is only 5% more RPM than 2400, but the cylinder pressure for a given % of power is higher when RPM is lower.

[Bob_Belville]

Because my scimitar prop's rpm restriction is 2350-2550 (over 24"), I generally cruise @ 2550, even when I'm too high to pull over 24".

 

@2550 (with the PF exhaust ?), I find I don't use RAM air until I get to 9000'

At 8000 Tuesday MP was 21.6 and the 930 said I was @ 70% power w/o ram air. OAT 48F.

On the return flight @ 9000 I used RAM and was still showing 70% @ 21.6/2550

 

My only issue is oil temp in climb. @ WOT & 2650 and 125k ias the OilT got to 227 and it is not summer yet. CHTs were OK, under 400 F during climb.

@ cruise, trimmed to 8.7 GPH LOP, 66% power, temps were 197 & 381/364/384/356. (152k TAS)

[Marauder]

Bob -- weren't you dealing with this oil temp problem before your engine rebuild? Any chance the 930 probe is off? Do they put the probe in the same place as the factory one?

[gjensen]

My oil temp will creep up in climb as well if I do not dial back the prop to 2500. In cruise, WOT and 2500 my oil temp will be between 200 and 210 on the factory guage whether ROP or LOP, all CHT's stay below 340. I am currently adding oil temp to the JPI for a more accurate reading.

[orionflt]

I have been experimenting with pulling my RPM's down to 2000, I have done it a few times now and comparing part of the flight at 2500/WOT and 2000/wot the smoothness is about the same the FF is 2 gph less and my airspeed has been dropping by around 4 Kts. as stated above that is not a lot of time difference when taking a trip, but it is a heck of a fuel savings. since I am still in the testing comparing stage I haven't kept the RPM low for the entire cruise portion so I can not do a time comparison for a point A to B trip. I guess the biggest trick to this testing is finding the most efficient blade angle to RPM in order to get the most efficiency out of the engine/prop combo

[Bob_Belville]

Bob -- weren't you dealing with this oil temp problem before your engine rebuild? Any chance the 930 probe is off? Do they put the probe in the same place as the factory one?

Chris, yes, I've been fussing with OilT ever since I got the 930. At the annual last August, just before the prop strike, we had sent the cooler and the vernatherm valve to Pacific for overhaul. The cooler is relocated to the high position behind cyl 4. Pacific said there was some coking (my term) but temps don't seem to be improved. I frankly don't remember where the probe is. I've had that conversation with Lynn but I've slept several times since then. OTOH, the engine builder was not concerned with oil temps below 225 in climb. Too cool is more harmful with respect to moisture from his experience. And the CHTs are OK so I'm going to take a wait and see. I'm still on straight mineral oil with only 17 hours since the engine was overhauled. Temps might come down a little when I switch to Phillips 20-50 XC.

 

Do you know if the probe can be either before of after the cooler? I would suppose the downstream temp ought to be several degrees cooler than upstream. One more interesting datum point, since the engine was reinstalled cyl 4 CHT runs ~20F cooler than the other 3. Before the overhaul 1,2,4 were close with #3 ~20F hotter. Baffle seals are pretty much the same, we replaced all last summer. I had the lower cowl repair during the tear down. The left cowl flap was is rough shape so exit air flow has probably been changed. The CFs are pretty effective. Opening CFs full open at cruise seems to drop OT ~6F at a cost of maybe 2 knots. 

[Marauder]

Bob - not sure where the probe sits. In my case with the 830, for all I know, it is sharing the probe with the factory gauge. The location certainly will impact the reading. If I get some time today I will pull the installation manuals and see where they want it installed. Neither my factory or the JPI oil temps get much above 200, even in the summer. It doesn't mean that the temp of the oil isn't reaching 212 degrees somewhere, just not at where the measurements are made.

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[orionflt]

Chris and Bob,

the probe is on the back of the engine installed in the filter/vernatherm housing. I'd post pics but I am working at the job that pays my bills.....eerrr well i'm suppose to be working.....

 

Brian

[PTK]

...Prop RPM at 2500 (2400 in the Eagle Only)

...And I have yet to fly a Mooney that is as smooth at 2200 or 2300 RPM as it is at 2500. So why do it? So try cruising at 2500 RPM. If you have an Eagle,2400. Everything else is giving away performance (our most valuable Mooney asset) for nothing in return.

2450 RPM, WOT, LOP all day long.  Smooth as silk.

 

I agree with Bob Kromer. Not as smooth at lower RPM.

[orionflt]

pulled a pic off the net. this is a remote filter installation, you can see the temp probe right on the top

 

[Bob_Belville]

Chris and Bob,

the probe is on the back of the engine installed in the filter/vernatherm housing. I'd post pics but I am working at the job that pays my bills.....eerrr well i'm suppose to be working.....

 

Brian

Brian, in that area, near the v-valve, I suppose the probe could be "seeing" oil either upstream or downstream the cooler. Do you know which?

[orionflt]

up stream of the cooler, the down stream side will be returning to the sump 

 

Brian

[ArtVandelay]

I agree with Bob Kromer. Not as smooth at lower RPM.

I didn't notice any difference in vibration, but thought it was quieter at lower RPM (2300).

[Bob_Belville]

This is the only pic I find that shows the spin on adapter. This region gets blocked by mags and vacuum pump after everything is on the plane. The probe and cooler hoses are not on yet but I suppose it matches the pic orianflt posted. 

[orionflt]

yep, that is it, they just pulled that silver cap and installed the probe.

[DaV8or]

I guess we should all bear in mind that many here are flying with different props. Not all Js have the same prop. I'm not sure what prop Kromer was referring to. For what it's worth, I typically fly around 2400 rpm because it seems more pleasant in the cockpit too me. I have to say I haven't done a bunch of testing to see how it compares NMPG, but I doubt there is a load of difference. Running 2500 is OK too. I have nothing against it.

 

Keep in mind that Bob also said this in the article-

 

 

Three Choices of Mixture Settings
I think you really have three choices of mixture settings with a normally aspirated engine. They are 1) 50 degrees rich of peak EGT (Exhaust Gas Temperature), 2) peak EGT, 3) 50 degrees lean of peak EGT. My choice? 50 degrees rich of peak EGT. All the time. It's the best combination, in my opinion, of best power mixture (100 degrees rich of peak EGT) and best economy mixture (peak EGT). Some pilots like to fly at peak EGT --that's okay, you save a little gas but loose some performance. Lean of peak EGT is okay too, but only if you're flying a fuel-injected, normally aspirated engine and have a good set of balanced fuel injectors installed. Operating lean of peak EGT is impossible if your fuel injectors aren't balanced to "squirt"equal amounts of fuel into each cylinder -- one or two cylinders are going to get too lean before the others, resulting in a very rough running engine.

 

I don't think I will run my engine where he suggests, nor do I believe that you really only have 3 choices.

[tomn]

This is the only pic I find that shows the spin on adapter. This region gets blocked by mags and vacuum pump after everything is on the plane. The probe and cooler hoses are not on yet but I suppose it matches the pic orianflt posted. 

Bob,

My oil temp prob for my new Insight G4 was installed in the oil passage port on the front of the engine behind the prop.  The 1/8 NPT plug was removed and the probe installed - has worked fine.