2700RPM Cruise

[201er]

Who here cruises at 2700RPM? What about at higher altitudes? Does 2700RPM cruise really put more wear on the prop/engine (aside from tach time)? How is operating at 2700 at altitude different than at sea level? Is it equally inefficient or because of TAS at DA, does it work differently higher?

 

I've stayed away from 2700 in cruise for now but sometimes wish I could actually make 75% power (especially when LOP) at higher altitudes. I'm wondering how much efficiency using 2700RPM specifically loses.

 

How's cruising LOP at 2700RPM? For example, at 6000ft is it better to make 75% by going 2700RPM or to be at 2500RPM and go ROP or whatever gets you 75%?

[jetdriven]

2700 RPM is more speed and more fuel burn.  Somewhere above 12K, it may be the same NMPG as 2500 RPM but with more speed. The only way to find out is to flight test it, as all the book numbers are ROP.

 

I wouldnt worry about wear at 2700 RPM vs. 2500 RPM. Its only 9% more RPM. Fixed pitch airplanes such as RV's routinely exceed 2800-2900 RPM and they last just as long. My A&P used to have an aerobatic clipped-wing taylorcraft (IO-360A1A) and he routinely ran it to 3200 RPM or higher on the downlines.  The elastic limit for crankshafts and rods is quite a bit higher than 2700 RPM.  Your engine is counterweighted and that is a concern, but at 2700 RPM it is also below limits. I have never read of a counterweight failure on an IO-360 Lycoming.

[carusoam]

Same or similar issue with IO550.

2700 rpm is great for getting off the ground, but fuel flow is quite high, sound level is also increased.

Overall, it is a great option to have.

Best regards,

-a-

[FloridaMan]

I run 2700 RPM at high altitude and with headwinds. In my case, I have the IO360-A1A that lacks the counterbalanced crankshaft. Since purchasing my plane, the vacuum pump (with 17 years and 1400 hours on it) went and the manifold pressure line for the gauge broke. I have a feeling that vibration contributed to both of those. I can't say for sure, but it most certainly feels like there's a lot less vibration up in the altitudes where the air is thinner at higher RPMs. 

 

IIRC, the Robinson R22 uses the IO360 and runs continuously at 3300 RPM. I have a feeling that the 2700 RPM limitation has a lot to do with prop tip speed and efficiency. Other aircraft have longer props and the tips come closer to supersonic at those RPMs, and have the potential to lose some efficiency. I don't think the prop on the Mooney is long enough for that to be much of a factor. 

 

Here's a prop calculator: http://www.kitfox.eu/technical/propcalculator/propprog.html 

[jlunseth]

My POH recommends 2600 - 2700 for the higher altitudes, but I think it is because the POH is for the original factory engine which had a fixed wastegate and a critical altitude of about 15k, so in the flight levels it was already running more like an NA engine and needed the RPM to make 100% HP.  The Merlyn increases the critical altitude quit a bit.  Mine is about 22,500.

 

I thought initially that the 2700 was also related to prop efficiency, so generally ran high RPM's at altitude, but I find it really does not make a lot of difference that I can tell, so am back to running around 2450-2500 up high.  I don't feel any less vibration though, about the same.  Lots less turbulence up there most of the time.

[aaronk25]

A guess but its prob cheaper to spin the io360 at 2700rpm up high and squeeze every ounce of power out of it than have the added expense of a turbo......although I really want one.

[Ren B]

Who here cruises at 2700RPM? What about at higher altitudes? Does 2700RPM cruise really put more wear on the prop/engine (aside from tach time)? How is operating at 2700 at altitude different than at sea level? Is it equally inefficient or because of TAS at DA, does it work differently higher?

 

I've stayed away from 2700 in cruise for now but sometimes wish I could actually make 75% power (especially when LOP) at higher altitudes. I'm wondering how much efficiency using 2700RPM specifically loses.

 

How's cruising LOP at 2700RPM? For example, at 6000ft is it better to make 75% by going 2700RPM or to be at 2500RPM and go ROP or whatever gets you 75%?

 

 

That would be nice, but remember even though you aren't making 75% power at 6000 feet or 8000 feet 2500RPM your true airspeed is better than 75% power at say 2000 feet

[201er]

Ok, well what's more efficient and/or faster?

 

6000ft 2500RPM 75% power

8000ft 2700RPM 75% power

 

Those are ballpark numbers but sound about right to me. Throttle is always WOT obviously. Above example is most likely ROP. What about an LOP example, what's going to be better?

 

4000ft 2500RPM 75% power

6000ft 2700ROM 75% power

[FloridaMan]

Is this not in your POH? I downloaded a POH for a '78 J somewhere and I use the data in it to supplement what the POH (which is like a pamphlet) in my 67F lacks (emergency procedures, et cetera). A quick look at the numbers seems to show the same speeds given the same power with a higher fuel burn at higher RPMs. From the POH charts, it looks like lower RPM and higher MP for a given power setting (ROP or LOP) gives you better economy than the higher RPM lower MP for the same power. 

[wishboneash]

For the M20J McCauley prop (74"), the prop tip speed is 594 mph at 2700 rpm and at 2500 rpm is 550mph. At 20C, the speed of sound is 768 mph. At a temperature of -40C, the speed of sound drops to 685 mph. Still plenty of margin at 2700 rpm. Speed of sound is independent of altitude to first order and only temperature matters.

[201er]

Is this not in your POH? I downloaded a POH for a '78 J somewhere and I use the data in it to supplement what the POH (which is like a pamphlet) in my 67F lacks (emergency procedures, et cetera). A quick look at the numbers seems to show the same speeds given the same power with a higher fuel burn at higher RPMs. From the POH charts, it looks like lower RPM and higher MP for a given power setting (ROP or LOP) gives you better economy than the higher RPM lower MP for the same power. 

POH is known to be wickedly wrong when it comes to speed or power figures. For example, the POH would suggest that flying at economy cruise vs best power yields the exact same cruise speed. This is obviously preposterous or why would anyone ever want to fly at best power?

[FloridaMan]

POH is known to be wickedly wrong when it comes to speed or power figures. For example, the POH would suggest that flying at economy cruise vs best power yields the exact same cruise speed. This is obviously preposterous or why would anyone ever want to fly at best power?

 

Gotcha. I admittedly have not flown enough nor do I have the equipment (no GEM) in my plane to check the validity of the POH data. I'll keep what you said in mind as I continue to fly. 

 

As for the prop tip speed, it was rationalization for the 2700 RPM limit being not a mechanical limitation on the engine, but possibly for other reasons, as stated. Some Cessnas have props up to 86" in diameter, which, anything over 2700 RPM could create some issues. I remember reading a pretty technical discussion on another forum regarding all of this a couple years back. 

 

[danb35]

For example, the POH would suggest that flying at economy cruise vs best power yields the exact same cruise speed. This is obviously preposterous or why would anyone ever want to fly at best power?

75% at best power is exactly the same horsepower as 75% at best economy--both are 75%, or 150 HP for our 200 HP engines.  With the same amount of power, the aircraft will perform exactly the same.

[201er]

75% at best power is exactly the same horsepower as 75% at best economy--both are 75%, or 150 HP for our 200 HP engines.  With the same amount of power, the aircraft will perform exactly the same.

Yeah but you can't measure that. POH has power settings of the same MP/RPM listed with same speed. Yet because the mixture is best economy vs best power, actual power will be different. Example from POH:

 

4000ft, 23.2", 2700RPM, 75% Power, 12.5GPH at best power yields 161KTAS

4000ft, 23.2", 2700RPM, 75% Power, 10.8GPH at best economy yields 161KTAS

 

This is why I said the POH isn't credible. In reality one of these is more than 75% (or one is less) but the POH doesn't account for this.

[wishboneash]

Yeah but you can't measure that. POH has power settings of the same MP/RPM listed with same speed. Yet because the mixture is best economy vs best power, actual power will be different. Example from POH:

 

4000ft, 23.2", 2700RPM, 75% Power, 12.5GPH at best power yields 161KTAS

4000ft, 23.2", 2700RPM, 75% Power, 10.8GPH at best economy yields 161KTAS

 

This is why I said the POH isn't credible. In reality one of these is more than 75% (or one is less) but the POH doesn't account for this.

 

I don't get this either. Forget the airplane and all other things being equal, if you look at purely from the engine & prop speeds (and using the IO360-XXX app)

 

4000ft, 45F, 75% best power is      22.0", 2700 rpm FF=12.6gph

4000ft, 45F, 75% best economy is 22.9", 2700 rpm FF=10.5gph

 

I plan to try this and see what TAS I get for both. Can't be the same "75%" power, can it? Why would I ever use the "best power" setting? How about the EGT, CHT and oil temps?

[jetdriven]

Is this not in your POH? I downloaded a POH for a '78 J somewhere and I use the data in it to supplement what the POH (which is like a pamphlet) in my 67F lacks (emergency procedures, et cetera). A quick look at the numbers seems to show the same speeds given the same power with a higher fuel burn at higher RPMs. From the POH charts, it looks like lower RPM and higher MP for a given power setting (ROP or LOP) gives you better economy than the higher RPM lower MP for the same power. 

 

Lower RPM is more efficient from a NMPG standpoint but slower. Part of the savings is because the prop is more efficient in lower RPM ranges, and part of it is because you have less total drag due to flying slightly slower.  Much like LOP, same thing there.  Eventually, you get to 2000 RPM, 15,000 feet, and 25 LOP. Man! Look at that fuel flow! 22 NMPG!  But then the realization that 94 KTAS is going to take forever to get anywhere, and with a headwind, you are actually losing efficiency.   You have to pick which stop to get off the train, same with low RPM and % power.

[Hondo]

Who here cruises at 2700RPM? What about at higher altitudes? Does 2700RPM cruise really put more wear on the prop/engine (aside from tach time)? How is operating at 2700 at altitude different than at sea level? Is it equally inefficient or because of TAS at DA, does it work differently higher?

 

I've stayed away from 2700 in cruise for now but sometimes wish I could actually make 75% power (especially when LOP) at higher altitudes. I'm wondering how much efficiency using 2700RPM specifically loses.

 

How's cruising LOP at 2700RPM? For example, at 6000ft is it better to make 75% by going 2700RPM or to be at 2500RPM and go ROP or whatever gets you 75%?

 

It sounds like you want to fly fast efficiently.  

 

Flying 75%, 2700 RPM at 8000 yields 169 KIAS with a 10.8 GPH FF at 25 ROP.

Flying 60%, 2400 RPM at 8000 yields 156 KIAS with a   8.6 GPH FF at 25 ROP.

That is 13 KIAS faster, but at a cost of 2.2 GPH, or about $1.00 per knot.

In hot weather running at 75% power could make if difficult to keep CHTs between 300 and 360 for cruise and prevent closing cowl flaps.

Cowl flap drag reduces airspeed by about 3 KIAS for half and 7 KIAS for full open, so running at high power can be counter  productive.

In this situation, running LOP reduces FF, CHTs and airspeed, but most of the speed lost can often be regained by closing the cowl flaps.

Running at 2700 RPM in cooler weather may make it harder to keep CHTs above 300, which is recommended to prevent lead fouling.

If so, reducing RPM can raise CHT above 300.

 

Power lost to friction has a cost of about .1 GPH per 100 RPM from 45% to 75% power.  Therefore, higher RPM results in a greater percentage

of power lost to friction at lower power settings, making lower RPM more advantageous at lower power settings. Also, at any given power setting,

MP + RPM/100 is a constant. In other words, a reduction in RPM is offset by an increase in MP.

 

To efficiently fly fast in a J, I fly WOT, 30 LOP at 8000 to 12000 and 2400 to 2200 RPM.

[Jamie]

I've just started playing with settings other than "2700rpm 23in" (75%) in cruise, and I like the results so far.  "2200rpm, 22in at 8000ft" is my new favorite. I get right around the book values (145-150 kts TAS) at a little over 9 gph.  At 2700rpm, 23in, it takes nearly 13gph. Could be I'm not leaning enough, but remember it's an io-390, not an io-360. It takes 12.9 to keep the indicated CHTs at 380F (well, the bar under '400F' on the GEM 600).

 

I'm willing to go 10-15 knots slower for a 30% reduction in fuel used -and- it's a lot quieter.

 

I do NOT understand why there have to be so many published settings. Or maybe it's the table itself. I feel like there's a column missing. I get that there are multiple combinations of (rpm, mp) that will achieve a given % horsepower. What I don't have enough information to do is pick one over another.

 

According to my POH, the following settings give me 55% at 8000ft:

• 2700, 17.8 mp, 10.1gph
• 2600, 18.6 mp,  9.8 gph
• 2400, 19.8 mp,  9.5 gph
• 2200, 22.0 mp,  9.2 gph

For those settings, range and endurance are max at the lowest rpm. That's it... that's all I see to choose one over another.  So if all else is equal (and it can't be) why not just list a table of (horsepower, altitude) with the single set of optimal settings for a given horsepower at a given altitude?

[jlunseth]

I would be cautious about flying at "best power" according to the old Mooney POH's. Best power in most cases is likely to be right at peak, or not very far from peak and definitely in the "red box."

[Hondo]

"I do NOT understand why there have to be so many published settings."

 

 

In 1980-81, Mooney didn't have digital engine monitoring systems or GPS.  

 

Take a look an an Ovation POH to see the difference.  

The Ovation POH has two pages for cruise power settings, one 50 ROP and one 50 LOP.

There is even a note which says add .4" MP for each 10C above std day temperature and

subtract .4" MP for each 10C below std day temperature.

With some work, similar tables could be created for the J.

 

Generally, the lowest RPM required to produce a given power is the most efficient.  

That 2200 RPM, 22 MP at 8000 sweet spot we both like, tops out at 55% power.

At 8000, increasing RPM to 2400 extends power to 64%, 2600 to 71%, which gives

higher power for climb or more speed.

[testwest]

Jamie

 

Your 22 square performance results match my Benchmark results exactly....

 

but why not run WOT/2200? With a little bit of ram rise you should get 23.5 " MAP (give or take) and nearly 10 knots faster at an almost identical MPG.

[testwest]

Ask, and ye shall receive! From Benchmark

 

6000ft 2500RPM 75% power (note WOT is about 76% power standard day)

 

TAS 163.4 knots, FF 14.7 gph, NMPG 11.1, CAFE Score No-Wind 8374 using Best POWER stock power setting

 

8000ft 2700RPM 75% power

 

TAS 166.1 knots, FF 15.4 gph, NMPG 10.82, CAFE Score No-Wind 8331 using Best Power.

 

Pretty much a wash, the effects of winds at differing altitudes are far greater than these different power schemes.

[M016576]

I would be cautious about flying at "best power" according to the old Mooney POH's. Best power in most cases is likely to be right at peak, or not very far from peak and definitely in the "red box."

Only at lower altitudes (higher %HP's)

[Jeev]

Only at lower altitudes (higher %HP's)

 

I agree.  Below say 70% power definitely 65% there is effectively no "red box".  I routinely fly my J at peak since since my cruise altitudes are rarely less than 8500ft or 10000ft if IFR out here in the SW.  I find its a nice spot to run as I give up too much speed at 20ish LOP and above 10k my J isnt happy with LOP. 

[aaronk25]

I run at peak egt at 4000ft wide open throttle chts 360s. If it climbs to 380 maybe run 5 lop or just a bit, but I have baffles in great shape and filled all the gaps in the lower front cowl with hight temp silicone.

Faster cruise gives more IAS and more cooling.

Screw the red box. Run as hard as you want just keep chts below 380-400.

I also run 350 hours a year, have 1500 hours on these cylinders and have 75/80 compressions.

Running hard dosent kill engines, heat does.