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Which is the right RPM


NicoN

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ACcording to the POH there are many combinations of RPM and MAP for the same engine power.

Recently, I tried using different power settings, all in low altiftudes and flying ROP.

Using a combination of low RPM/higher MAP caused nearly no difference in IAS, but the CHT was significantly lower than the same power setting with higher RPM. Also FF was slightly lower.

When CHT is the most important parameter, then low RPM should be better for the engine.

But I also learned, that lower RPM tends to move the point where the maximum cylinder pressure occurs moves closer to TDC compared with high RPMs.

The reason behind should be, that the process of bruning the fuel stays constant in time. So, the crankshaft simply moves less way in that time. (Not my idea).

 

What to do ?

 

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I think the best RPM depends on your:

1.  Magneto timing.  A engine that is timed at 25 BTDC probably needs to run at a higher RPM than one that is timed at 20 BTDC.

2.  Propeller length.  Longer propellers need to run at lower RPM to get maximum efficiency.

3.  MP.  Higher MP requires higher RPM.  For example, you wouldn't want to run your engine at 30" and 2000 RPM.  That would exceed engine design limits.  For a normally aspirated engine, while it is OK to run a little over square (maybe a couple inches) you don't want to be too much over square.

4.  TAS.  Higher true airspeed requires a higher RPM to obtain maximum prop efficiency.  As noted, higher RPM causes higher friction loss, but it offset somewhat by higher prop efficiency.

I'm a bit surprised by your findings.  I've found that when my -A3B6D engine CHT is high, I can usually drop it by 5 or 10 degrees by increasing the RPM by 100.

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I think the best RPM depends on your:
1.  Magneto timing.  A engine that is timed at 25 BTDC probably needs to run at a higher RPM than one that is timed at 20 BTDC.
2.  Propeller length.  Longer propellers need to run at lower RPM to get maximum efficiency.
3.  MP.  Higher MP requires higher RPM.  For example, you wouldn't want to run your engine at 30" and 2000 RPM.  That would exceed engine design limits.  For a normally aspirated engine, while it is OK to run a little over square (maybe a couple inches) you don't want to be too much over square.
4.  TAS.  Higher true airspeed requires a higher RPM to obtain maximum prop efficiency.  As noted, higher RPM causes higher friction loss, but it offset somewhat by higher prop efficiency.
I'm a bit surprised by your findings.  I've found that when my -A3B6D engine CHT is high, I can usually drop it by 5 or 10 degrees by increasing the RPM by 100.


I was talking to orionflt about distance flying and we discussed lower RPMs. Normally at altitude I fly max MP I can obtain and 2500. The vibration is less at 2500 than 2400.

I decided to try running at 2000 RPM and found like the original OP, I really hadn’t lost much airspeed. It was a one time trial and I will repeat it tomorrow to see if what I was seeing was real or not. It obviously was a lot quieter but to see less than 5 knots difference really perplexed me.


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There's a whole bunch of people here who have attended the Advanced Pilot Seminars on engine operations that will probably chime in.

  • Conventional wisdom -- Don't run oversquare
  • New idea -- run MP high and RPM low as much as practical as long as the overall power is below 65%
  • When running LOP, the primary factor in determining % power is your fuel flow.
  • When running ROP, the primary factor in determining % power is your air flow (MP x RPM)
  • lower RPM has less frictional losses

 

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1 hour ago, Marauder said:

 


I was talking to orionflt about distance flying and we discussed lower RPMs. Normally at altitude I fly max MP I can obtain and 2500. The vibration is less at 2500 than 2400.

I decided to try running at 2000 RPM and found like the original OP, I really hadn’t lost much airspeed. It was a one time trial and I will repeat it tomorrow to see if what I was seeing was real or not. It obviously was a lot quieter but to see less than 5 knots difference really perplexed me.
 

For the first 3 years of owning the plane I was running 24/24.   Last year the compression checks were 79/80 on all 4 cylinders. (this is on an old engine build)    This will generally produce 135 knots over the ground.  I have the 2100-2300 red zone limitation.   On a couple of flights where I have just been poking around the county or so I thought   I was running 2000 rpm.    Still was doing 135 knots  (various heading directions).     Is this possible?

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1 hour ago, Marauder said:

 


I was talking to orionflt about distance flying and we discussed lower RPMs. Normally at altitude I fly max MP I can obtain and 2500. The vibration is less at 2500 than 2400.

I decided to try running at 2000 RPM and found like the original OP, I really hadn’t lost much airspeed. It was a one time trial and I will repeat it tomorrow to see if what I was seeing was real or not. It obviously was a lot quieter but to see less than 5 knots difference really perplexed me.


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When comparing lower vs higher RPM and CHT I think you have to consider power.  If you are high enough that you are full throttle (>6500' for me) then reducing RPM will in fact reduce CHT because the power is reduced.  Each 100 RPM will be a bit less than a 4% reduction in power.  But since airspeed is related to power in a squared (or is it cubed) relationship, a small reduction in power will result in an even smaller reduction in speed.  If you really want to compare two settings, make your initial setting and wait for the speed to stabilize.  Note the IAS and CHT's.  Then reduce the RPM but increase the MP.  Start with about 1" for each 100 RPM.  Wait.  If the IAS is different, adjust the MP until you get the same IAS, then note the CHT's.

And of course you need to take into account updrafts and downdrafts.  I've seen my CHT's vary by as much as 15F within a few minutes even though I haven't touched the power.  In an updraft my plane has to descend through the airmass to maintain altitude.  That means my IAS increases and my CHT's will drop.  During the downdraft the opposite happens.  As long as the IAS remains steady you aren't getting up or down drafts.  Changing IAS indicates you are getting up/down drafts and the test is probably invalid.

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27 minutes ago, Yetti said:

For the first 3 years of owning the plane I was running 24/24.   Last year the compression checks were 79/80 on all 4 cylinders. (this is on an old engine build)    This will generally produce 135 knots over the ground.  I have the 2100-2300 red zone limitation.   On a couple of flights where I have just been poking around the county or so I thought   I was running 2000 rpm.    Still was doing 135 knots  (various heading directions).     Is this possible?

You can never use groundspeed, especially on different days to compare speeds.  IAS would be the speed to use while being aware of up/down drafts.

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Interesting question. A few points to consider:

1. Propeller efficiency varies as both TAS and RPM, and propeller efficiency is generally plotted as a function of advance ratio J = V/nD, where V is TAS, n is RPM and D is diameter. So, the optimum rpm varies with airspeed. Generally, lower rpms for lower airspeeds and higher rpms for higher airspeeds should yield the greatest efficiency.

2. The purpose of a constant speed propeller is to broaden the peak of the efficiency curve over a wider range of advance ratios. This means that at normal cruise speed, the efficiency isn't overly sensitive to rpm.

3. Any rpm/map combination that is in the engine manual is okay to use. For the Lycoming IO-360A, the range at sea level is 1800/25" to 2700/28.5" depending on power output desired. 

4. When comparing different rpm/map settings, it is important that the airspeed be the same for both. This is the only way to know that the power output is the same.

5. As noted, rpm/map combinations affect engine power and efficiency as well as the propeller.

6. Often it comes down to finding a rpm/map combination where the engine runs smoothest. A lot of Lycoming four-bangers don't run as smoothly at lower rpms.

7. On a normally aspirated engine, as you climb you will run out of manifold pressure at WOT and then will have to increase rpm to maintain power.

 

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Really depends on the objective of the day...

  • Max speed...
  • Max efficiency...
  • Min noise...
  • min vibration...
  • min CHTs or TIT...

For the NA IO550 Best Cruise with the TopProp and 310hp STC recommends 2550rpm.

That is probably an aerodynamic efficiency highlight for the prop... the FF increases more than the TAS does...

PP thoughts only, not an aerodynamicist....

Best regards,

-a-

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I run higher RPM at higher altitudes--2300 down low, 2400 at mid altitudes, 2500 up high.

To go slow (on approach, in the pattern, flightseeing or flying witb Cessnas), I generally set 2300 and reduce the throttle as needed / desired.

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5 hours ago, carusoam said:

Really depends on the objective of the day...

  • Max speed...
  • Max efficiency...
  • Min noise...
  • min vibration...
  • min CHTs or TIT...

For the NA IO550 Best Cruise with the TopProp and 310hp STC recommends 2550rpm.

That is probably an aerodynamic efficiency highlight for the prop... the FF increases more than the TAS does...

PP thoughts only, not an aerodynamicist....

Best regards,

-a-

I also run 2500-2550 RPM in the M20J with the mccauley prop. 

Edited by jetdriven
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OP has a turbocharged motor, so he gets the luxury (or curse) of choice.  On our long trips, we're usually above 10,000' in our J, I've usually got the RPM's up to 2600 or 2650 so, otherwise we get too slow.  It's weird, 2650 rpm doesn't bother me but 2700 rpm feels uncomfortable for extended periods in terms of noise and vibration.  If the weather and bumps permit, I'll go lower and see if I can get down to 2400 RPM, but it just hasn't seemed to happen often.

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5 hours ago, jetdriven said:

It’s not quieter. In fact it’s louder for the residents under the flight path because the aircraft is lower at every point downrange.  This is only a 200HP airplane. Pulling even 10HP off reallly affect climb. 

I was talking about RPM in cruise, not climb.  And it's not 10 less HP, it would be 3.7 HP on takeoff

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Thank you for all your postings.

The question came up on a flight in really low altitude (4000ft) with varying power settings and experimenting with the different ways to achieve the same %power.

And indeed, the difference with a lowRPM/HighMAP vs HighRPM/lowMAP was having lower CHT (about 15°) and nearly no speed penalty with the lowRPM variant.

The engine felt a little quieter,. Of course, I do not expect good climb rates with this setting. And also, i should try the same in much higher altitudes

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21 minutes ago, NicoN said:

Of course, I do not expect good climb rates with this setting. And also, i should try the same in much higher altitudes

I would not recommend climbing at these power settings. In the turbo Mooneys it's best to climb with take-off power, all the way to cruising altitude. Only then, once leveled off, set cruise power LOP.

With my 252 I leave everything full forward all the way to cruising altitude. That might be 6000 ft or it might be 26,000 ft. It's the same thing. Full power all the way to altitude, then lean for LOP cruise.

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2 minutes ago, gsxrpilot said:

I would not recommend climbing at these power settings. In the turbo Mooneys it's best to climb with take-off power, all the way to cruising altitude. Only then, once leveled off, set cruise power LOP.

With my 252 I leave everything full forward all the way to cruising altitude. That might be 6000 ft or it might be 26,000 ft. It's the same thing. Full power all the way to altitude, then lean for LOP cruise.

Paul, I suppose your 252 has an automatic wastegate that limits the turbo boost. Are you climbing at "100%" power all the way into the flight levels? Does the excess fuel keep CHTs cool enough? 

(It's been a long time since I flew a turbo.)

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I usually cruise at 2400rpm.  Like Tom said, less RPM=less friction.  I've never tried cruise at 2000 rpm.  I might do it just to see what it's like.  That's a difference of 16--20% in rpm depending on if you cruise at 2400 or 2500.  That's gotta mean something to the motor.

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3 minutes ago, Bob_Belville said:

Paul, I suppose your 252 has an automatic wastegate that limits the turbo boost. Are you climbing at "100%" power all the way into the flight levels? Does the excess fuel keep CHTs cool enough? 

(It's been a long time since I flew a turbo.)

Yes, the 252 has the automatic wastegate. And so, yes, I climb at 100% power all the way to the flight levels. FL260 is the highest altitude I've been to, but I was still at 100% power when reaching FL260. 

My CHT's stay nice and cool (cowl flaps open) when climbing at full power. I'm usually at about 800 ft/min and CHT's are all in the low 300's. 

Once leveled off in cruise in the high flight levels, I usually have to keep the cowl flaps cracked open just a bit to keep the CHT's under control. The air is just too thin to carry away much heat. 

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Yes, the 252 has the automatic wastegate. And so, yes, I climb at 100% power all the way to the flight levels. FL260 is the highest altitude I've been to, but I was still at 100% power when reaching FL260. 

My CHT's stay nice and cool (cowl flaps open) when climbing at full power. I'm usually at about 800 ft/min and CHT's are all in the low 300's. 

Once leveled off in cruise in the high flight levels, I usually have to keep the cowl flaps cracked open just a bit to keep the CHT's under control. The air is just too thin to carry away much heat. 

What is your airspeed(s), I assume you keep airspeed up more than Vy to keep the engine cool?

 

 

Tom

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8 minutes ago, ArtVandelay said:

What is your airspeed(s), I assume you keep airspeed up more than Vy to keep the engine cool?

Yes, at 800 ft/min I'm usually at 120 IAS down low and that goes up as I gain altitude. 

It's also worth mentioning that the 252 is really well cooled. With cowl flaps open, I don't think there is anything I can do to get the CHT's over 380. Even climbing a Vx, it runs  cool when full rich and cowl flaps open.

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