How to decide MP and RPM settings

[Code90]

As a low time pilot cruising along in our 77 Lance, I had a power setting dilemma. Looking at the back of our sun visor, I had picked out the settings for 65% power. My question is how to decide which setting to use. At my altitude, at 65%, there were three different setting combinations for MP and RPM.
I don't have the numbers here to be more specific. But what is the difference between using a setting of 22MP, 2300RPM vs a higher MP and lower RPM? Should one setting give more speed? Is one less stressful to the engine?

 

[Apache123]

Power out is power out. If it's within the safe operating limits of your engine, lower RPM will cause less wear and consume less fuel. I recommend the Pelican's Perch articles: http://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks182081-1.html

 

[flyingron]

Matter of preference. Higher MP / lower RPM is a quieter and a bit more efficient. Depending on what engine you have, you may prefer the higher rpm for lubrication purposes.

 

[Stewartb]

Every airplane's engine/prop has it's own sweet spot. Listen to yours. Find the settings that give you the best output and happy temps with the least vibration.

 

[Sam D]

Power out is power out. If it's within the safe operating limits of your engine, lower RPM will cause less wear and consume less fuel. I recommend the Pelican's Perch articles: http://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks182081-1.html

Second the Deakin recommendation. In my Lance (NA), if I'm going cross country, it's wide-open throttle and prop full at takeoff then prop back to 2500 in the climb (for the neighbors). Throttle stays wide open until some point in my descent at destination. Prop is pulled back as suggested above.

 

[txflyer]

I think the over squared myth has been busted.

Lower RPM's means the engine turns millions of revolutions less over time, which translates to longevity.

 

[Code90]

Throttle stays wide open until some point in my descent at destination.

Curious about this part. I have always done TO with wide open throttle, prop forward, climbed at 25/ 2500, then looked at back of visor to set cruise to 65% settings. At higher altitudes, it ends up being about wide open throttle. Are you saying that at lower altitude (say 5000') you still fly wide open throttle in cruise?

If so, why and what is the benefit? Are you leaning like crazy to not burn 20 gph+ ? What kind of fuel burn are you getting?

 

[Henning]

Find the RPM that gives you the smoothest operation and go from there. Then pull back the mixture until the engine power falls off, now add mixture until the power surges forward and the engine finds its it's voice. While the description is ephemeral, you will recognize it quite obviously and suddenly as you advance the mixture gently. This happens to correspond with the point on the combustion graph where the bottom of the 'nose' starts rising from the LEL limits. This also happens to coincide with the upper end of the 'heel slope' of the torque band.

This is where your engine is "fat, dumb, and happy", it will run at this power setting with minimum intervention for as long as you feed it fuel. You will see nice moderate CHTs, very little and light colored exhaust residue, and see the most work done for your fuel dollar. Since your power is restricted by fuel flow, Manifold Pressure no longer is significant of power produced. Look in the chart for the TAS/altitude metric for that percent power, TAS will tell you how much power you are making when operating LOP. Vibration and CHT are the primary damaging components to your aircraft, keep both to a minimum.

 

[BillTIZ]

With the C182RG, it's pretty much WOT from takeoff, not pulling 25"MP above pattern altitude. Most cruise is above 8500MSL so it is WOT. Pull the RPM back to 2300 for the cockpit noise factor, then lean until the engine stumbles, then slightly richer. No EGT gauge.

For the T-41, it's WOT for takeoff, reducing mixture for best power per the placard for altitude. Normally about 14-15goh. The T41 pulls 2700 rpm for takeoff, at 500agl it's rpm back to 2500. At our altitude MP will already be below 25". At pattern altitude reduce the fuel to top of the green arc, about 12gph. Reaching cruise altitude, above 7500MSL it's WOT, rpm about 2350, lean to about 1385F on the EGT. Yields about 9-10gph and 125KTAS. The T-41 pulls 210HP for takeoff.

 

[Stewartb]

2700? Interesting. My old Hawk XP made 210 hp at 2800 rpm. Great performer on tires with an 80" seaplane prop!

 

[poadeleted20]

Once I decide what %HP I want, I generally use the MP/RPM combination with the lowest RPM on the chart for that %HP. This keeps it quieter and extends engine life, but doesn't do much for fuel consumption (the internal engine efficiencies don't change significantly between say 21"/2600 RPM and 25"/2200 RPM). Other than that, anything on the chart is safe and acceptable. You don't want to go "off the chart" on the high MP/low RPM side lest you create too low an RPM for the MP and thus compromise your preignition/detonation margins, and this may limit the use of wide open throttle at lower altitudes.

 

[bobmrg]

As a low time pilot cruising along in our 77 Lance, I had a power setting dilemma. Looking at the back of our sun visor, I had picked out the settings for 65% power. My question is how to decide which setting to use. At my altitude, at 65%, there were three different setting combinations for MP and RPM.
I don't have the numbers here to be more specific. But what is the difference between using a setting of 22MP, 2300RPM vs a higher MP and lower RPM? Should one setting give more speed? Is one less stressful to the engine?

Bottom line, you are choosing between speed and fuel savings...you pay for higher speed with fuel. Lindbergh taught Doolittle's Raiders to use high MP, low RPM.

Bob Gardner

 

[poadeleted20]

Bottom line, you are choosing between speed and fuel savings...you pay for higher speed with fuel.

If you're cranking out 65% power in a typical light GA engine, you're not going to see any significant change in either speed or fuel consumption within the MP/RPM ranges you'll find in the POH charts.

Lindbergh taught Doolittle's Raiders to use high MP, low RPM.

I think you're referring to Lindbergh's work with USAAF P-38 pilots in the Pacific later in the war, not the training and mission done by then-Lt. Col. Doolittle's people in B-25's in early 1942. With those turbosupercharged Allison V-1710 engines in the P-38, the ability to pack 60 inches of manifold pressure into the cylinders changes the situation a lot. The difference between, say, 21 and 25 inches just doesn't produce that level of improvement in specific range.

 

[Sam D]

Curious about this part. I have always done TO with wide open throttle, prop forward, climbed at 25/ 2500, then looked at back of visor to set cruise to 65% settings. At higher altitudes, it ends up being about wide open throttle. Are you saying that at lower altitude (say 5000') you still fly wide open throttle in cruise?

If so, why and what is the benefit? Are you leaning like crazy to not burn 20 gph+ ? What kind of fuel burn are you getting?

Lower down I'd use a lower MP setting but I just don't cruise there that often.

The Deakin articles are really good on this topic.

 

[BillTIZ]

2700? Interesting. My old Hawk XP made 210 hp at 2800 rpm. Great performer on tires with an 80" seaplane prop!

The T41 is continous rated at 210HP, from what I recall, the Hawk XP had to be pulled back after 5min at max power?

The T-41B is the Army version of the Hawk XP, built before Cessna released the XP to the general public.

 

[Stewartb]

They're all variants of the Reims Rocket, aren't they? I never really knew the history. The engine is rated at 210hp from the factory. Cessna de-rated it to 195hp by limiting revs. The Isham STC includes the 5 minute full power/2800 rpm limitation. Not like anyone needs to run to WFO for longer than that. TCM took no exception to the STC with respect to warranty or TBO and sadly I had experience with that. I only commented because I thought the engine was rated 210Hp @ 2800rpm. I was surprised that the T41 might be different. Good airplanes in any case.

 

[ahkahn]

I am very interested in following this thread now that I just got my new Lance, as well.

In my transition training, I am WOT on takeoff, then at about pattern altitude, I back off to 25/2500 and lean back until EGT's rise decently, then climb out. When at altitude, I've been backing off to 2400 RPM and adjust MP accordingly (usually 75% for me). I then lean using the LeanFinder on the EDM-800, then go about 75 ROP. I have run it around 20 LOP, but still not comfortable enough to keep that up regularly.

Still not confident this is the best plan, but it's what I'm doing. I have an email into the seller who has been very helpful, as well.

 

[flyingron]

This keeps it quieter and extends engine life,=

Unless you have an IO-550. Continental recommends keeping the RPMS at 2400 or higher for lubrication efficiency.

 

[midlifeflyer]

The T41 is continous rated at 210HP, from what I recall, the Hawk XP had to be pulled back after 5min at max power?

You recall correctly.

 

[Henning]

Bottom line, you are choosing between speed and fuel savings...you pay for higher speed with fuel. Lindbergh taught Doolittle's Raiders to use high MP, low RPM.

Bob Gardner

I think you're referring to Lindbergh's work with USAAF P-38 pilots in the Pacific later in the war, not the training and mission done by then-Lt. Col. Doolittle's people in B-25's in early 1942. With those turbosupercharged Allison V-1710 engines in the P-38, the ability to pack 60 inches of manifold pressure into the cylinders changes the situation a lot. The difference between, say, 21 and 25 inches just doesn't produce that level of improvement in specific range.

IIRC The first airmen he worked with were Chenault's Flying Tigers in the their P-40s, basically doubling the combat/patrol range.

 

[Bill Watson]

Curious about this part. I have always done TO with wide open throttle, prop forward, climbed at 25/ 2500, then looked at back of visor to set cruise to 65% settings. At higher altitudes, it ends up being about wide open throttle. Are you saying that at lower altitude (say 5000') you still fly wide open throttle in cruise?

If so, why and what is the benefit? Are you leaning like crazy to not burn 20 gph+ ? What kind of fuel burn are you getting?

For my flying (RV10 IO540) I don't cruise below 5k, 6k is minimum. Practically all flying is point to point at 7k to 15k. The goal is to get there efficiently while hopefully maintaining long engine life.

For those flights, WOT and prop forward to altitude. I do lean during climb by holding EGTs constant above pattern altitude (easy and safe with good engine instrumentation).

I cruise WOT, prop back to some sweet spot and at some LOP setting based on the speed vs economy desired.

I descend WOT with the prop back and mixture leaned a bit further until I need to slow.

 

[flyingcheesehead]

I am very interested in following this thread now that I just got my new Lance, as well.

In my transition training, I am WOT on takeoff, then at about pattern altitude, I back off to 25/2500 and lean back until EGT's rise decently, then climb out. When at altitude, I've been backing off to 2400 RPM and adjust MP accordingly (usually 75% for me). I then lean using the LeanFinder on the EDM-800, then go about 75 ROP. I have run it around 20 LOP, but still not comfortable enough to keep that up regularly.

Still not confident this is the best plan, but it's what I'm doing. I have an email into the seller who has been very helpful, as well.

Ouch... I think this is probably the worst plan! No knock on you, it's what your average CFI will teach you to do. But, while CFI's seem to know everything when you're a student or low-time pilot, they really don't. And most of them have never OWNED an airplane either, so they don't really care about things like engine management. The seller may not be any better in this regard.

Learn from an engine expert: Read those Pelican's Perch (John Deakin) articles linked to earlier in the thread.

A few things:

1) No reason to back off the MP to 25". Most engines run just as well at WOT and 2500. I do generally back off to 2500 after takeoff to keep the airport neighbors happy. Deakin's articles have more technical info on why it's better to be WOT in the climb.

2) For climb mixture, take a glance at what your EGT's are shortly after takeoff, and then just slowly lean in the climb to maintain the same EGT's. My Mooney has a cheat here: There's an additional single-probe EGT with a "climb" range marked on it. IME, though, on planes without that particular feature, you'll be wanting EGT's in the mid-1300s for the climb.

3) Try 65%. The difference between 65% and 75% for me is only 10 knots, but it costs me an extra 5 gph, and the engine will last a lot longer at 65% (keep an eye on your CHTs, again Deakin has lots of info but you want to keep them well below 400ºF).

4) 75 ROP is going to give you some of the highest CHTs and shorten your engine life considerably. It sounds like you've tried 20 LOP, which is a much better place to be - Why are you "still not comfortable enough to keep that up regularly"?

Hope this helps you make your engine last a long time.

 

[flyingcheesehead]

Unless you have an IO-550. Continental recommends keeping the RPMS at 2400 or higher for lubrication efficiency.

Source?

The last thing I changed was to start running at 2300 instead of 2200 because of a notice from Conti that 2200 can cause the crank to go out of balance over time or somesuch. I haven't seen anything about running at 2400...

Thanks in advance!

 

[coloradobluesky]

I've been running my 0360 Lycoming at 20"/2000rpm leaned to just rich of roughness and getting 5.5gph, which for my local flights of 1/2 hour or so works just fine.

It all depends.

I bring the throttle back first to 18" and when I bring the prop back to 2000rpm, the manifold pressure goes up to 20" ALL BY ITSELF! How cool is that!

 

[Henning]

The one thing people fail to recognize with the Lindbergh doctrine with regards to operating most modern aircraft engines is along with the use of minimum RPM, a speed around 1.3 L/D max was also a parameter. At this speed you limit ICP greatly. ICP is the most destructive force to your engine unless it gets out of balance. If you want to increase speed, you have to increase power. You can do that by increasing RPM or increasing ICP. The easiest way on the engine is to increase RPM until you get a prop tip speed of .9M. That will require the least ICP and provide the least residual heat (which becomes CHT) for the extra power. All our direct drive aircraft engines operate at the very bottom of an efficient RPM range for their piston diameter and fuel properties. Unless you have a balance/harmonics issue, there is no great increase in strain on the engine parts anywhere in the operating RPM range.

RPM should be chosen by where the engine runs smoothest primarily, and this will typically be closer to the top of the green arc than the bottom. You gain both ignition timing and ICP reduction with the higher RPM. The small gain in fuel efficiency that you will gain pulling 2200rpm with a 12gph LOP fuel flow vs a 2450rpm with 12gph LOP fuel flow will not be significant. However the extra CHT you will end up with using the lower rpm may be. If you can stay below 350°CHT regardless how you run it, it probably won't make any difference as long as it isn't shaking.

 

[RichNY]

Source?

The last thing I changed was to start running at 2300 instead of 2200 because of a notice from Conti that 2200 can cause the crank to go out of balance over time or somesuch. I haven't seen anything about running at 2400...

Thanks in advance!

TCM SB07-08A

 

[flyingcheesehead]

The one thing people fail to recognize with the Lindbergh doctrine with regards to operating most modern aircraft engines is along with the use of minimum RPM, a speed around 1.3 L/D max was also a parameter. At this speed you limit ICP greatly.

Airspeed? How does changing airspeed affect ICP??

 

[Henning]

Airspeed? How does changing airspeed affect ICP??

You don't need as much power/torque to maintain a lower airspeed. Torque=ICP.

 

[flyingcheesehead]

TCM SB07-08A

Continental's site no longer lists that SB... Is it possible they changed their mind?

The one I was talking about is CSB09-11A, which mentions some 520's that had counterweights let go and recommends 2300+ RPM but doesn't mention lubrication performance at all.

 

[jesse]

Ouch... I think this is probably the worst plan! No knock on you, it's what your average CFI will teach you to do. But, while CFI's seem to know everything when you're a student or low-time pilot, they really don't. And most of them have never OWNED an airplane either, so they don't really care about things like engine management. The seller may not be any better in this regard.

Learn from an engine expert: Read those Pelican's Perch (John Deakin) articles linked to earlier in the thread.

A few things:

1) No reason to back off the MP to 25". Most engines run just as well at WOT and 2500. I do generally back off to 2500 after takeoff to keep the airport neighbors happy. Deakin's articles have more technical info on why it's better to be WOT in the climb.

2) For climb mixture, take a glance at what your EGT's are shortly after takeoff, and then just slowly lean in the climb to maintain the same EGT's. My Mooney has a cheat here: There's an additional single-probe EGT with a "climb" range marked on it. IME, though, on planes without that particular feature, you'll be wanting EGT's in the mid-1300s for the climb.

3) Try 65%. The difference between 65% and 75% for me is only 10 knots, but it costs me an extra 5 gph, and the engine will last a lot longer at 65% (keep an eye on your CHTs, again Deakin has lots of info but you want to keep them well below 400ºF).

4) 75 ROP is going to give you some of the highest CHTs and shorten your engine life considerably. It sounds like you've tried 20 LOP, which is a much better place to be - Why are you "still not comfortable enough to keep that up regularly"?

Hope this helps you make your engine last a long time.

Stupid CFI here...

What Kent writes is good advice, although I would ignore paying any attention to an absolute EGT value because it will vary wildly between aircraft and engine monitor installations. EGT is all about relative. Absolute value means nothing.

 

[Henning]

Sure, I think prohibitions of "over-square ( MP > RPM )" may have been from the radial engine days. I've found big Continentals (470, 520, 550) quite happy 2" "over-square" and LOP (in cruise). Eckalbar has discussed this at length in terms of "volumetric efficiency," etc.

The main concerns that limit engine life are excess fuel flow creating carbon build up on the valves and in the ring lands, and high CHT. Bottom ends letting go are much less common issues. As far as the extra wear and tear across 2000 hrs between 2200rpm and 2500rpm is next to no difference on the rotating assembly from the extra rotation. The reciprocating forces are all still well within the design parameters for the parts used.

 

[flyingcheesehead]

You don't need as much power/torque to maintain a lower airspeed. Torque=ICP.

Okay, but it'd be a lot easier to talk about MP/RPM than ICP, since we don't have an ICP lever. In addition, what you say above only applies for unaccelerated (steady-state) flight.

The easiest way on the engine is to increase RPM until you get a prop tip speed of .9M. That will require the least ICP and provide the least residual heat (which becomes CHT) for the extra power.

Reference?

I can find all kinds of references saying max power out of the prop is around .9M, but none mentioning its effect on ICP or CHT. Max power usually isn't where ICP/CHT are lowest...

 

[ahkahn]

Ouch... I think this is probably the worst plan! No knock on you, it's what your average CFI will teach you to do. But, while CFI's seem to know everything when you're a student or low-time pilot, they really don't. And most of them have never OWNED an airplane either, so they don't really care about things like engine management. The seller may not be any better in this regard.

Learn from an engine expert: Read those Pelican's Perch (John Deakin) articles linked to earlier in the thread.

A few things:

1) No reason to back off the MP to 25". Most engines run just as well at WOT and 2500. I do generally back off to 2500 after takeoff to keep the airport neighbors happy. Deakin's articles have more technical info on why it's better to be WOT in the climb.

Good tip. I did read some of Deakin's articles, and it makes sense why MP is not the end-all be-all of engine performance.

3) Try 65%. The difference between 65% and 75% for me is only 10 knots, but it costs me an extra 5 gph, and the engine will last a lot longer at 65% (keep an eye on your CHTs, again Deakin has lots of info but you want to keep them well below 400ºF).

My engine has been running very cool, even at 75%. I'm under 300 degrees for CHT's.

4) 75 ROP is going to give you some of the highest CHTs and shorten your engine life considerably. It sounds like you've tried 20 LOP, which is a much better place to be - Why are you "still not comfortable enough to keep that up regularly"?

The main reason I am still not comfortable is because the guy that sold me the plane owned it for around 15 years. He ran the old engine up to 2800 hours before overhaul, and the current engine had 500 hours on it when I purchased the plane last month. It tested out at 79/80 across the board. He's been doing something right, and said he runs it between 65-75 ROP, so I am very hesitant to mess with a good thing.

On a different note, he just got back to me an said that ran it at 24/2400 in a hurry and would do 23/2300 at a leisurely pace.

Thanks Cheesehead for your input! Go Pack Go!

I also hope this helps the OP, as well.

 

[flyingcheesehead]

Stupid CFI here...

What Kent writes is good advice, although I would ignore paying any attention to an absolute EGT value because it will vary wildly between aircraft and engine monitor installations. EGT is all about relative. Absolute value means nothing.

This. The temp reference I gave was just an "in my experience" value, what should be used is what you see shortly after takeoff. Deakin discusses this, too.

And Jesse is not a stupid CFI... Jesse is the kind of CFI I wish everyone could have. Unfortunately, it seems like the vast majority of CFIs are only in it to fill their logbook with enough numbers to move on to their next job, and thus they don't really care about things like piston engine management.

 

[flyingcheesehead]

My engine has been running very cool, even at 75%. I'm under 300 degrees for CHT's.

Hmmm... It's possible to run them too cool as well. Most people will tend to run them too hot, not too cool, so the cool end doesn't get mentioned nearly as much. Just be sure you're keeping your CHTs well *above* 250ºF, otherwise the lead scavenging agents in the fuel don't work and you risk fouling things up inside the engine.

The main reason I am still not comfortable is because the guy that sold me the plane owned it for around 15 years. He ran the old engine up to 2800 hours before overhaul, and the current engine had 500 hours on it when I purchased the plane last month. It tested out at 79/80 across the board. He's been doing something right, and said he runs it between 65-75 ROP, so I am very hesitant to mess with a good thing.

I would bet that he flew it a lot - IE, on a regular basis. If you fly twice a week and keep your CHTs below 400 in climb and 380 in cruise, your engine will probably last a very long time. OTOH, if you let it sit a lot and don't fly for 6+ weeks at a time, you probably won't make it to TBO even using "perfect" engine management techniques.

So, go fly! It's good for the airplane!

 

[flyingron]

Source?

http://tcmlink.com/ektron_cf_share/servicebulletins.cfm#

Looks like they backed down from 2400 for the 550 to 2300 for almost all the six cylinder engines.

 

[coloradobluesky]

I really don't know the limit of my Lycoming 0360 oversquare, but I don't go more than 5 difference. IOW, 2000rpm and 25" would be my max (the 5 spread).

The other way, theres no limit. I could go 20" and redline (2800 I think).

 

[flyingcheesehead]

The easiest way on the engine is to increase RPM until you get a prop tip speed of .9M.

Oddly enough, I can't get anywhere near that. At sea-level standard temp of 59ºF, max RPM and normal cruise airspeed, my tips are at about M0.758. At an OAT of -20ºF (at altitude when it's super-cold like it is now), they're still only at M0.823.

Ironically, none of the prop tip speed calculators I found on the Internet are accurate - None of them take translational velocity into account, only rotational. So, they show it as even lower - M0.714 at standard temp.

I did find this interesting article on prop performance, though, and it has all of the correct formulas too, even though their prop performance calculator is one of the incorrect ones : http://www.pponk.com/HTML PAGES/propcalc.html

 

[TUPilot]

I really don't know the limit of my Lycoming 0360 oversquare, but I don't go more than 5 difference. IOW, 2000rpm and 25" would be my max (the 5 spread).

The other way, theres no limit. I could go 20" and redline (2800 I think).

I personally just stick with the AFM power settings on the IO-360... I normally go WOT and max RPM up until 500 to 1000 feet (depends on how noise sensitive the area is) then drop to 25" to 26" and 2500 RPM for climb.

 

[ahkahn]

Hmmm... It's possible to run them too cool as well. Most people will tend to run them too hot, not too cool, so the cool end doesn't get mentioned nearly as much. Just be sure you're keeping your CHTs well *above* 250ºF, otherwise the lead scavenging agents in the fuel don't work and you risk fouling things up inside the engine.

Yep, I've been getting concerned about that. I've been seeing between 250 and 280 degrees, except for cylinder #5 running at 320 degrees. In all fairness, OAT has been between 0 and 20 degrees Fahrenheit, so that may be a contributing factor, for now.

I would bet that he flew it a lot - IE, on a regular basis. If you fly twice a week and keep your CHTs below 400 in climb and 380 in cruise, your engine will probably last a very long time. OTOH, if you let it sit a lot and don't fly for 6+ weeks at a time, you probably won't make it to TBO even using "perfect" engine management techniques.

Your bet is correct! He had been averaging between 200-250 hours per year. His record year was over 300 hours. Flew the s..t out of it!