How to decide MP and RPM settings

coloradobluesky said:
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).
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The manual for my old K35 Bonanza (Continental IO-470) had an easy-to-read chart showing the minimum allowable rpm for a given manifold pressure. That one didn't recommend less than 1900 rpm @ 22", or less than 2200 rpm @ 24".

No such chart that I can find for the O-360. The POH for the C-172RG (O-360-F1A6) doesn't state the limits per se, but the performance charts don't list any power combinations more than 2" oversquare.

I bet few pilots of airplanes with fixed-pitch props realize how much "oversquare" they operate on nearly every takeoff and climb ...

:eek: :D :yikes:

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I've been told that the pressures in the cylinder go up with higher manifold pressure, but I don't have any proof of that. It is getting more air and more gasoline at higher throttle. Higher throttle means higher manifold pressures. Makes sense to me.

My dad (and he had been an automotive engineer) used to tell me the least wear on my car engine was to put it in top gear and lug it at low speed. You get the best gas mileage that way. That was with a manual transmission of course. Also the least wear way of warming the engine up. Least rpms win.
 
I go WOT until cruise altitude and then pull everything back at once, throttle, rpm and lean (in that order). Then I check ALL my settings, flap settings zero, all the knobs and gizmos get touched and verified
 
ahkahn said:
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.



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



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.
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This is surprising to me. I ran a saratoga for quite a few years and I wouldn't see CHT's below 300 unless I was LOP or extremely rich. 75* ROP would get me 350's to 370's at 65% at altitude.

Eggman
 
flyingcheesehead said:
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.



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...
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You have three ICP levers!:lol:
 
What do you have for instrumentation for measuring the CHT? If it's what came with the plane I'd have little faith in it being accurate to any degree and would suggest you get a proper engine monitor.
 
jesse said:
What do you have for instrumentation for measuring the CHT? If it's what came with the plane I'd have little faith in it being accurate to any degree and would suggest you get a proper engine monitor.
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Not sure if you're asking me, but I had a JPI EDM-700.
 
ahkahn said:
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.



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!
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You might consider this...

Below 8000 feet, no significant headwind: 2400 RPM, MP at 75% per POH for best power mixture (or less). Lean until all cylinders are at least 30F lean of peak. This will give you an actual HP output of something around 60%. For a very low fuel burn.

Below 8000 feet, headwinds: Go ahead and run it hard, 75% power and all cylinders at least 100F ROP. Monitor CHT's

Above 8000 feet, WOT, 2400 RPM and 50F ROP. That is a sweet spot for higher altitude cruising where you want to make as much HP as possible without the need to burn extra fuel for cooling and detonation avoidance.


I'm not a Lance pilot. Does it have cowl flaps? IMO The ideal cowling situation would have you needing to crack the cowl flaps slightly open on a normal temperature day to keep the engine cool enough (all CHT's below 380) when running at best power, 75% mixture.

On the same day, the LOP economy cruise should stay cool enough with the cowl flaps closed.

From what I understand, the more airflow you have through the cowl, the more drag on the airplane. If your engine is cooling too well, you might be leaving some airspeed on the table. Something to evaluate when it gets warmer this spring.
 
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coloradobluesky said:
I've been told that the pressures in the cylinder go up with higher manifold pressure, but I don't have any proof of that. It is getting more air and more gasoline at higher throttle. Higher throttle means higher manifold pressures. Makes sense to me.

My dad (and he had been an automotive engineer) used to tell me the least wear on my car engine was to put it in top gear and lug it at low speed. You get the best gas mileage that way. That was with a manual transmission of course. Also the least wear way of warming the engine up. Least rpms win.
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Manifold pressure and cylinder pressure are not the same thing and do not go hand in hand.

Attempt to oversimplify:
Manifold pressure tells you, how much air is waiting to get into the cylinders at any given time.
This pressure is then converted into cylinder pressure by adding fuel, ignition and compression. The cylinder pressure is then converted into rotational force = torque.
You can increase or decrease your cylinder pressure for any given manifold pressure by your mixture lever, and to some extent RPM lever. Fuel is much more important than RPM though when measuring ICP since we cannot control ignition angle (why oh why??!!), assuming your camshaft and cylinder head flow are roughly in the ballpark for the rpm range.(flat torque curve)

I believe your father was an automotive engineer a long time ago, The low speed myth was alive in the 70's. We know way better nowadays.
 
Ive also heard the best way to warm up a cold engine is to run it a minute, leave it off for 5 and then restart it. The heat from the one minute warms things up after soaking through the metal. Now that one Ive never done -- my starter only has so many starts!

But I do see a lot of pilots start up, taxi, runup and takeoff without getting the oil in the green, or even to register at all! I taxi around for 5 minutes to warm it up myself.
 
Here is a tip:

Fly the airplane at 6500 feet full throttle at 2400 rpm, let it stablize and check fuel flow and TAS.

Then increase the rpm to 2500. Check the same.

Then increase to 2600 [most POH prohibit sustained ops this high- but check anyway] then drop it down to 2300 - and do the same thing.

You will notice a significant change from 2400-2500 only in terms of fuel flow. You MAY gain 2, maybe 3 knots, but you ll see at least a 10% iincrease in fuel flow.

The sweet spot is where the motor runs smoothest - for me is 2425 rpm on the Horizon tach. Much less vibe and harmonics than at 2400 or 2500.

If I'm down low I'll run it 25" and 2300rpm just cause is smooth and running a little cooler and with less fuss and bother and quieter.

You'll notice that the difference between WOT @ 2400rpm and 24" and 2400rpm is negligible when it comes to fuel flow. When you get above where the throttle matters 7000' or so - then you should just set it for the smoothest RPM and WOT.


You will also notice that the difference between max sustained power in many motors and the 2400rpm 70% or so power is measured in fuel flow and not speed - you'll be up in 17gph and getting 155ktsa in a Lance, whereas WOT and 2400 you'll see 151. and 14gph - you're reducing your operating cost $15 an hour and losing a whole 4mph. . . . .

My Comanche shows me 160ktas @ 2500 rpm, WOT and 16.5gph - whereas if I drop it down to 2400rpm and WOT at max power I can lean to 14gph and see 157ktas - sometimes in warmer air 13.5. i'm saving 2.5gph and losing 4 mph or so. I not running it WOT and 2500 EVER.
 
As you lower the rpm, manifold pressure goes up without adjusting anything else. You leave the mixture the same does the ICP go up?

The ICP pressure, that's BEFORE the combustion in the cylinder right? The pressures DURING combustion must be HUGE! Or is it the pressure during combustion? I dunno.
 
coloradobluesky said:
Ive also heard the best way to warm up a cold engine is to run it a minute, leave it off for 5 and then restart it. The heat from the one minute warms things up after soaking through the metal. Now that one Ive never done -- my starter only has so many starts!

But I do see a lot of pilots start up, taxi, runup and takeoff without getting the oil in the green, or even to register at all! I taxi around for 5 minutes to warm it up myself.
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No, that obviously is not the best way to warm anything up.

Do you boil water by cutting out the burner after a minute, leave it off for 5, and then start again?
 
coloradobluesky said:
Why not? You are putting the maximum amount of fuel and air in the cylinder (this causes the maximum amount of heat to be produced) and turning the minimum number of rpms thus heating the cylinder up in the least number of rpms, doing the least amount of damage by running a cold engine vs a hot engine.
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No, that does not cause the maximum heat. Also, you do NOT want to produce maximum amount of heat when you are warming an engine up.
Your comment assumes 100% hydrocarbon combustion, which does not happen.

You also forget things like friction, piston clearance/slap etc.
 
Maximum amount of fuel produces maximum amount of heat. That one is pretty basic.
 
coloradobluesky said:
But I do see a lot of pilots start up, taxi, runup and takeoff without getting the oil in the green, or even to register at all! I taxi around for 5 minutes to warm it up myself.
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Depending on the gauge, minimum oil temperature spec by the engine manufacturer might not register on a crappy 1980's cessna (piper, whatever) gauge.

I flew up to the great white north this weekend, the JPI said the oil temp was a balmy 90F while the stock gauge was pegged out cold.

Above maybe 20F, starting and letting it idle at 1000 RPM for 2 min, then low power taxi, pre takeoff checks and then run up should be enough running to get the temperature warm enough for a full power takeoff.

Below that temperature you should really be pre-heating
 
James_Dean said:
This is surprising to me. I ran a saratoga for quite a few years and I wouldn't see CHT's below 300 unless I was LOP or extremely rich. 75* ROP would get me 350's to 370's at 65% at altitude.

Eggman
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I have an EDM-800, but also have a LoPresti Howl Cowl which may also be making a difference.
 
coloradobluesky said:
Ive also heard the best way to warm up a cold engine is to run it a minute, leave it off for 5 and then restart it. The heat from the one minute warms things up after soaking through the metal. Now that one Ive never done -- my starter only has so many starts!

But I do see a lot of pilots start up, taxi, runup and takeoff without getting the oil in the green, or even to register at all! I taxi around for 5 minutes to warm it up myself.
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If it's running, I won't shut it down, but if it fires the prime load then dies, I will give it a minute or two for the heat produced to soak its way out, then reprime with even more fuel and get it to run in a couple of blades.
 
Bobanna said:
,
No contest about this (^), but I thought we were talking about engine efficiency in various RPM and MP settings. BSFC increases at lower engine speeds, propellers are more efficient at lower speeds, LOP is actually beneficial (for reasons that you adduced) at lower engine speeds, etc. Temperatures are also easier to control, generally. Of course, numerous other factors affect engine longevity, but my remarks represent the benefits of the (putatively destructive) over-square operation. Blessings
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Efficiency is an overall effect of cost per mile as well as just fuel consumption. While the increase in BSFC at minimum RPM may save you another 20¢ a mile over running max RPM, if it costs you a set of cylinders or valves before overhaul. That may not be a savings.
 
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So, when you get up to high alt w/ a normally aspirated plane (say, 10-14k) of course you'll be running WOT, but would it be acceptable to run the prop at 2500 or even 2600? (Talking a Comanche here). Still certainly below 65% at that altitude.
 
dans2992 said:
So, when you get up to high alt w/ a normally aspirated plane (say, 10-14k) of course you'll be running WOT, but would it be acceptable to run the prop at 2500 or even 2600? (Talking a Comanche here). Still certainly below 65% at that altitude.
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Yep, run redline if you want so long as it's smooth.
 
What are cylinder pressure curves?

For some general reading about manifold pressure here's a good primer.
http://www.advancedpilot.com/downloads/prep.pdf

So we know that the higher the MP, the better the engine is breathing, or the better it's packing air into the cylinder. Assuming we maximize the combustion process by adding just the right amount of fuel we can best utilize that air. It is not a factor of adding more fuel because that doesn't necessarily make more power. With the available air we need to meter the fuel to get ideal mixture for the desired result, whether power, heat control, etc. Coincidentally we're limited in our control of power. Our airplanes are rated for max power at relatively low RPMs yet we have to run balls to the wall to achieve the max rated RPMs. The only way we can demonstrate it is by maximizing MP at the target RPM. Othwise referred to as WFO (wide open).

Engine break-in instructions always advise to use high power settings. The reason is to assure higher cylinder pressures which forces the rings against the cylinder walls and helps to seat rings. Lower pressures are known to promote cylinder glazing. I believe this idea is universally accepted and is probably the best demonstration of high power/higher MP resulting in higher cylinder pressures. Wanna make that interesting? Break in a Cub motor using a climb prop. No control of MP and by virtue of a long flat prop the cylinder pressures aren't as high as they would be with a coarse cruise prop. And the rate of glazed cylinders shows it.

More good reading. EPI has some excellent engine theory articles.
http://www.epi-eng.com/piston_engine_technology/volumetric_efficiency.htm
 
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One more thing. To the notion that reduced RPM reduces wear? I got rid of my Hobbs Meter a long time ago. I manage my airplane by tach time. Tachometers are made to clock tach times appropriately for the aircraft they're installed in. Tach time accrues relative to your airplane's average operating RPM. In my 180 an hour at 2300 will register one hour tach time. An hour at 1250 will read as a half hour tach time. Over the years it takes to run your engine out you can earn a few bonus hours by operating at lower RPMs but it won't add up to much. My old and wise mentor taght me to set my power settings by my butt. Feel the airplane. Adjust the knobs to make the least amount of vibration. That'll extend the life of the engine and the airframe.
 
Unless you operate in destructive parameters or let the engine rot from neglect, the bottom end of any modern GA recip is good for 5000hrs. Here's how an engineer who designs engines put it to me when I asked him about duty ratings: "It's like this, the engine is good for XXXXX gallons of fuel, put it through there in a 1000 hrs or 10,000, you'll be due to go through the engine."

You don't really change the stress levels greatly until you start getting above 4200rpm, above that you are causing increases in wear and tear, but truly most bottom end damage and stress failures come from low power and high speed in descent. When you start spinning the engine with the prop and the throttle is closed, you greatly increase the stretch forces on the rod bolts. This is the most common cause of catastrophic bottom end failures of recip engines. Every engine I exploded at the drag races exploded after the trap when I lifted my foot.

The difference between 2000rpm and 2700rpm in LOP cruise as measured in wear and tear across 3000hrs of steady use will be negligible.
 
coloradobluesky said:
yeah, hang science. Go by the old guy.
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My old friend had 40,000+ hours. Owned the same 180 for 45 years when I did my last BFR with him. He had tons of cool stories, like flying a DC-6 in heavy clouds in the Juneau area and had all 4 engines quit. Those were fun BFRs. I miss him. He knew more about engines and airplanes than anybody I've met but you still have time to impress me. :wink2:
 
I don't really know the answers. But conducting experiments and getting results and analyzing them is the way to find out. I suspect that a lot of stuff we deem important doesn't matter that much.

What is intuitive is not always right. Take low rpms in a car for example. Going up a hill, its intuitive to shift down and get higher rpms. The car is more responsive, has better acceleration that way. But for maximum efficiency, less wear on engine, less fuel consumption, the best thing to do is leave it in high gear and floor it. It might not matter all THAT much, but over long term it will.

Planes are different, theres not as much accelerating, stop and go. WE tend to fly at a contstant speed for a long time....

Interesting stuff though
 
coloradobluesky said:
I don't really know the answers. But conducting experiments and getting results and analyzing them is the way to find out. I suspect that a lot of stuff we deem important doesn't matter that much.

What is intuitive is not always right. Take low rpms in a car for example. Going up a hill, its intuitive to shift down and get higher rpms. The car is more responsive, has better acceleration that way. But for maximum efficiency, less wear on engine, less fuel consumption, the best thing to do is leave it in high gear and floor it. It might not matter all THAT much, but over long term it will.

Planes are different, theres not as much accelerating, stop and go. WE tend to fly at a contstant speed for a long time....

Interesting stuff though
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Until it rattles and knocks a hole through the piston (Ben, care to provide your pic?). There are limits to what we can do. The best gauge of how close to that limit we are that we currently have in use is CHT. CHT tells you how much excess heat you are producing. If you can keep CHT down around 340°, you are doing well. If you are doing this LOP, you will make TBO without mechanically needing to top the engine (there is always an AD wildcard.:rolleyes2:). If you do it ROP, you will have valve and seat issues as well as ring washout/seal issues mid time from all the carbon deposits and excess raw fuel.

If you are pushing hard with a 370° CHT, you are pushing as hard as you really want to be pushing. Any gains above that are not worth the costs in fuel or reliability. Every doubling in speed requires a square increase in power, and you add a factor on top of that since prop slippage is a percentage factor.

Most Naturally Aspirated planes don't have a big problem with this as power drops with the atmosphere's ability to take away the heat. Turbo engines up high often CHT is limiting.

BTW, I had not been able to break 370° CHTs running 10°LOP/WOT/2675 RPM in the 310 even in climb in a Florida summer.
 
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Cylinder pressure curve would be a graph of the pressure in the cylinder. You could vary the throttle and the rpm a see the effect.
 
flyingcheesehead said:
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 :rofl:: http://www.pponk.com/HTML PAGES/propcalc.html
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On the RV-12 with the 912IS and Sensenich 2A0R5R70E, you would have to be doing 7800 engine RPM (3210 prop RPM).
Dunna thin Rotax would be agreeable to that, altho' it's ok with Sensenich
 
coloradobluesky said:
Maximum amount of fuel produces maximum amount of heat. That one is pretty basic.
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not true - only if it burns it all. . . .if there is excess fuel [as when the mixture is too rich for stochioastic combustion - then you're cooling the cylinders with the excess fuel.
 
Stewartb said:
My old and wise mentor taght me to set my power settings by my butt. Feel the airplane. Adjust the knobs to make the least amount of vibration. That'll extend the life of the engine and the airframe.
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coloradobluesky said:
yeah, hang science. Go by the old guy.
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I'm not sure science would disagree with the old guy here. Vibration can cause a lot of damage over time.
 
coloradobluesky said:
Cylinder pressure curve would be a graph of the pressure in the cylinder. You could vary the throttle and the rpm a see the effect.
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The best indicator you're going to get is divide torque by surface area and apply whatever factors are required. Either that or you're going to need to put a live pressure gauge on each cylinder. If you work off peak torque using a strain gauge on the mount, you can get a lot of performance tuning information.
 
Henning said:
The best indicator you're going to get is divide torque by surface area and apply whatever factors are required. Either that or you're going to need to put a live pressure gauge on each cylinder. If you work off peak torque using a strain gauge on the mount, you can get a lot of performance tuning information.
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How do you measure torque on an airplane engine installed in the aircraft?

That strain gauge you're talking about would depend on a calibrated engine mount (strain gauges measure distortion, not force or torque), and measures thrust, not torque. You can apply a lot of torque to a prop spinning at 5000 RPM, but it won't make much thrust if the prop is stalled.

If you truly want to know peak torque, it's rather precisely peak RPM for a fixed pitch prop. For a constant speed prop, the best measure is related to energy production (EGT), calibrated by the airspeed indicator. I.e., find the EGT that gives you the best indicated airspeed. If you relate it to peak EGT, it will be less sensitive to remaining variables like OAT and altitude.
 
Look at what the aircraft POH recommends? That's my starting point learning a new aircraft.
 
Blueangel said:
Look at what the aircraft POH recommends? That's my starting point learning a new aircraft.
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A lot of POHs are written with terrible recommendations for power settings, though - The manufacturers need to hit the cruise speed numbers the marketing department is using!
 
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