Leaning and Engine Performance (split topic)

[jrenwick]

I am just wondering if operational methods could contribute to this problem? I lean pretty much--can you damage exhaust or intake valves by over leaning? I use 100 LL, fly it about 100 hrs annually & run it @ 2500/2600 rpms on trips. Suggestions?

I am not an expert on this, but you can find what I believe is expert advice from Electronics International, a manufacturer of electronic engine analyzers (EGT, CHT and other things displayed as a bar graph). See "The Pilot's Manual for Leaning and Diagnosing Engine Problems" at http://www.buy-ei.com/The_Pilots_Manual_by_EI.htm . Caveat: this is marketing material for their product.
They say that exhaust valve life is a function of the temperatures your engine runs at, and they recommend leaning so as to keep the sum of the two temperatures below 1825 degrees F for each cylinder, and all cylinder head temperatures below 400 degrees F. One way they recommend doing this is to lean to the point where the leanest cylinder is 100 degrees F lean of peak. Another is lean-of-peak operation, where the richest cylinder is running 30 to 80 degrees lean of peak. This is hard to do without an engine analyzer, of course.
As you slowly lean an engine from full rich, EGT, CHT and power output will be increasing. The first of the three to reach a peak and start decreasing is the power output, and this occurs around 100 degrees rich of peak EGT. Next the CHT peaks, and this happens around 50 degrees rich of peak EGT, depending on the engine make and model (Continental and Lycoming have published graphs for these values). It can be dangerous to run lean of the peak power point but rich of peak EGT, because that's where CHT is highest, EGT is close to peak, and exhaust valve cooling is worst.
I have also heard a very knowledgeable engine mechanic in Minneapolis say that at 65% power or less, you can't hurt an engine by leaning, no matter where you set the mixture.
Best Regards,
John

[N1478D]

I have heard radio chatter where the pilot says that he has leaned for altitude, and then he is descending or ascending a couple of hundred feet or so without adjusting his/her mixture, even though they are adjusting the throttle. The way I was instructed, and my interpretation of the operating manual is that anytime the throttle is moved, you readjust the mixture. Same for changing altitude. I have heard more than one pilot say that that is the way they are operating - set the mixture and then have throttle and/or altitude changes. So, there are at least two different leaning methods being used. Don't know enough to say that one method is wrong, or one is more right than the other, but it does confuse the issue somewhat.

[jrenwick]

I fly my 170 with an EI engine analyzer. I notice that whenever I change the throttle setting, the EGT changes: more throttle, higher EGT; less throttle, lower EGT. This makes sense, because EGT is a direct indicator of power.

While leaning my engine, the first cylinder to peak is #5, typically, about 1385 degrees at 2350 RPM. After peak, I richen the mixture to 1285 degrees on #5. With each throttle change after that, I could easily adjust the mixture to maintain 1285 degrees -- BUT -- would the peak EGT have been the same at the new throttle setting? If not, 1285 is no longer the desirable 100 ROP on the leanest cylinder. This should be easy enough to determine, and I'll put that on my to-do list for a future flight. Has anybody else done this experiment?

Best Regards,

John

[GAHorn]

Just to be comparing apples to apples, the Continental cylinders are numbered starting at the one closest to the firewall on the co-pilot's side as number one. The rearmost cylinder on the pilot's side is number two....and so on until the right front cylinder is #5 and the left front cyl. is #6. (All the odd-numbered cyls are on the right side, and the even numbered cyls are on the left.)
The C-145/O-300 engine installations in 170's typically have their #2 cylinders as the leanest running and the hottest CHT. That is why the factory placed the CHT probe on that cylinder. It is also why single-probe EGT's are recommended on that cylinder.
This engine has such an "agricultural" induction system, and such an inaccurate fuel atomization system (carburetor) that digital engine analyzers are overkill for it, in my opinion. (It's the old "measure it with a micrometer, mark it with a grease-pencil, cut it with a hatchet" technological solution.) The mixture control for this installation is so crude that the original set-up (no EGT, utilizing RPM indications with a fixed-pitch prop for leaning judgment) is about the most cost effective method available.
TCM states that best power is obtained 100 degrees rich of peak. The only problem with our installations is "Which cylinder do we run 100 degrees rich?", ...for it's impossible to get a carbureted engine to run the majority of it's cylinders equally as for as mixture and temps are concerned. This is because some of our cylinders are farther away from the carb in their induction pathways than others and on top of that, the rubber-hoses/hose clamp construction of that induction system is less than efficient/accurate than the digital engine analyzers capbility.....It's the same trick of measuring the hatchet-cut with the micrometer.
If we consider that at typical cruising altitudes, (say 2500 or above), the recommended cruising RPM (2450) will result ONLY in power settings of 65% or less..... And that ANY leaning operation performed at less than 65% is not harmful to our engines (per the manufacturer).... Then what instrumentation could we purchase, or what method could we practice, that would do any better than to lean for maximum rpm? (The 170 Owner's Manual recommends to lean slowly until max rpm, then enrichen until the first sign of RPM loss. This will result in the best-power mixture setting that will give the maximum amount of horsepower for that throttle setting.)
There's a popular movement these days toward digital engine analyzers and "lean of peak" operations. There are some knowlegeable folks in the industry that endorse such practices. It works very well with more sophisticated engines such as fuel injected, turbo-charged engines in higher-end installations. In those systems digital instrumentaton can be operated more accurately. The only problem is that in OUR installations (C-145/O-300 engines with fixed pitch props), such instrumentation is unuseable to it's full capability because the installation itself is incapable of such accuracy.
I personally prefer to keep to the original recommendations and instrumentation, and I believe it's fully adequate within the limitations of these airplanes (and it lets me watch things outside the cockpit more.) At least that's my recommendation because I believe that's the most accurate answer to the question of how to lean/operate this engine most efficiently.

[N1478D]

If we consider that at typical cruising altitudes, (say 2500 or above), the recommended cruising RPM (2450) will result ONLY in power settings of 65% or less..... And that ANY leaning operation performed at less than 65% is not harmful to our engines (per the manufacturer)....

This is one of the terms that is confusing to me - 65% power setting. At 2500 feet, we can certainly obtain more than 1755 RPM (65% of 2700 RPM redline), actually, we can obtain the full 2700 RPM. Logically, running at 2450 would be a 91% power setting to me. Pardon my ignorance, sure some of you are laughing, but I really don't understand. Who came up with this power setting scale and what instrumentation did they use to derive it? If it is relative to altitude, then it does not seem very helpfull, because our only power measurement device is the tach. And, related to the subject of leaning, at different RPM settings at whatever altitude, the position of the mixture will be different for each power setting at that altitude, so it seems that it is part of the operation of the airplane to change the mixture setting any time the throttle is changed Am assuming that anytime you are running 2700 RPM you are running at 100% of the engine capability as marked on the tach. Would enjoy all comments, and would really like to be educated on this power setting thing. So, 1350 RPM wouldn't be a 50% power setting?

[lowNslow]

Joe, your prop is nothing more than a couple of small wings, and just like the wings on your 170 the drag (or effort required to move thru the air) increases as the SQUARE of the velocity. I.e. double the speed = four times the drag (effort). Using your math the old C145 would be producing 43 Hp at just an 800rpm idle.

[GAHorn]

Well, here goes a "short" attempt at this subject.
On a STANDARD DAY (59 degrees F, 29.92" Hg baro, at Sea Level) at 2700 RPM our engines are making 100% power (145 h.p.). As density altitude increases, that same RPM results in less h.p. because the engine loses efficiency in thinner atmosphere. Examples: 2700 rpm @ 2500' equals only 123 hp (85%), and at 5000' 2700 rpm equals only 109 hp (75%) and at 7500' 2700 rpm equals only 95 hp (66%).
Since the recommended cruising rpm is 2450, and since at Sea Level that equals 106 hp (73% ), by the time one gets to 2500' 2450 rpm only equals 95 hp (66%). At 5000' it equals only 87 hp (60%).
Charts giving all this information is found in Section V, Operational Data in the Owners Manual, Titled "Cruise Performance with Lean Mixture".

For simplicity's sake, 65% power at Seal level is about 2350 rpm, at 2500' it's 2450 rpm, at 5000' it's 2525 rpm, at 7500' it's 2600 rpm, at 10,000' and above it's unobtainable. (The maximum power charted at 10K and above is 55% which is about 2500 rpm.)

If you are getting 2700 rpm above 7500' in climb or level flight with a standard prop then your tachometer is likely inaccurate.

[N1478D]

Uhmm! That seems like an odd way of measuring power up in the air cruising around. Don't fly at sea level too much, even here in Texas. Guess if you were given the job of coming up with some charts to put in a manual that would be one way of doing it. Probably not going to start thinking that 2450 RPM is any less than 90% of my available power setting, unless maybe I am below sea level in Death Valley. Still have not seen the value of knowing how much horsepower you have at whatever altitude, and then going thru some conversion process, or looking in a chart, to figure out a different base line than 100% at redline, it all still seems relative to whatever power you have available. But, thanks for explaining where it came from.

With my math, you leave the horsepower out of the equation. Who knows how much horsepower one has? And who really cares? At 800 RPM you are using 30% of your available RPM. The panel does not have a horsepower guage in it. If the airplane is at an altitude that only allows 2500 maximum RPM, then that is my 100% power setting at those conditions.

Of course, if you are using MMO, AND you did the hokey pokey, and your dog didn't bite you that morning, maybe you are getting 145 horsepower with your wheels touching the waves. OH, but wait, the salt spray would be clogging up the air filter, but wait, if you had a M & M filter, they do better when they are clogged up and you would be making 150 horsepower. Just trying to be funny. Ran a K & N filter on the diesel truck and after spending the bucks on it I was sure the truck was running even better that second 100,000 miles with all of the dirt on that filter.

[johneeb]

George,
How much horse power is your Ford-9N putting out at 75% on a sea level standard day?
Johneb

[GAHorn]

Answer: Almost as much as my 170!
If we check out the Type Certificate Data Sheet (TCDS) on our airplanes, we'll see that at Sea Level, min/max static rpm, the standard propeller must generate between 2230 and 2330 for the DM prop, or between 2250 and 2350 for the MDM prop. According to the power charts previously mentioned, this equates only to between 83-94 hp (58-65% pwr).
Comparing this to the TCM engine/prop/power-curves it is 118 hp or about 80% power.
Why the difference? Because the cruise charts take into consideration the resultant gains due to the forward speed of the airplane while the TCM charts do not. (Static rpm will increase when the airplane accelerates, and to maintain constant rpm at 2230-2330 will require a reduction of throttle/power.)
In any case, the point is that during takeoff we do not get 145 horspower because the engine only generates that hp at 2700 rpm.
If we were to re-pitch our propellers to obtain 145 hp/2700 rpm for takeoff, then in order to avoid exceeding red-line inflight we'd have to reduce throttle and fly around at pretty low power settings....and that wouldn't get us anywhere very promptly and would cost us a huge penalty in fuel per mile.
(Joe would have to just stay in the pattern, and shoot touch-and-goes all week.)

[GAHorn]

... Probably not going to start thinking that 2450 RPM is any less than 90% of my available power setting, unless maybe I am below sea level in Death Valley. Still have not seen the value of knowing how much horsepower you have at whatever altitude, and then going thru some conversion process, or looking in a chart, to figure out a different base line than 100% at redline, it all still seems relative to whatever power you have available. But, thanks for explaining where it came from.

With my math, you leave the horsepower out of the equation. Who knows how much horsepower one has? And who really cares? At 800 RPM you are using 30% of your available RPM. The panel does not have a horsepower guage in it. If the airplane is at an altitude that only allows 2500 maximum RPM, then that is my 100% power setting at those conditions....

Perhaps one should think of 2450 as being 90% RPM....but not 90% hp. The value of knowing how much hp you have at whatever altitude is: That way you know how much fuel burn to expect with a lean mixture....and how many miles in which that will result (or put another way, how much endurance you'll have at any particular power setting, i.e., MPG.) Percent power means Percent (HORSE)power....not percent rpm. Horsepower is a unit measure of work performed and therefore can be quantified with BTU's or other measures of energy (such as fuel consumption.) Simple RPM measurements cannot (because of all the variables.)
If you really want to know these things, then you might photocopy the chart from the Owners Manual, laminate it, and include it on the back of your checklist (along with the T.O./Landing performance charts). THEN you'll have a really useful checklist!

[N1478D]

Well, I don't think we are talking apples to apples. Fully understand what horsepower is, and understand that it was used to come up with the charts. Once we are flying our airplanes, we don't have a clue as to how much horsepower we have at any given second, all we have is a tach, the view out the window, throttle control, mixture, and airspeed indicator. If the throttle is all the way in and it doesn't look like you are going to clear the trees, then you know you don't have enough horsepower for that situation. If you are crusing along at 5,000 feet at 2450 RPM, you have all the horsepower you need. It seems like an unnecessary complication to include horsepower in RPM settings while in flight because it cannot be measured in our airplanes. 65% power setting sounds like it would be close to 1/2 of your available throttle control, which isn't the case. In less you have a red airplane down in Central Texas, now it makes sense. Have everybody else set their throttle to 65%, and then you might be able to keep up with them. Gee George, hope I didn't blow your plan, that was very clever and bet it was working out for you.

That would sure complicate my fuel burn management system that's in place vs switching to a system to include varying horsepower amounts at different altitudes and atmospheric conditions. Gee, all I have to do now is be on the ground after 3 hours of flight time and fill her back up and am pretty sure she will never be outta gas up in the air. But, let's see, flying along at 9,000 feet, temp is 33, density atltude is 9400, so square the velocity, no, cube root of the . . ., I think I would run outta fuel before I could ever do the math to compute how much horsepower I had, and then it would be changing again. Think I'll stick to the clock, looking at the guages, and inside the tanks.

[mvivion]

I would respectfully point out that the value of a six probe egt/cht isn't restricted to simply allow you to lean your engine more efficiently, though they will do that, even with a carbureted engine.

They will also tell you when one cylinder isn't pulling its share, when things are starting to go bad, and help the mechanics to trouble shoot the problems.

As to being able to even out the EGT's enough to lean effectively with a multi probe gauge, here's a little test:

Take your multi probe equipped, carbureted engine out there and run it up to a "normal" lean. Now pull on partial carburetor heat, a little at a time, and watch the graphic display of the EGT's. Now lean again.

You'll find that with partial carburetor heat on, you will raise the induction temperature enough to help equalize the atomization of fuel, which will help to level the egts off. Not perfect, but closer, and much better for leaning.

This should be done as a regular practice ONLY with a carburetor air inlet temperature sensor, though, because you do not want to raise the carb inlet temp into the icing range, and it's easy to do. The trick is to raise the inlet temp to just OVER the freezing level.

I know, now you are allowing unfiltered air into your carb. So what? You will only do this in flight anyway, and if there's that much stuff up there, you probably aren't flying anyway.

Just some thoughts.

Mike Vivion

[GAHorn]

Hot air is "less dense" air...and so costs you in loss of hp. That's why high density altitudes cause a loss of performance. The proof of that statement is: The hotter air is less dense, therefore contains fewer molecules of oxygen and consequently the mixture is too rich....therefore re-leaning becomes necessary in order to reduce the fuel to more efficient levels. Less fuel equals fewer BTU's equals less power.
It's certainly true enough, a six-probe EGT gauge is certainly more informative than a single probe, as is a 6-probe CHT more informative than a single-probe, especially for troubleshooting. It's unlikely in my opinion that digital gauges are significantly better (for the money) than analog ones in this installation for troubleshooting, but maybe.
As for simple leaning of this engine EGT's (digital or analog) are of no particular advantage. Max RPM is obtainable with or without them.
(This might be a personal-preference idiosycrasy of mine. I had digital EGT in my C-206 and it about drove me nuts, especially at night with the constant "dance" the numbers did. 1455, no-1456, no-1454, no-1458, and on and on and on. I thought it pretty distracting when all I wanted to know was when max EGT was reached and I wanted to run 25-degrees generally richer. But if other gauges in the cockpit were digital, then perhaps it wouldn't have stood out like a sore thumb so.)
I gave it away and bought an analog EGT.

Joe: "...we don't have a clue as to how much horsepower we have at any given second, all we have is a tach, the view out the window, throttle control, mixture, and airspeed indicator."

That's not correct, Joe. We have more than a clue, we have a power chart. And an altimeter, and a temperature gauge. All to be used in order to set power correctly.

Joe: "...If the throttle is all the way in and it doesn't look like you are going to clear the trees, then you know you don't have enough horsepower for that situation. If you are crusing along at 5,000 feet at 2450 RPM, you have all the horsepower you need. It seems like an unnecessary complication to include horsepower in RPM settings while in flight because it cannot be measured in our airplanes. 65% power setting sounds like it would be close to 1/2 of your available throttle control..."

In order to know if you're going to clear the trees before it's too late.....we consult the takeoff/landing distance charts in Section V of the Owners Manual (or read the same chart in the Approved Flight Manual) ...which already/automatically takes into consideration the power charts/density altitude....and give the distances required. Quite right, if you are cruising at 2450 rpm/5,000' you may not be overly concerned about it, ...BUT...it might interest you to realize that (by consulting the chart) you could climb up to 7,000', obtain the same % power setting, take advantage of the winds and save fuel! Without an understanding of that chart you'd not have that advantage, you'd simply be feeling like the victim of circumstances. As for 65% sounding like it should be close to 1/2 throttle,...consider this: If the air at 5,000' is thinner than at sea level, then less fuel can be completely burned in the same volumetric engine cylinders. (The cylinders don't change size, but the density of air inside them sure did.) So since we're burning less fuel, then we're making less power. You'll have to push that throttle MORE than 1/2 way in just to maintain the SAME power.
And THAT's the value of the charts, to give us a way to determine/predict such things.
If you continue to just use RPM instead of relating it to the power charts, Joe, ...you'll never keep up with the rest of the fleet!

Anyways, simple RPM alone is not the measure of % power any more than simple passenger count is the measure of take-off weight.

Scenario: "Let's see now, last winter when I took off from this location I cleared the trees by 20 feet, and I only had full fuel and 3 little kids with me...that's 4 people total, right? So I only have 4 people today too! So no problem! (Must not be important that they're 4 fatties and the temperature is 100 and the density altitude is 7,000. This engine will still make 2350 rpm for takeoff, so the power must be the same! HP doesn't count...only RPM counts! Right?) H e r e w e g o o o o !! %@%$#%$# TREES!!!!

[PostScript: Just FYI: There actually IS a guage that will show a direct readout of power, called a BMEP gauge. It was found in the big old airliners of a bygone era, the DC-6's/7's, Connies, etc. It utilized an oil pressure transducer which read torque at the propeller, and had automatically compensated for temp/altitude (density altitude) and gave a direct reading which I think was referred to as Brake Mean Equivalent Power. Much too complicated and expensive an installation for a C170 though.]

[N1478D]

Come on now Geoge, look outside of your paradym!

First off, you are starting off all of your calculations using the imaginary number of 145 horsepower. Even if our airplanes were new, how many of them would have exactly 145 horsepower?

Second, I have never seen a pilot looking at the trees, whip out his calculator or chart and say, "OK, I have 122 horsepower, that will do it!". Owners practice and know their airplane and it's capabilities. Renters look at charts and they are either not allowed by the rental agreement or they just don't know the airplane well enough to go to tight places.

Cessna was probably required by the regulations to provide charts. Even with interpolation for the different atmospheric conditions we fly in, the charts would have a hard time accomadating all of the different ways airplanes are maintained, fuel quality, prop nicks, tire sizes, etc, etc. Those same charts say it takes 3 times as much runway to land than it really does.

It's like the accountant that eventually comes to believe that the real reason every one is there at the corporation making widgets is to help him count the widgets, not to make them. Those charts are just documentation George, you don't have to defend them. Take a survey and find out how many owner pilots refer to them.