Leaning for Best Power?
 

The other day I flew in and around the Vic / NSW mountain range. Quite a bit different from the majority of flying I've done as a PPL since obtaining my licence only a few years ago.

As part of this trip I did a stop and go at Mt Hotham (elevation circa 4,300'). Even though this question has occurred to me before, it wasn't until I did the takeoff at Mt Hotham that I truly wondered that perhaps I have missed something in my training.

As I took off to the north west the terrain gently climbs. I was pretty light with half fuel and only 2 pax in a 180hp cessna 172. I realise I was at 4,300' to begin with and density altitude was closer to 6 - 6.5k feet, although was surprised about the climb performance.

So I have two questions I'm hoping some of you very experienced guys / girls out there can help me with:

1) I assume that I should have tested for max power before taking off - by applying full power and leaning until best power is indicated?

2) Assuming the answer to 1 is yes, then when climbing I was always taught to full rich, full power and climb to desired level then power back (if appropriate based on height) then lean. If it makes sense to lean at say a takeoff at a density altitude 6,000', should I be leaning as I climb above 5,000 ft (up to max 10,000' VFR limit)?

Thanks for your help.

If you have an engine monitor, the procedure is:

(1) Take off at sea level on a ‘standard’ day.

(2) Note the EGT of a cylinder (I usually monitor the EGT and CHT of my hottest cylinder) just after take off.

(3) During the climb, lean every couple of minutes to get to the same EGT on that cylinder (no need for absolute precision - close is good enough).

When taking off at higher density altitudes:

(1) Lean to until you reach the same EGT as you noted at #2 above.

(2) Do #3.

This assumes that your engine’s fuel flow has been set up properly.

If you maintain full rich for the entire climb, you are steadily losing power compared with the mixture that is an optimal compromise between power and being kind to your engine.

As the APS people say: It’s now how hard you run your engine, but how you run your engine hard.

Thanks Lead Balloon - I have no individual cylinder monitor, just the general CHT and EGT monitor for the engine as a whole.

In that case, use the single-point EGT reading. Provided you are always using the same reference instrument/temperature - as you are bound to do if you have only have one - it’s better than nothing.

Thanks, I'll give that a go.

I've got nothing, just Tacho and my ears and eyes.

Suggestions LB, please?

Kaz

Where is our old friend Jabawocky..? He's normally all over this stuff like a rash..!

Kaz - full throttle, stand on the brakes, lean until it sounds like it's working better and off you go. Continue climbing - simple.

You *could* climb at full power and full mixture till your desired altitude, however you'll definitely be losing some power wasting fuel by running too rich, so the advice above is good. The approach you've given is certainly a simple method that it sounds like the school has been teaching it's PPL's.

The approach of testing full static power is an interesting one and I'm assuming not many of us would have done that before, or had to do it before, however it does seem quite valid. My only concerns though are that if you're pushing the envelope that far that you need to do this, then you may have been outside the safe and legal envelope for the aircraft performance. Or... you're panicking a little as performance had degraded not that badly, it just possibly scared you as you're not used to that level of performance. Where would you have drawn the line if you didn't get full rpm that you'd get at sea level during your static run-up?


If it's any consolation, I was listening to a couple of 172's with 3-4 pax and camping gear taking off out of the recent Mt. Beauty fly-in, heading east towards the high ground. They were both climbing and one pilot said to the other, "is your stall warning going off yet" and the other replied, "almost constantly" !!! The latter made the decision to turn around, gain some height and then clear the higher ground more safely.

It's always worth checking the POH. From a C172 POH:

Prior to takeoff from fields above 3000 feet elevation, the mixture
should be leaned to give maximum RPM in a full throttle, static
runup.

Also know the best rate and angle of climb, from the same POH 74-72 KIAS best rate and 62-67 KIAS best angle depending on altitude.

(Figures may be dependent on model, check your own POH.)

Lots of useful information in the POH. Its use is very poorly taught.

Mixture use by John Deakin author of The Pelicans Perch.
 

Suggest you read the complete set of engine handling articles by John Deakin author of the Pelican's Perch series. Excellent article there on mixture use.
https://www.avweb.com/news/pelican/182084-1.html

What On eyre said!:ok:

SOP when operating out of mountain strips in PNG.

I'm too new to post a URL! Google "lycoming service instruction 1094 pdf". I found it very useful.

John deakin stuff really helped me when I was flying cessnas around. Good read for anyone in GA

Ironically, although this procedure may well result in the engine delivering around the maximum power it can in the circumstances, that outcome may not be optimal.

When you go ‘balls to the wall’ on a properly set up, normally aspirated piston aero engine at sea level in standard conditions, the engine is not delivering the maximum power that could be delivered by the engine. It’s actually (or should be, if the engine’s set up properly) richer than the mixture that would deliver maximum power.

You could suck more power out of the engine, by leaning. But that would be bad.

The optimal ‘high power’ sea level settings for normally aspirated piston aero engines are richer than the mixture that would deliver maximum power. That’s for the good of the engine.

At aerodromes at really high density altitudes there’s probably no mixture setting that could result in damage to a properly set up, normally aspirated piston aero engine. But at 3,000 to around 6,500’, badness can happen if operating for extended periods at the mixture that produces the maximum power output for the engine.

It's an AUSTER! Mechanical brakes NBG over about 2000 rpm :ok::O

Thanks for that and I seem to be doing it right. Not up in the mountains much but often high on a hot day and have to progressively lean as I climb.

Kaz

[QUOTE=StickWithTheTruth;10019990]

The approach of testing full static power is an interesting one and I'm assuming not many of us would have done that before, or had to do it before, however it does seem quite valid. My only concerns though are that if you're pushing the envelope that far that you need to do this, then you may have been outside the safe and legal envelope for the aircraft performance. Or... you're panicking a little as performance had degraded not that badly, it just possibly scared you as you're not used to that level of performance. Where would you have drawn the line if you didn't get full rpm that you'd get at sea level during your static run-up?
/QUOTE]

I was well within the envelope and I would describe my state of mind as panicking. Climbing out of YHOT with terrain slightly rising at a density altitude of about 7,000', hence climb performance was vastly different from sea level ops, brought to the front of mind my question about leaning as you climb because I never have done it. If it does produce more power without damaging the engine, then a greater climb rate would have given me more peace of mind.

With only a few hundred hours under my belt I still constantly think about the what ifs of an engine failure in my flight planning, before flight on takeoff, just after takeoff and the whole flight! As I said, given the terrain was rising and my climb performance was sub (sea level) standard, my margin of safety (for a glide to an appropriate level) was not increasing as quickly as I am use to.

P.S. You have to be joking (or he was) about the 172 pilot with the stall warning consistently going off?!?!?! I flew into Mt Beauty the other day and back to Porepunkah. There's no way I'd do a straight out departure in any direction except straight out the Kiewa Valley. Circle around the airfield to gain height and enjoy the incredible scenary - most spectacular place I've flown into!

If you’re really interested in getting optimal performance from your engine, only fly aircraft with an all-cylinder monitor (after you’ve read and absorbed the content of all of John Deakin’s articles that Centaurus has conveniently linked for us). I lean during the climb to maintain the ‘balls to the wall’ sea level EGT on my hottest cylinder. With the fuel system and timing set up properly, all CHTs remain (and can be seen on the monitor to remain) below my ‘caution’ temperature of 200c.

Interestingly, the ‘steam driven’ original single point CHT gauge rarely reaches the bottom of the green arc, much less get anywhere redline. I reckon I’d have to get the cylinders to melting point before that gauge would redline.

So LB don't you think your original single point gauge just might be faulty !!

They weren't joking about the stall warning. It was mildly concerning to hear it.

No I don’t.

I know from the sensor location, and the way in which I run the engine, that the bottom of the green is about the right indication.

LB fair enough - so what does the redline indicate ?
I have no argument with multipoint EGT and CHT - have this on my RV. But many older aircraft just had basic CHT and EGT if any at all.
EGT is not the problem - Excessive CHT does the damage. I just lean to give smooth running with hottest cylinder CHT no more than my self imposed maximum of 380 degrees F - considerably less than manufacturers max. EGT is not relevant.
I like to keep it simple. IO-360 with one electronic and one normal mag using above gives me 26 l/hr at economy cruise 2200 rpm 140 kt tas.

I think the redline on the old CHT gauge is 460F (238C). My monitor alarm is set to 200C (392F) - a nice round number that is, like your 380, considerably less than the gauge redline. I’ve seen 210C once, on a very hot day just after take off, but anything over that I’m back on the ground, one way or the other.

Yes - EGT is not the problem, but rather CHT.

However, EGT provides some good reference points that assist in keeping CHT under control. Knowing when EGT peaks is very important, because 40 to 50F rich of that is the mixture at which you will give an engine its hardest (and unnecessarily hard) beating. And to return to the specific issue raised by this thread - knowing the EGT ‘balls to the wall’ at sea level on a standard day gives you the reference for leaning during the climb, as well as the reference to lean to when taking off from a field at higher density altitudes.

26lph and 140kts TAS: A beautiful set of numbers!

What aircraft type are you getting 140knots from at 26lph? almost sounds too good to be true unless you are pulling off LOP and you're quoting figures from 8,000ft+ for TAS.

I get get a TAS of around 135-140 at 9,500ft on 20-21 lph with a 400kg+ payload, just sayin' :-)

SWTT - RV9A And get 140 TAS from 4000 ft up.

So that's your cruise / low power economy setting?

SWTT - correct. I can also stooge around all day at 90 kts burning 18 l/hr (or less). Or go fast 155-160 at 29-30 l/hr.
Bit hard to beat a 9A.

thepilotadviser,

Although I fully agree with the above posters re the usefullness of having a multi-cylinder engine monitor, I gather that you are mainly flying rented club or school aircraft that don't always have an egt gauge fitted? Assuming you don't have a turbocharger, the short answer to your questions 1 and 2 are "yes", to both. On a normally aspirated, fixed pitch aircraft at a high elevation field on a hot day, you won't hurt a thing by leaning until you see the RPM peak. Don't do that down low or with a turbocharged engine though.

What speed were you flying? Flying too fast will also greatly decrease climb performance. If climb performance was an issue, you probably should be targeting around 70-75 KIAS, or even slower e.g. 65 for best angle to clear terrain.

Is there evidence to support the idea that the same mixture always gives the same EGT? (Actually we know it doesn't - one ignition inoperative will raise EGT, detonation will lower EGT, but lets ignore malfunctions.)

I would be slightly surprised if the EGT was always the same - my guess would be that lower RPM, lower manifold pressure and lower air density would each lower EGT to some extent. That is just a guess though, I would be very interested in actual data.

Ah, back in time your single point EGT was adjusted by a screw on the back of the gauge.
You'd turn that to get your particular engine to peak at the star on the gauge face as you're leaning.
Only then does your gauge accurately reflect 25 degrees per division, and only at that point on the guage.
It's probably not like that now. Everyone is so much smarter these days.

Well obviously you’ll get different EGTs for the same mixture if you change MP or RPM (or timing or turn off a mag).

The data comes from the engine monitor and comparative climb performance.

If you take off in an aircraft at sea level with a normally aspirated aero engine at mixture full rich, pitch full fine and throttle wide open, and touch nothing during the climb, the mixture will become more and more rich as you climb, because the manifold pressure becomes lower and lower. The further rich you go the less power you are producing (on this part of the power curve). There is no doubt or controversy about this.

If you instead take off and touch nothing, other than the mixture so as to lean during the climb to get around the same EGT as you got at sea level on take off, you’ll get more power out of the engine than you would if you’d stayed full rich. The CHTs will remain nice and cool. The CHTs on the engine monitor and the rate of climb compared with full rich shows this.

I see it every time I take off and climb to a ‘decent’ height. SOP is I don’t touch the throttle or RPM (unless it’s early in the morning and near a populated area, in which case I’ll pull the RPM back a bit until I’m clear) and lean every minute or so during the climb to get back to the ‘target’ EGT. If I go full rich during the climb, the rate of climb goes down compared with the rate of climb when the mixture is set to achieve the ‘target EGT’.

PS: One correction to the above. Some engines have an altitude compensating fuel pump which automatically leans to match the density altitude. It does, automatically, what I’m trying to do by leaning manually to the ‘target’ EGT.

andrewr -
I was climbing out at about 70-75 knots. Climb was still positive and from memory at least 300 - 400' per minute. As it was quite windy and not head on I wanted to keep some airspeed up my sleeve. I was positively climbing, albeit slower than normal due to terrain also increasing my density altitude.

I have a digital EGT / CHT in my aircraft. I'll give Desert Goats approach a try and see how I go.

In reading these comments which are very helpful thanks, I have noted that I am perhaps not paying enough attention to my CHT on climb out. The old dial is always in the green but my digital one does indicate over 400f regularly on climb out. Perhaps I'mm climbing too hard too regularly?

If you have a look at the article that Centaurus linked: https://www.avweb.com/news/pelican/182084-1.html you’ll see near the end a graph and some discussion about the relationship between strength of the material cylinders are made of and temperature. Protracted periods above 400F are not good for cylinder longevity.

Are you getting those temps even with the mixture full rich?

Thanks Lead Balloon. I plan on reading that article in detail this weekend. Yes, I'm seeing plus 400 with full rich. Ignorantly I have not been monitoring this closely so cannot say for how long or exactly how high I have seen this. Will definitely be paying attention now. Again, in training it was ful throttle, full rich and away you go until you reach altitude. As I now have a new digital CHT to compliment my old dial I am seeing more accurate read outs I guess. The old dial one never gets too close to the red.

Are your head temps sitting above 400 on all cylinders regularly, or just one?

On my aircraft engine, mixture has absolutely ZERO affect on CHT. CHT is about airflow, thus angle of climb and airflow. No matter how lean I set it, the CHT's do not rise, in fact they sometimes drop.

Is this not the norm for a Cessna?

Are you saying that your CHT stays the same whether the mixture is set e.g. 20c ROP versus e.g. 100C ROP? That would be an extraordinary outcome.

The data show that if you continue to lean from around 20c ROP your CHT will start to reduce. That could explain why they “sometimes drop”.

What probe/monitor brand do you have?

It's an MGL TC03 12 channel with high grade sensors.

Hmmmm. And the engine and aircraft type?