Why Full Rich On Take Off?

Geico266

Geico266

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Geico
Long story but, I had the opportunity to talk to one of the founding partners at Ly-Con, CA. I asked him several questions about their IO-360 220 HP engine. Specificslly , can I run 91 octane mogas in that engine and the answer was yes. Another question was why do you need to run full rich on take off? He really could not answer it except to say " Just do it".

So I will ask the masses.... With today's engine monitors measuring every hole why do you you need to land or take off full rich? I can monitor each EGT and see that they are no where near peak, more like 1200f. If I run full rich on take off I'm at 1125f. :dunno: how does running the engine at full power on take off at with an EGT of 1200f hurt the engine? :dunno:

What say you?
 
The prevailing theory holds that extra fuel absorbs heat through evaporation. The CHT is what is controlled that way, not the EGT.

Some engine/carb/injection setups run way too rich, though. The R182 is a good example of that. Cessna specified a certain fuel flow for takeoff power and it's so rich that it will make smoke at higher DAs. It makes the engine really rough with the carb heat on, too. Wastes a lot of fuel.

Dan
 
Dan Thomas said:
The prevailing theory holds that extra fuel absorbs heat through evaporation. The CHT is what is controlled that way, not the EGT.

Some engine/carb/injection setups run way too rich, though. The R182 is a good example of that. Cessna specified a certain fuel flow for takeoff power and it's so rich that it will make smoke at higher DAs. It makes the engine really rough with the carb heat on, too. Wastes a lot of fuel.

Dan
Click to expand...

I monitor each CHT also. They never get above 350F.
 
Geico266 said:
I monitor each CHT also. They never get above 350F.
Click to expand...

That may be what is reading at that one particular location, however the issue is what to use in those extremely high pressure/friction situations over the entire engine system. The only currently available response - to keep the "shock" heat/cool to a minimum - is to use the fuel to do that.
The temperature variances will even be more evident with a lower octane fuel because your cylinders will have more unspent fuel that will retain heat but not disperse it in combustion or cause detonation.
It's simply a way to keep metal fatigue (stretch/shrink) to a minimum, reduce hot-spots specifically where those temp sensors can't see.
 
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In the auto world, you want rich mixtures under full throttle and high loads to prevent detonation. I can attest that detonation is the fastest way to ventilate an engine block. I would assume the same physics applies to airplane engines.
 
Running lean without load on an engine is one thing but upon climb you'd have a lot of load on the engine. Detonation tends to occur under load only. How would you know how much to lean an engine before you put the load on it?

I build turbocharged automotive race engines and we run them very rich 10.0 to 11.5 to 1 afr because if for some reason under boost the wastegate doesn't keep up you need that cushion of AFr being rich. naturally aspirated under load we'd run more like 12.5 afr. Under cruise this can be much leaner even into the 16.5 far range for fuel economy.


Going full rich on take off presumably would offer some cushion under load. At least that's my take on it. However Im looking at it from automotive race car point of view. It'd be nice to hear from an A&P on this question
 
I assume you mean full rich when taking off from below about 3000 ft msl?

My understanding from various mountain flying guides is that, absent POH guidance, in a normally aspirated engine one should try to lean for best performance at full throttle before taking off. Sparky Imeson's Mountain Flying Bible even suggests doing this leaning during the takeoff roll, but keep it a tad rich.
 
What do you want the engine & aircraft designer/manufacturer to say? What instrumentation is available and what is the pilot trained to do? What are the liabilities?

Answer those questions and you will have your answer.

And of course we don't use full rich for max power at high density altitudes with normally aspirated engines. The engine would be overly rich compared to the design point of sea level and the engine will make less than spec power at max power since less oxygen is available for combustion. The engine also "sees" lower pressures in the combustion chamber (lower power also) so the detonation margin is larger. The gasolline sellers take advantage of that and sell lower octane gasoline at higher altitude gas stations.
 
It is easier to say to run full rich on takeoff than it is to explain that you need to be at full rich above a certain power percent. Then you have to assume that every pilot that is going to be using your engine understands that power percent varies with DA and that they know how to calculate that. And once you open the door to leaning on takeoff, then you will inevitably have the guy the leans just a little too much but still manages to get it off the ground but maybe not over top of those trees at the end of the runway.

It is much easier to say "run full rich at takeoff".

That's my story, and I'm sticking with it (until Henning explains why I am wrong).
 
Turbo69bird said:
Running lean without load on an engine is one thing but upon climb you'd have a lot of load on the engine. Detonation tends to occur under load only. How would you know how much to lean an engine before you put the load on it?

I build turbocharged automotive race engines and we run them very rich 10.0 to 11.5 to 1 afr because if for some reason under boost the wastegate doesn't keep up you need that cushion of AFr being rich. naturally aspirated under load we'd run more like 12.5 afr. Under cruise this can be much leaner even into the 16.5 far range for fuel economy.


Going full rich on take off presumably would offer some cushion under load. At least that's my take on it. However Im looking at it from automotive race car point of view. It'd be nice to hear from an A&P on this question
Click to expand...

Sent you a PM. Thanks!
 
Who said you "had" to t/o full rich? It is recommended as stated above and for very good reasons.
Most importantly very high ICP (internal cylinder pressure) is developed on take-off. The FAA requires a detonation safety envelope for every engine that is controlled by mixture. Full rich accomplishes the safety without monitors or instrumentation. Provided there is no problem with timing or fuel flow, full rich and full power will not cause detonation and/or pre-ignition.
If you have the proper equipment and are able to monitor and react to the data it provides it is possible to operate the engine at less than full rich, just remember, high pressure and high temperature can put an engine into detonation in a matter of seconds. A cylinder can go into thermal runaway, melt and destroy itself and possibly other parts of the engine in less than one minute.
Please be careful........
 
When I was becoming familiar with my turbo 182, on a x-country, new to me, field, I had leaned on the ground, but apparently not enough to make it cough on TO. And, so I did inadvertently take off with the mix leaned so. After lift off, Climb rate was less than normal, but it was climbing. Even so, I was concerned something was not right. Scanning everything, I caught my mistake and promptly pushed in the mixture. Ah ha!, more power, and it began climbing like normal, all before reaching the departure end of the runway.
 
checklist, checklist, checklist....
You also would have caught that if you routinely time your t/o roll.....
 
bbchien said:
checklist, checklist, checklist....
You also would have caught that if you routinely time your t/o roll.....
Click to expand...

Wouldn't the time vary by DA (for NA engines) and also by weight?
 
above 70-75% power most engines lose the ability to carry away heat at a "best power" mixture setting. The fuel is to help with cooling...

.however I have always taken DA into account at takeoff...I always use about 90% of full rich at an average 1500DA where I take off. Sometimes the rpm gain in doing this can be 50-75
 
bbchien said:
checklist, checklist, checklist....
You also would have caught that if you routinely time your t/o roll.....
Click to expand...

That was about four years ago, with just a few hours after a 35 year break. And, I forgot to mention that my instructor, who normally teaches in a 172, either didn't notice anything wrong, probably because it was climbing more like one of those, or he was just waiting to see if I caught it. 300 hours later, I haven't done anything like that since!
 
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Lots of misinformation and old wives tales in this thread.
When you are at WOT, with the prop at highest RPM, your VSI is your horsepower meter, lean to highest reading to get best horse power.
Works at any altitude, and any engine.
Manufactures want you at full rich because they know that method is dummy proof.
 
Geico266 said:
He really could not answer it except to say " Just do it".
Click to expand...

I bet he could answer the question, he just chose not to. It may be due to liability concerns and to avoid the lengthy conversation that would follow. There is a lot more to the subject than a person could impart in a short conversation.
 
The 170 guy said:
Lots of misinformation and old wives tales in this thread.
When you are at WOT, with the prop at highest RPM, your VSI is your horsepower meter, lean to highest reading to get best horse power.
Works at any altitude, and any engine.
Manufactures want you at full rich because they know that method is dummy proof.
Click to expand...

So what proof do you have that there is "lots of misinformation" in this thread?

You forgot about heat by the way. It's not just about getting the highest horse power.
 
Yes, but I'm teaching the normal home port summer condition. You have to have SOME useable benchmark for near adequate performance.
 
Increasing use of engine monitors helps leaning To best power, which helps pilots maximize power in certain regimens. I use mine mainly in the climb when I reach about 4000 to start leaning aggressively to maintain the best climb rate since the climb is where we are the slowest and need to get to the 160knot cruise as soon as I can.
 
I use Mike Busch's procedure of using the engine monitor to lean during climb by maintaing the egt reading at TO power levels. As you climb and DA increases, by remaing full rich your egt's will drop due to an over-rich mixture. When they drop a couple bars, back out the mixture to return them to TO power levels. The procedure maintains power, and keeps cht's under control by not over-leaning the mixture, without the rigid " no leaning below 5k" rule of thumb.

Seems to work pretty well for my carbed Comanche.
 
Here's some fun homework if you like to dig in your POH... Some may have this data some won't.

Cessna 182P POH does.

1. When is it approved to operate continuously at peak EGT? What's the benefit?

2. What's the leaning technique used to determine the power tables in your POH?

3. While not listed in the Limitations section of the POH, what note about leaning is a printed limitation of mixture control on a Cessna 182P? (Hint: Normal Procedures Section of the POH.)

4. At my home airport (KAPA) what percentage of break horsepower is my O-470S producing on a standard temperature and pressure at full power? How about a 75 degree day?

5. Applying knowledge from question 1 and question 5... How would you approach leaning for a summertime takeoff here? Do you see a safe conservative method that provides plenty of detonation margin?

6. Extra credit, now it's a 32F day? Any difference?

Google "Cessna 182P POH", there's a number of free copies on the Net in PDF format if you want to play along. Or answer from your own POH.

I love POH diving. The answers are hidden right there in plain sight in the numbers. Think. :)
 
Engine monitor or not. Full or high power with a mixture set around max EGT is still in the "red box". Most planes don't allow engines to be run with max EGT until full throttle only achieves around 60% power. This of course is all in a general sense ..many variables with engine type
 
BobThePilot said:
So what proof do you have that there is "lots of misinformation" in this thread?

You forgot about heat by the way. It's not just about getting the highest horse power.
Click to expand...

All the proof I need is in Cruiser's post, It is very apparent that there are pilots here that do not understand LOP operations.

But I can't speak for the 170 guy.
 
Indeed HEAT (CHT!). I live in Florida and in the middle of the summer full rich is what it takes to keep CHTs below 380 in my Mooney on all cylinders on takeoff and initial climb to 3000. After that I start leaning. I have a JPI730 and have tried various combinations. Full rich to 3000 works best for me to keep all CHTs in the happy range.


Sent from my iPad using Tapatalk HD
 
Fwiw ,The latent heat of atomization is what removes heat in the intake tract, it works great on carbureted engines, but much less so with fuel injection.

I often wonder why I don't see AFR gauges and wide band O2 sensors in planes, Im guessing The LL fuels. EgT gauges are obviously seen instead.
 
Tom-D said:
All the proof I need is in Cruiser's post, It is very apparent that there are pilots here that do not understand LOP operations.

But I can't speak for the 170 guy.
Click to expand...

what did I state that was wrong? Please correct the misinformation for my benefit and others on this thread.

thanks,
 
Cruiser said:
what did I state that was wrong? Please correct the misinformation for my benefit and others on this thread.thanks,
Click to expand...

You said…..

Who said you "had" to t/o full rich? It is recommended as stated above and for very good reasons.
Most importantly very high ICP (internal cylinder pressure) is developed on take-off. The FAA requires a detonation safety envelope for every engine that is controlled by mixture.

Got a reference for that?

Full rich accomplishes the safety without monitors or instrumentation. Provided there is no problem with timing or fuel flow, full rich and full power will not cause detonation and/or pre-ignition.

That is the condition that will cause Detonation when the RPM is to low to allow the piston to move down the cylinder and relieve the pressure. think why you lower the MAP before you lower the RPM.


If you have the proper equipment and are able to monitor and react to the data it provides it is possible to operate the engine at less than full rich, just remember, high pressure and high temperature can put an engine into detonation in a matter of seconds.

Not when the RPM is high enough. this will not happen.


A cylinder can go into thermal runaway, melt and destroy itself and possibly other parts of the engine in less than one minute.

That is absolutely ridiculous. What is a thermal run away in recip? thermal runaway happens in nicad-batteries, and nukie plants, not in recipe engines.


Please be careful...…..

End quote

You need to obtain a copy of the manufacturers engine operation manual, get the terms right, and follow the engine power charts.

and remember there is no difference between a power setting that creates a 1400 degree EGT RICH
and one that creates a 1400 degree EGT LEAN.
 
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Tom-D said:
You said…..

Who said you "had" to t/o full rich? It is recommended as stated above and for very good reasons.
Most importantly very high ICP (internal cylinder pressure) is developed on take-off. The FAA requires a detonation safety envelope for every engine that is controlled by mixture.

Got a reference for that?
Click to expand...

AC 33.47-1 Section 6e - in part
"Engine power settings through the engine operating envelope should
be tested to verify establishment of adequate detonation margins. The engine
should not demonstrate limiting detonation at fuel flows down to 12 percent
leaner than the minimum fuel flow recommended by the engine fuel flow chart."
 
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Cruiser said:
FAR 33.47-1 Section 6e - in part
"Engine power settings through the engine operating envelope should
be tested to verify establishment of adequate detonation margins. The engine
should not demonstrate limiting detonation at fuel flows down to 12 percent
leaner than the minimum fuel flow recommended by the engine fuel flow chart."
Click to expand...

Also learn the difference between a FAR and an AC.

The FAR only says

33.47 Detonation test.
Each engine must be tested to establish that the engine can function without detonation throughout its range of intended conditions of operation.

IOWs when you operate with in the engine power charts you will not detonate. That's one of the reasons we can only pitch a prop to a certain limit and it must produce an RPM that is with in the limits given the Type Certificate

The AC is not regulatory. it is merely a suggested method. acceptable to the FAA.
 
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touche! My mistake in typing FAR instead of AC.

You asked for a reference, I gave you an FAA acceptable one. Will you agree that my original statement was NOT misinformation?
 
Cruiser said:
touche! My mistake in typing FAR instead of AC.

You asked for a reference, I gave you an FAA acceptable one. Will you agree that my original statement was NOT misinformation?
Click to expand...

Your statement..

Who said you "had" to t/o full rich? It is recommended as stated above and for very good reasons.
Most importantly very high ICP (internal cylinder pressure) is developed on take-off. The FAA requires a detonation safety envelope for every engine that is controlled by mixture.

Where does the regulation state any thing about a safety envelope? or any thing about the mixture control?

The engine in test must prove that it will run with in parameters of the design with out detonation. That is the only requirement.

The regulation was written that way to allow for all the different methods of providing fuel/air ratio to the combustion chamber.
I'll give ya a "B"
 
Jim Logajan said:
My understanding from various mountain flying guides is that, absent POH guidance, in a normally aspirated engine one should try to lean for best performance at full throttle before taking off. Sparky Imeson's Mountain Flying Bible even suggests doing this leaning during the takeoff roll, but keep it a tad rich.
Click to expand...

I've done it during takeoff roll (it only takes a couple of seconds), but find that it's easier to do it during run-up, then just leave it there. I have better uses for that 100 or so feet of runway than playing engineer.
 
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Tom-D said:
You said…..


A cylinder can go into thermal runaway, melt and destroy itself and possibly other parts of the engine in less than one minute.

That is absolutely ridiculous. What is a thermal run away in recip? thermal runaway happens in nicad-batteries, and nukie plants, not in recipe engines.
Click to expand...

While the use of the term "thermal runaway" may not be the best choice of words, it effectively describes the rapid rise in CHTs that can exceed 2.5°F/sec when an engine is in severe detonation.
The term is used by:
Mike Busch, Savvy Aviator
John Schwaner, SkyRanch Engineering
and Airmotive Engineering Corp. All recognized as leading aviation experts. I think I am in good company.

I will not use it in the future since it is "technically" incorrect and I do not want to spread misinformation.
 
Tom-D said:
Your statement..

Who said you "had" to t/o full rich? It is recommended as stated above and for very good reasons.
Most importantly very high ICP (internal cylinder pressure) is developed on take-off. The FAA requires a detonation safety envelope for every engine that is controlled by mixture.

Where does the regulation state any thing about a safety envelope? or any thing about the mixture control?

The engine in test must prove that it will run with in parameters of the design with out detonation. That is the only requirement.

The regulation was written that way to allow for all the different methods of providing fuel/air ratio to the combustion chamber.
I'll give ya a "B"
Click to expand...

WHEW! Glad I am not spreading misinformation :)
 
The 170 guy said:
Lots of misinformation and old wives tales in this thread.
When you are at WOT, with the prop at highest RPM, your VSI is your horsepower meter, lean to highest reading to get best horse power.
Works at any altitude, and any engine.
Manufactures want you at full rich because they know that method is dummy proof.
Click to expand...

With or without flaps?

Yeah - I know - just sayin' it's not entirely accurate. To be sure, you need full engine instrumentation. "Full Rich" is a catch-all that covers 90+% of the situations and gets repeated early and often. As is usually the case, the real answer to the question is "it depends." If you don't what it depends on, then you need to go with the simple method you were taught as an ab-initio student until you learn more about it.

I rarely use full rich on takeoff - but I did last week. Conditions called for it. I know what those conditions are. Do you?
 
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