Flying over square
Back to the oversquare!
I once had a C172 that a previous owner had installed a MP gauge. That machine had a static RPM of about 2240.
Shock horror the MP was just over 29" on most days according to my instructors who first taught me the "rules" of operating an aircraft with constant speed propellers that engine should have been toast!! However that machine used to fly parachute and glider tow operations and was full throttle day in day out and did well over 3000 hours.
That was when I realised my instructors didn't have a clue
The BE76 is a classic in that the AFM has the climb power of full throttle and 2600 or 2700 RPM. However the instructors were told by the CFI that climb is 25/25 and that is what they taught. Even after I managed to demo them that if you fly it iaw the AFM the CHT would comfortably sit at 380 degrees. At 25/25 the CHT was just over 460.
I have never heard of anyone not having the confidence to run a fixed pitch aircraft at full throttle (without knowing it being oversquare!!!!!)
Shock horror the MP was just over 29" on most days according to my instructors who first taught me the "rules" of operating an aircraft with constant speed propellers that engine should have been toast!! However that machine used to fly parachute and glider tow operations and was full throttle day in day out and did well over 3000 hours.
That was when I realised my instructors didn't have a clue
The BE76 is a classic in that the AFM has the climb power of full throttle and 2600 or 2700 RPM. However the instructors were told by the CFI that climb is 25/25 and that is what they taught. Even after I managed to demo them that if you fly it iaw the AFM the CHT would comfortably sit at 380 degrees. At 25/25 the CHT was just over 460.
I have never heard of anyone not having the confidence to run a fixed pitch aircraft at full throttle (without knowing it being oversquare!!!!!)
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Aww give me a break Jaba, I got out of turbo's and into oversquare, because I don't want any more pm's from Jas.
and you never replied
I'm devastated, i really am a SNAG you know.
Even after I managed to demo them that if you fly it iaw the AFM the CHT would comfortably sit at 380 degrees. At 25/25 the CHT was just over 460.
If you climbed at 25/25 like the aero club told you (never mind the CP or the Ops manual or the POH)
I always thought the 25/25 was a compromise power setting taught by flying schools for passenger comfort reasons and a little preparation for more advanced types with METO/MCP limits. Once you attain a reasonably safe altitude increase speed and reduce power to reduce noise, vibration and body angle to make a more pleasurable experience for the paying customer, a sort of cruising climb. The higher speed offsets the reduced full rich cooling being close to 75% power (normally aspirated) its pretty close to a high cruise setting.
As an instructor it did annoy me that students did not understand that this 25/25 setting was not a limitation. Full power was available in any situation if required and should be used to achieve performance targets. The amount of times a student when asked to demonstrate a max angle/rate climb set 25/25 was very dissapointing.
The whole over square thing has no merit whatsoever and does nothing for engine life if it does not accidentally match with a recomended setting anyway.
Shock cooling will have little effect on perfectly formed materials as even heating and cooling will most likely occur (as designed). Unfortunately, most aircraft parts in general these days are far from perfect, any small defects/imperfections in a high stress area will be made weaker by large, sudden variations in temperature. Over time this may accelerate a failure such as cracking etc... Continued operation of an engine outside of its normal temperature range whether hot or cold will cause more lasting damage than sudden changes within that range.
As an instructor it did annoy me that students did not understand that this 25/25 setting was not a limitation. Full power was available in any situation if required and should be used to achieve performance targets. The amount of times a student when asked to demonstrate a max angle/rate climb set 25/25 was very dissapointing.
The whole over square thing has no merit whatsoever and does nothing for engine life if it does not accidentally match with a recomended setting anyway.
Shock cooling will have little effect on perfectly formed materials as even heating and cooling will most likely occur (as designed). Unfortunately, most aircraft parts in general these days are far from perfect, any small defects/imperfections in a high stress area will be made weaker by large, sudden variations in temperature. Over time this may accelerate a failure such as cracking etc... Continued operation of an engine outside of its normal temperature range whether hot or cold will cause more lasting damage than sudden changes within that range.
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43inches.....what an appropriate name, lots of MP there 
Please sir, wipe that OWT from you memory. Full rich or richer mixture means more fuel to air ratio, your statement suggests that fuel is doing cooling. Maybe I am reading more into your words, but please understand the injested fuel does not do any cooling at all. far from it, it is the source of all heat!
J
The higher speed offsets the reduced full rich cooling being close to 75% power (normally aspirated) its pretty close to a high cruise setting.
J
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Full rich or richer mixture means more fuel to air ratio, your statement suggests that fuel is doing cooling. Maybe I am reading more into your words, but please understand the injested fuel does not do any cooling at all. far from it, it is the source of all heat!
SShhhh I can hear T28D coming, I've gotta run.
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trent
unburnt fuel out the exhaust.....I have never seen that happen. Not once. Have you? If so please explain.
I have seen unburnt fuel drip all over the nose gear from fitting the dreaded anti theft device lock and fuel drains back out through the throttle body and air intake.
.........leaves a nice tell tale residue.
T28 is after you hey....I gotta watch
unburnt fuel out the exhaust.....I have never seen that happen. Not once. Have you? If so please explain.
I have seen unburnt fuel drip all over the nose gear from fitting the dreaded anti theft device lock and fuel drains back out through the throttle body and air intake.
.........leaves a nice tell tale residue.
T28 is after you hey....I gotta watch
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Correct me where i err please.
Fuel dictates the size of the fire....................................
LOP. Your fuel is lower, so the fire is smaller, so less heat can be generated. You are also in a zone that the oxygen levels permit total burn of the fuel.
Peak. You are burning everything max, but you are opening the door to detonation and things like that.
ROP. You are rich, so if fuel is having a cooling effect, it certainly isn't doing it at the point of measurement. ROP is a hotter temp than PEAK. What is happening? the unburnt fuel is being burnt in the exhaust, causing a rise in what the probe is seeing.
Maybe a reword on the terminology being used need be considered, or at least clarified.
These days it seems to be taught as Best Power, Best Endurance. I know jaba, t28d and trent can relate em back, but can the others.
Cheers
Jas
Fuel dictates the size of the fire....................................
LOP. Your fuel is lower, so the fire is smaller, so less heat can be generated. You are also in a zone that the oxygen levels permit total burn of the fuel.
Peak. You are burning everything max, but you are opening the door to detonation and things like that.
ROP. You are rich, so if fuel is having a cooling effect, it certainly isn't doing it at the point of measurement. ROP is a hotter temp than PEAK. What is happening? the unburnt fuel is being burnt in the exhaust, causing a rise in what the probe is seeing.
Maybe a reword on the terminology being used need be considered, or at least clarified.
These days it seems to be taught as Best Power, Best Endurance. I know jaba, t28d and trent can relate em back, but can the others.
Cheers
Jas
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Jaba
Simplest description I can offer is to think of what an exhaust system 'Backfire' is.
I'm sure you've seen engine tailpipes with that dark sooty telltale on the inside of the pipe. Unburnt fuel. Much more noticeable with large diesels engines with knackered injectors, thick black smoke, not completely burnt fuel in carbon form.
Peak engine power comes at a close to 12.5 :1 air/fuel ratio, and best economy at 15.5 :1 ratio. However petrol will burn anywhere from 20 : 1 to 8 : 1. If an overly rich mixture between 12.5 : 1 to 8 :1 (which will develop less power than peak mixture 12.5:1) then unburnt fuel will travel through the OTTO cycle.
BUT, I am not my wife and therefore by definition, don't know everything.
Jas, I agree with everything except
Think EGT guage and richening the mixture from peak EGT, the EGT will decrease. That is the unburnt fuel removing heat from the cylinder.
*Where I have used the term 'unburnt fuel' please substitute 'not completely burnt fuel', to be more precise.
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Found another interesting article - Mixed up about fuel mixtures
Simplest description I can offer is to think of what an exhaust system 'Backfire' is.
an incomplete burn which causes the fumes to explode in the exhaust system
Peak engine power comes at a close to 12.5 :1 air/fuel ratio, and best economy at 15.5 :1 ratio. However petrol will burn anywhere from 20 : 1 to 8 : 1. If an overly rich mixture between 12.5 : 1 to 8 :1 (which will develop less power than peak mixture 12.5:1) then unburnt fuel will travel through the OTTO cycle.
BUT, I am not my wife and therefore by definition, don't know everything.
Jas, I agree with everything except
ROP is a hotter temp than PEAK
*Where I have used the term 'unburnt fuel' please substitute 'not completely burnt fuel', to be more precise.
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Found another interesting article - Mixed up about fuel mixtures
Using a typical air-cooled aircraft engine running at normal power, with the throttle in a fixed position, if fuel is slowly added to the stoichiometric mixture (0.067) the added fuel will have a cooling effect and the combustion gas and cylinder head temperature will decrease.
Last edited by Trent 972; 18th Oct 2011 at 17:20. Reason: add in a reply to Jas
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Jas, keep reading mate....you almost have this sussed out.
Fuel cooling, sure you can generate some evaporative cooling effect with fuel but not in your engine, pour some on your arm and wave it about, it will feel a fraction cooler than the rest of your arm but bugger all really.
Now for the facts. Going richer does NOT mean fuel is cooling. It means a less violent bang thru a more lethargic mixture.
It has the effect of retarding timing, not the spark timing just the flame front. Thus lower value of PPP and further from TDC. Hence lower CHT, and a bit less power for all that extra fuel.
Remember in our fixed spark timing engines the way we can control the combustion event is by mixture control. In your car, especially with EFI lots of things are done differently. Even the old Holden engines (old for me) had vac advance.
Trent..".no that fuel is burnt. And the reason for that flame spitting you see on many race cars is not relevant to this discussion, as good as I am at thread drift I think we should leave this tread with an educational trail for pilots not rev heads. Even though some of us are both
Fuel cooling, sure you can generate some evaporative cooling effect with fuel but not in your engine, pour some on your arm and wave it about, it will feel a fraction cooler than the rest of your arm but bugger all really.
Now for the facts. Going richer does NOT mean fuel is cooling. It means a less violent bang thru a more lethargic mixture.
It has the effect of retarding timing, not the spark timing just the flame front. Thus lower value of PPP and further from TDC. Hence lower CHT, and a bit less power for all that extra fuel.
Remember in our fixed spark timing engines the way we can control the combustion event is by mixture control. In your car, especially with EFI lots of things are done differently. Even the old Holden engines (old for me) had vac advance.
Trent..".no that fuel is burnt. And the reason for that flame spitting you see on many race cars is not relevant to this discussion, as good as I am at thread drift I think we should leave this tread with an educational trail for pilots not rev heads. Even though some of us are both
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Jaba, I know you're a GAMI fan, so perhaps a quote from Bill Cunningham (Powermaster Engines) on the GAMI website might interest you. CAUTION article contains the words 'unburned fuel', continue at your own risk.
ps. evaporation only happens with a difference in humidities. Pour petrol on your skin while you're standing in the rain and you will be wet and have a higher calorific value, but not cooler.
Unburned fuel molecules (denser than air molecules) absorb heat from the combustion and transport that heat through the exhaust process.
At least that was what I was taught in my OWT classes.
Bill Cunningham, founder of Powermaster engines in Tulsa, Oklahoma, said the key to keeping cylinders healthy is keeping them cool. That can be accomplished with a rich mixture setting that pours unburned fuel through them, or a very lean setting that restricts fuel and creates less heat in combustion.
ps. evaporation only happens with a difference in humidities. Pour petrol on your skin while you're standing in the rain and you will be wet and have a higher calorific value, but not cooler.
Unburned fuel molecules (denser than air molecules) absorb heat from the combustion and transport that heat through the exhaust process.
At least that was what I was taught in my OWT classes.
Last edited by Trent 972; 18th Oct 2011 at 21:49. Reason: will someone let me know if Clinton says something intelligent, because he's still 'ignored' here!
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Right Clinton. When the engine is leaned, there is less unburnt fuel and the combustion chamber is hotter. Check a spark plug and you'll see it is white ash under those conditions. Lean produces maximum power but it is a very fine balance between power and damage. Rich produces less power but gives more reliability due to the fuel cooling the combustion area. Check a spark plug and you'll see a film of unburnt black carbon under those conditions.
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Clinton you are a funny dude I did not think you folk down there had a sense of humor, but I was wrong.
Trouble is someone is going to believe that little gem
Trent
You are almost king of the wind ups, now being a clever chap, how about you explain to the viewers why that "play on words" almost says but does not quite an OWT.
Think about it this way, is that black soot unburnt? No, it's black and sootie, it is burnt, but maybe not fully combusted. So what is it Mr Cunningham is trying to tell people, especially those who lived a diet of OWT's and believe fuel cools
I am sure you will wrk it out.
PS this iPad is great.....just not for debates on pprune
I did not think you folk down there had a sense of humor, but I was wrong.Trouble is someone is going to believe that little gem
Trent
You are almost king of the wind ups, now being a clever chap, how about you explain to the viewers why that "play on words" almost says but does not quite an OWT.
Think about it this way, is that black soot unburnt? No, it's black and sootie, it is burnt, but maybe not fully combusted. So what is it Mr Cunningham is trying to tell people, especially those who lived a diet of OWT's and believe fuel cools
I am sure you will wrk it out.
PS this iPad is great.....just not for debates on pprune
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Jas, I agree with everything except
Quote:
ROP is a hotter temp than PEAK
Think EGT guage and richening the mixture from peak EGT, the EGT will decrease. That is the unburnt fuel removing heat from the cylinder.
Quote:
ROP is a hotter temp than PEAK
Think EGT guage and richening the mixture from peak EGT, the EGT will decrease. That is the unburnt fuel removing heat from the cylinder.
Freudian slip. WTF was i thinking....lord only knows
Unburnt Fuel/not completely burnt fuel (charge) is really just a play/choice of words, meaning pretty much the same thing
Rare thing to see todays car's with that lovely fawn coloured deposits in the exhaust. Maybe thats a lead thing.
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Jaba, I can see us getting closer (not in a biblical sense though )
Black and sooty and sticky, that's why it sticks to the inside of the exhaust. As opposed to light grey (ok, fawn then) and not sooty like fully combusted petrol.
Don't have an iPAD
, bloody kids think it's too techo for me and won't let me use it.
In my very best 'Pauline' PLEASE EXPLAIN Mr. Cunninghams use of 'unburned fuel'. You tell me that's not possible, remembering GAMI have used the article on their website.
)Black and sooty and sticky, that's why it sticks to the inside of the exhaust. As opposed to light grey (ok, fawn then) and not sooty like fully combusted petrol.
Don't have an iPAD
, bloody kids think it's too techo for me and won't let me use it.In my very best 'Pauline' PLEASE EXPLAIN Mr. Cunninghams use of 'unburned fuel'. You tell me that's not possible, remembering GAMI have used the article on their website.
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Walls Of China
Lean produces maximum power but it is a very fine balance between power and damage. Rich produces less power but gives more reliability due to the fuel cooling the combustion area.
I must say that I'm really enjoying this thread and some of the associated articles, but thought I'd just post the below quote from the GAMI article for those who have a tendency to just take other peoples advice on face value and then consider it fact.
I'm by no means arguing the merit of LOP operations, but just wanted to ensure some of the uninitiated don't go running an engine LOP when it is not set up to do so.
As for "flying over square" - the R985 (supercharged radial) I operate does 36.5/2300 @ Max and climbs out at 30/2000, 'nuff said
Not all GA engines can run safely LOP. Carbureted engines, for example, lack precise fuel/air metering systems and typically run rough and lose power LOP. And electronic engine monitors that show cylinder head and exhaust gas temperatures for every cylinder are necessary for safe LOP operations.
As for "flying over square" - the R985 (supercharged radial) I operate does 36.5/2300 @ Max and climbs out at 30/2000, 'nuff said
As what Trent said regarding fuels. Fuel air mixture will determine how much of the charge is burnt in the time available during the power stroke. "Peak" EGT is just that, the peak exhaust temperature. If you richen the mixture from that position you gradually flood the cylinder with fuel until you eventually end up with a rich cut, this will result in progressively cooler EGT.
Best Power allows a slightly higher density charge to enter the cylinder and therefore produce more power. The excess fuel above peak mixture allows better cooling reducing damage caused by excessive combustion heat at higher power settings.
Mixture burns slower lean so engine may run slightly hotter, EGT is cooler however. EGT is really a fuel management tool, whats going on in the engine is more reflected in CHT and oil temps.
Detonation and preignition can happen at any operating mixture setting due to a number of reasons. At high power and running peak you just dramatically increase the chance of it in a healthy engine. Lycoming actually recomends operating at peak during cruise as long as proper cruise powers are adhered to (less than 75% normally aspirated and less than 65% for super/turbo charged).
ROP. You are rich, so if fuel is having a cooling effect, it certainly isn't doing it at the point of measurement. ROP is a hotter temp than PEAK. What is happening? the unburnt fuel is being burnt in the exhaust, causing a rise in what the probe is seeing.
LOP. Your fuel is lower, so the fire is smaller, so less heat can be generated. You are also in a zone that the oxygen levels permit total burn of the fuel.
Peak. You are burning everything max, but you are opening the door to detonation and things like that.