Rough engine on run-up
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AlexUM - wasn't calling you into question, just suprised..
Crash one - think it's been covered pretty comprehensively, but yes, water was causing a missfire somewhere on the system for that on one mag, hence the roughness; i.e. it wasn't working *consistently* on that mag. Eliminating it from the equation leaves smooth running, at a (very) slightly reduced power. I'm not suggesting that's normal behaviour - in this case the dousing simply upset something that was probably failing anyway.
Yes, you do get better burn with both mags, but it's perfectly possible to design an engine that burns well with one (some cars, notably alfa romeo have run twin-plug installations). A two plug system running on one will most likely be worse than a one plug system running on one, but dropping one mag out does not make that much difference - IIRC about 200rpm from the cruise setting in this case.
Crash one - think it's been covered pretty comprehensively, but yes, water was causing a missfire somewhere on the system for that on one mag, hence the roughness; i.e. it wasn't working *consistently* on that mag. Eliminating it from the equation leaves smooth running, at a (very) slightly reduced power. I'm not suggesting that's normal behaviour - in this case the dousing simply upset something that was probably failing anyway.
Yes, you do get better burn with both mags, but it's perfectly possible to design an engine that burns well with one (some cars, notably alfa romeo have run twin-plug installations). A two plug system running on one will most likely be worse than a one plug system running on one, but dropping one mag out does not make that much difference - IIRC about 200rpm from the cruise setting in this case.
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AlexUM - wasn't calling you into question, just suprised..
Update on the engine: Student on solo X-C. Went up to 8500ft and leaned out. On the way home forgot about the red knob (checklist?
) and flew a couple of patterns practicing his short-fields before full stop for about 45min. Can this single-time misadjustment be the reason? We need two new pistons and the corresponding valves 
Cheers,
Alex
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Mark, Never thought you were
Update on the engine: Student on solo X-C. Went up to 8500ft and leaned out. On the way home forgot about the red knob (checklist?) and flew a couple of patterns practicing his short-fields before full stop for about 45min. Can this single-time misadjustment be the reason? We need two new pistons and the corresponding valves
Cheers,
Alex
Update on the engine: Student on solo X-C. Went up to 8500ft and leaned out. On the way home forgot about the red knob (checklist?
) and flew a couple of patterns practicing his short-fields before full stop for about 45min. Can this single-time misadjustment be the reason? We need two new pistons and the corresponding valves Cheers,
Alex
HOWEVER, if the student 'sort of leaned' (at 8500 you need quite a bit of red knob back in most planes) they could have gotten to say 150-200 ROP (back to a reasonable performance level). In this case, when doing pattern work, they could have been at full power at peak EGT, deep deep in the Red Box and could have crisped the cylinders in a few minutes let alone 45 minutes.
The engine makes 100% power fairly rich of peak with a lot of partially burned fuel. Same RPM and Throttle setting, but with the mixture set to peak and you will pull much more than 100% out of the engine - and that is not a great idea for longevity
Another cause of burnt cylinders is prelonged climbs at low airspeeds on hot days. This will be exacerbated by engine baffles which are in poor condition, which seems to the case in many trainers. Since most trainers do not have a cylinder head temperature guage the first indication of engine overheating will be high oil temp, a lagging indicator, assuming the pilot is even monitoring the oil temp in the climb, a big assumption based on what I have seen with most PPL's....and their instructors.... By the time you see redline oil temps the cylinders will be well on their way to being cooked.
Another scenario which I think is relatively common arises from the fact that the student rarely leans the engine during their training. When they get their PPL and start going places they will lean the engine and then forget about it. Therefore if they decide to climb they wil probably just shove the throttle full in. To avoid this I make a big point in the after solo phase of the PPL that the student notes the mixture position (and indicates this to me by touching the mixture knob) before adjusting power for any flying outside the circuit. This to inculcate the habit of always linking throttle movements to the mixture setting, which will avoid the scenario mentioned above.
Another scenario which I think is relatively common arises from the fact that the student rarely leans the engine during their training. When they get their PPL and start going places they will lean the engine and then forget about it. Therefore if they decide to climb they wil probably just shove the throttle full in. To avoid this I make a big point in the after solo phase of the PPL that the student notes the mixture position (and indicates this to me by touching the mixture knob) before adjusting power for any flying outside the circuit. This to inculcate the habit of always linking throttle movements to the mixture setting, which will avoid the scenario mentioned above.
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Alex.
Going back to our posts about static RPM on 13th May I flew yesterday and rejected the takeoff when the RPM was much less than it should have been. Static should have been at least 2350 but was only 1900.
Fixed prop, shortish runway, no room for messing around on the climbout etc.
Rejected takeoff, the plugs pulled out and cleaned, back in and it was fine...after a very thorough run up with me and the engineer.
This is why you should do those final checks as you are rolling.
RPM
T&Ps
Airspeed
Have fun!
G
Going back to our posts about static RPM on 13th May I flew yesterday and rejected the takeoff when the RPM was much less than it should have been. Static should have been at least 2350 but was only 1900.
Fixed prop, shortish runway, no room for messing around on the climbout etc.
Rejected takeoff, the plugs pulled out and cleaned, back in and it was fine...after a very thorough run up with me and the engineer.
This is why you should do those final checks as you are rolling.
RPM
T&Ps
Airspeed
Have fun!
G
Last edited by gijoe; 26th May 2010 at 19:09.
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This is why you should do those final checks as you are rolling.
RPM
T&Ps
Airspeed
RPM
T&Ps
Airspeed
Originally Posted by Pull what
So with a performance take off on a short strip you would do those checks while rolling?
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Pull what - It should be the work of a split second to glance inside and note these during the takeoff roll - any takeoff roll - do you not always check that the ASI is alive anyway?
Unless there's a confusion and you're thinking that you check them rolling down the runway prior to starting the takeoff roll?
I'm personally pondering how you detect a loss of power with a CS prop - other than shaking, banging, or dramatic loss of power, likely to be quite insidious..
Unless there's a confusion and you're thinking that you check them rolling down the runway prior to starting the takeoff roll?
I'm personally pondering how you detect a loss of power with a CS prop - other than shaking, banging, or dramatic loss of power, likely to be quite insidious..
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I just had to chuckle as I find myself in exact the same situation....
Pelican's Perch #46:<br>"But My Mechanic Says ..."
First thing our mechanic said was "Somebody running it too lean again!".
Cheers,
Alex
Pelican's Perch #46:<br>"But My Mechanic Says ..."
First thing our mechanic said was "Somebody running it too lean again!".
Cheers,
Alex
Leaning will do the following for you: Cleaner cylinders, less carbon deposits/build-up, cleaner sparkplugs and prevent valve sticking. Its not about fuel savings, its about running your engine properly and extending its operating life.
On the ground, lean until you can barely reach 1200rpm. At this power setting its absolutely impossible to damage the engine because of agressive leaning. At full rich the fuel/air ratio is WAY above what is required for such power setting so dont be affraid to lean on the ground. The overly rich mixture is designed to provide sufficient engine cooling at TO power on a low density altitude day. Leaving the mixture fully rich will result in incomplete combustion --> more carbon. Another advantage of using such an agresive leaning on the ground is that it will prevent you from taking off with a leaned out mixture, wich could lead to severe detonation if a low density altitude condition exists.
Unfortunatedly all of Deakins analysis are based on big bore, fuel injected engines with very reliable fuel injectors and engine monitoring instruments. At least it seems that for that kind of set up, he has managed to go beyond the recommended TBO with his engines following more or less the recomendations I've given above.
Remember, do not lean with a high power setting unless you have the appropriate monitoring gauges CHT/EGT for all cylinders and can efficiently monitor them.
I used to fly at Embry-Riddle Florida, where (as aproved by the POH) we would start leaning at 1000ft AGL to the top of the green arc on the F/F gauge (12GPH) and 1/4 of turn for each 1000ft there after on XCs. The 12GPH mark would result in about 1" of mixture out. Of course this was an approved procedure, so I definitively do not encourage you doing the same on a 152. I simply wanted to show that its not always "DO NOT TOUCH THE RED MISTERIOUS LEVER OR BAD THINGS WILL HAPPEN"
As a final advise, take a GOOD look at the CHT/EGT/SFC/POWER chats, you can learn something there for sure.
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Last edited by palou89; 26th May 2010 at 16:57.
For non turbocharged engines you should get about 1/2 inch less than field barometric MP on takeoff (field baro is what the MP guage reads when the engine is not running).
Obviously in both cases you would also check that you have full redline RPM as well.
On a related topic one area that does not IMO often get the attention it deserves is noting the fuel flow on takeoff in aircraft with fuel injected engines. All light aircraft engines depend on excess fuel flow to help cool the cylinders at full power. The fuel flow should be right at the redline maximum value (or even a little bit over) at full power. If it is even a little bit low it should adjusted by an engineer. Prolonged operation without suficent fuel flow will damage the cylinders.
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Like McGoo said, why not?
Soft field takeoff = don't stop at the end.
Shortish = probably enough time to start rolling and look at everything.
...it actually wasn't that short but practice etc. and I was only trying to get the point across...by putting it into context...hoping that Alex would remember it...
Anyway...
G
Soft field takeoff = don't stop at the end.
Shortish = probably enough time to start rolling and look at everything.
...it actually wasn't that short but practice etc. and I was only trying to get the point across...by putting it into context...hoping that Alex would remember it...
Anyway...
G
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What effect would (say) a burnt valve, fouled plug etc., have on that manifold pressure? My gut feel says nothing.. the MP gauge is reading just that - MP. If the engine is spinning at the full RPM, the MP wouldn't be affected by the amount of power the engine is generating, it's still pumping the same volume of air, right?
Less sure for a turbo'd engine, but the logic should be the same.
P.S. Ditto the fuel flow - both measure the inputs, and don't really give a clue whether there's power being produced, so long as there's enough oomph to get the prop into the governing range.
Less sure for a turbo'd engine, but the logic should be the same.
P.S. Ditto the fuel flow - both measure the inputs, and don't really give a clue whether there's power being produced, so long as there's enough oomph to get the prop into the governing range.
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Indeed, the subject of the original post appears to have been a fixed pitch prop, as such there is rarely a MP gauge, or really that much need for one. What is your point?
Static RPM is meaningful as a measure of engine output ONLY if you have a fixed pitch prop. I was pondering the manifestation of the same problem with a CS prop due to personal interest (being as I tend to fly behind CS props). Big Pistons Forever provided a suggestion, which to be honest, doesn't make sense (to me). As far as I can see, there is no way of objectively observing the power output when a wobbly prop is fitted. Perhaps someone knows better.
I don't think it's terribly off-topic for the thread.
Static RPM is meaningful as a measure of engine output ONLY if you have a fixed pitch prop. I was pondering the manifestation of the same problem with a CS prop due to personal interest (being as I tend to fly behind CS props). Big Pistons Forever provided a suggestion, which to be honest, doesn't make sense (to me). As far as I can see, there is no way of objectively observing the power output when a wobbly prop is fitted. Perhaps someone knows better.
I don't think it's terribly off-topic for the thread.
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I don't think it's terribly off-topic for the thread.
Mark123
In large radial engine aircraft engine power (Torque) was directly measured and indicated on seperate dedicated (BMEP) guages. However there is no guage in a light aircraft which directly measures engine power. This is because for a non turbo/supercharged simple flat 4 or 6 cylinder light aircraft engine running normally, there is a very strong correlation between the induction manifold pressure and the power output. POH cruise charts show the percentage of power developed at a variety of RPM and MP combinations. Most will have a column for % of power and corresponding MP's at maximum RPM....or the takeoff condition. If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower....which IMO would be a good reason to abandon the takeoff.
Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.
In large radial engine aircraft engine power (Torque) was directly measured and indicated on seperate dedicated (BMEP) guages. However there is no guage in a light aircraft which directly measures engine power. This is because for a non turbo/supercharged simple flat 4 or 6 cylinder light aircraft engine running normally, there is a very strong correlation between the induction manifold pressure and the power output. POH cruise charts show the percentage of power developed at a variety of RPM and MP combinations. Most will have a column for % of power and corresponding MP's at maximum RPM....or the takeoff condition. If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower....which IMO would be a good reason to abandon the takeoff.
Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.
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If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower...
But suppose there's no problem in the inlet manifold whatsoever and the throttle is wide open. At max RPM the air is being sucked in normally so you have a slight pressure drop vs. ambient exactly like you would expect in a full throttle situation. But there's another problem. Say the engine is only running on one set of magnetos (or even less). Or there's a fuel flow restriction (e.g. dirty filter) which causes the engine to run too lean, but still (just) develop max RPM with the prop fully fine. Or something else internally to the engine. You wouldn't see that through an MP reduction or rise, would you?
In the case of a dead mag I can say from personal experience that the engine will not develop full MP because I aborted a takeoff in a twin last year after noticing in the "good engine check" the MP needles were split. It turned out the mag coil had died. As for a too lean mixture two things will happen
1) The engine fuel flow is so restricted that there is not enough to support full combustion at max power and the engine will start to run rough. or
2) There is enough fuel to support full combustion but not at the over rich condition necessary to allow proper cylinder cooling at high power.....in which case you will get abnormally high CHT's assuming you did not catch the problem during the runup.
Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.
However since from what I have observed over many years instructing most PPL's do not
1) Actually check the field baro indication prior to start up to see if the MP guage is reading a sensible number.
2) Do not know what they should be looking for in the runup particularly when it comes to testing the mixture control,carb heat,mags and cycling the prop
3) If they look at all at the engine guages at the start of the takeoff (which many do not) look but do not see.
So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.
Finally if the aircraft is not accelerating normally it doesn't necessarily mean that the engine is not producing full power which points out the importance paying attention to what the aircraft is telling you and aborting (early) any takeoff which doen't feel right and for short fields having a physical go no go point allready established in your mind.
1) The engine fuel flow is so restricted that there is not enough to support full combustion at max power and the engine will start to run rough. or
2) There is enough fuel to support full combustion but not at the over rich condition necessary to allow proper cylinder cooling at high power.....in which case you will get abnormally high CHT's assuming you did not catch the problem during the runup.
Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.
However since from what I have observed over many years instructing most PPL's do not
1) Actually check the field baro indication prior to start up to see if the MP guage is reading a sensible number.
2) Do not know what they should be looking for in the runup particularly when it comes to testing the mixture control,carb heat,mags and cycling the prop
3) If they look at all at the engine guages at the start of the takeoff (which many do not) look but do not see.
So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.
Finally if the aircraft is not accelerating normally it doesn't necessarily mean that the engine is not producing full power which points out the importance paying attention to what the aircraft is telling you and aborting (early) any takeoff which doen't feel right and for short fields having a physical go no go point allready established in your mind.
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Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.
Frankly guys if you are bound and determined to find some possible combinations of factors which could result in an indication of full MP and RPM and yet have the engine developing significantly reduced power I am sure you will probably be successfull.
So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.
So my recommendation is rather than worry about the .1 % probabilty problems...... concentrate on the basics so they get done properly on every single takeoff.
In the case of a dead mag I can say from personal experience that the engine will not develop full MP because I aborted a takeoff in a twin last year after noticing in the "good engine check" the MP needles were split. It turned out the mag coil had died.
However, I cannot understand how it would make a difference for a normally aspirated engine - if it's turning full RPM, it sucks a set amount of air. The MP gauge has no clue whether the air is being used to oxidise fuel or not. If you could spin the prop at redline rpm on the starter with both mags off, it would show the same MP as it would running normally, and probably the same fuel flow - but CHT / EGT would be zero.
However, I guess in thinking it through I've answered my own question, the only thing that will tell me something is wrong is the seat of my pants.. EGT might give some clues - if it reacts fast enough (and yes, I know to look for a rise in EGT on one mag..)