The Mysterious Mixture Control

[zinfinion 0]

Having never flown a real A/C it's hard for me to know how accurate flight is.

 

Other than being possible to run well while leaned too much (30% mix at 2000' for example) I'm quite impressed with how it's been modeled in Flight. Though apparently you can run fuel injected engines quite lean and just get a loss of power rather than the roughness you would experience with a carbureted engine.

 

I'm no pilot either, but all the real world things I've read line up rather well with what's in game.

[fastcarfastplane 0]

nice write up...

 

in the 152 i fly, i lean til it starts to misfire, then screw the nob in til it smooths out, then an extra turn for good measure....

 

in the skyhawk i fly, it has an egt guage temp, pull out til it starts to spike, then put it back in....

 

 

 

edit:

 

one thing i've never understood...in all the POH, and other stuff you read, and the way you're taught, AND on every checklist, they refer to fully rich as "full power" or "mixture set to best power"....the reason that don't make sense is, because of a long time of working on engines, building race engines, and drag racing cars(legally)...anyone can tell you, the leaner you go, the more power you make. unfortunately it also makes more heat, but you can compensate with higher octane fuel, which in turn burns a bit cooler to avoid detonation...i.e, which is why high compression motors use high octane fuel. heat is bad mmmmkay.

[RoboRay 126]

Ok, I just made a 180nm flight in the Maule and did some experimenting...

 

I wanted to find the minimum-altitude for WOT/LoP in the Maule, and if the "sum 48" number for combined manifold pressure and RPM equating to 75% power is correct on this engine, it's doable down to about 4000' with the checklist cruise RPM of 2300, as I was showing 25" MP at that altitude. I remained at the altitude for the flight, to explore the "worst case scenario."

 

From fuel flow, peak seemed to occur at about 60% on the mixture control. Cylinder head temp there hovered around 238°F +/-2 (I was playing with the mixture a bit trying to pin down the exact peak, and it never quite stabilized). I don't recall the exact oil temp, but it was well over 200°F (but not in the red).

 

A mixture setting of 38% (well lean of peak) established a cruise speed of about 142kts. Cylinder head temp stabilized at 224°F, oil temp at 189°F. Fuel flow was 12.1 gph.

 

A slightly slower cruise at around 136kts should have yielded 11gph or so, and lower temps as well, but I've always cruised at about 142kts with traditional leaning, and I didn't want to be conservative for this test, anyway.

 

I don't know how well these numbers match up to real-world performance, but they do seem to behave according to my expectations and, right or wrong, the engine does run smoothly far lean of peak. For now, unless I'm below 4000', my throttle is going to stay wide open.

 

[Bobsk8 12,735]

nice write up...

 

in the 152 i fly, i lean til it starts to misfire, then screw the nob in til it smooths out, then an extra turn for good measure....

 

in the skyhawk i fly, it has an egt guage temp, pull out til it starts to spike, then put it back in....

 

 

 

edit:

 

one thing i've never understood...in all the POH, and other stuff you read, and the way you're taught, AND on every checklist, they refer to fully rich as "full power" or "mixture set to best power"....the reason that don't make sense is, because of a long time of working on engines, building race engines, and drag racing cars(legally)...anyone can tell you, the leaner you go, the more power you make. unfortunately it also makes more heat, but you can compensate with higher octane fuel, which in turn burns a bit cooler to avoid detonation...i.e, which is why high compression motors use high octane fuel. heat is bad mmmmkay.

 

When I started flying back in the late 70's the 152 was using 100 LL fuel. If you taxied with full rich, the plugs would lead foul, ( this is in South Florida), and this would show up when you did the mag check. Get a drop of 300 rpm or more, and you either had to clear the fouled plug by leaning the mixture at around 2100 rpm, and then try the mag check again, or taxi back to the ramp to have a mechanic pull the plug. So what we did , was lean it out while taxing to the active runway, and then prior to takeoff go to full rich. Sometimes we would lean it a tad too much during taxi, and the engine would quit. If you had a passenger, this would make their hair stand up straight when they would see and hear the prop suddenly stop spinning. You would quickly have to explain to them why this happened, but a few of these first time fliers were not too thrilled with having the engine quit while you were taxiing...

 

I actually took off one day, and got to 1,000 feet agl, and the engine began running really rough, cause one of the plugs had lead fouled. I think I had a 300+ drop on one mag, at that point. I tried clearing it by leaning while flying around the airport traffic area, and couldn't and told the tower that I needed to land, which I did. Never had that problem with the C-172 which I flew alot.

[scotchegg 4,186]

Interesting post. TBH in all my simming years I never worried much about complex engine management (ducks) but ironically Cliffs of Dover got me into it and I'm beginning to appreciate the added dimension it can add to the immersion. Kind of a shame to note a lot of it isn't modelled in Flight.

 

Also another embarrasing question, is there a tutorial / explanation about prop percentages somewhere? I assume it's talking about prop pitch and I think I understand the theory, but in CLOD getting the pitch wrong can blow your engine quite quickly, but it seems to have 0 effect on the engine in Flight unless I'm missing something?

[RoboRay 126]

Interesting post. TBH in all my simming years I never worried much about complex engine management (ducks) but ironically Cliffs of Dover got me into it and I'm beginning to appreciate the added dimension it can add to the immersion. Kind of a shame to note a lot of it isn't modelled in Flight.

 

Also another embarrasing question, is there a tutorial / explanation about prop percentages somewhere? I assume it's talking about prop pitch and I think I understand the theory, but in CLOD getting the pitch wrong can blow your engine quite quickly, but it seems to have 0 effect on the engine in Flight unless I'm missing something?

 

With a Constant Speed Prop, the actual prop pitch is controlled automatically, driving the prop to the correct angle for your selected RPM, speed, current power demands, and probably some other variables.

 

What you are doing with the somewhat-misnamed "prop control" is simply selecting your desired engine RPM for the governor to attempt to maintain. The actual prop pitch will be changing on its own throughout the flight, to stay at the target RPM.

 

CLOD probably had Variable Pitch Props that do control the blade angle directly.

[scotchegg 4,186]

Thanks Ray, that's clear.

[zinfinion 0]

@RoboRay: My initial post was indeed slightly misleading when I said:

 

At lower cruise altitudes the WOT no longer applies and you need to throttle back to 75% or less power...

 

What I should have said was: At lower cruise altitudes the WOT no longer applies unless power can be reduced to <75% through RPM alone. If this can be done through RPM alone, the throttle can be left wide open. But if power is still above 75% after reducing to cruise RPM, then throttle will need reduced as well. (Obviously above 8000', since an engine can never make more than 75% power, any combination of throttle, prop, and mix can be used without worrying about harming the engine.)

 

Given that sea level MP is 29.6" and it drops roughly an inch every 1000', if you cruise at 2000 RPM in the Maule you can run WOT/LoP pretty much from 1000' on up.

 

@scotchegg: I'm no expert on warbirds but that sounds like an overspeed situation perhaps. Constant speed props use the governor to prevent both underspeed and overspeed from occurring.

[DMagick 0]

I try to watch the fuel flow. But mostly just listen to the engine. The problem with that is it doesn't seem like altitude matters much in Flight, or at least very little difference. I find myself constantly going into settings to make sure it's still on manual control. It says to lean the engine at above 5000 ft. But I find that even at a few hundred feet or maybe just 1000 the mixture can be leaned to about 25% before there is significant sputter. I mean, it just feels like the entire % range is wasted in a sense... like there is 2 settings. Full rich for take off and landing, and leaned at about 25%ish.... while flying at pretty well any altitude.

 

Any pilots out there, is this really how quickly you can lean? Or do you go in stages like acutally leaning a little bit at a time as your altitude climbs above 5000 ft?

 

I also don't notice much of an effect at relatively low altitudes, but when you get a bit higher, around 10k ft. for most of the planes, or 15k-20k ft. for the P51 (and I assume the other warbirds) you really have to lean or else you start losing power.

 

While flying up to McKinley in the Stearman I had to bring the mix down to 20% when I was at around 20000ft. Below 15% the motor would start to sputter, above maybe 35 or 40% I would start to lose a lot of power, and the engine would cut out at really high levels. So there is a pretty narrow range of useful fuel mix levels if you get high enough.

 

The problem is that most of the flights I make are way too short to justify getting up very high, and all the non-warbird planes struggle above 10k anyway. So aside from managing fuel flow for longer flights, or heavier loads, closely managing fuel mix doesn't seem all that important.

[RoboRay 126]

Thanks Ray, that's clear.

 

Glad to help. You may also find this interesting...

 

http://www.avweb.com/news/pelican/182082-1.html

[zinfinion 0]

So aside from managing fuel flow for longer flights, or heavier loads, closely managing fuel mix doesn't seem all that important.

 

My main benefit from all this is I can reliably hit 75% power (lower if above 8000') through adjusting throttle and RPM (which also gets me to a consistent IAS, weather aside [also great for timing legs flying VFR in mountain passes]), and then when I set the mix for that combination of power and altitude I know roughly what my fuel flow will be allowing me to be very precise in the amount of fuel I take on overloaded cargo runs.

 

Obviously it's up to everyone just how deep they want to go with all this. I find for me that it helps me "click" better with the planes and gives me more mastery over them, and reduces a number of things that were previously complete unknowns.

 

You may also find this interesting...

 

One of these days I will sit down and read all of his articles. I swear I spend more time learning than flying. :lol:

[RoboRay 126]

What I should have said was: At lower cruise altitudes the WOT no longer applies unless power can be reduced to <75% through RPM alone. If this can be done through RPM alone, the throttle can be left wide open. But if power is still above 75% after reducing to cruise RPM, then throttle will need reduced as well. (Obviously above 8000', since an engine can never make more than 75% power, any combination of throttle, prop, and mix can be used without worrying about harming the engine.)

 

Given that sea level MP is 29.6" and it drops roughly an inch every 1000', if you cruise at 2000 RPM in the Maule you can run WOT/LoP pretty much from 1000' on up.

 

That is the way I understood it. I selected 2300rpm simply because that is what Flight's M-7 cruise checklist specifies.

 

Unless I'm misunderstanding, with throttle wide open and mixture full rich, the engine should be at 75% rated power at 2300rpm and 4000' (yielding 25" MP), making this as hard as the engine should be continuously run in level flight. That's why I chose it, to see how high the temps might go at peak in the lowest altitude appropriate for the checklist cruising RPM (at 75% power), then to see how the temps would drop leaned back to my normal cruising speed (well below 75%).

 

[zinfinion 0]

...with throttle wide open and mixture full rich, the engine should be at 75% rated power at 2300rpm and 4000' (yielding 25" MP)

With throttle wide open and mixture between full rich and peak EGT. Page 3-6 of the IO-540 manual (and practically every other engine manual, page number aside).

 

 

With regards to the highlighted bit. That is not percent engine power. That is percent of percent engine power. So if you have manifold and RPM set to 75% power, if you set mix to RoP as shown in the Max Power Range, you will get 100% of 75% power. If you set mix to LoP in the Best Economy Range you will get 93% of 75% engine power for an actual 69.75% engine power. You can also see the Fuel Consumption drops tremendously from full rich to Best Economy (hence the name). With modern tuned injection engines there is really no benefit to running RoP unless you absolutely need 100% of a given power percentage.

 

If you want to check out full rich never being max power, not even at sea level, start at Honolulu, follow the same steps as my Bradshaw experiment and you will see RPM rise from 2628 at 100% mix to around 2649 at 75% mix, after which it drops again (obviously indicating full power at 75% mix). The mix levers are set to provide excess fuel at 100% mix for engine cooling during climb (100% power at sea level when at max RPM, declining as altitude is gained), so 100% mix will never be max power. At sea level you would NEVER lean to max RPM as cooling from the excess fuel is far more important. As well, ram air + windmilling (new one, learned that from the article you just linked) will quickly make up the 72 RPM short of 2700.

 

At Bradshaw however the engine is already far below 100% power due to altitude and the air is cooler, so excess fuel is not needed as much. That said, at high altitude you lean to max RPM and then stop pulling on the lean knob. The max RPM lean range actually goes quite a bit lower, down to around 40% mix if I recall correctly. That would not be wise to takeoff at.

 

And I got way more long winded than I intended to but hopefully all this helps.

[Mirtma 0]

Good reading!

[jcomm 5,939]

Whow, from my laptop readings in this and other threads, here at the sunny beach, all I can say is that, just for it, MS FLIGHT was already a great investment...

 

I'll get a new license guys when I get back home, start a new career - with zero crashes as my main goal, like in RL - and buying only one cockpitless aircraft - the Cub that comes with Hawaii...

 

Meanwhile I got another fan into the group thx to my temporary - as usual - quarell with MS FLIGHT... He's making good use of the full install, cockpitless included :-)