Cessna Corvalis Question

[rddaos]

Quick question on leaning the mixture for fuel economy. 

I've read the POH and the tutorial that come with the Cessna 400.  My understanding is that the peak TIT temperature is indicated by the blue bar on the TIT gauge, and I should operate about 50 degrees lean of peak (at power settings less than 65%).  In fact, best power is reached at 1625 degrees.  Well, the blue bars are right in the middle of the red (danger) zone, at about 1800 degrees.  50 degrees lean of this "peak" is 1750 degrees, which is still in the red zone.  Moreover, the blue bars don't seem to change position as I change the throttle setting, they're stuck at 1800 degrees.  So even if I'm at 50% of power (throttle), I cannot operate at 50 degrees lean of this peak.  Also, it appears as I move the mixture setting, I peak out in temperature - not at the blue bars, but at a significantly lower temperature, so I'm not sure if the blue bars truly represent the peak temperature.  Any suggestions?  If it's as simple as adjusting mixture to hit 1625 degrees, that's pretty easy.  But something tells me there's more to it.  I'm a little foggy on how this leaning procedure works as you can tell  ;D

[JGregory]

Quick question on leaning the mixture for fuel economy. 

I've read the POH and the tutorial that come with the Cessna 400.  My understanding is that the peak TIT temperature is indicated by the blue bar on the TIT gauge, and I should operate about 50 degrees lean of peak (at power settings less than 65%).  In fact, best power is reached at 1625 degrees.  Well, the blue bars are right in the middle of the red (danger) zone, at about 1800 degrees.  50 degrees lean of this "peak" is 1750 degrees, which is still in the red zone.  Moreover, the blue bars don't seem to change position as I change the throttle setting, they're stuck at 1800 degrees.  So even if I'm at 50% of power (throttle), I cannot operate at 50 degrees lean of this peak.  Also, it appears as I move the mixture setting, I peak out in temperature - not at the blue bars, but at a significantly lower temperature, so I'm not sure if the blue bars truly represent the peak temperature.  Any suggestions?  If it's as simple as adjusting mixture to hit 1625 degrees, that's pretty easy.  But something tells me there's more to it.  I'm a little foggy on how this leaning procedure works as you can tell  ;D

rddaos,

Here is the quick answer...

First, make sure you are established in cruise and your throttle and prop settings are where you want them.  Then, switch to the "ASSIST" page.  What you want to do is lean the mixture at a "steady" rate (TIT will increase) until you reach peak and then continue to lean (TIT will decrease) until you are approximately 50-90 degrees lean of peak...preferably about 1625 degrees but this will vary.  You should NEVER run in the YELLOW or RED but can reach those temperatures momentarily as you are adjusting for LOP operations. The blue bars will indicate the peak temperature that was reached during the "current" adjustment.  To reset the bars simply close and re-open the "ASSIST" page.

Hope this helps.

Jim

[rddaos]

Thanks for clearing that up.  I actually wasn't watching carefully enough at first when I first hit peak and the blue bars were set to their max.  So subsequently, I couldn't understand why the bars were frozen.  Now it makes sense.  ;D

-rddaos

[Lukasz]

LOP operations are much more, than pure TIT control. If you want to know more about it, I recommend reading at least the following articles:

http://www.avweb.com/news/pelican/182085-1.html

http://www.avweb.com/news/pelican/182179-1.html

http://www.avweb.com/news/pelican/182176-1.html

http://www.avweb.com/news/pelican/182583-1.html

http://www.avweb.com/news/pelican/183094-1.html

http://www.avweb.com/news/pelican/pelicans_perch_84_mixture_cht_194816-1.html

There is also another fantastically informative and interesting series fromthe same author, started with http://www.avweb.com/news/pelican/182102-1.html

Also there is very good Power Point slideshow, true "LOP at a glance". 100% scientific data!

http://www.eaa42.org/misc/gregs_show.ppt

The best part is, most of this stuff works in X-Plane, especially well in Corvalis Now I'm gonna bug Goran for JPI engine monitor for Sundowner

[rddaos]

Thanks for the information.  But now my cylinder head temperatures went bonkers, and I can't figure out how to get them to come down.  I'm sitting on the runway trying to figure it out.  I've tried running full power and full rich, I've tried running full power and lean of peak.  Temperature is just continually creeping up.  What's the best way to reduce the CHT, please?  Who knows, maybe I have to actually be in the air flying around for the CHT to come down.  Gonna give it try.   

Back to mixture settings.  I've done some reading on mixture control, and it seems most of the articles are long and complicated while presenting very little useful information.  i HAVE, however, taken away a few key points from my reading, i.e. at high power settings, the engine creates lots of heat and the mixture should be rich to cool the engine, but at low power settings, you can run lean of peak for fuel economy.  Oh yeah, and also that during descent, I may need to enrich the mixture since the air density goes up.  But other than that, mixture controls are still a mystery.  By the way, what's the purpose of adjusting the propeller control and manifold pressure?

[Lukasz]

Thanks for the information.

No problem I hope you've found the articles useful!

What's the best way to reduce the CHT, please?  Who knows, maybe I have to actually be in the air flying around for the CHT to come down.  Gonna give it try.

The best way to reduce CHT, in case of air cooled engine, is to put them against a strong airflow of (preferably) cool air. And the easiest way of doing that is to fly the plane So you're on a right track. But if they're too hot in-flight, than either it's a malfunction of some kind, or (more likely) you're pushing engine too hard. If that is the case, you have to reduce power or reduce climb angle to improve airflow.

Another X-Plane specific thing is that you can have only adjustable cowling flaps - either manual or automatic. Fixed cowling, like found on Corvalis, isn't simulated. So planes like it usually have manual cowling set to default value of 75% and left alone. Still, you can accidentally manipulate cowling flaps, causing excessive engine warming or cooling.

Back to mixture settings.  I've done some reading on mixture control, and it seems most of the articles are long and complicated while presenting very little useful information.

Long and complicated usually means useful. Aviation isn't about flying - it's about getting to know how to fly safely and efficiently and only then about how to put this knowledge into practice You've come to a conclusion, that the articles you've read contain very little information. Maybe that's correct, but only after you've read them all, the pieces of puzzle begin to fall into places and suddenly the full picture emerges. You can't solve puzzle with only half of the pieces, right?

i HAVE, however, taken away a few key points from my reading, i.e. at high power settings, the engine creates lots of heat and the mixture should be rich to cool the engine, but at low power settings, you can run lean of peak for fuel economy.

That is correct. As a general rule of thumb, over 65%* of power you want mixture to be as rich as possible. At full takeoff power it's as much as 200-250F ROP (rich of peak). Below 65% of power it doesn't really matter, where you put the mixture, because it won't hurt the engine. The best place for LOP operations is between 50-80F, but in reality you could go as far as 200F LOP and the engine would still be running, though at so low power setting, that horizontal flight wouldn't be possible. And I'm speaking about real life here!

* That's a value as described by Continental. Lycoming says 75%, but real life (and X-Plane) testing have shown, that 65% is better and more "safe".

There are good reasons for that, backed up with data, but for a more thorough explanations I'll send you to the links I've provided. They are very friendly-written, engine management can't be described in a easier way.

Oh yeah, and also that during descent, I may need to enrich the mixture since the air density goes up.

Not necessary, as the engine is probably on low power setting. Besides, running rich at low power settings leads to fouling of sparkplugs inside cylinders. That's the reason behind excessive leaning while on the ground, during warming up and taxiing. Another technique is to run full power for 30 seconds once in every 15 minutes of low power - rich mixture flight. The best way is to enrich before final or as necessary (like engine running roughly), other than that leave the mixture alone on descends.

But other than that, mixture controls are still a mystery.

Only before reading about it

By the way, what's the purpose of adjusting the propeller control and manifold pressure?

These are not simple questions, that can be adressed by a single post on forum. It's a part of pilot's education and training. The articles I've linked to, also cover this areas of plane/engine management.

At a glance, throttle is a mean to reduce engine's power by restricting airflow into the cylinders. The less air - the less fuel can be burned - the less energy is freed - the less power produced. As a fact, engines are the most happy when they can run full rated power, providing there is enough cooling. From engineering point of view, running with partially open throttle is very similar to having fouled air filter. Which is obviously bad.

Mixture in reality does the same. The leaner mixture - the less fuel introduced - the less fuel burned... you get the point. As a side effect, by setting mixture in accordance with outside air pressure, you can maintain desired fuel to air ratio, regardless of altitude (to a point, but you get the idea). Oh, by the way, a saying that you can lean only over 3000ft is a bullshit. You can and should lean on every altitude.

So, what's the best way to operate an engine (that is capable of such "treatment")? You guessed right - full throttle and lean mixture for lower power. That's exactly what is written on last slide in Power Point presentation I've linked to. WOT-LOP-SOP-FTW! Or something like that

But what does mean, that engine is "capable"? It has to be equipped with two devices: balanced fuel injection nozzles and advanced engine monitor. Both aren't really needed in X-Plane, from software point of view, but it's mandatory to have them in real life to perform LOP and if you want to simulate it, then don't attempt LOP on planes that are carbureted** or doesn't have engine monitor with a separate display of CHT, EGT, OIL and so on for each of the cylinders.

** The carbureted engines aren't totally "busted" here, but explaining it in detail is beyond of the scope of this anyway too long post. Again, I'll refer to the articles of my guru, John Deakin (and other great column writers fom avweb).

Ah, yes, the propeller. Fine pitch (blue lever forward) is for maximum power and noise. Coarse (blue lever midway) is for reasonable cruising (read - the best speed). Even more aft is for slow, but efficient cruising (read - max range). The more aft the lever is, the quieter the propeller. If you want to fly realistically, thne don't push the prop lever forward on final approach. Descend and approach with prop on cruise settings. Push it only in two cases: taxi out of the runway or go-around. The same goes for mixture. Leave it and push forward on go-around. The proper "escape sequence" is red forward, blue forward, black forward, hold level flight to pick up some speed (at least Vx or Vy or even more) and only then begin a climb out.

Of course, there is a lot more to the three colourfull magic levers, but you know what to do in order to know more

"And now, we'll show you the proper movie" If you've endured through this post, then you deserve a prize for patience. Here you go:

Only for Corvalis TT or similar, turbocharged, fuel injected, LOP capable plane! It's only a rough guide - consult POH! (and Corvalis happen to have one of the best if not the best manual in X-Plane world)

These procedures would be more or less different for other types of planes, like carbureted or fixed prop or not LOP capable.

flight leg = throttle = propeller = mixture

warm up & taxi = idle = forward = as lean as possible without rough engine running

take off = full forward = full forward = full forward

climb = full forward = -100 RPM = full forward (adjust vertical and horizontal speed for best cooling, 150 KIAS would be good)

cruise = full forward = 2400-2300 = LOP to 15GPH (fuel flow, gallons per hour)

descend = leave prop alone, lower power to maintain safe speed with either throttle or mixture, watch CHT!

approach & landing = as required = at cruise setting = at cruise setting (or as required)

taxi to ramp = idle = forward = lean as much as possible

shutdown = idle = forward = lean to cut-off

That's how I fly, watch the engine instruments and airspeed

[LA]

I'll throw some more info in the pot....

In my case, it's a carbureted engine with a constant speed prop. Since my airport's altitude is 4607' msl, I'll crank over the engine with the red knob full forward, but pull it out about 5/8" just after engine start. I'll adjust it again during the runup. I only have a CHT & EGT for cylinder #3. This is usually the hottest cylinder on a Lycoming setup such as mine, but it's not always the case. Since I don't have fuel injection, nor CHT & EGT to monitor each cylinder, I won't be trying to run LOP.

In flight, I'll be leaning the "old fashion way". Lean until the engine runs rough, and then about three turns rich. This will put my EGT at about 1325 F. Bare in mind, that EGT's can differ, just with variations of where the probes are installed on the exhaust pipe. For me, a smooth running engine & 1325 F. means I'll have clean & un-fowled spark plugs. It's probably 100 degrees ROP, but I can't say for sure.

Since I usually fly at higher mountain altitudes, I'll never go full rich on landing. After takeoff, I usually pull the prop knob back a bit, to lower the sound levels. Cruise is usually between 2350 & 2450 rpms. My engine prop combination has limitations for vibration between 2000 & 2250 rpms. I just can't run continuous between those rpms. 2000 rpm is to slow for my taste.

One option for lower power settings is to keep the black knob full forward, and use lower rpm (blue knob) to fly slower. I don't prefer that method, and will just pull manifold pressure (black knob) back. I'll adjust the red knob (lean/rich en) for different power settings, as explained above.

For landing, the constant speed prop is an excellent speed brake, even while left in the cruise position. As said in another reply, it doesn't need to be pushed in on final approach. But if I'm doing a go-around, or touch and go............then it needs to be pushed in. You can really tell a lack of takeoff power, other wise. But only push it full forward, when the plane has slowed down on final. Otherwise, it's like jamming a motorcycle into low gear, at the wrong time!

And finally, for best results, running LOP really requires an engine monitor for each cylinder, as well as fuel injection. Carbed engines just have to many variances between each cylinder. Those that really get into running LOP, will often change fuel injection orfices, to get the most even cylinder temps possible.

LA

[rddaos]

What's going on is a simple thermodynamic combustion process that produces shaft work.  If it were explained as such, I would probably get it.  Thanks for pointing out that the throttle is merely a valve restricting air flow.  That is VERY important information, whithout which, an explanation a mile long still wouldn't make sense to me.  I'd like to be able to reason my way through the mixture settings from a physics standpoint.  The problem is, there are still (i'm guessing) about 3 more puzzle pieces missing from my knowledge.  So please allow ME to explain to YOU how the Corvalis mixture settings OUGHT to work, and then you can point out the variables I'm failing to account for.

Throttle: Limits the amount of air (oxygen) available for reacting with fuel.  The throttle has 2 sides - the side open to the atmosphere, and the engine side.  When the cylinder head moves to draw air into the combustion chamber, the throttle valve blocks some of the air from moving from the atmosphere side to the engine side.  Therefore, the cylinder head actually draws a partial vaccuum into the cylinder, i.e. the air in the cylinder is less dense, i.e. contains less oxygen.  Therefore, the maximum engine power is achievable with a wide open throttle. Now why an open throttle helps to cool the engine, I can't say.  If cooling is solely attributable to excess air diluting the hot combustion gases (CO2 + H2O), then leaning the mixture should give the same result.  Somehow, in addition to restricting the amount of air that gets into the combustion chamber, the throttle must ALSO be preventing air from passing over the engine block.   

Mixture control: self-explanatory.

Propeller pitch control?  A mystery.  Don't know why it matters.  If the throttle is 1/2 way open, and the mixture is 3/4 toward full-rich, then the air & fuel flow to the combustion chamber is fixed, i.e. the amount combustion energy is fixed, i.e. the amount of power (thrust) the propellers can generate is fixed.  You would THINK then that increasing the pitch would simply make the props harder to move through the air, and there would be a corresponding decrease in rpm - the end result being the increase in pitch is cancelled by a decrease in rpm.  Of course, this assumes the propeller can spin at a different rpm than the engine crankshaft, and that the EGT is unaffected by changing the pitch. 

Exhaust gas temperature.  The hotter the EGT, the more energy wasted to the atmosphere, i.e. the less combustion energy converted to spinning the propeller.  However, one can't tell solely from the EGT how efficiently the engine is running because the mixture effects the EGT as well - hence one watches for the PEAK in EGT.     

One more gap in my knowledge.  Does running rich REALLY provide enough excess fuel to cool the engine at high power settings solely by virtue of the fact that the unburned fuel itself soaks up enough heat to cool the EGT significantly?  Is that the total picture??  If so, wow!

[Lukasz]

There is an additional fuel nozzle, which supplies additional amount of fuel to the engine, but only when throttle is fully open. There is a mechanical linkage that enables that. And yes, that excessive amount of fuel soaks away heat from the engine, cooling it in the process. That's why you have to run full throttle during climb, for that additional fuel makes up for less airflow around cylinders and helps keeping CHT within acceptable limits.

Throttle does nothing to the airflow cooling cylinders from outside. Only airspeed and cowling shape affects that method of cooling.

In a piston engine, there is a direct and constant gear linkage between engine shaft and propeller, be it 1:1 or other gear ratio. Therefore any RPM reduction with propeller lever also affects engine PRM. The engine still "wants" to run at the RPM set by throttle and mixture, but the propeller slows it down to the value set with propeller governor. The slower engine turns, the less power it produces.

So the throttle is set, but the engine turn slower that it should for that setting, which means that it can't suck air fast enough from behind throttle valve, to maintain appropriate "vacuum level". That enables more air moving from before throttle valve to behind throttle valve, causing increase in manifold pressure. By playing with propeller you also affect the engine itself, that's why it changes it's power setting also.

While it is possible to run "oversquare" (in fact it's good for engine, to a point), lowering prop RPM too much while maintaining high power setting can damage the engine. A good rule of thumb for GA planes is no more that 4" oversquare, but consult POH to be sure.

Lowering prop RPM has few uses. It increases propeller efficiency at higher airspeeds, it reduces engine wear, it makes less noise and vibration and indirectly it enables to fly farther. RPM may be lower, but it's the prop blade pitch that is important here. And since it's easier for a pilot to control propeller by it's speed rather that pitch, then that's why it's done like that.

"By changing cruise RPM, we are doing the same thing. A high RPM may produce the maximum power, but at a cost in drag (and fuel). Just like any airfoil, the prop has its optimum angles, too, but data on these are not readily available to the pilot, and they are often overridden by the need for the most efficient airspeeds of the airplane. But generally speaking, for any given power, the lowest possible RPM will reduce friction within the engine, and this may be the most important parameter. Some props are probably more efficient at a specific RPM." - from "Pelican's Perch" column.

EGT in reality measures quality of fuel to air ratio. Before EGT gauges, pilots adjusted mixture by observing exhaust flame colour - which indicated temperature and ratio efficiency. Bright blue flame was an indication of a perfect ratio, as a peak on EGT gauge is today. In reality air doesn't ideally mix with fuel, so you have to introduce a little bit of extra fuel, hence the maximum power is achieved around 50-80F ROP, and not at the peak itself. By the way, you need to adjust mixture and EGT after each altitude and power change, as the peak moves up and down, reflecting the influence of the rest of variables.

Yes, you need rich mixture and full throttle to have as much as possible fuel to soak the heat. However too much mixture leads to loss of power or even carburetor flooding. So rich mixture doesn't always mean the red lever fully forward. Rich - lean is subjective and is judged by EGT for given conditions.