Sanity check. Once leaned at altitude, it shouldn't matter if you change RPM, if you haven't changed altitude?? I never need to do this in normal ops, but my break-in procedure suggests a change in RPM every so often. Break-in is leaned rich of peak.
RPM vs LOP
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If you change the amount of air (throttle control) you will need to change the amount of fuel (mixture control) if you want to keep the same proportionate mixture.
IOW, if you give it more air (throttle) without also giving it more fuel (mixture), it becomes more lean.
So you adjust both controls.
IOW, if you give it more air (throttle) without also giving it more fuel (mixture), it becomes more lean.
So you adjust both controls.
Let'sgoflying!
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and this can be verified by the engine charts in your poh. It will usually show %bhp for each p.a./mp/rpm
The engine automatically adjusts fuel delivery to account for the change in air intake as throttle position or RPM is changed. That’s the basic function of a carburetor, for example.
However, aircraft engines generally have fixed ignition timing, which means that if the engine is slowed there is a little more time in each cycle for the charge to burn. Therefore if you reduce RPM the effect is like enrichening the mixture a tad, dropping EGT slightly. The effect is likely less if you are rich of peak to begin with.
However, aircraft engines generally have fixed ignition timing, which means that if the engine is slowed there is a little more time in each cycle for the charge to burn. Therefore if you reduce RPM the effect is like enrichening the mixture a tad, dropping EGT slightly. The effect is likely less if you are rich of peak to begin with.
I'm now flying at 7 or 8 thousand, so no change in throttle typically. Engine is fuel injected. Does that change the equation?
I've been running roughly 90-100 deg ROP at 2500rpm. I was thinking pulling it back to 2300rpm for 5 or 10 minutes is not going to be all that different a mixture at stable altitude. Based on resultant fuel flow, it seemed about what I had expected.
I've been running roughly 90-100 deg ROP at 2500rpm. I was thinking pulling it back to 2300rpm for 5 or 10 minutes is not going to be all that different a mixture at stable altitude. Based on resultant fuel flow, it seemed about what I had expected.
A change in either rpm or manifold pressure will change the amount of air moving through the engine. The fuel rate will need to be adjusted to match. It may not need much but I would anticipate something.
Dan Thomas
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Carburetors and mechanical fuel injection automatically adjust the fuel flow for the airflow. Sometimes it's not all that linear, but it does change it. It would be nearly impossible to manage the engine if we had to be working two knobs constantly. What would happen, for instance, when you open the throttle for takeoff and the fuel flow didn't rise?
Altitude and air temperatures are the chief reasons for mixture adjustments. Density altitude. In the takeoff and initial climb, the mix usually needs to be richer to keep the CHTs within reason and to prevent detonation. In cruise, and leaned as required, if you are changing altitude with cruise climb or descent, you should be watching the EGTs and managing the mixture.
Some of you need to study this: https://aerotoolbox.com/carburetor/
And this: https://www.aircraftsystemstech.com/p/fuel-injection-systems.html
Get rid of the misconceptions.
Altitude and air temperatures are the chief reasons for mixture adjustments. Density altitude. In the takeoff and initial climb, the mix usually needs to be richer to keep the CHTs within reason and to prevent detonation. In cruise, and leaned as required, if you are changing altitude with cruise climb or descent, you should be watching the EGTs and managing the mixture.
Some of you need to study this: https://aerotoolbox.com/carburetor/
And this: https://www.aircraftsystemstech.com/p/fuel-injection-systems.html
Get rid of the misconceptions.
Rgbeard
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Any time I change anything, it causes me to scan all Temps and Pressures including fuel flow. And of course, that scan is ongoing through the flight.
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tsts4
Pattern Altitude
IMO, don't worry about constantly adjusting the mixture for your break-in. Set the mixture up where you need it to be at the highest RPM you plan to use, then change the RPMs at will. Just keep an eye on the CHTs as heat is the enemy of a good break-in.
Stewartb
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Engine break-in is usually done at lower altitudes. You’re goal is to maintain high cylinder pressures.
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What's the various combinations for 75% on your engines. I'm guessing there's a 2,300 RPM answer and a 2,500 RPM answer.
Alternate between those, and enjoy.
Alternate between those, and enjoy.
jesse
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What this guy says. The carb/fuel injection attempts to maintain the same-ish ratio while you adjust the throttle, and they do this fairly well.
As a pilot, you may want a different fuel/air ratio for different power settings and that is primarily why you adjust the mixture sometimes while making power changes. A real-life scenario might be wanting a much richer mixture while at full-power and a much-leaner mixture while at reduced power in cruise.
Ed Haywood
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In theory, there is no difference between theory and reality. In reality, there is.
If I change power settings or altitude, I check my lean. Can't hurt.
If I change power settings or altitude, I check my lean. Can't hurt.
jesse
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This all depends. Is there a turbo? What altitude are you at? Constant speed prop? Etc.
For example. In a turbo airplane:
- If you are leaned way out because you are crusing at 55% power, and suddenly increase to 100% power, you absolutely will need to increase the mixture (and do so prior/while making that power change). As you need a richer fuel mixture at 100% power.
- If you are leaned for 80% power, and decrease to 50% power, you would probably want to lean the mixture some more yet to save fuel. You can more safely run at leaner mixtures while at lower power settings.
Key detail: The above is not because your fuel system doesn't maintain the same fuel/air ratio at different power settings (like others incorrectly said). It is because you NEED/WANT a different fuel/air ratio at different settings and you as a pilot need to command that. In a car, your ECU is constantly changing the fuel/air ratio based on the environment, engine load, and other factors.
If you are not in a turbo airplane, fixed-pitch prop, and lets assume you are cruising at 8,000 ft:
- Most likely you'd be WOT, and leaned as appropriate for that. If you reduced RPM, you want to lean out a little more, as you can get by with a leaner fuel mixture. Leaner fuel mixtures (in the right scenario) are better for your plugs, and save you money.
- Lets say you were at 40% power, leaned for that, and decided to increase to WOT...Then you might want to go a little richer.
- That all said, without a turbo, at 8,000 ft..It doesn't really matter what you do with the mixture knob as you won't make enough power to hurt anything.
Every scenario will result in a slightly different answer. There are thousands of scenarios you will encounter as a pilot. What you should do will change depending on the equipment, and countless other factors. There is no "short cheat list" for what you should do. You need to understand the relations of power, mixture, and temperature. Once you really understand your engine you will know what you should do.
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This is key. To get the high power/high pressure you want for the break-in, it's best to fly low, wide-open throttle (if your POH allows), and with a rather rich mixture (say, 150-200 deg F ROP) to stay outside the "red box". From there, slight RPM changes affect conditions a little bit, but I think you'd be overthinking things if you tried to re-lean after each slight RPM change.
- Martin
Dan Thomas
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As others have said, 8000 is too high to get 75% on a normally-aspirated engine. You need to be down lower. The POH for your airplane hass all the dope on that. Check the cruise charts. You DO have a POH, right?
If the engine is run at lower power levels than recommended, the break-in will not go well. The cylinders and rings will glaze and stop wearing to mate with each other, and the engine will use oil too much its whole life.
If the engine is run at lower power levels than recommended, the break-in will not go well. The cylinders and rings will glaze and stop wearing to mate with each other, and the engine will use oil too much its whole life.
schmookeeg
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Yep. As low as practicable (I usually do 2500' MSL), I fly the thing wide open throttle, 200dF ROP, and change RPM to a random value between 2500 and 2700 every 10 minutes. I don't re-lean on the RPM changes, since the fuel controller is supposed to do the "basic adjustment" for me, and this is hardly a precision operation. If I end up in the yellow arc, I may climb another 1/2000' or so, but I try hard not to.
Usually you can watch the rings break at around the 30-45mn mark with dropping CHTs.
Usually you can watch the rings break at around the 30-45mn mark with dropping CHTs.
Thanks for the altitude advice. As stated above, I flew the first several hours below 5,000ft and at max cruise RPM, as instructed. Now the published procedures say to alternate between 65% and 75% power, until 10 hrs of flight time, at up to 8000 max altitude. My plane is a 1999 and I believe they stopped publishing percent of power curves prior to that. There are a variety of acceptable power setting pages, but they only list fuel flow and speed an various RPM, lean conditions and altitude.
I believe we lose about 3% of horsepower for every 1,000ft of altitude. I've only done 8,000ft for 2 hours, at max cruise RPM and 100ROP. I'm just guessing that was between 65% and 75%. Altitude alone took her down to 75% and I was running her as hard as the book allows.
I believe we lose about 3% of horsepower for every 1,000ft of altitude. I've only done 8,000ft for 2 hours, at max cruise RPM and 100ROP. I'm just guessing that was between 65% and 75%. Altitude alone took her down to 75% and I was running her as hard as the book allows.
Dan Thomas
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Stewartb
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Electronic FI can control mixture based on inputs from sensors. Airplanes use mechanical injection. There are a couple of exceptions in the exp world, but even there mechanical injection is more common.
jesse
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I don't have a POH for a '99 Beech 58 to say what is in there. But just some random guesses based on experience flying similar things. I assume you have a IO-550-C?
25" & 2500 RPM will probably be about 80% power ROP and 75% LOP
23" & 2300 RPM will probably be about 70% power ROP and 65% LOP
So cruise somewhere around there, change things up a bit, and you should be good. Reality is, at this point, you're probably over-thinking it. I think if you just flew as you normally flew while respecting the 8,000 ft limitation you'd be doing what they wanted with the power settings.
Without a computer constantly calculating all the changing environmental factors, it is hard to know exactly what your % power is. In the right region is good enough for this stuff.
Ummm, what are you flying?
schmookeeg
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sarangan
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Actually, in the case of a carburator, increasing the air flow will increase the fuel flow, but not in a linear fashion. The mixture knob is there to correct for this nonlinearity. This follows from the orifice plate effect.
http://sarangan.org/aviation/2020/10/02/what-exactly-does-mixture-control-do/
Clip4
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If you change the RPM, you change manifold pressure.
Dan Thomas
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The orifice plate (throttle plate) simply controls the amount of air, or more correctly, the velocity of the air passing though the venturi. It's that velocity that creates the pressure differential that forces fuel out of the bowl, through the main jet, and out of the nozzle, which is in that venturi. It operates in an acceptably linear fashion. If it didn't, we'd have engines running so lean at 1/4 throttle that they quit, and so rich at full power that they'd quit. Or vice-versa. But they don't quit, because that mixture stays relatively constant throughout the throttle travel. Only at idle do we need idle fuel outlet ports near the edge of the mostly-closed throttle plate, because there isn't enough velocity through the venturi to draw any fuel through the main nozzle.
Now, that velocity is the same whether we are at sea level or at 15,000 feet, for any given throttle setting. But the density of air, the weight per cubic foot of it, is much lower at altitude due to the much lower atmospheric pressure lowering the density of the air. Even at sea level, a hot day will reduce that density. So we need to adjust the mixture manually to get the air/fuel ratio back closer to what the engine needs.
Dan Thomas
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The RSA injection system used on Lycomings:
The Continental system used on those engines:
See any computers there anywhere?
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Llewtrah
Not sure that’s right.
The mixture knob adjusts the ratio of fuel to air, to compensate for less air (density) as one climbs.
The throttle controls the volume of the fuel-air mixture (set by the mixture knob and the altitude), to control the power for that mixture ratio.
For a fixed altitude as described, once the mixture is set, the throttle varies the power - not the fuel-air ratio, I’m pretty sure.
Stewartb
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The mixture control in surprisingly simple. The fuel bowl has a low chamber that feeds the main jet. The feed is controlled by a simple barrel ith a slot. When the slor aligns with the jet feed, you’re full rich. Pulling the mixture rotates the barrel and reduces the slot area to feed the jet, so restricts available fuel. Pull it all the way and the slot rotates until no fuel is available. Simple and effective.
Clip4
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Fuel injection in a piston airplane is purely a mechanical system. The computer that controls the mixture in a piston airplane is called the pilot.
Stewartb
Final Approach
EFI is up and coming in the exp ranks. I have friends that run EFI so lean of peak they need pilot-adjustable louvers to block airflow to keep CHTs warm when LOP. Give it 30 years and it’ll trickle into the standard category.
Not sure what you find fishy. It's listed at the bottom of each of my posts. 1999 Baron 58 and, as another asked to confirm, it has TCM IO-550-C engines. Two of them, believe it or not.
This is probably true. I've typically been running at different altitudes and power settings for each leg, which are usually 30-45 minutes, and not messing with them. My last leg was nearly 2 hr, which seemed a bit long to leave at one setting, so I pulled 2500 rpm back to 2300 rpm for a bit. I was running fairly rich 100ROP and the new lower fuel flow at 2300 was still a tad higher than I'd have expected to lean ROP at that power setting. Both were WOT. I'm sure it was fine, as both power configurations were probably a bit excessively rich, but it is what got me thinking. In 30 years of flying, I've never felt the need to change RPM mid-cruise. Not even on several past break-ins, for which I had never read every word of the manufacturers instructions either.
What I'd really like to do is get this break-in over with, by taking a good long flight. Say 2.5-3.0 hours each way. That would be too long to set and forget.
Yes, I have a JPI for each cylinder. I didn't really start to pay close attention to each CHT, until about 3 hours in. All were running in the 325-340 range, with one exception running at 380, when at cruise and leaned 100ROP. I have a funny feeling I missed it.
Well, sorta.
(I really oversimplified things in my first post, rather than taking the OP down a rabbit hole, but given the technical nature of the forum that was probably a mistake.)
In a carburetor, air passes through a venturi, creating a vacuum which draws fuel into the air. The throttle is regulating this flow, hence indirectly regulating the amount of vacuum and thereby how much fuel is drawn in. Thus the mixture does change with throttle position and sorta/kinda works okay.
The problem is that the amount of vacuum created by the venturi (and thus fuel flow) is proportional to the square of the speed of the flow through the venturi, whereas we'd like the mixture change to be closer to linear. The mixture will therefore only be "correct" at one throttle position. Automobiles that use carburetors have multiple fuel jets to tweak the mixture change to be more linear as throttle position varies.
In airplanes we use a mixture control to do this.
As an extreme example to illustrate the OP's question, consider ground operation of an airplane. Altitude is 100% constant. At idle on the ground, we lean the mixture; otherwise it will be much too rich and in time will foul the sparkplugs. When we go to full throttle to begin the takeoff roll, however, we must go full rich or the engine will be starved for fuel and shut down. Altitude is constant, but we're changing the RPM dramatically and we must also change the mixture setting.
Ed Haywood
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If all goes well, will be doing this for the first time in about 2 months. I have been putting off ordering an engine monitor. Gonna get off my wallet and do that today. Sounds like 4 cylinder CHT is extremely helpful for that process.
Dan Thomas
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Yes, I missed that footer.
I found an online AFM for the 2005 B58. No percent given in the power charts, as you say. Then I checked the Continental M-O manual, and it has the same charts, no percent power given. The one manual I couldn't find free on the 'net was the IO-550-C Operator's manual, but I suspect that it, too, won't have the stuff.
I wonder why they did that. Do they think today's pilot's don't understand percent power?