Wide Open Throttle

[2nd505th]

Seen some threads on this but may have missed any applicable to my aircraft: A-152 O-235-L2C, no EGT. Tach redlines at 2550, though variant shows:

Power 115 hp (86 kW) at 2700 rpm, alternate ratings of 115 hp (86 kW) at 2700 rpm, 112 hp (84 kW) at 2600 rpm, 110 hp (82 kW) at 2550 rpm and 105 hp (78 kW) at 2400 rpm

Not sure why I cannot run at 2700 or why they show these variants (airframe related)?

I'm new to this concept. It was an A&P that told me:

Throttle forward from takeoff to cruise, then lean, stay WOT.

He said engine seals up nicely at high RPM. If oil pressure stays high it means the hot thinner oil is not getting past seals. Its good for the engine and saves fuel. Also said never idle below 1200 rpm.

My questions that would apply to my AC:
1) So stay WOT and lean to stay out of red on the tach and or until slight needle drop?
2) Applicable to any altitude?
3) What is the procedure to descend - lean w/o throttling back?

Appreciate any answers.

 

[RussR]

The general advice for WOT through cruise is useful, but generally only in constant-speed-prop airplanes where you can also control the rpm. In a fixed-pitch prop, you can only get away with doing this if you're climbing to a high-enough altitude to where the engine can't make enough power to maintain redline rpm anyway.

Leaning at WOT isn't going to noticeably reduce rpm until it REALLY starts to reduce rpm because the engine is starving. So that's not an answer. Mixture isn't an RPM control.

Besides, running with WOT in a fixed-pitch prop at low altitude is really loud.

 

[wilkersk]

Is the A&P telling you to lean at high power settings? Or, is he telling you to stay WOT till you get to cruise altitude, then set cruise power, and then lean?

Without CHT gauges, I'd be very cautious about aggressively leaning above about 65% power.

The Red Box

As far as getting your C-152's RPM up to 2,700, when you level off, leave the throttle full open as you trim the aircraft for level flight. Once you're trimmed out, watch your RPM. This is where you'll get your highest RPM and your fasted airspeed. If you're just playing around below 3,000' and want to see how fast your airplane will go, try this. Once you get all trimmed out and RPM approaches the max continuous limit, reduce the throttle 50 rpm or so and leave the mixture full rich. Its good to know what your airplane and engine are capable of.

 

[2nd505th]

Is the A&P telling you to lean at high power settings? Or, is he telling you to stay WOT till you get to cruise altitude, then set cruise power, and then lean?

Without CHT gauges, I'd be very cautious about aggressively leaning above about 65% power.

The Red Box

As far as getting your C-152's RPM up to 2,700, when you level off, leave the throttle full open as you trim the aircraft for level flight. Once you're trimmed out, watch your RPM. This is where you'll get your highest RPM and your fasted airspeed. If you're just playing around below 3,000' and want to see how fast your airplane will go, try this. Once you get all trimmed out and RPM approaches the max continuous limit, reduce the throttle 50 rpm or so and leave the mixture full rich. Its good to know what your airplane and engine are capable of.

Not my A&P, don't really have a steady one. But he said WOT and lean it back at cruise. I wasn't sure if he meant lean to stay below redline or just to conserve fuel.

 

[mondtster]

Seen some threads on this but may have missed any applicable to my aircraft: A-152 O-235-L2C, no EGT. Tach redlines at 2550, though variant shows:

Power 115 hp (86 kW) at 2700 rpm, alternate ratings of 115 hp (86 kW) at 2700 rpm, 112 hp (84 kW) at 2600 rpm, 110 hp (82 kW) at 2550 rpm and 105 hp (78 kW) at 2400 rpm

Not sure why I cannot run at 2700 or why they show these variants (airframe related)?

Let’s start with this part. What does the limitations section of your flight manual say about maximum engine speed? Where should the redline on the tach be? What horsepower is the engine rated at? Are there any STCs applied that would affect this?

 

[Jim Carpenter]

To your question about maximum rpm, here's a link to the Type Certificate Data Sheet for your 152 (you have to scroll through to the appropriate section.)
DEPARTMENT OF TRANSPORTATION (faa.gov)
This details the original certification specs, note that your O235-L2C is limited to 2550 as a redline. As you guessed, this has to do with propeller and airframe. The other rpm specs are for other airplane models, like the 112hp/2600 for Tomahawks, etc. So, even though the engine itself can go higher, in your aircraft it cannot.
I agree with others, I'm not sure what the A&P is getting at, but at level cruise, with WOT, you may be bumping past redline rpm, so you'd want to back off the throttle a bit first, then lean for altitude, if needed. Quick question, just for clarity, at what altitudes are you usually operating? (home base elevation, average cruise altitude, etc.)

 

[Dan Thomas]

Fixed-pitch-prop airplanes are typically propped for redline RPM at full throttle in level flight at sea level. As you climb, the prop drag is less, but so is the HP, so the redline at WOT is typical. I'd be surprised if you got more than redline in that 152.

And never idle below 1200? That's nonsense. Some of it comes from checklists that say to reduce RPM to 1000 or something like that after the runup. It doesn't mean you should taxi at 1000 or 1200; that's a good way to burn out your brake pads and discs and eat up tires, and it pulls the nose down so that the prop picks up a lot of junk with that much airflow and strong prop tip vortices. Wrecks the prop.

Engine manuals say to set the idle at, typically, somewhere between 600 and 700 RPM. If the engine manufacturers didn't want the engine running below 1200, that would be their idle setting.

The A&P is not the final authority on engine operation. The engine and airframe manufacturers are. The 152 POH says this about starting:



It says the same thing for after the runup. 1000 RPM or less. They don't say how much less, do they?

For taxiing it says this:



At 1200 RPM you'll have too much speed or you'll be using a lot of brake.

This is the O-235/O-290 Operators Manual, published by Lycoming. Read it and see if you can find in there anywhere that says that the engine shouldn't be run below 1200 RPM. https://www.lycoming.com/sites/default/files/O-235&O-290 Operator Manual 60297-9.pdf

 

[mondtster]

Fixed-pitch-prop airplanes are typically propped for redline RPM at full throttle in level flight at sea level. As you climb, the prop drag is less, but so is the HP, so the redline at WOT is typical. I'd be surprised if you got more than redline in that 152.

And never idle below 1200? That's nonsense. Some of it comes from checklists that say to reduce RPM to 1000 or something like that after the runup. It doesn't mean you should taxi at 1000 or 1200; that's a good way to burn out your brake pads and discs and eat up tires, and it pulls the nose down so that the prop picks up a lot of junk with that much airflow and strong prop tip vortices. Wrecks the prop.

Engine manuals say to set the idle at, typically, somewhere between 600 and 700 RPM. If the engine manufacturers didn't want the engine running below 1200, that would be their idle setting.

The A&P is not the final authority on engine operation. The engine and airframe manufacturers are. The 152 POH says this about starting:

View attachment 104898

It says the same thing for after the runup. 1000 RPM or less. They don't say how much less, do they?

For taxiing it says this:

View attachment 104899

At 1200 RPM you'll have too much speed or you'll be using a lot of brake.

This is the O-235/O-290 Operators Manual, published by Lycoming. Read it and see if you can find in there anywhere that says that the engine shouldn't be run below 1200 RPM. https://www.lycoming.com/sites/default/files/O-235&O-290 Operator Manual 60297-9.pdf

Dan, I’m reading between the lines here but based on the OP’s recent thread(s) asking about an engine overhaul I’m going to guess that some of the idle statements are directed at engine break in. I’m also guessing some of the other things concerning wide open throttle may be as well. In that context the original post makes more sense.

 

[Dan Thomas]

Dan, I’m reading between the lines here but based on the OP’s recent thread(s) asking about an engine overhaul I’m going to guess that some of the idle statements are directed at engine break in. I’m also guessing some of the other things concerning wide open throttle may be as well. In that context the original post makes more sense.

I don't see any mention of a freshly-overhauled engine. The definitive procedures for break-in are here: https://www.lycoming.com/sites/default/files/Lycoming Reciprocating engine Break-In and Oil Consumption.pdf

This document applies more to field-overhauled engines, not factory overhauls. The factory runs them in a test cell for about 20 minutes before shipping, and the test flight comes after installation with minimal ground-running.

The instructions are to run it at 1000 RPM until the oil temp comes up some, then at 1500 for 15 minutes, and at full power for 10 seconds. Nowhere does it tell one to never idle it at 1200 minimum after the initial break-in runup and flight. They do warn against power-off descents to avoid ring flutter, but even at power-off the RPM will be at around 1300 due to windmilling.

 

[mondtster]

I don't see any mention of a freshly-overhauled engine.

Right, which is what makes this challenging to create a decent response. I’m guessing on this based on other recent threads the OP has created. I’m betting this is either a new engine or one that has had recent cylinder work.

Regardless, I feel that a thorough review of aircraft limitations coupled with some engine theory research might best answer the OP’s questions.

 

[2nd505th]

To your question about maximum rpm, here's a link to the Type Certificate Data Sheet for your 152 (you have to scroll through to the appropriate section.)
DEPARTMENT OF TRANSPORTATION (faa.gov)
This details the original certification specs, note that your O235-L2C is limited to 2550 as a redline. As you guessed, this has to do with propeller and airframe. The other rpm specs are for other airplane models, like the 112hp/2600 for Tomahawks, etc. So, even though the engine itself can go higher, in your aircraft it cannot.
I agree with others, I'm not sure what the A&P is getting at, but at level cruise, with WOT, you may be bumping past redline rpm, so you'd want to back off the throttle a bit first, then lean for altitude, if needed. Quick question, just for clarity, at what altitudes are you usually operating? (home base elevation, average cruise altitude, etc.)

Usually I'm somewhere around 3k and 5k msl.

 

[wilkersk]

Not my A&P, don't really have a steady one. But he said WOT and lean it back at cruise. I wasn't sure if he meant lean to stay below redline or just to conserve fuel.

Read that article in the link. I think you might get something out of it.

WOT until you reach cruising altitude, level off, let the airplane accelerate. Then, reduce throttle to a cruise power setting, then lean. At least, that's how I was taught.

For descent, this is what I do: push the nose over and use throttle to manage RPM (there's no need to pull the throttle to idle). Push the mixture in as you descend. I will stick half the amount I leaned back in on initial descent, and then stick the rest back in (full rich) as I descend through around 3,000'msl.

 

[2nd505th]

Right, which is what makes this challenging to create a decent response. I’m guessing on this based on other recent threads the OP has created. I’m betting this is either a new engine or one that has had recent cylinder work.

Regardless, I feel that a thorough review of aircraft limitations coupled with some engine theory research might best answer the OP’s questions.

My question does not have to do with an overhaul or cylinder though a new cylinder is to occur next week. It really has to do with the best way to treat the engine in flight. After talking to this A&P it seemed it would be better to fly near red-line than 2300-2400 rpm like I usually do. I fly that way because I'm usually never in a hurry, just joy riding - figured I'm putting less stress and saving fuel. But maybe that is not the best approach during cruise.

 

[Salty]

As long as your rpms aren't in a red zone, run it as slow as you want in cruise. If you get it below 65% HP, lean it as much as you want without worry.

What's "best" is to keep the head temperatures down. Running low power does that as long as you aren't climbing at Vx the whole time.

 

[mondtster]

My question does not have to do with an overhaul or cylinder though a new cylinder is to occur next week. It really has to do with the best way to treat the engine in flight. After talking to this A&P it seemed it would be better to fly near red-line than 2300-2400 rpm like I usually do. I fly that way because I'm usually never in a hurry, just joy riding - figured I'm putting less stress and saving fuel. But maybe that is not the best approach during cruise.

Then I would ignore what this mechanic has to say and focus on what the flight manual tells you. Particularly in the limitations section.

 

[Dan Thomas]

This is the 1980 152 POH cruise chart:


The dashed lines are power settings not recommended for cruise, or at least not leaned as per the "Conditions" statement at the top. So nothing over 76% power should be aggressively leaned. You can get destructive detonation doing that. Lycoming changed their policy on leaning a few years ago from doing whatever you want with the mixture below 75%, to the same but below 65%.

The "Recommended Lean Mixture" is outline in Section 4:



Sometimes when climbing to higher altitudes you need to lean for smoothness, but that won't be lean of peak.