Idle rpm

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woxof

Pre-takeoff checklist
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woxof
In an post about having the throttle closed for start on another forum, someone wrote....

"This technique is valid, but the throttle should be advance to 1000 as soon as oil pressure is register (most horizontally opposed engines) Two reasons. 1. Most engines the cam lobes are only lubricated by what is sprayed past pressure lubricated bearings such as the cam and crank etc. Thus low rpm and low oil pressure will not lube the cam lobes well at low idle. 2. The cam is most highly loaded at low rpm/idle."

Have never heard this information. Is it correct for your typical airplane engine?
 
Digging through my engine info, supposed to idle my engine at >1000 RPM
 
Continental (at least on the 550) recommends 600 RPM, so I don't buy "someone on another forum"'s supposed "universal truth."
 
Right after start, I go to 1000RPM. Lyc O360
 
In my Lycoming 0-360 I’m at 900 after start up. Then I lean and bring it down to 900 again. I stay below 1100 in taxi until my my lowest CHT is 250. No science, just my norm. Takes forever on a 32 degree day in the winter.
 
If you have to idle at 1000 RPM you're going to burn out a lot of expensive brake linings and discs.

The engine manufacturers specify idle RPMs at around 650 or so, not 1000. If 650 was going to eat cams, we'd see a 1000 RPM spec.
 
If you are starting a Cirrus, immediately after the engine fires, you need to set warmup idle to no less than 2000 RPM. I have learned this by observation alone and have not consulted a POH.
 
Dan Thomas said:
If you have to idle at 1000 RPM you're going to burn out a lot of expensive brake linings and discs.
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I will set it to idle at 1000 rpm and lean aggressively, but I have no problem pulling the power all the way out to slow down before applying brakes to stop.
 
My Lycoming manual says to start and maintain airframe manufacturer's specified idle until oil pressure stabilizes, then advance to 1000 RPM. My idle is set to 600 RMP. I've always used 1200 for warming up my Continentals.
 
Crack the throttle for start and you’re automatically around 800-1000rpm right after start-up.
 
Dan Thomas said:
If you have to idle at 1000 RPM you're going to burn out a lot of expensive brake linings and discs.

The engine manufacturers specify idle RPMs at around 650 or so, not 1000. If 650 was going to eat cams, we'd see a 1000 RPM spec.
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Not on my plane. It takes way more than 1000 RPM to get the thing to move.
 
The mechanic should adjust the carburetor for 6-700 rpm or whatever the service manual specifies but the pilot should increase the rpm during operation to whatever the POH says to reduce fouling.
 
My Lycoming manual says 1,000 - 1,200 RPM. I target 1,000 RPM.
 
I'm not an expert on aircraft engines, but every pressure lubricated engine I've ever been around feeds oil through the lifters to lube the cam and the rockers. I find it VERY hard to believe cams in aircraft engines would be splash lubricated.

I believe (and I could be wrong) the reason for running 1000rpm after starting is to get some heat into the cylinders and spark plugs to prevent lead fouling.
 
Jim K said:
I'm not an expert on aircraft engines, but every pressure lubricated engine I've ever been around feeds oil through the lifters to lube the cam and the rockers. I find it VERY hard to believe cams in aircraft engines would be splash lubricated.

I believe (and I could be wrong) the reason for running 1000rpm after starting is to get some heat into the cylinders and spark plugs to prevent lead fouling.
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The cam bearings get oil but the lobes rely on splash oil...
 
Jim K said:
I'm not an expert on aircraft engines, but every pressure lubricated engine I've ever been around feeds oil through the lifters to lube the cam and the rockers. I find it VERY hard to believe cams in aircraft engines would be splash lubricated.

I believe (and I could be wrong) the reason for running 1000rpm after starting is to get some heat into the cylinders and spark plugs to prevent lead fouling.
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The Lycoming cam is above the crank and the lobes get most of their lube by oil thrown off the crank. Continental has it underneath when it gets lots of oil. Oil leaking out of the lifter/lifter bore clearances can't be relied on to reach the cam. There was an aftermarket cam availabe that was drilled to carry oil from the camshaft bearings out to the lobes, something Lycoming could have done a long time ago. They chose instead to go with roller lifters about 12 years ago.

That said, oil is being flung off that crank at 650 RPM, too. The crank just barely misses the camshaft. Most wear happens at startup when the cam is pretty dry. And Lyc's cam, being high in the case, close to the wall and much cooler when things are cold, gets moisture condensing on it when oil pan heaters are used. Rusts it. Higher idle won't fix that.

One mistake a pilot can make is starting and letting the RPM run up high instantly. Oil pressure isn't up yet and nothing is getting any oil. Higher power means higher loads on everything, and with no oil that just wears things out faster.

The Cenrilube cam in a lyc case. Look how close the crank comes to the camshaft. When the rods are on the crank it gets even tighter in there. Even at 650 RPM the centrifugal forces on the oil coming off the crankshaft are going to be far higher than gravitional force. Oil will be flying everywhere in there. In the flight school I asked the students and instructors to idle at closed throttle, 650-700 RPM, until temps started coming up. I had every engine go to TBO. I must have replaced at least a dozen timed-out engines in my time there.

See the tiny holes in the Centrilube cam lobes?

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Welp, there you go. I guess I'm used to inline and v engines where the oil drips out of the lifter bores onto the cam. Obviously the horizontal engines don't benefit from that.
 
Jim K said:
I'm not an expert on aircraft engines, but every pressure lubricated engine I've ever been around feeds oil through the lifters to lube the cam and the rockers. I find it VERY hard to believe cams in aircraft engines would be splash lubricated.

I believe (and I could be wrong) the reason for running 1000rpm after starting is to get some heat into the cylinders and spark plugs to prevent lead fouling.
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It is. The camshaft and lifter bodies are splash lubricated. After starting, it takes a few rev’s before getting any oil to the camshaft.
 
Where do hydraulic lifters get their oil? How does it become pressurized to travel through the push rods to the rocker arms?
 
CAT IO-520 Overhaul Manual

8-47. STARTING PROCEDURE. (post-overhaul)
a. Open throttle to approximately 900 to 1200 RPM position.

goes on to describe running it at 1200 for the 5 minutes.
 
TSIO-550 Ops & Install Manual 7.3.3
normal start
4. Mixture.................................................................FULL RICH
5. Throttle.................................................................900-1000 RPM
 
Mike Busch, an AOPA article:

“Once the engine fires, resist the urge to rev it up. Let the engine idle at minimum rpm (typically 600 to 800) for at least a minute or two until the engine warms up a bit and all the splash-lubricated components (especially the cam lobes and tappet faces) have had a chance to get thoroughly drenched with engine oil. Then you can throttle up to higher rpm, but do so slowly and cautiously. The stress on the cam-to-tappet interface rises sharply with increasing engine speed, so premature throttle-up of a dry-started engine will damage those components”
 
I’ve read break-in instructions that said to advance throttle to 1800 rpm immediately to reduce stress on the cam lobes and lifter bodies. My new engine’s service manual says to slow idle until oil temp reaches 140*, then advance. That makes no sense to me and I wouldn’t have the time to wait.
 
The name of this thread is "Idle RPM."

That means only ONE thing. Throttle pulled all the way back
 
Stewartb said:
I’ve read break-in instructions that said to advance throttle to 1800 rpm immediately to reduce stress on the cam lobes and lifter bodies. My new engine’s service manual says to slow idle until oil temp reaches 140*, then advance. That makes no sense to me and I wouldn’t have the time to wait.
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That just highlights the fact that they really can't find a justification for elevated RPM. And I'd like to know the basis for claiming that cam and lifter loads are lower at higher speeds. At higher power there is more pressure remaining in the cylinder that makes the exhaust valve harder to open. The valve spring loads don't change. More cam speed means more friction and heat, though a very low speed would give more time for the oil to squeeze out of the interfaces.
Aviation is full of engine mythology. A lot of it is spread by people with no real maintenance experience.
 
Dan Thomas said:
If you have to idle at 1000 RPM you're going to burn out a lot of expensive brake linings and discs.

The engine manufacturers specify idle RPMs at around 650 or so, not 1000. If 650 was going to eat cams, we'd see a 1000 RPM spec.
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what engine manufacturer? mine specifically states 1000-1200 for idle. you're not doing any damage to brake linings or discs by just sitting there.
 
kgruber said:
The name of this thread is "Idle RPM."

That means only ONE thing. Throttle pulled all the way back
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ah ok. so the question is what rpm r u at when the throttle is pulled all the way back, not what rpm u set it to to let ur engine warm up.......
 
eman1200 said:
what engine manufacturer? mine specifically states 1000-1200 for idle. you're not doing any damage to brake linings or discs by just sitting there.
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A lot of light airplanes will taxi and even accelerate at 1000 RPM. As an instructor and mechanic I saw plenty of worn brakes when students were taught that 1000 RPM was the thing to do. They ride the brakes, and a lot of PPLs do too.
 
These pictures are of the remains of a C-182's gear when the pilot taxied from the GA ramp to RWY 26 at KMOT. A distance of about 1½ SM and downhill darn near the entire distance.

As I tell my students: While taxiing, either brakes or power. Not both at the same time, unless you are trying to tighten a turn, then only one brake to get the turn started.

16956.jpeg 16955.jpeg 16954.jpeg 16953.jpeg 16952.jpeg 16957.jpeg
 
Lycoming O-320
9-25. IDLE SPEED AND MIXTURE ADJUSTMENT - (Except Simmonds Fuel Injector). See figures 9-1 and 9-2. With engine thoroughly warmed up, check magneto drop-off. If the drop-off is excessive, check for fouled plugs. If drop-off is normal, proceed with idle adjustment. Close the throttle, engine should idle at approximately 600 RPM (turbocharged engines, approximately 1000 RPM). If the RPM increases appreciably after a change in the idle mixture adjustment during the succeeding steps, readjust the idle speed adjustment to restore the desired RPM.
 
smv said:
These pictures are of the remains of a C-182's gear when the pilot taxied from the GA ramp to RWY 26 at KMOT. A distance of about 1½ SM and downhill darn near the entire distance.

As I tell my students: While taxiing, either brakes or power. Not both at the same time, unless you are trying to tighten a turn, then only one brake to get the turn started.

View attachment 87445
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Wow. Look at that axle sticking up 90° to the gear leg. Hot enough to melt that adapter casting. There goes the leg's heat-treatment.

Aircraft brakes use a metallic friction compound. I used to find little burrs sticking up out of the brake disc's surface, caused by dragging the brakes and heating them enough to start the pad's metal bits welding to the disc. Then those burrs start eating the pad.
 
Automotive and aviation engines are kinda different.

A typical old-school car V engine might have a number of things running off the cam: distributor, fuel pump, oil pump. Running the engine at high RPM with cold oil can put high stresses onto the cam “area” because of the oil pump (cold viscous oil). Example: cam gear driven oil pump will knife-edge the cam gear and oil-pump driven distributor gear with inattention to cold oil startup RPM in old AMC & Buick V6 & V8 engines circa 1960s-1987.

Plus, oil has not had a chance to splash lube the valvetrain, including the cam lobes, lifter, rockers, etc. Forget loading- how many times does rubbing of two surfaces occur before lube gets there? At high RPM, you MAY get more scraping cycles before lube. No real difference in engine types.

So between loads imposed on parts caused by viscous cold oil, and the slow flow of oil and splash lube, high RPM at start up makes low mechanical sense in any engine, or at least old-school “automotively”.
 
Dan Thomas said:
Wow. Look at that axle sticking up 90° to the gear leg. Hot enough to melt that adapter casting. There goes the leg's heat-treatment.
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He was lucky to not have lost the entire airplane. Not like it was February and he was trying to get the oil warmed up... This happend in June (a few years ago)!
 
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