Cycling the prop

MachFly

MachFly

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MachFly
What's the point of cycling the prop three times?

I know there are a lot of people who do it three times and I could never understand why. Theoretically cycling it just once should give you the same amount of information.


Thanks
 
edit: google gave me this :)

You'll always want to refer to your POH for specific operating procedures, but we'll discuss general principles. First, you may have been taught to cycle the prop three times during the runup. This is a holdover from DC-3 and B-17 days. With most modern engines and propellers, cycling the prop once to ensure the system works is good enough, except on very cold days when you'll want to cycle that thick prop oil.
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http://www.flyingmag.com/technique/tip-week/constant-speed-prop-basics
 
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MachFly said:
What's the point of cycling the prop three times?

I know there are a lot of people who do it three times and I could never understand why. Theoretically cycling it just once should give you the same amount of information.


Thanks
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Magic and three is a biblical number.



This goes back to the days of big round engines when you wanted to cycle some warm oil into the hub to reduce the chance of blowing out a prop-seal with the prop under full load and high oil pressures to change the pitch (that's at least the explanation some old people who flew those planes gave me :wink2: ). But those are feathering props that are hooked up the other way around from single engine CS props.

The other reason is that it is a ritual. On the first cycle you observe the drop in rpm, the second time the drop in MP, the third time you check for the drop in oil pressure. Now, you can hear the drop in rpm, the drop in MP has no real diagnostic value and the drop in oil pressure doesn't really tell you anything earth-shattering either. You could sprinkle holy water on the engine and it would have the same effect.
 
weilke said:
On the first cycle you observe the drop in rpm, the second time the drop in MP, the third time you check for the drop in oil pressure.
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Uh, isn't it an increase in MP? At least it is with my plane.

Um, yeah, I do three as that is how I was taught. But for me, one of the three is to watch the windscreen for drops/spittle of oil.

And, it works. Once I did see spittle of oil on the windscreen after cycling the prop, and the A&P checked it out and said the seals were starting to leak. Off the hub went for a rebuild :sad:.
 
Bill Jennings said:
Uh, isn't it an increase in MP? At least it is with my plane.

Um, yeah, I do three as that is how I was taught. But for me, one of the three is to watch the windscreen for drops/spittle of oil.

And, it works. Once I did see spittle of oil on the windscreen after cycling the prop, and the A&P checked it out and said the seals were starting to leak. Off the hub went for a rebuild :sad:.
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I also do it three times as it was how I was taught as well. Each cycle gives a focus to a new area of the panel and/or environment. It's not like it takes that much longer and it makes me feel better to do it that way.

You over in Chattanooga Bill? Drop on in to BGF and visit!
 
There's also service bulletins for props that spend much of their time at low pitch that start to exhibit small leaks at the hub to "deep cycle" them more aggressively and often to move the seals through their full range of motion to even them up and stop the leak.
 
I also do it because that is what I was taught but every time think to myself "What am I doing? This doesn't make sense." :lol: Once really should be enough.
 
Thou shall counteth to three, to four shall thy not count, nor to two unless it shall be followed by three. Three is the number to which thou shall count.

Usually I do it twice.
 
Five is right out. Once the number three being the third number be reached, ignitest thou thy starter and mags and thy engine, being good in my sight, shall starteth

Interesting how many things we do "just because we were taught".
 
Once is enough. Usually on the roll while taxiing for takeoff. Gravel doesn't favor stationary runups or multiple prop cycles and there's no advantage in doing more than one unless your hub is so cold the first time had a significant delayed response.
 
Stewartb said:
Once is enough. Usually on the roll while taxiing for takeoff. Gravel doesn't favor stationary runups or multiple prop cycles and there's no advantage in doing more than one unless your hub is so cold the first time had a significant delayed response.
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:no: :no: :no: :no: Had I waited to do that, I would I have killed my engine.

There I was.....

I was getting ready to depart Knoxville, TN. I started my engine, checked the gauges on start up. All looked well, oil pressure good, fuel pressure good, etc... Go to cycle prop, and it is ssssslllllloooooooowwwwww to drop. Between the time I checked the oil pressure gauge and went to prop cycle my oil cooler blew. The tip-off was that the prop cycle and lack of RPM drop. Had I waited until takeoff, all the oil would have been gone, and I would have been buying a new engine out of my own pocket. No thanks.
 
So you cycle the prop at start-up, not on the run-up pad near the runway? How many guys do that? None that I know.
 
Stewartb said:
So you cycle the prop at start-up, not on the run-up pad near the runway? How many guys do that? None that I know.
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Not immediately after start up, but before I leave the ramp.
 
I look for oil/fuel on the ground when pulling away from my parking spot.

Then again after the run-up
 
weilke said:
Magic and three is a biblical number.



This goes back to the days of big round engines when you wanted to cycle some warm oil into the hub to reduce the chance of blowing out a prop-seal with the prop under full load and high oil pressures to change the pitch (that's at least the explanation some old people who flew those planes gave me :wink2: ). But those are feathering props that are hooked up the other way around from single engine CS props.

The other reason is that it is a ritual. On the first cycle you observe the drop in rpm, the second time the drop in MP, the third time you check for the drop in oil pressure. Now, you can hear the drop in rpm, the drop in MP has no real diagnostic value and the drop in oil pressure doesn't really tell you anything earth-shattering either. You could sprinkle holy water on the engine and it would have the same effect.
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Okay I see. I did some research (actually read the same article Rigged4Flight quotes) and found that some radial aircraft needed it but couldn't find why.

Are there any engines/props that are commonly used in the modern world that require this?




Regarding the comments that said to do it in the cold. How cold does it need to be?
I've had to start an O-520 in -20C weather not too long ago, I assume it would have been a good idea to cycle it three times then?
 
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This is what I was taught.

1. Look for first sign of oil pressure drop, return to high rpm.
2. repeat #1
3. Look out the windshield and do full cycle. Look for any oil spray on the windshield.
 
It's a bit different with a Cirrus. Though it has a conventional constant speed prop and governor, it does not have a dedicated prop control. The cable that would normally go to a prop control goes to a cam mechanism at the power lever.

There's just one "Power Lever". You have to go to pretty high power to hit the "cam" to get into the prop operating range.

This was one area where a lot of pilots started omitting the prop check. Seemed like awfully high power to sand blast your flaps. Or delaying it until the takeoff roll where "2700 rpm - CHECK" became a de facto prop check.

Looking at the one SR22 POH I have, it does not seem to mention any prop check or "cycling" as part of the Run-up/Before Takeoff checklists. I wonder if that's still the case.
 
I do 3, but was taught:

1. Watch oil pressure.
2. Watch RPM.
3. Listen for something to sound odd.

I have been doing it the same way for over 20 years and never once questioned it.

The POH states during run up: Propeller - EXCERCISE to obtain 300 to 400 RPM drop.
 
This is consistent with what I was taught.

Keep Propeller Cycling To A Minimum

It is absolutely necessary to cycle a constant speed propeller before flight. Not only does this operation exchange cold thick oil in the propeller with warm thinned oil from the engine, it also ensures that the propeller and propeller governor are functioning properly. The amount and frequency of this operation is what is at issue. When we purposely deep cycle our propellers, that is cycle them down more than 400 RPM, we cause more vibration and chatter on the engines counterweight system than we do if we only cycle the prop down only 300 RPM. This increased amount of vibration and chatter can cause increased wear on the engine's counterweight bushings and pins. The fit of these pins and bushings is what allows the counterweight system to absorb a very specific frequency of harmonic vibrations within the engine. If the fit between these components is compromised we are said to have "detuned" the counterweight system and thus we have severely limited the counterweight system's ability to absorb it's intended frequency of vibrations. These vibrations, when not absorbed, can cause many vibration related maintenance problems. Anything from catastrophic engine or propeller failure to minor cracking of engine baffles and cowlings is a possible result of detuned counterweights. By keeping the frequency and RPM that we cycle the propeller to a minimum, we can maintain the counterweight systems ability to absorb it's intended frequency of vibrations and limit any problems related to detrimental harmonic engine vibrations while still testing the propeller and propeller governor properly.
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http://tcmlink.com/fiddefault.aspx?cgroup=MATTITUCK&cpagename=GOODHABITS
 
MachFly said:
What's the point of cycling the prop three times?

I know there are a lot of people who do it three times and I could never understand why. Theoretically cycling it just once should give you the same amount of information.


Thanks
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Think of it as stretching before a workout
 
Reedster said:
I do 3, but was taught:

1. Watch oil pressure.
2. Watch RPM.
3. Listen for something to sound odd.

I have been doing it the same way for over 20 years and never once questioned it.
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Again, that's why I do it three times...so I can move my focus around, not just hope I caught everything with one cycle.
 
Henning said:
Thou shall counteth to three, to four shall thy not count, nor to two unless it shall be followed by three. Three is the number to which thou shall count.



Usually I do it twice.
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The Holy Prop-Cycle of Antioch? From the Book of Powerplants, Chapter 5? ;)
 
Crap, I only do it twice. Am I unsafe?

first cycle to check the gauges (Tach, MP & Oil Pressure are all in one glance)

Second to look outside for oil mist from the prop. Now that I think about it, The oil mist will blow back to the windscreen whether I cycle once or twice...and it's not going anywhere after I return to idle. Oh well, stupid habits die hard...
 
I do it constantly in flight. It makes a cool "whom whom whom" sound. :D

Actually my primary instructor taught me 3 times to circulate the cold oil out. But my current instructor, who did my complex endorsement, said once and not to far. Just enough to hear that it works.

The suggestion to run it through it's whole range to exercise the seals makes sense, but the quoted article about loosening the fit of the counter balance parts also make some sense. What's the right answer? (Aside from the obvious "read the POH". No offense to Piper/Cessna et. al. but we've learned that they didn't know as much about engines or longevity as we'd like.)

John
 
jsstevens said:
The suggestion to run it through it's whole range to exercise the seals makes sense, but the quoted article about loosening the fit of the counter balance parts also make some sense. What's the right answer? (Aside from the obvious "read the POH". No offense to Piper/Cessna et. al. but we've learned that they didn't know as much about engines or longevity as we'd like.)



John
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Probably depends on if you ask engine folks or prop folks. ;)

Physics might indicate that if there's something out of balance (extra mass on one side than another) inside an engine, spinning it faster isn't going to fix it. Might feel smoother but it's still screwed. If there really is a magic low RPM that'll hurt things, it'll happen at idle too, let alone when the prop is pulled back at run up RPM.

No one recommends not idling a fixed pitch prop engine anywhere I've seen.

People DO recommend dynamic prop balancing.

Which one do you think causes more damage/stops more damage? ;) Pick your poison.
 
Henning said:
Thou shall counteth to three, to four shall thy not count, nor to two unless it shall be followed by three. Three is the number to which thou shall count.

Usually I do it twice.
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:lol: I should watch that movie again. Haven't seen it in a year or two.

I do it three times... Because, you know, superstition.
 
In CA I'll pull the prop once, up here in the tundra, three times.
 
JasonM said:
This is what I was taught.

1. Look for first sign of oil pressure drop, return to high rpm.
2. repeat #1
3. Look out the windshield and do full cycle. Look for any oil spray on the windshield.
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That's not good, that's setting up for corrosion. Cycle that sucker all the way back and let it run the full travel. The only time that sucker ever gets to full swing is when you do it on the ground unless you pull them all the way back for descent and leave the power in.
 
Dave Theisen said:
Does your prop know where it is? :)


Sent from my iPad using Tapatalk
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Just the difference between my preflight on a 60f day and a 0f day.

I also check for ice on my breather tube for a quick turn flight and a few other little things.

I'll pull it all the way back, just long enough for the prop to grab and out it back forward.
 
Henning said:
That's not good, that's setting up for corrosion. Cycle that sucker all the way back and let it run the full travel. The only time that sucker ever gets to full swing is when you do it on the ground unless you pull them all the way back for descent and leave the power in.
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Yeah baby.....and give those counter weights a good shaking. :yikes::hairraise::yikes::rofl:
 
If you don't do exactly three you'll crash. Simple as that. :thumbsup:

I've noticed a slightly delayed response on the first pull with relatively cold oil and engine. I see the greatest oil pressure drop on the first one and less on the next two. If you do three you make sure it behaves consistently and responds instantly on pulls two and three.
 
petrolero said:
I've noticed a slightly delayed response on the first pull with relatively cold oil and engine. I see the greatest oil pressure drop on the first one and less on the next two. If you do three you make sure it behaves consistently and responds instantly on pulls two and three.
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:yeahthat:

Probably could just do two, but with the delay sometimes on the first one with a cold engine I'd at least like to do 2 on a cold day (see ice in you're beard cold), but ain't like the third is going to hurt chit.
 
I learned to cycle the prop three times to check RPM, oil pressure, MP but twice is enough if you can multitask and check oil pressure and MP on the second cycle.
 
Blueangel said:
I learned to cycle the prop three times to check RPM, oil pressure, MP but twice is enough if you can multitask and check oil pressure and MP on the second cycle.
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What exactly are you gaining by observing the MP pressure increase? Honest question...I've never been able to think up a reason to look at the gauge during this check. I mean yeah, it'll go up, but what benefit do you get observing that change with your eyes on the gauge?
 
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jesse said:
What exactly are you gaining by observing the MP pressure increase? Honest question...I've never been able to think up a reason to look at the gauge during this check. I mean yeah, it'll go up, but what benefit do you get observing that change with your eyes on the gauge?
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The same with the oil pressure drop. What does it tell me ?

Probably worth noting that there is a big difference between testing the prop on a single with a CS prop and a twin with a feathering prop:

In the typical small-plane CS prop:
A. There is very little or no 'cold oil' in the hub. If the prop rests against the fine-pitch stops, the cylinder is pretty much empty and even if you dont cycle the thing, warm oil will get into it once it needs to pitch towards coarse during takeoff roll.
B. 'Testing' the rpm changing function at 1700 tells you nothing about the governor/prop systems ability to maintain a set RPM under full power. The flyweights can be frozen in place and the engine will still make 'wroom wroom' on the 1700rpm test just from the speeder spring pushing down on the pilot valve.

One could do away without the runup test. If it works on takeoff roll, that's good enough. Worst case, you fly a lap around the pattern with the prop in fine pitch.

In the feathering prop otoh:
C. You do have to get the cold thick oil out of the hub by exercising the system, as it may not move into fine pitch otherwise and may not reach rated hp on takeoff.
D. In the feathering prop, you have to test the feathering function as this is a critical system.

For the most part, we do the three times exercise because 'its tradition!'. A tradition that made it into small airplane GA because generations of flight instructors flew planes with feathering props that required a test of the system.
 
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