Prop Sync'ing

Banjo33

Banjo33

Line Up and Wait
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Banjo33
I'm putting this in the Mx section, as it is either a user issue or a MX issue. Airplane is a '66 Twin Comanche (IO-320s).

My issue is: once established at cruise altitude (it actually starts sooner when I pull the props back to 2500 on climb out), my process is to pull MP back to desired power setting, adjust prop rpm to desired, big pull back on both mixtures, fine tune each mixture with the EGT, then manually sync the props. What I'm seeing is a fuel flow difference of about 1-1.5 GPH and prop rpm split of about 100-150 rpm. My MEI has a couple thousand hours in Twinkie's and accepted this, but I just want to confirm my procedures AND the end result are normal/optimal. A 1.5 GPH split results in a loss of about 4.5 gallons of usable fuel over a 3 hour flight...that's about 45 minutes of fuel over the course of that time and throws off the accuracy of my shadin FF gauge! Part of me wants to tweak the mixtures to even out the FF, but this seems somewhat counterproductive to proper leaning for optimal engine health. Additionally, once I'm complete with my adjustments, I can feel the left engine very slightly pulling the left wing over, indicating it is producing a little more thrust than the right side.

I should mention that the mechanic has flown this airplane quite a bit too and wasn't concerned with it. So the MEI is good, the mechanic is good, now I can't help but think I am doing something wrong?
 
Are you synching the engines by ear, or by the tachs? If you synching them by ear to eliminate the harmonic warble, your real RPM is matched. If you have a difference in fuel flows at that point, you have a pressure/mixture differential between the engines, and the difference in RPM is in the tach.
 
Sync'ing by ear, ignoring the tach until they engines sound "smooth." The tach indicates the split once I'm complete.
 
Henning said:
Are you synching the engines by ear, or by the tachs? If you synching them by ear to eliminate the harmonic warble, your real RPM is matched. If you have a difference in fuel flows at that point, you have a pressure/mixture differential between the engines, and the difference in RPM is in the tach.
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Agreed..
 
N801BH said:
Agreed..
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I'm not sure what's being said here? Is it that there's a discrepancy between the two tachs or that there's a difference in power output between the two engines? Fwiw, at idle (on deck) and full power on takeoff and climb out, the tachs are pretty much identical.
 
Banjo33 said:
I'm not sure what's being said here? Is it that there's a discrepancy between the two tachs or that there's a difference in power output between the two engines? Fwiw, at idle (on deck) and full power on takeoff and climb out, the tachs are pretty much identical.
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The discrepancy is with the tachometers. If the warble is smooth, the RPMs are the same regardless how much power is being made. Un matched tachs are very common, they need recalibration usually every 7-10 years, most planes never get one.
 
Henning said:
The discrepancy is with the tachometers. If the warble is smooth, the RPMs are the same regardless how much power is being made. Un matched tachs are very common, they need recalibration usually every 7-10 years, most planes never get one.
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Yup....
Ear's don't lie.... Tachs do....
 
Henning said:
The discrepancy is with the tachometers. If the warble is smooth, the RPMs are the same regardless how much power is being made. Un matched tachs are very common, they need recalibration usually every 7-10 years, most planes never get one.
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Ok, I wasn't aware of that. Even if they are good at each end of the spectrum?

What does re-calibrating them entail? Do they remain installed or need to be removed? And is it normally something an A&P can do or do they get sent out?
 
If your props are synched by ear and they still indicate a 150RPM difference, there is a problem with the tachometers.

Have you calibrated your fuel "K" factors? If not it is "garbage in----garbage out."

To me, it sounds like there is a need to calibrate all the settings on your engine monitor.
 
kgruber said:
If your props are synched by ear and they still indicate a 150RPM difference, there is a problem with the tachometers.

Have you calibrated your fuel "K" factors? If not it is "garbage in----garbage out."

To me, it sounds like there is a need to calibrate all the settings on your engine monitor.
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There is that too... Every Flow Scan transducer I have worked with needed a K Factor tweak to match actual fuel flow..

I check mine every 500 gallons run through my plane..
 
kgruber said:
If your props are synched by ear and they still indicate a 150RPM difference, there is a problem with the tachometers.

Have you calibrated your fuel "K" factors? If not it is "garbage in----garbage out."

To me, it sounds like there is a need to calibrate all the settings on your engine monitor.
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There's no engine monitor, just the Shadin FF computer. I've not personally calibrated it (only have about 7 hours in the plane), but when flying with the mechanic, he mentioned that it was "spot on." I'll look into what this entails regardless.
 
Have you confirmed that the fuel burned matches what the fuel flow indication says is being burned? Like a few others have mentioned, the fuel flow instrument may need some adjustment.

I never get too concerned about a difference in the tach indication, they'll be matched if you sync them by ear.
 
mondtster said:
Have you confirmed that the fuel burned matches what the fuel flow indication says is being burned? Like a few others have mentioned, the fuel flow instrument may need some adjustment.

I never get too concerned about a difference in the tach indication, they'll be matched if you sync them by ear.
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I have not. I have so few hours in this plane so far, checking the accuracy has been low on the list of priorities. Definitely will be something I pay better attention to on the next fill up.
 
So, we have determined that the tach's are different. We have not yet determine just how different the manifold pressure gauges or the fuel flow sensors are.
 
Exactly. Which is easier to check...MP gauges or tachs? FF is up to me and should be pretty easy. I'd think the tachs would be next with a backlight or hand held tachometer. If the tachs check good, then by default it's the MP gauges?
 
Banjo33 said:
Ok, I wasn't aware of that. Even if they are good at each end of the spectrum?

What does re-calibrating them entail? Do they remain installed or need to be removed? And is it normally something an A&P can do or do they get sent out?
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How do you know they are good? Just because the needles match don't mean the RPMs do most twins full forward will not be in synch and you have a fast warble like this: http://youtu.be/__ZNDLyx-IY

Are the engines aurally in synch at full throttle when the tachs are?

The mechanical tachs you can take to any speedometer shop for calibration. What they do is adjust the spacing between the drive magnets.
 
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Calibrate the tachometers first and then check the prop governor too! Obviously there is a calibration discrepancy. They were probably set from those erroneous tachometers.
 
Henning said:
How do you know they are good? Just because the needles match don't mean the RPMs do most twins full forward will not be in synch and you have a fast warble like this: http://youtu.be/__ZNDLyx-IY

Are the engines aurally in synch at full throttle when the tachs are?

The mechanical tachs you can take to any speedometer shop for calibration. What they do is adjust the spacing between the drive magnets.
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On takeoff, if one engine were producing more (or less) thrust than the other, would that not create some directional problems on takeoff roll? At idle and at full power, both tach needles are exactly in-line with each other. Once I adjust prop rpm to my desired cruise rpm, that's when the need to sync the props becomes obvious. Doing so (audibly), results in the split.

I haven't noticed any directional issues on takeoff or landing rollout or indications of out of sync props during these phases of flight...and the tach needles are pretty much married up during these phases of flight. Granted, I may not notice an audible difference on takeoff as I'm not actively listening for it. Still, I would assume there would be some directional issues at full throttle on the takeoff roll if there were an rpm split between the engines.
 
Banjo33 said:
Ok, I wasn't aware of that. Even if they are good at each end of the spectrum?
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they probably aren't. If they are off at a mid-rpm then they can't be accurate at redline. odds are someone has mis-set the prop gov redline per the inaccurate tach
 
JHW said:
they probably aren't. If they are off at a mid-rpm then they can't be accurate at redline. odds are someone has mis-set the prop gov redline per the inaccurate tach
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That doesn't explain why there's no directional issues on takeoff roll even though the tachs are married up? Would there not be a thrust differential?

Actually, after further thought, maybe you are.

Assuming there is a mis-adjustment, when one prop momentarily over speeds, the prop governor will pull that prop's pitch back in order to eliminate the overspeed condition. This would mask the misadjustment on one tach at full throttle (both audible AND differential thrust). In this case, it's less of a prop governor issue and more likely a tach issue, as was previously discussed.

To test it at high power, I would actually need to push the throttles up to something short of redline to eliminate the prop governor's ability to interfere, keeping the needles married, and THEN I would experience some directional issues. I don't think I'm quite up for that, so I'll get with the mech and see if he has a light gun.
 
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Banjo33 said:
On takeoff, if one engine were producing more (or less) thrust than the other, would that not create some directional problems on takeoff roll? At idle and at full power, both tach needles are exactly in-line with each other. Once I adjust prop rpm to my desired cruise rpm, that's when the need to sync the props becomes obvious. Doing so (audibly), results in the split.

I haven't noticed any directional issues on takeoff or landing rollout or indications of out of sync props during these phases of flight...and the tach needles are pretty much married up during these phases of flight. Granted, I may not notice an audible difference on takeoff as I'm not actively listening for it. Still, I would assume there would be some directional issues at full throttle on the takeoff roll if there were an rpm split between the engines.
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No, until the loss of thrust is significant you don't really notice, don't worry about anything but SOUND at this point, the physics of sound don't lie about the RPM, sound is the reference standard.

Always listen to your engines, that is your primary early warning of failure.
 
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Banjo33 said:
I have not. I have so few hours in this plane so far, checking the accuracy has been low on the list of priorities. Definitely will be something I pay better attention to on the next fill up.
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That's what I figured. I'd plan a trip that takes a couple of hours to get somewhere and compare the actual fuel burned to what the fuel flow indicator says then.

Banjo33 said:
Exactly. Which is easier to check...MP gauges or tachs? FF is up to me and should be pretty easy. I'd think the tachs would be next with a backlight or hand held tachometer. If the tachs check good, then by default it's the MP gauges?
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What I would do is to do a trip at an altitude where you can leave the throttles wide open, then sync the props. Then adjust your mixture and monitor the fuel flow and compare it to the fuel burned. Assuming the throttles go all the way to the stops manifold pressure should be the same, and the props will be the same if you do it by ear. Then all that is left is the fuel flow indication.
 
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