rpm issue

Aztec Driver

Aztec Driver

Line Up and Wait
Joined
Mar 7, 2005
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Elizabethtown, PA
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Bryon
I noticed an issue with my right engine at takeoff power today. All checks at runup were as normal as always. Right engine has always been a little slower to go toward feather than the left, but always excercised well at high power.
Today, I noticed that at full takeoff power, the right engine rpm's had dropped to 2400-2450 rpm. When I pulled the power back to climb power, the rpm's increased to normal. I had thought that I didn't have them all the way forward, but it happened again on the next takeoff. At normal climb power, everything is acting proerly. At full power, I was unable to increase the prop rpm at all.
Is this a malfunction or maybe an adjustment of the prop governor?
 
Sure sounds like a malfunction. Of course, if there's no yaw, and the engines aren't "beating," the malfunction could be in the tach, but I wouldn't be flying that plane that way until I sorted that out, as you could be getting significantly less than full power on the right engine, and if the left one quits, you'd be in more trouble than you'd normally expect.
 
Ron Levy said:
the malfunction could be in the tach
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Bryon,

Is there a noticeable difference in pitch between the two? (Not blade pitch, musical pitch) That big of a difference should be very audible, and it won't be "beating" any more at that point. If you're a musician at all - The difference between the one on full throttle and the one that's 2400-2450 will be larger than a minor second, but smaller than a minor third. (If you've got perfect pitch, the left engine will be between E and F, while the right engine at 2400-2450 will be between a D and an E flat. ;))

If you can't hear a difference, I would guess that it's a tach problem. If you don't think you're capable of hearing a difference, I volunteer to fly it for you to check. :D
 
It is not a tach issue. It has a definite difference in rpm. I don't need perfect pitch to notice it. From climb power or lower, though, everything returns to normal. On the other hand, this bird has always been a difficult one to sync the engines. Left engine has always pulsed up and down 25 rpm after power reduction from the day I bought it. And yes, Ron, it is not developing full power. I thought I did notice a very small yaw on one takeoff while testing things out. And I don't plan on flying it more than figuring out what is going on. The problem does not show up at ground runup or at any power and prop setting other than full power flat pitch and climbing at takeoff. If I level off at full power, rpm increases slightly, I think. Something I have to check to get more information. More details as they come in......
 
My thought is either the engine's not making power (which would be a fuel issue or a spark issue most likely, and probably quite noticable) or the prop governor is somehow screwing up. When was the last time the plugs and mags have had a good look at? How about last time the nozzles were cleaned? And are you indicating proper fuel flow?
 
Aztec Driver said:
It is not a tach issue. It has a definite difference in rpm. I don't need perfect pitch to notice it. From climb power or lower, though, everything returns to normal. On the other hand, this bird has always been a difficult one to sync the engines. Left engine has always pulsed up and down 25 rpm after power reduction from the day I bought it. And yes, Ron, it is not developing full power. I thought I did notice a very small yaw on one takeoff while testing things out. And I don't plan on flying it more than figuring out what is going on. The problem does not show up at ground runup or at any power and prop setting other than full power flat pitch and climbing at takeoff. If I level off at full power, rpm increases slightly, I think. Something I have to check to get more information. More details as they come in......
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I'm wondering if you have sticking blades. The high thrust of full power might be jamming them in the hub. When was the prop last apart?

Dan
 
Symptoms of the typical prop gov failure.
 
Ted DuPuis said:
My thought is either the engine's not making power
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The engine would have to be awfully low on power for the engine to be that much below proper high RPM with the blades on the low pitch stops. That would produce an awfully noticeable yaw. Try it on your plane some time -- set both on full power, and then throttle back one until the RPM drops to 2450. Then go back to full throttle, and (at a high enough altitude not to overboost the engine, or with the throttles both back to about 26 inches or less) pull the prop back to 2450. You'll see what I mean about the difference in yaw between a governor rolling back and losing so much power the engine won't turn full RPM even with the blades at min pitch.

or the prop governor is somehow screwing up.
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That's way more likely if he's only getting a slight yaw and the airplane is performing near normally.
 
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Two notes:

1) There's a big difference between doing that on the ground and in flight. Ground is more accurate for Bryon's situation, if I'm understanding him correctly that he's noticing this on takeoff. On the ground, you'll need significantly more power to keep that 2450 RPM.

2) I agree the prop governor is more likely. But when diagnosing a problem, it's best to first think of the possible causes, because it's not always the obvious one.
 
Aztec Driver said:
Today, I noticed that at full takeoff power, the right engine rpm's had dropped to 2400-2450 rpm. When I pulled the power back to climb power, the rpm's increased to normal.
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Byron, I want to understand this. You're climbing at, say, 30" mp and getting 2550 rpm (normal for the Aztec, IIRC) on the left and 2450 on the right. When you reduce mp to say, 25" for climb, you're saying the right engine rpm increases to 2550 (before you touch the prop control). Is that right?
 
rotorhead1026 said:
Byron, I want to understand this. You're climbing at, say, 30" mp and getting 2550 rpm (normal for the Aztec, IIRC) on the left and 2450 on the right. When you reduce mp to say, 25" for climb, you're saying the right engine rpm increases to 2550 (before you touch the prop control). Is that right?
Click to expand...

Close.

Actually, it is 2600 normal rpm (2700 red line) for both. But, essentially, yes, on takeoff climb, my rpm on the left is 2600 and the right is 2450. When I bring the manifold pressure back to 25", the right prop increases to 2600 rpm and matches the left. I need to do some more testing, but what I recall is: right engine 2450 rpm, manifold pressures on both engines equal, fuel flow slightly lower on right engine. Not a lot of time at full power, and lots of things happening at that time, so I was concentrating more on making sure it wasn't going to shut down on me. I've already been there and done that, I have no desire to do it again. Weather permitting, I will go out later this week and do some full power ground runups and see what else I can find. Just thought I would consult the board to see what things I can look for.

On the takeoffs where I experienced this, it didn't seem to be lacking in performance, but indications were off normal, and I thought I sensed a small yaw towards the right engine on one takeoff, enough to actually take notice.
 
Aztec Driver said:
Actually, it is 2600 normal rpm (2700 red line) for both.
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Shouldn't they go to full redline on takeoff? :dunno:

Deakin sez:

http://www.avweb.com/news/pelican/182179-1.html said:
With all aircraft engines, use full available rated takeoff horsepower for all takeoffs. I cannot think of a single exception. You are usually not being kind to your engine when you use less, and you may very well be mistreating it when you do so. I see warbird operators using a lot less power all the time, and I think they're wrong. You can tell 'em, but you can't tell 'em much. If the engine manufacturer has published data for an alternate power setting for takeoff, that's fine, too. Otherwise, stick to the full rated power. That means full throttle (for normally aspirated engines), full redline RPM, and mixture full rich at sea level, or leaned appropriately for altitude.

While I'm grumbling, get your tachometer checked, too, many are not accurate. You want full redline RPM for takeoff, no more, and no less. It's important.
Click to expand...
 
Aztec Driver said:
Close.

Actually, it is 2600 normal rpm (2700 red line) for both.
Click to expand...


Okay. I see you changed airplanes - Aerostar now (or is it the Twin Comanche in your picture?). I've never seen anything like that, but I think I'd call a propeller shop first and at least run the issue past them before I did anything else.
 
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Ted DuPuis said:
1) There's a big difference between doing that on the ground and in flight. Ground is more accurate for Bryon's situation, if I'm understanding him correctly that he's noticing this on takeoff. On the ground, you'll need significantly more power to keep that 2450 RPM.
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Not that much more. You see full RPM well before you get to full throttle, even on the ground.
 
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Ron Levy said:
Not that much more. You see full RPM well before you get to full throttle, even on the ground.
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Not in many of the twins I've flown. On the low pitch stops it can take more torque than available to reach max RPM statically, even at full throttle.
 
Aztec Driver said:
Close.

Actually, it is 2600 normal rpm (2700 red line) for both. But, essentially, yes, on takeoff climb, my rpm on the left is 2600 and the right is 2450. When I bring the manifold pressure back to 25", the right prop increases to 2600 rpm and matches the left. I need to do some more testing, but what I recall is: right engine 2450 rpm, manifold pressures on both engines equal, fuel flow slightly lower on right engine. Not a lot of time at full power, and lots of things happening at that time, so I was concentrating more on making sure it wasn't going to shut down on me. I've already been there and done that, I have no desire to do it again. Weather permitting, I will go out later this week and do some full power ground runups and see what else I can find. Just thought I would consult the board to see what things I can look for.

On the takeoffs where I experienced this, it didn't seem to be lacking in performance, but indications were off normal, and I thought I sensed a small yaw towards the right engine on one takeoff, enough to actually take notice.
Click to expand...

This definitely sounds like a prop or governor issue but the increase in RPM with a decrease in MP is peculiar. I could see how a problem in the blade bearings could prevent the pitch from increasing at max thrust and then allow blade rotation when the MP was reduced but the props should already be at min pitch before the throttles are advanced at the beginning of the takeoff roll and any sticking ought to cause excess RPM, not under RPM.

I'm thinking that either there's some weird issue in the prop control linkage that's limiting the travel at the governor end, perhaps as a function of engine movement or something is preventing proper oil flow into the governor or allowing too much oil to escape from the governor output.

One diagnostic approach would be to attach a pressure gauge to the output of the governor and see what that looks like when the problem occurs. I think this is the same pressure as would feed an accumulator and if so that would be a convenient point to tap in. You could compare the left engine with the right engine as well.
 
Ron Levy said:
Not that much more. You see full RPM well before you get to full throttle, even on the ground.
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Ron, that's simply not true for every twin, or every plane with a constant speed prop. And in the case of both of the twins I fly, that's blatantly wrong.
 
flyingcheesehead said:
With all aircraft engines, use full available rated takeoff horsepower for all takeoffs. I cannot think of a single exception.
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That's where Deakin is wrong one more time,,,,,, the war bird operators can't get 115/145 any more so they can't run full horse power.

all the big radials are limited on how MAP they can run on 100LL
 
Ron Levy said:
Not that much more. You see full RPM well before you get to full throttle, even on the ground.
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Agreed, even the old beaver /985 will reach max RPM before WOT occurs.
 
Aztec Driver said:
I noticed an issue with my right engine at takeoff power today. All checks at runup were as normal as always. Right engine has always been a little slower to go toward feather than the left, but always excercised well at high power.
Today, I noticed that at full takeoff power, the right engine rpm's had dropped to 2400-2450 rpm. When I pulled the power back to climb power, the rpm's increased to normal. I had thought that I didn't have them all the way forward, but it happened again on the next takeoff. At normal climb power, everything is acting proerly. At full power, I was unable to increase the prop rpm at all.
Is this a malfunction or maybe an adjustment of the prop governor?
Click to expand...

Did you notice any oil pressure differences, at full power?

I believe your prop gov. isn't porting enough oil pressure to the prop to hold it on the low pitch stop during full power, and CTM is pulling the prop off the stops.

How old is the prop gov?

my advice, swap the prop gov. from one side to the other see if the discrepancy follows the prop gov.
 
Tom-D said:
That's where Deakin is wrong one more time,,,,,, the war bird operators can't get 115/145 any more so they can't run full horse power.

all the big radials are limited on how MAP they can run on 100LL
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True - And you'd think Deakin, with his warbird experience, could have "thought of" that. :dunno:
 
flyingcheesehead said:
True - And you'd think Deakin, with his warbird experience, could have "thought of" that. :dunno:
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Another point: I don't know much about the big old radials of yore, but the only time I've seen a lower power setting hurt an engine is when someone fails to clear the trees at the end of a runway.
 
Tom-D said:
Agreed, even the old beaver /985 will reach max RPM before WOT occurs.
Click to expand...

On the Aztec and 310, you won't, at least on the ones I fly.

It varies from plane to plane.
 
Update....

Did some checking and testing today. Appears to be a tachometer issue. Full power runup yields proper rpm, and all of a sudden the right tach falls off to 2400 with no change in pitch or sound from engine. Goes back up just as suddenly, again, with no pitch changes. Hopefully, this will be a "relatively" lower cost repair. Maybe it is time to install the new digital tachs?
 
Aztec Driver said:
Maybe it is time to install the new digital tachs?
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Personal choice. I like analog tachs better than digital, but it would be nice to get rid of the 20' long tach cable and replace it with a few wires. What would be really nice would be an analog electronic tach. You know, like what cars have.
 
Aztec Driver said:
It is not a tach issue. It has a definite difference in rpm. I don't need perfect pitch to notice it. From climb power or lower, though, everything returns to normal. On the other hand, this bird has always been a difficult one to sync the engines. Left engine has always pulsed up and down 25 rpm after power reduction from the day I bought it. And yes, Ron, it is not developing full power. I thought I did notice a very small yaw on one takeoff while testing things out. And I don't plan on flying it more than figuring out what is going on. The problem does not show up at ground runup or at any power and prop setting other than full power flat pitch and climbing at takeoff. If I level off at full power, rpm increases slightly, I think. Something I have to check to get more information. More details as they come in......
Click to expand...


Coil in the mag breaking down at high power. It won't show on a mag bench test. Nice thing about operating most twins is that you can swap parts side to side for diagnostics.
 
Henning said:
Coil in the mag breaking down at high power. It won't show on a mag bench test. Nice thing about operating most twins is that you can swap parts side to side for diagnostics.
Click to expand...
Bryon has changed his mind (after further testing) about the tach:

Aztec driver said:
Update....

Did some checking and testing today. Appears to be a tachometer issue.
Click to expand...
 
Henning said:
Coil in the mag breaking down at high power. It won't show on a mag bench test. Nice thing about operating most twins is that you can swap parts side to side for diagnostics.
Click to expand...

It will more likely that it is a lead breaking down under full power, same symptoms as a C-150 that gave me fits.
 
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