Why can't I get more than 2100rpm

T

Tsreynolds71

Filing Flight Plan
Joined
Aug 24, 2015
Messages
2
Display Name
Display name:
T Reynolds
Good afternoon I have 65 Cessna 210. I am having problems getting my Rpm above 2100 rpm. I flew one night to south Texas and flew great and on return I took off from airport and leveled off and I rotated the prop back to cruise and it went down to 2100 rpm and I couldn't get it to go any higher. Everything sounds normal. When I test fly it seems to run great on takeoff. The prop does feel real sensitive but it could be my imagination since I'm paying close attention and trying to figure what else it could be. An a/p told me to replace the prop gov which I did and it didn't change.
I was wondering if anyone else ran into this same problem.
 
It's possible something has slipped on the prop (blades at wrong angle, stops misaligned). This is one of the Macaulay props? You should stop dealing with moronic "poke at it with a stick" mechanics and find a reputable prop shop to look at it.
 
If everything sounds correct, maybe it is the Tachometer.

I just replaced mine because it was reading high.

Just a thought.

Flav
 
Old mechanical tacs love to read low. Find someone with a Prop Tach and check it.
 
Your takeoff would be pretty lackluster at 2100 rpm but you state yours is okay. That alone would make me suspicious of the tach. And the mechanic's advice for you to change the prop governor.
 
Norman said:
Old mechanical tacs love to read low. Find someone with a Prop Tach and check it.
Click to expand...

Or you can do a quick "sanity" check using AC powered lights at the airport. AC is 60Hz. The mercury or sodium lights at your airport "blink" at that frequency. Position your airplane so you can see the propeller in the light form one of these and set it so it appears to stop. Then read the tach. If you're seeing something other than a multiple of 60 (1200, 1800), you've got a tach problem. Verify with a prop tach.

John
 
Normally I might tend to agree that if take off and climb performance is fine that the RPM is likely higher than the 2100 indicated and the tach could be at fault. However the portion in bold below leads me to think it might be prop related instead.

The way I read it is that he reduced the RPM to 2100 from a higher unspecified RPM (it would be nice to know what RPM) and then he was unable to increase RPM again beyond 2100. Unless the tach decided to go out of adjustment by a few hundred RPM at that very moment, I do not think it is the tach. Sounds like a failure in the prop or its control system. It therefore would be either the prop governor (already replaced), the prop itself or the prop cable adjustment. I would double check the rig though that should have been done when the governor was replaced. If the rig is good, take it to a prop shop. If you do not have a prop shop on field, I would have your A&P remove it and send it to the shop. I would be reluctant to continue to fly it as is.

However if you are able to test the tach with a strobe or by slaving in another tach, it would not be a bad idea to do so before pulling the prop. But I am betting on a prop issue.

Tsreynolds71 said:
Good afternoon I have 65 Cessna 210. I am having problems getting my Rpm above 2100 rpm. I flew one night to south Texas and flew great and on return I took off from airport and leveled off and I rotated the prop back to cruise and it went down to 2100 rpm and I couldn't get it to go any higher. Everything sounds normal. When I test fly it seems to run great on takeoff. The prop does feel real sensitive but it could be my imagination since I'm paying close attention and trying to figure what else it could be. An a/p told me to replace the prop gov which I did and it didn't change.
I was wondering if anyone else ran into this same problem.
Click to expand...
 
jsstevens said:
Or you can do a quick "sanity" check using AC powered lights at the airport. AC is 60Hz. The mercury or sodium lights at your airport "blink" at that frequency. Position your airplane so you can see the propeller in the light form one of these and set it so it appears to stop. Then read the tach. If you're seeing something other than a multiple of 60 (1200, 1800), you've got a tach problem. Verify with a prop tach.
Click to expand...

Well, um, sort of. The lights blink twice per cycle, once on the positive peak and once on the negative peak. Since props are measured as revolutions per MINUTE, we've got to convert 120 pulses per second to minutes, or 7200 blinks/minute.

And we've got either two or three blades on the prop, any one of which will appear motionless.

The math isn't difficult; there are quite a few googles that will give you major and minor strobe effects with metal vapor or fluorescent lights.

I agree with the contingent that say it is a tach problem. 2100 RPM will give a very sluggish takeoff performance and horrible climb. Mechanical tachs are known for this sort of failure mode.

Jim
 
Last edited:
weirdjim said:
Well, um, sort of. The lights blink twice per cycle, once on the positive peak and once on the negative peak. Since props are measured as revolutions per MINUTE, we've got to convert 120 pulses per second to minutes, or 7200 blinks/minute.

And we've got either two or three blades on the prop, any one of which will appear motionless.

The math isn't difficult; there are quite a few googles that will give you major and minor strobe effects with metal vapor or fluorescent lights.

Jim
Click to expand...

Thanks for the "rest of the story". You are correct and I was in a hurry when typing. (And on my first cup of coffee!)

John
 
Thank you all for the reply. I checked the prop cable over and everything was tight. It goes all the way to each stop on the governor.

@skydog I was actually 2700 on take off and brought it back to 2500 for climb and then after I leveled off for cruise I was setting it for 2350 and it dropped down to 2100 and I couldn't get it to go higher than that.
I was thinking it could prob be the tach also but wanted to see if anyone had any advice.

again thank everyone for the advise.
 
Does the engine sound change when you think the rpms should be increasing?
 
60 cycles/second * 60 seconds/min = 3600 cycles/min

1800 revs/minute * 2 stop appearances/rev (because its a two blade prop) = 3600 appearances/minute.

So a two blade prop will appear to stop once at 1800 rpm

If the light blinks at 120 cycles per second, it will appear to stop TWICE (180 degrees apart) at 1800 rpm.

Ill let the reader figure out a 3 blade prop.
 
coloradobluesky said:
60 cycles/second * 60 seconds/min = 3600 cycles/min

1800 revs/minute * 2 stop appearances/rev (because its a two blade prop) = 3600 appearances/minute.

So a two blade prop will appear to stop once at 1800 rpm

If the light blinks at 120 cycles per second, it will appear to stop TWICE (180 degrees apart) at 1800 rpm.

Ill let the reader figure out a 3 blade prop.
Click to expand...


Not quite. With a fluorescent or metal gas discharge lamp (sodium vapor, etc) you get TWO pulses per cycle, one at the positive peak and another at the negative peak, so the PRF is 7200, not 3600.

There are a MULTITUDE of combinations for which the prop will appear motionless, some of them not immediately obvious.

Here is the link to the 2011 thread in this forum that we discussed the matter:

http://www.pilotsofamerica.com/forum/archive/index.php/t-42166.html


Jim
 
Read the post again. I address this in the statement:

If the light blinks at 120 cycles per second, it will appear to stop TWICE (180 degrees apart) at 1800 rpm.

You are confusing everyone with this 120 cycles per second. Whether its 60 or 120 it will appear to STOP at 1800. If it is 120, it will appear to STOP TWICE at 180 degrees apart.

But you will only see one because the other one will be blocked by the cowling.

Just go out and try it for petes sake.
 
Last edited:
If the rpm and the light dont sych up, the prop wont appear to stop entirely, it will appear to rotate, slowly. Sometimes appear to rotate backwards. If you dont see how this is so, think about it some more. Blink 1. That Blink 1 makes the prop appear to exist in that position. Blink 2 makes it appear in the new position. Blink 3 etc

If they synch up the prop appears to STOP. That is the magic!
 
Last edited:
Yeah, well I'M the guy that taught your girlfriend how to do that thing you like...
 
Why not get a TruTach and use a real airplane tool to map your true RPMs across the tach's operating range?
 
With mechanical tachs the deviation from indicated is rarely linear so it's worthwhile to note those deviations at more than one RPM setting. Most good mechanics have optical tachs. Ask to borrow one.
 
Stewartb said:
With mechanical tachs the deviation from indicated is rarely linear so it's worthwhile to note those deviations at more than one RPM setting. Most good mechanics have optical tachs. Ask to borrow one.
Click to expand...

If you know the approximate near where your cruise setting is, do you really care what the deviation near idle will be? So if you know your WOT deviation, you know near everything you need. Unless something is totally wack, it should be less the lower on the tach you are.

I know mine reads ~100 low at 2400, so I just wing it from there.
 
I want to know precisely what my engine RPMs are at the top and bottom of the green arc and at full throttle.
 
You must log in or register to reply here.
Back
Top