Prop RPM on Takeoff

[Shadrach]

On 9/4/2019 at 6:47 PM, kortopates said:

Your original tach was never an approved source for setting the RPM limits either. We're required to use a much more accurate source such as an approved optical tach so that RPM can be set accurately not to exceed redline. You really want to ignore it now because you couldn't tell it was overspeeding before? How much error becomes important?

As an example, I just flew with a client who's OEM RPM was reading 120 RPM higher than actual (opposite issue). He wondered why is performance was so far below book. A 50 year old instrument can be pretty far off without getting re-calibrated. 

Modern engine analyzers give us accurate data, generally at the same accuracy test gauges do. Its a shame not to make use of it.

 

I’ve used an optical tach to periodically verify actual RPM for years. Rpm millisecond fluctuates of ~1% in take off and climb not audible nor visible on an analog tach even when the governor was new.  I’m looking for guidance on setting RPM because I would think there would a +/- spec. Getting precise redline under all conditions would be a challenge.

[Tcraft938]

2 hours ago, Jim Peace said:

I have the same setup except 1000 hours SMOH...I get 2650 rpm soon after full power is applied sometimes it will touch 2700 later on in the takeoff roll or initial climb.

Thank you very much.  Helpful information.  From what I understand, it is better to be a little under the 2700rpm max than over it unless the overage is nominal and very short duration while the governor adjusts itself.

[carusoam]

Ross,

Wouldn’t that rpm spec be resident on the TCDS?

Expect that the redline is written with a spec of +0\-some number...?

The STC for my engine states one limit @2700rpm...

So that would be not 2700.001 in transition...

Now, one would have to take into account how accurate the tach has to be to provide good data regarding rpm.

Best regards,

-a-

[kortopates]

4 hours ago, Shadrach said:

I’ve used an optical tach to periodically verify actual RPM for years. Rpm millisecond fluctuates of ~1% in take off and climb not audible nor visible on an analog tach even when the governor was new.  I’m looking for guidance on setting RPM because I would think there would a +/- spec. Getting precise redline under all conditions would be a challenge.

We're actually not trying to set it so that stays on redline on takeoff since because of the rpm variation that will guarantee it exceeds redline. Instead we try to set it so that it will be very near redline without actually exceeding redline. At Savvy we don't recommend adjusting it up unless it's more than 40 rpm low, but if its exceeding redline on every takeoff by 10-20 or more, it should be turned down. Pilots naturally want the maximum performance out of their engine so they tend to fixate on wanting to see it at redline. But the bigger concern should be on not exceeding limits and longevity.   

[N201MKTurbo]

This was not a problem with stock mechanical tachs. The line is 10 RPM wide. As long as it pointed somewhere near the red line on takeoff we were all happy! Now that everyone has a digital tach, we are all crazy about a few RPM here and there. 

[Shadrach]

1 hour ago, kortopates said:

We're actually not trying to set it so that stays on redline on takeoff since because of the rpm variation that will guarantee it exceeds redline. Instead we try to set it so that it will be very near redline without actually exceeding redline. At Savvy we don't recommend adjusting it up unless it's more than 40 rpm low, but if its exceeding redline on every takeoff by 10-20 or more, it should be turned down. Pilots naturally want the maximum performance out of their engine so they tend to fixate on wanting to see it at redline. But the bigger concern should be on not exceeding limits and longevity.   

IIRC my optical tach shows 2664rpm with very brief excursions down to 2638rpm.  It almost always dances between exact two numbers. It drives me crazy.

[PT20J]

I think we are getting overly precise about something that's not that accurate. Ten or twenty RPM is less than a 1% variation -- probably pretty good for a hydromechanical control system. If there is a tolerance, it would probably be in the governor overhaul manual. When I had my prop resealed and governor overhauled, the combination ran up to redline in flight with no adjustments on the newly rebuilt Lycoming IO-360-A3B6.

Here is what McCauley has to say about it:

 

[carusoam]

In the mid nineties... Mooney used a digital tach with an analog display... the needle moves in clicks of about 20rpm..?

It is a beautiful device compared to bouncing analog needles or always changing digits.... 

Add that to the discussion of accuracy and/or precision...

Best regards,

-a-

[211º]

You can check a tach very accurately by doing a run-up at night under the mercury vapor or sodium ramp lights. Since they flicker at 60 hertz, the prop will freeze at multiples of 120 (think stroboscopic effect). The prop will appear to stand still at exactly 1800 rpm.

From here: http://www.cessna172club.com/forum/ubbthreads.php?ubb=showflat&Number=139838

[chriscalandro]

For those with the PCU5000 the magic number is 4 threads from the plate as measured by a fingernail on the pilot side. Your mileage may vary. 
 

Now when someone searches as I did, an answer shall appear. 
 

also, those mechanical tachs? Turns out they are garbage for accuracy. Mine was under a year old and was off by about 100rpm (reading high). Now that I have a digital gauge I spent the time today to get it right. 
 

your welcome,

Chris heneverhelpsanybody Calandro

 

[carusoam]

Holy cow Chris,

You're  getting more and more helpful every day.

What happened?

There is another thread around here where somebody was looking for this information... like yesterday...

@Flybeech21 Kevin, have you seen this thread?  See if this is familiar at all?

PP thoughts not a mechanic...

Best regards,

-a-

[jetdriven]

Since installing a JPi830 we have noticed that our 6 year old SLI mechanical is indeed within one needle width of correct. It’s not cheap, but it’s accurate.  

[JimB]

Mine goes to 2703-2708 during takeoff. Think that's enough to warrant adjusting it? That's less than .5 percent.

[Guest]

Here is Lycoming guidance on an overspeed.

Clarence

https://www.lycoming.com/sites/default/files/SB369S Engine Inspection after Overspeed.pdf

[chriscalandro]

5 hours ago, JimB said:

Mine goes to 2703-2708 during takeoff. Think that's enough to warrant adjusting it? That's less than .5 percent.

That’s within the error of the gauge itself. If you can manage that small of an adjustment you have a lot more patience than I. 

[MBDiagMan]

Everything forward in takeoff as the manual says should result in 2700 for takeoff roll and climbout.