RPM Restrictions

[Pinecone]

4 hours ago, Ragsf15e said:

Yeah, that’s the issue on instrument approach.  I like gear and half flaps which means ~2300 rpm to hold 90mph.  I guess I could either go with gear up and no flaps with 2000rpm or gear, full flaps and slip down the ils with 2400rpm? 

More RPM and Less MP or More MP and Less RPM.  Both can lead to the same amount of power

[takair]

7 hours ago, Ragsf15e said:

I’ve got a 3 blade speed brake, maybe that’s the difference?  I’ll try 100mph and see if she’ll hold 100mph at 2351?

I’m surprised the three blade has the same restriction…. That can open up another can of worms. 

[Ragsf15e]

13 hours ago, Pinecone said:

More RPM and Less MP or More MP and Less RPM.  Both can lead to the same amount of power

On final, mp is pulled back enough that rpm is no longer controlled by the governor (at high rpm).  Rpm becomes dependent on the mp like a fixed pitch prop.  It just happens that some of the props also have a yellow range right where that mp for holding a good airspeed on final gives you that specific rpm.  You probably don’t have this combination with your setup.
You can either add mp and go faster or reduce it and go slower, but you can’t change rpm by itself because it’s on the stop. Edit- I guess you can still slow it down which probably works but would feel weird!

edit: maybe i get what you meant… probably kind of tongue in cheek, but pull the prop back to slow it down and add in some mp?  

[Ragsf15e]

7 hours ago, takair said:

I’m surprised the three blade has the same restriction…. That can open up another can of worms. 

Yeah I don’t have the exact numbers in front of me because they’re slightly different, but it results in a similar issue in the pattern.  I mostly notice on final.

[47U]

Prop restriction on my 3-blade McCauley…

 

[Pinecone]

On 3/28/2023 at 8:50 PM, Ragsf15e said:

edit: maybe i get what you meant… probably kind of tongue in cheek, but pull the prop back to slow it down and add in some mp?  

Exactly. 

Leave putting the prop full forward until short final.  Or just teach yourself Red, Blue, Black for go around.

[outermarker]

Dynamic Regulatory System (faa.gov) 

• SAIB Number:
  • CE-06-62 is targeted at Piper PA-28R-200. However, Paragraph (c) notes Mooney E & F IF YOU HAVEN'T COMPLIED WITH AD 77-12-06R2.  AD 77-12-06R2 is superseded by 2002-09-08 which is superseded by 2007-26-09. 
  • I went to a Piper Forum and found this discussion about what does "avoid continuous operation" mean.
  • My Cherokee 180 had the same RPM restriction from 2150 to 2350 RPM. On a visit to the Sensenich factory years ago, I asked the same question about how long one could be in this area. It was explained to me in graphic detail, literally.
    
    The answer was "the effects [stress] are cumulative. The less time [in the restricted range] the better." I was given a copy of a graph showing RPM points where vibration set up standing waves in the blade causing the tips to bend rapidly! It was explained that the prop and the crankshaft are a system with respect to these vibrations, and the hollow crankshaft of the O-360-A3A was an issue! It was hollow to accommodate a constant speed prop which the Cherokee did not have.
    
    Eventually, an AD was issued to require periodic inspection of the hollow crank interior for corrosion pits. I eliminated both the RPM restriction AND the crank inspections by switching to an O-360-A4A at overhaul time. The O-360-A4A, unlike the A3A, has a solid crankshaft, and the type certificate for the Cherokee 180 also specifies this alternative engine!

[carusoam]

Two things from way back….

That often caused confusion….

1) Avoid continuous operation….

2) Destructive harmonic vibrations…

 

Following pic… from a 1965 M20C

1) The yellow range is originally marked on the instrument face… 2 - 2300

2) It got updated prior to Y2K with an increase yellow range…

3) Oddly, the yellow range is colored red 

4) The updated rpm range is literally a piece of red tape on the surface of the instrument’s lens…. The parallax in the pic is big.  It is easy to read 100 rpm low or high depending on the angle you see the instrument from…

 

PP thoughts only, not a mechanic…

Best regards,

-a-

[47U]

9 hours ago, carusoam said:

Following pic… from a 1965 M20C

1) The yellow range is originally marked on the instrument face… 2 - 2300

2) It got updated prior to Y2K with an increase yellow range…

3) Oddly, the yellow range is colored red 

4) The updated rpm range is literally a piece of red tape on the surface of the instrument’s lens…. The parallax in the pic is big.  It is easy to read 100 rpm low or high depending on the angle you see the instrument from…

I’m not surprised that my tach wasn’t the only one with the wrong ‘restricted operation’ range markings.  Technically, aren’t improper range markings not airworthy?  (And who is responsible for airworthiness?)  

When the 3-blade McCauley replaced the 2-blade on my ‘63C in the early ‘90s, the range markings on the tach were not changed per the STC.  It wasn’t that hard to do, and really cheap for a range marking sheet, .00295 amu at Wag-Aero.  

 

[carusoam]

2 hours ago, 47U said:

I’m not surprised that my tach wasn’t the only one with the wrong ‘restricted operation’ range markings.  Technically, aren’t improper range markings not airworthy?  (And who is responsible for airworthiness?)  

When the 3-blade McCauley replaced the 2-blade on my ‘63C in the early ‘90s, the range markings on the tach were not changed per the STC.  It wasn’t that hard to do, and really cheap for a range marking sheet, .00295 amu at Wag-Aero.  

 

This amount of info is what makes MS so powerful!!!

Best regards,

-a-

[Planegary]

I have the original tach in my bird. Should I abide by the limits on the tach as displayed as has been done in the past or follow the guidance of the portable digital tach that I sometimes use?  The tachs have about a 100 RPM difference at 2350 where the original tach reads slower than the digital tach

[outermarker]

In my case I have complied with AD 77-12-06R2 which is superseded by 2002-09-08 which is superseded by 2007-26-09. So, I'm leaving mine at 2150 - 2350.

[Planegary]

I just don’t know which tach I should follow. The old one that has been followed for decades or the newer more accurate one to more accurately comply with the prop restrictions

[jaylw314]

On 3/27/2023 at 7:08 PM, MikeOH said:

Really???

I've had my F for 5 years and have NEVER had need to operate below 2400 rpm, and other than the slight annoyance on downwind I described earlier this has been a complete non-issue for me.

It's good for whale watching so you don't bother the big critters as much, or site-seeing low over noise-sensitive areas, but that's about it

[Vance Harral]

42 minutes ago, Planegary said:

I just don’t know which tach I should follow. The old one that has been followed for decades or the newer more accurate one to more accurately comply with the prop restrictions

While I wasn't on the team that determined the restricted RPM range, I've been involved in similar efforts.  The kind of engineering analysis that goes into this sort of thing almost certainly included guard-banding to account for tachometer accuracy and precision characteristics.  Furthermore, the nature of the concern is long-term cumulative effect, so it's not like 2351 RPM is OK forever and 2350 RPM is immediately catastrophic.

Putting those two thing together, it really doesn't matter which tachometer you reference to comply with the restricted range, unless one of them is grossly off by hundreds of RPM and therefore is not even airworthy.

 

[Planegary]

Thanks for the clarification. I didn’t want to cause any damage out of ignorance

[carusoam]

3 hours ago, Planegary said:

Thanks for the clarification. I didn’t want to cause any damage out of ignorance

Two important things come from this discussion…

1) The limitations are defined by one engineering source with great accuracy… and then applied to the tachs we had on day one…

2) Those day one tachs are all calibrated before they get installed…

 

Then real life takes over…

3) After day one… accuracy of the tach has probably drifted… a couple of percentage points…

 

4) When you see your tach is off by 100rpm, that is about 4%

5) If you see your tach needle bouncing…  this is a sure sign of inaccuracy caused by wear…

 

This is important…

6) You want your tach to be as accurate as feasible… get it calibrated.

Or get it updated enough to be calibrated… including the spinning cable that delivers the ‘data’.

 

How you know how right the data is…

7) Set your power with MP and RPM… use your WAAS source combined with CloudAhoy… check the OAT…

8) Calculate your T/O distance and initial climb rate…

9) Use CloudAhoy to measure your T/O and climb rate…

 

10) If your numbers don’t agree… there is probably some benefits to having your instruments working like they did on day one…

Contact your instrument shop for that…. 
 

Next steps…

11) Use various sources of MP and RPM… compare to the calculated….

12) know the issues of the MP gauge are kinda similar… mechanical that ages… and that weep hole that may have a crack radiate out from it…

 

For comparison…

13) 200 rpm on my IO550 is 30 excess hp… or about 10% of the total hp…

The IO360 works the same way…  remember that 4% we were discussing above?

14) When I use the extra 10%, My T/O run goes from 1200’ to 800’…   
 

Eye opener…
15) If I am expecting to be off the ground in 800’ using full power… and something causes me to lose 100rpm… like a bad rpm setting…

16) I might only be off the ground in 1k’ instead… (linear math used for simplicity, Mooney flying is never linear) 

 

Summary….
17) Sure our instruments can be calibrated… to real numbers.

18) It is best to use real numbers from your installed primary equipment…

19) You can probably supplement primary instruments with a lowish cost JPI….

 

Not covered…

20) What is the acceptable accuracy of our old mechanical instruments… 

21) There is probably a range that is acceptable, and calibration is the way to get them back working properly again….

22) When the stuff is worn beyond calibration… they are easy to OH, or replace with fancy digital stuff….

23) Expect proper instrumentation to work wonders… actual T/O distances meeting book numbers are not magic.  They are real world engineering, repeatable, useable things…

24) Then collect real data with what you have… prove that when you set power properly… you get  the same T/O result…

 

What happened to my M20C…

25) something broke the welds of its engine mount…. Probably those vibrations we discussed…

26) Last I flew it was to an MSC… where they pointed out the engine being supported by 3 tiny bolts… not four.

27) Along the way… the tach cable wore. The needle started to oscillate and wind up exceeding redline… there is a post at the end of the range… the needle hits the post and pops off…

Calibration was checked when the needle was put back on… for low dollars…

28) Along the way… the line to the MP gauge broke at the weep hole… getting the weep hole replaced took calling the factory to find what drill to use…  vibration made the weep hole crack….

 

So… if you can… go with as much accuracy as you can afford….

1) Your T/O distance is important when trees are growing larger in the windshield….

2) Avoiding the actual ranges that need to be avoided… will keep things from braking sooner than expected…

 

I could have done much better… but there was no MS around at the time…

Getting an engine mount OH’d is a regular process that I was not familiar with, and I didn’t recognize two broken welds…

The big hint… the engine sag was getting worse… and was different from day to day!   

Posting a pic early would have made this a non-event…

Know that engines and their mounts… don’t fix themselves…. If something looks like it improved on its own…. Look closer.

PP thoughts only, not a mechanic or instrument guru…

How was that?  

Best regards,

-a-

[Hap]

On take off, at what altitude are most folks dialing their props back? 

 

64 M20E

[Ragsf15e]

1 hour ago, Hap said:

On take off, at what altitude are most folks dialing their props back? 

 

64 M20E

Uh oh, can of worms!

You gonna get lotta different answers.  Many people climb at 2700 and leave it there. Others follow the checklist which might have you reduce power pretty Far - 25/25?  
Me? I pull mine back to 2600 and climb at full throttle/2600 all the way until level off.  I make that 2600 pull about 1000’.

The engine (io-360) is rated for 2700 continuous though, so pulling rpm is reducing power…

[EricJ]

5 hours ago, Hap said:

On take off, at what altitude are most folks dialing their props back? 

 

64 M20E

+1 there's gonna be a variety of opinions on this.   I don't pull the prop until reaching cruise altitude, unless there's some compelling reason like heat or needing to conserve fuel to pull it back before that.

[Hank]

6 hours ago, Hap said:

On take off, at what altitude are most folks dialing their props back? 

 

64 M20E

Full throttle, 2700 RPM from the runway to cruise altitude, leveled off and speed no longer increasing. Then I set my desired power and lean the mixture.

The part you didn't ask:  power reduction for descent.

To descend, I push the yoke for 500 fpm and spin the trimwheel to hold the descent without me holding the yoke. Because my engine is carbed, I periodically reduce throttle and richen the mixture to maintain my cruise MP & EGT settings. I reduce the throttle to slow to flap speed, and go to Takeoff Flaps ideally as I'm entering downwind. 

[MikeOH]

WOT and 2600, leaning all the way to keep sea level takeoff EGT