M20F RPM Restriction

[TNIndy]

Would someone remind me why my 67 M20F has the "WARNING" not to cruise  within the range of 2100 and 2350 RPM's? Thanks

[RLCarter]

Sets up harmonic vibrations is what I was told

[RobertGary1]

Yea it’s a harmonic thing. I have the a3b6 balanced engine and now no restrictions. 

[MBDiagMan]

I try to avoid that range under any conditions even if not cruising.  The JPI turns the RPM numerals red in that range which is a great reminder.  On approach though, it sometimes has to operate in that range for a minute or two.

[Browncbr1]

I thought it is torsional vibration from the OEM prop.  I have a top prop, which changed the restriction to between 2350-2550 only over 24”

[TNIndy]

Thanks for the info everyone. I thought it was related to the OEM prop also but can't seem to find my notes. We are setting up my Dynon system and set that range to show red for now. 

[M20F]

6 hours ago, TNIndy said:

Thanks for the info everyone. I thought it was related to the OEM prop also but can't seem to find my notes. We are setting up my Dynon system and set that range to show red for now. 

Maxwell set it to yellow on mine.  It is cautionary and in my case when I want to fly say 23/25 I can clear a caution light (yellow).  Red you can clear the flashing but it stays solid red.  Could distract from more important things.  

[Guest]

8 hours ago, TNIndy said:

Thanks for the info everyone. I thought it was related to the OEM prop also but can't seem to find my notes. We are setting up my Dynon system and set that range to show red for now. 

It’s a type certificate limitation for the propeller when installed on certain Lycoming engines.

Clarence

[TNIndy]

Thanks, that's what I was looking for. I'll check the prop log. Setting the range to yellow rather than red sounds like a good idea also.

Finally getting close to completing the install.

 

[PT20J]

Double check, but it looks like the TCDS requires a red marking on the tach.

[TNIndy]

Thanks, I wonder why the type certificate shows 2000-2350 and the POH shows 2100-2350? I won’t be operating continuously between 2000 and 2100 so it really doesn’t matter, just curious. 

[RobertGary1]

On 5/29/2022 at 10:24 PM, PT20J said:

Double check, but it looks like the TCDS requires a red marking on the tach.

Any change in prop or change to a3b6 stc will supersede this. 

[A64Pilot]

8 hours ago, TNIndy said:

Thanks, I wonder why the type certificate shows 2000-2350 and the POH shows 2100-2350? I won’t be operating continuously between 2000 and 2100 so it really doesn’t matter, just curious. 

Mistakes are made, often it’s as simple as that.

Personally I’d use the TCDS data, it’s likely more accurate but mostly because it’s most conservative. It’s likely more accurate as it’s in list format and a list is easier to check than possibly hundreds of pages of POH

Edited May 31, 2022 by A64Pilot

[Igor_U]

I am almost 100% certain that restriction is 2100-2350. That is how my original Tach was marked as it is now my engine monitor (CGR30).

[carusoam]

The avoidance range is related to the specific engine and prop combination….

If you have a different prop installed other than the original…. It is highly likely that the range no longer applies… or is different than the original…

The ranges also changed over production year…

In 1965… the avoidance range was colored red… which is non-standard for today’s color schemes…

So… check with your mechanic, engine builder, prop builder to see how to proceed…

Its nice to see the statement regarding continuous operation…. Worded to indicate avoiding this range during cruise…

It has caused a few discussions around here about what continuous operation really means… 

PP thoughts only, not a mechanic…

Best regards,

-a-

[tony]

On 5/31/2022 at 11:37 PM, carusoam said:

The avoidance range is related to the specific engine and prop combination….

If you have a different prop installed other than the original…. It is highly likely that the range no longer applies… or is different than the original…

 

Thats correct, I have a three bladed prop and the range is shifted accordingly.  I thought it was a Lycoming restriction caused by a resonance in the crank that flowed into the TDCS.  

[A64Pilot]

It’s a combination of engine and prop usually, but not always.

Even turbines that run butter smooth it’s common to have avoid ranges, very many turbines idle is set so high so you can’t get down to that RPM is one way to handle it.

The wicked one in turbines is called reactionless mode, you never feel any vibe, it’s reactionless, but the prop is tearing itself apart. 

https://flightsafety.org/ap/ap_apr94.pdf

Point is as much as possible avoid, the avoid ranges. My 1500 to 2000 is right where I want to be on approach, so I’m still learning to stay at 2000 then idle on short final.