RPM for 1 to 1 Tach Time

[Pinecone]

What is the RPM where the Tach Time equals real time?   M20K 252.

Having a JPI 830 installed and have to set that number.  It would be nice if it showed the same as the installed hour meter.

[N201MKTurbo]

What RPM do you usually cruise at?

[Pinecone]

Yes.

Too new with plane to have settled in.

But, as installed, there is some RPM that the Tach time equals the actual time.   With mechanical tachs there are standard numbers.  Some of which are not normal cruising RPMS, like 2333. 

[N201MKTurbo]

1 hour ago, Pinecone said:

Yes.

Too new with plane to have settled in.

But, as installed, there is some RPM that the Tach time equals the actual time.   With mechanical tachs there are standard numbers.  Some of which are not normal cruising RPMS, like 2333. 

The mechanical tachs are limited by the available gear ratios inside. There is no such limitation with electronic tachs.

If I had to guess I would set it to 2400.

[Pinecone]

Thanks. I understand that.

I am asking what Mooney used in the 252.  I can set it at 3000 RPM and have all the inspection intervals increase.

 

[PT20J]

I think the honest thing to do is to set it to the rpm Mooney originally.

I would contact Frank Crawford (Manager Technical Resources at Mooney) fcrawford@mooney.com (830) 792-2920 and he can probably look it up. I’ve found Frank very helpful in the past for documentation questions.

Skip

[Pinecone]

Thanks.  I called, he was not at his desk, so I emailed.

 

[Hector]

For the C model it is 2500 RPM. Not sure if Mooney used different RPM for different models.


Sent from my iPad using Tapatalk

[carusoam]

Check the POH…

Some of the newer models got more than a paragraph when it comes to instrumentation…

it might have been included in the tach description…

PP guesses only… call Frank to make contact.

Best regards,

-a-

[Pinecone]

I already checked there, to try to figure out how the hour meter in the plane worked to record time.

It is driven off the tach at a rate proportional to the RPM.    But even that was hard to find.  The actual RPM for 1 hour tach = 1 hour actual is not in my POH

[Jim Peace]

On 9/22/2022 at 12:28 AM, Hector said:

For the C model it is 2500 RPM. Not sure if Mooney used different RPM for different models.

where did you see that?  When my JPI900 for my 64c was programed 2400 rpm was used.  I think we found it marked on the old tach or we called mooney etc...almost 5 years ago so I forget....

[PT20J]

The factory mechanical tach in my 1978 M20J was 2300 rpm. 

[A64Pilot]

Some tachs have that info on a data tag on the tach, maybe a shop that overhauls tachs would know.

I would suspect all Mooney’s with very similar RPM ranges would be the same, better, less expensive and easier to stock one tach as opposed to several.

[Will.iam]

On 9/23/2022 at 7:00 AM, Pinecone said:

I already checked there, to try to figure out how the hour meter in the plane worked to record time.

It is driven off the tach at a rate proportional to the RPM.    But even that was hard to find.  The actual RPM for 1 hour tach = 1 hour actual is not in my POH

Did you ever get a response from mooney on what the official tach number were used for 252’s?

[Pinecone]

No, I did not.

I set it to 2500 and it is close.  I am going to try 2525.

[philiplane]

1 to 1 tach time RPM is usually lower than you might think. On 2575 max RPM constant speed prop engines, it is only 2292 RPM, almost the same as a 2700 RPM engine with a fixed pitch prop, which is 2297 RPM. The highest ratio I have seen for a non-geared engine is 2400 RPM. 

The 1 to 1 time is not really a cruise average, it's an average for a one hour flight including the lower RPM of descent and landing.

Your tach would normally show the ratio on the data plate. 

Edited October 31, 2022 by philiplane

[EricJ]

8 hours ago, Pinecone said:

No, I did not.

I set it to 2500 and it is close.  I am going to try 2525.

It may say on the back of the tach.   I think mine does.

[Pinecone]

Maybe if I have a reason to pull the tach.

I can tracking the original tach hours and the JPI, and will adjust so that they match

As I stated, I set the JPI to 2500, and it reads a bit high.

This is a 252 converted to Encore.  So original red line was 2700, now 2600,

[EricJ]

6 minutes ago, Pinecone said:

Maybe if I have a reason to pull the tach.

This is one of those places where a $20 flexible USB endoscope earns its money.

[jaylw314]

55 minutes ago, EricJ said:

This is one of those places where a $20 flexible USB endoscope earns its money.

If you're a luddite and still have one of those flip phones with a camera, those are a reasonable alternative  I managed to get some serial #'s off some instruments with a standard smartphone, but it's pretty tight behind the right panel

[Pinecone]

I emailed Frank again and he sent me the chart below.  But the quick answer is, the 1 to 1 tach time is 2500 RPM.

 

 

[PT20J]

This is for the models with electric tachs. I believe my 1978 J with a mechanical tach was 1:1 at 2300 rpm. My J with the B&D electric tach was 1:1 at 2500.

[47U]

Light Plane Maintenance did an article back in the ‘90s, maybe?  They said the number for the ‘2300’ tachs was 2310 for it to be timed 1 for 1.  They explained how you could verify the accuracy of the tach by timing how long it took to turn over 1/10 of an hour on the hour meter, which is gear driven and shouldn’t have any slippage like the needle.  At 2310 rpm, it should take 6 minutes to turn over 1/10 of an hour.  Crude, I know, but this was way before iPhone apps and cheap handheld tach sensors.

[Pinecone]

I changed the ratio to 2500 in my JPI, and after a couple of short flights, it seems to be off a tiny bit. Maybe 2510 is the key.