The Cessna 120-140 Association Forum

Just bought a 140A with the C90-14F. I’m going through a 140 learning curve now. Anyway at some point, nothing in the log book, someone put a McCauley CM7144 and re-pitched it to either 46 or 48. It’s a little hard to determine from the stamp. It seems to work fine but I’m only getting about 85-90 IAS @ 2400 RPM in Cruise. In my research I’m finding that McCauley specs either a CM7152 or 54. I guess someone was looking for better Takeoff/Climb performance? Anyway, I guess my question is two part: 1. Is it okay to continue running with the 7146/48? 2. What would be the best option to run? I live in Prescott, AZ at 5000’.

Thanks in advance for you reply/input...
Boyd Gallaher

OK, Assuming you are talking about a McCauley 1B90-CM7144 repitched to 46 or 48.
I am also assuming that your prop is stamped for Max 90 HP. After looking at the C-140A TCDS and the McCauley TCDS (P-842) you should be good to go EXCEPT for one little thing. Per TCDS 5A2 rev. 22 (C-140A) you still have to have a static RPM within the limits shown. Not over 2350, not under 2000 rpm. (Which maybe hard to check at your current location (elevation) and time of year (summer in Arizona).
This of course is JMHO. Steve

If the static RPM falls within the limits and you're happy, great, then leave it alone, but if you find a problem, then calibrate your tachometer which can often be inaccurate!

Static RPM is fairly immune to temperature and elevation as the factors that make the prop easier to turn also reduce engine power.

I have a C-90 and a 71-52 prop, although the length is actually only 70. Works great. With 48 pitch I'd expect you'd be spinning your wheels, so to speak. Get the tach checked first, though; they're notoriously inaccurate at this age.

5099 wrote: ↑Tue Jul 24, 2018 9:30 pm OK, Assuming you are talking about a McCauley 1B90-CM7144 repitched to 46 or 48.
I am also assuming that your prop is stamped for Max 90 HP. After looking at the C-140A TCDS and the McCauley TCDS (P-842) you should be good to go EXCEPT for one little thing. Per TCDS 5A2 rev. 22 (C-140A) you still have to have a static RPM within the limits shown. Not over 2350, not under 2000 rpm. (Which maybe hard to check at your current location (elevation) and time of year (summer in Arizona).
This of course is JMHO. Steve

Thank you Steve very helpful...

8279 wrote: ↑Wed Jul 25, 2018 4:55 pm

5099 wrote: ↑Tue Jul 24, 2018 9:30 pm OK, Assuming you are talking about a McCauley 1B90-CM7144 repitched to 46 or 48.
I am also assuming that your prop is stamped for Max 90 HP. After looking at the C-140A TCDS and the McCauley TCDS (P-842) you should be good to go EXCEPT for one little thing. Per TCDS 5A2 rev. 22 (C-140A) you still have to have a static RPM within the limits shown. Not over 2350, not under 2000 rpm. (Which maybe hard to check at your current location (elevation) and time of year (summer in Arizona).
This of course is JMHO. Steve

Thank you Steve very helpful...

You're welcome but, the points about the tach are very true and worth thinking about.
But, if it were me and I was not making a logbook entry, a simple static run done on your own would be enough.
Someone has already done the paperwork of installing the prop so, check it out and enjoy the flight.
Best of luck, Steve

Agree with all the above. We have a C90 with a 71 50 northern Il. seems to be the best all around flying.

5099 wrote: ↑Wed Jul 25, 2018 8:10 pm

8279 wrote: ↑Wed Jul 25, 2018 4:55 pm

5099 wrote: ↑Tue Jul 24, 2018 9:30 pm OK, Assuming you are talking about a McCauley 1B90-CM7144 repitched to 46 or 48.
I am also assuming that your prop is stamped for Max 90 HP. After looking at the C-140A TCDS and the McCauley TCDS (P-842) you should be good to go EXCEPT for one little thing. Per TCDS 5A2 rev. 22 (C-140A) you still have to have a static RPM within the limits shown. Not over 2350, not under 2000 rpm. (Which maybe hard to check at your current location (elevation) and time of year (summer in Arizona).
This of course is JMHO. Steve

Thank you Steve very helpful...

You're welcome but, the points about the tach are very true and worth thinking about.
But, if it were me and I was not making a logbook entry, a simple static run done on your own would be enough.
Someone has already done the paperwork of installing the prop so, check it out and enjoy the flight.
Best of luck, Steve

It is a 1B90 CM7148 but re-pitched to 44. I did a static check this morning using a digital tach showing 2350 +or-. So I guess I’m good until I can either find a 7152 or repitch this guy. I checked with McCauley tech support and they told me there is no limits on re-pitching if you do a dye penetrate check afterward? Anyway, I want to thank you and everyone who have contributed in my propeller education.

8279 wrote: ↑Thu Jul 26, 2018 8:14 pmIt is a 1B90 CM7148 but re-pitched to 44. I did a static check this morning using a digital tach showing 2350 +or-.

Right on the ragged edge! I doubt the prop pitch is really 44, though. I'd be surprised if you could meet the static limit with a 44 inch pitch on a healthy C90.

Keep in mind you'll have a huge difference in performance between a 44 and a 52.

Right on the ragged edge! I doubt the prop pitch is really 44, though. I'd be surprised if you could meet the static limit with a 44 inch pitch on a healthy C90.

Keep in mind you'll have a huge difference in performance between a 44 and a 52.
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I’m seeing about 91-93 IAS in cruise 2400 RPM smooth air at 6500’ MSL which I’m guessing is about 10 or so MPH on the slow side. I did find in the logs (Airframe) where this prop was repitched back 1991. Like I said I just bought it so I’m on a 140 learning curve and appreciate all the input and help I can get. I live at 5045’ field elevation. This little guys does pretty good up here on takeoff and climb and I sure wouldn’t want to lose that for an extra 10 MPH in cruise I guess. I’m guessing that’s what they had in mind back in 1991? McCauley say a 50 is Climb 52 is Standard and 54 is cruise for this airframe engine combo. It did start life with a 52. I guess I’m thinking maybe a 50 or 52 (still thinking about it) and around 2200 static RPM and maybe a little longer ground run on takeoff and a little less rate of climb but maybe 105 IAS in cruise? What do you think John? Anybody else?