The Cessna 120-140 Association Forum

So I have a McCauley CM7148 on my 140 with a C90-12F. First, do the last 2 digits indicate that its pitched to 48? If so, that means it would be a "extra climb" prop, correct? Considering the apecs for the 140 say a 50 is a climb prop, 52 is standard, and 54 is cruise. Is that an accurate summary? That would explain why my IAS is only about 95 mph at cruise at 5500'.

8164 wrote: ↑Sun Oct 20, 2024 6:33 pm So I have a McCauley CM7148 on my 140 with a C90-12F. First, do the last 2 digits indicate that its pitched to 48? If so, that means it would be a "extra climb" prop, correct? Considering the apecs for the 140 say a 50 is a climb prop, 52 is standard, and 54 is cruise. Is that an accurate summary? That would explain why my IAS is only about 95 mph at cruise at 5500'.

You are understanding the "numbering" correctly. A better estimation of your installation would be to do a good static run up and see where your RPM lies. Also, be certain and use either a digital or calibrated tach to accurately measure your RPM. Even the little "cheapie" Remote control airplane tachs work well. You may not want to trust your installed tach. Gage your results against the RPM range given in the type certificate to tell whether you have a "super" climb prop.

I have a 7152 on my C90-14F and I always thought it was a cruise prop. I have no documentation to back that up. Seems to be nicely matched to the C90 in that it will just barely redline (2475rpm) wide open level at around 2000msl and 65F. Sometimes I do wish for a little less pitch to help climb...something like 7151. But that all depends on how strong running your particular C90 is.

There are tachometer apps for our smartphones. I use one called video tachometer.
Josh

Our C-90 1948 was delivered with a 71-54 in 1948 we have found the best performance for all around use is a 71-50. When we flew Chicago to Tucson we used a friends 71-46 it was a big difference and greatly needed out west at 10,500. It depends on your use of flying ? around the patch on hot days with friends ? climb prop , you travailing with light loads a 7150-54. Also the aircraft weight is a factor , have fun thanks for posting on the Forum. Jeff

Keep in mind there is a static RPM specified for each airframe/engine/prop combination, and if you can't meet it, the plane is legally unairworthy!

For a land plane with that prop and a C90 the limits are not over 2350 and not under 2050. (Actual RPM, of course.)

For climb performance you want to pitch toward the upper limit, and, for cruise towards the lower limit.

There is no specific pitch requirement, but there is a length requirement. A 71" prop can be as short as 69-1/2" and a shorter prop acts like a finer pitch, so keep that in mind.

Some great information here. Thank you for the responses. I learned something