Re: Propeller Repitch
Posted: Wed Apr 01, 2020 5:26 pm
My earlier comment regarding the EM prop came about because I relied upon memory rather than documentation when I posted. And here I am doing it again...but with somewhat more recollection.
The EM and MDM props used a more efficient blade profile/design than did the DM. This allowed the EM/MDM to have slightly greater (coarser/higher) pitch then the MDM for similar performance. This is signified by the greater “standard” pitch in the EM/MDM. I.E.: The DM7653 was selected by Cessna as the “standard” propeller, giving the best compromise between field-length performance vs cruise performance.
Cessna designated the DM7651 as a “climb” prop (better field length performance at the expense of cruise performance).
Cessna designated the DM7653 as the “standard”, and the DM7654 as the “cruise” prop.
When the improved EM/MDM blades were developed, approximately 2” greater pitch in 76” dia. blades were given those designations, IE, MDM 7655 became “standard” while MDM7656 became the “cruise” prop.
This is what led me to the earlier (misguided) question of gehfer, as he referred to his MDM7655 as a “standard”prop... which was not in agreement with the EM data according to the Assoc’n STC. In other words, the EM and the MDM blades are identical...but the McCauley data increases pitch by 2” for the same performance as the earlier DM blades.... HOWEVER the Assoc’n STC did not follow that logic, instead retaining the earlier DM performance-data for the STC-approved EM prop (The Assoc’n STC did not accommodate/recognize the improved performance of the MDM/EM blades when authorizing the EM installation.)
There’s another thing I notice about the propeller data with reference to the Type Certificate Data Sheet (TCDS). It has been repeated here (by some) that the ONLY static rpm allowed is 2230-2330 and the TCDS is used as “proof” of that belief. The statements either outright or by implication, that props of lesser/lower/finer pitch cannot be allowed on the airplane because they will not conform to the TCDS as if the TCDS is paramount over all other things.
Clearly that cannot be true because that would not accommodate various STCs etc that DO allow variance with the TCDS. Further, to install or re-pitch a prop to something less than the 7651 ordinarily TCDS-approved does NOT require further approval than to use an airworthy prop model ... [edit: Regardless of pitch, if it can meet another basis of approval than the TCDS.} In other word, if you live on a 500 foot strip and need a 7647 prop (or some other pitch) to feel good about your performance... and if you find that your static RPM suddenly jumps to 2500 ....you are NOT in violation of some law prohibiting it by the TCDS.
Don’t be surprised however to discover that you are unable to maintain full-open throttle during cruise ...because you are NOT authorized to overspeed the 2700 RPM limit of your C145/O300 engine! You will be cruise RPM limited and you will turn more revolutions per mile and burn more fuel per mile than previously...just as if you kept your car in low-gear all the time.
My present engine (O-300-C) with an EM7655 prop (which I had previously thought a cruise-prop per McCauley-implied and Assoc’n data) I had planned to replace with a EM7653 prop to bring the airplane into “standard” configuration (because I believed the TCDS and Assoc’n STC data)... I now realize is probably best left alone for “standard” performance.
Hope that was sufficiently confusing.
The EM and MDM props used a more efficient blade profile/design than did the DM. This allowed the EM/MDM to have slightly greater (coarser/higher) pitch then the MDM for similar performance. This is signified by the greater “standard” pitch in the EM/MDM. I.E.: The DM7653 was selected by Cessna as the “standard” propeller, giving the best compromise between field-length performance vs cruise performance.
Cessna designated the DM7651 as a “climb” prop (better field length performance at the expense of cruise performance).
Cessna designated the DM7653 as the “standard”, and the DM7654 as the “cruise” prop.
When the improved EM/MDM blades were developed, approximately 2” greater pitch in 76” dia. blades were given those designations, IE, MDM 7655 became “standard” while MDM7656 became the “cruise” prop.
This is what led me to the earlier (misguided) question of gehfer, as he referred to his MDM7655 as a “standard”prop... which was not in agreement with the EM data according to the Assoc’n STC. In other words, the EM and the MDM blades are identical...but the McCauley data increases pitch by 2” for the same performance as the earlier DM blades.... HOWEVER the Assoc’n STC did not follow that logic, instead retaining the earlier DM performance-data for the STC-approved EM prop (The Assoc’n STC did not accommodate/recognize the improved performance of the MDM/EM blades when authorizing the EM installation.)
There’s another thing I notice about the propeller data with reference to the Type Certificate Data Sheet (TCDS). It has been repeated here (by some) that the ONLY static rpm allowed is 2230-2330 and the TCDS is used as “proof” of that belief. The statements either outright or by implication, that props of lesser/lower/finer pitch cannot be allowed on the airplane because they will not conform to the TCDS as if the TCDS is paramount over all other things.
Clearly that cannot be true because that would not accommodate various STCs etc that DO allow variance with the TCDS. Further, to install or re-pitch a prop to something less than the 7651 ordinarily TCDS-approved does NOT require further approval than to use an airworthy prop model ... [edit: Regardless of pitch, if it can meet another basis of approval than the TCDS.} In other word, if you live on a 500 foot strip and need a 7647 prop (or some other pitch) to feel good about your performance... and if you find that your static RPM suddenly jumps to 2500 ....you are NOT in violation of some law prohibiting it by the TCDS.
Don’t be surprised however to discover that you are unable to maintain full-open throttle during cruise ...because you are NOT authorized to overspeed the 2700 RPM limit of your C145/O300 engine! You will be cruise RPM limited and you will turn more revolutions per mile and burn more fuel per mile than previously...just as if you kept your car in low-gear all the time.
My present engine (O-300-C) with an EM7655 prop (which I had previously thought a cruise-prop per McCauley-implied and Assoc’n data) I had planned to replace with a EM7653 prop to bring the airplane into “standard” configuration (because I believed the TCDS and Assoc’n STC data)... I now realize is probably best left alone for “standard” performance.
Hope that was sufficiently confusing.