Propeller Repitch

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GAHorn
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Re: Propeller Repitch

Post by GAHorn »

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. :lol:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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n2582d
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Re: Propeller Repitch

Post by n2582d »

gahorn wrote:... 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 ... REGARDLESS of pitch. 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. ...
George, This is news to me. I'd agree with you if that "strip" happened to be a canal and you had a 170A or B on floats with a McCauley 1A170 or a McCauley 1C172/MDM but not if it was a landplane.
Propeller
(a) McCauley 1A170
Static r.p.m. at max. permissible throttle setting:
Landplane: Not over 2330, not under 2230
Seaplane (Models 170A and 170B): Not
over 2525
, not under 2300.
No additional tolerance permitted.
How do you understand "no additional tolerance permitted."? Now the issue is more cloudy when you've installed a McCauley 1A175-DM prop in accordance with Kenmore's STC SA111NW which doesn't list any specific length, pitch or static rpm requirements. In the TCDS the McCauley two-position prop also does not list static rpm requirements although it does specify certain length and pitch parameters.

I'd be interested to know what Kenmore has to say about static rpm limits with this STC. Is the 2700 max. rpm on the TCDS for the O-300 and in the Airplane Flight Manual the upper limit for static rpm in this case? If so, as soon as you took off with full throttle you'd be exceeding 2700 rpm. Part of what is required to be checked during an annual or 100 hour inspection is static rpm IAW FAR 43.15 (C)(2)(i):
Each person approving a reciprocating-engine-powered aircraft for return to service after an annual or 100-hour inspection shall, before that approval, run the aircraft engine or engines to determine satisfactory performance in accordance with the manufacturer's recommendations of—

(i) Power output (static and idle r.p.m.)
The only "manufacturer's recommendation" I see for static rpm is in the Owners Manual which is 2230-2330 rpm (1952 C-170B Owners Manual - not a CAA approved document) and in the Airplane Flight Manual.
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Gary
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Bruce Fenstermacher
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Re: Propeller Repitch

Post by Bruce Fenstermacher »

gahorn wrote:Cessna designated the DM7653 as the “standard”, and the DM7654 as the “cruise” prop.
The DM7655 is cruise, not 7654.

According to McCauley:
MDM 7652 is climb, 7654 is standard and 7656 is cruise on a 170 with the C-145/0-300.
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gfeher
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Re: Propeller Repitch

Post by gfeher »

The McCauley Propeller Application Guide (MAG), Rev 3, on pp. J-31 to J-36, lists the 1C172/MDM7653 as climb, 1C172/MDM7655 as standard, and 1C172/MDM7656 as cruise for the 170, 170A and 170B with the C-145/O-300.
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
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GAHorn
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Re: Propeller Repitch

Post by GAHorn »

n2582d wrote:...George, This is news to me. I'd agree with you if that "strip" happened to be a canal and you had a 170A or B on floats with a McCauley 1A170 or a McCauley 1C172/MDM but not if it was a landplane. ....How do you understand "no additional tolerance permitted."?


Well, Gary, I have great respect for your opinions and experience, and I’d have preferred that you’d have considered the alternatives available to aircraft owners when modifying their airplanes. ANY modification violates the published TCDS.

Rhetorical Question: Surely you are not suggesting that ALL departures from the TCDS are illegal?

The TCDS is applicable ONLY to UN-modified airplanes. I thought I made that inference when I commented about STCs (but should also have included other legal methods of modifications...I was attempting brevity....apparently an error when someone can turn the dogs loose.) :wink:

I hope it’s understood by all readers that: 1- i am a proponent of FAR compliance, including proper documentation and approvals for modifications, and 2- that any changes from the TCDS/Production Certificate of the airplane needs a basis of approval.

In the case of a prop pitch-change, the TCDS does not specify the pitch of the props mentioned, nor does the TCDS limit the props which can be installed on the airplane WITH PROPER APPROVAL. I thought it obvious that if one re-pitches their prop to the example I offered... that a basis of approval must be obtained. I’m not suggesting that an owner can remove his prop, clinch it up in the shop vise and use a pipe-wrench to give it a different twist.

However, if one DOES have their prop altered by a CRS/Prop-Shop to a pitch other than those mentioned in the McCauley/Cessna pubs, then the re-installation will have to be done by someone qualified and certificated to approve that reinstallation. DOH.
The airplane will NO LONGER meet the performance criteria published in the AFM (another req’d document) but the work can be legally accomplished if FARs are followed, and that was my point [I made that edit}.
Bruce Fenstermacher wrote:
The DM7655 is cruise, not 7654.

According to McCauley:
MDM 7652 is climb, 7654 is standard and 7656 is cruise on a 170 with the C-145/0-300.
Bruce, While I may have made a “typo” (can’t recall where I got that pitch info at the moment),.... I think you may be mixing apples and oranges. (DM vs MDM)

There are several publications which indicate that Cessna “approved” the DM7653 as the standard prop and that the MDM and EM series increased pitch in an analogous comparison, but those pubs, like Owners Manuals, are not always “approved” or “acceptable” data.
According to the McCauley Application Data there was no “cruise” (DM) prop designated by Cessna (although some commercial publications state otherwise.)

It would be good to point out to everyone that making pitch changes should not be undertaken lightly. The approved propellers for the airplane are of specified configuration and pitch changes must be in accordance with approved data. Propellers are a critical component of the airplane. A consideration some might overlook is vibratory-harmonics that propeller subjects to itself and the engine and the rest of the airplane. There are RPM ranges that most propellers simply cannot withstand, and some (I’m thinking of some Sensenich models here) RPM ranges cannot be tolerated except in transition, without fear of serious failure. The loss of a few inches of propeller-tip in-flight (or a crankshaft counter-balance conflict) can prove disastrous.

But there’s no reason to fear re-pitching your prop to achieve better performance if approved data and procedures are followed. I certainly hope no one thinks I was recommending otherwise.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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n2582d
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Re: Propeller Repitch

Post by n2582d »

gahorn wrote:I hope it’s understood by all readers that: 1- i am a proponent of FAR compliance, including proper documentation and approvals for modifications, and 2- that any changes from the TCDS/Production Certificate of the airplane needs a basis of approval.
... But there’s no reason to fear re-pitching your prop to achieve better performance if approved data and procedures are followed. I certainly hope no one thinks I was recommending otherwise.
Thanks for the clarification George. Take a look at FAR 43.15 (b):
Click to enlarge
Click to enlarge
In your earlier post it appeared you were suggesting one could reduce the pitch of a propeller listed in the TCDS beyond the limits for static rpm listed there. A cursory look at McCauley Propeller Systems Application Guide appears to show static limits which are higher than the TCDS limits for land planes but these are for aircraft on floats as indicated by the “F” in the “Type” column. On the Table of Contents page it says:
Click to enlarge
Click to enlarge
Like you clarified, the owner needs to make sure the propeller/engine/aircraft conforms to the TCDS or properly altered condition — which in this case means STC.
Last edited by n2582d on Thu Apr 02, 2020 7:14 pm, edited 1 time in total.
Gary
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GAHorn
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Re: Propeller Repitch

Post by GAHorn »

n2582d wrote:... or properly altered condition — which in this case means STC.
Or “Field Approval” such as is the case of at least one B-model owned by one of our Members I’ve flown with, which had a “floatplane” approved propeller installed on his landplane. (He moves the airplane back and forth on wheels or floats.). In his case, his airplane has dramatically shortened runway length requirements as a landplane, but at the penalty of slower cruise speeds.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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