----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----At 08:31 PM 1/5/2005, William R. Bayne wrote:
I take it all back. McCauley explicitly limits the C85-1A/B90 combo to lengths between 68.5" and 71".
Where?
In the TCDS for the prop, P-842
NOTE 9. Special Limits.
Table of Propeller-Engine Combinations
Approved Vibrationwise for Use on Normal Category Single-Engine Aircraft
No one in the FAA in their right mind would issue a field approval, much less an STC, that doesn't meet those limits.
I'm not sure I want to go there...but keep in mind that refer to members of an unelected agency comprised of career bureaucrats that has already achieved every politician's dream...unlimited (personal) authority without meaningful (personal) responsibility!
But he real point is that they aren't going to but their butts on the line for something that's prohibited by the manufacturer.
Like I said before, get it cut down or replace it. I've seen too many C85's with broken center main bearing webs and/or broken crankshafts to risk chancing it.
How many have you seen and, of those, would you attribute all to propeller-induced stress or could other factors have initiated said failure(s)? Most of us lack such personal experience to draw from.
4 cases and 2 cranks, IIRC, over the years. I don't attribute them to anything, but since there is a vibration concern expressed by McCauley, I wouldn't press my luck by going test pilot status. One of the cranks broke in two and the engine kept running. Had a wierd vibration... All of the cases had been broken for an indeterminate time before the teardown.
My own, uneducated opinion is that the lower limit is of more concern than the upper limit...
Of what-static rpm, cruise rpm, allowable length? If the latter, how does that relate to our thread exploring the difference between a 71" prop or a 72" prop on a C-85?
Minimum length. Just a guess, based on analysis of the McCauley limits.
Forgive me, but I'm totally confused by your pronouncements here. That may result from the lack of source information to cross check.
A cursory check of a Univair catalog, No. 198, p. 202 (McCauley Installation List) shows a Taylorcraft 19 landplane fitted with a C-85-12 or 12-F (same as in the Ercoupes) had a CM7251 "Standard" prop and a CM7248 as a "Take-off Climb" prop recommended with a max. diameter of 72" and a min. diameter of 69.5".
Likely a typo. Consider the source (;~) The Taylorcraft TCDS says 71"
This references notes 1 and 2, which essentially confirms no rpm restrictions (from McCauley),
This is because originally there was an RPM range restriction (yellow arc), something like "Avoid continuous operation between 1950 and 2100 RPM". It was rescinded back in the '40s
and explaining that the 1A90/CF (elliptical tip) and the 1B90/CM (square tip) models of the same diameter and pitch are directly interchangable.
My point here is that, IF the FAA actually had to explain to someone exactly why they would not issue a one-time STC or 337 for a 7246CM on a Continental C-85-12 or 12-F, what would a logical reason be?
Length prohibited by McCauley for vibration considerations. Quite logical, IMHO.
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