----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----
Hi All, This issue is not "black and white" simple. As in life, there's considerable grey area". Propellers have a pretty considerable design stress limit "fudge factor". You also don't typically get have significant vibration or experience threatening harmonics with a well balanced and properly maintained engine/engine mount shock absorbers/propeller/spinner installation. Fred Weick said many times he originally believed 415-C owners would be happy with a 75hp engine and prop combination that could rev up and put out close to 85hp in cruise, so that's what was initially sold by the factory. The ships would turn up close to the 2575 rpm "limit" of the mechanically identical C-85 with C-75's smaller carburetor jet and longer prop (73"). He never mentioned considering vibration a problem with the wood Sensenich fitted as standard on most production coupes right after the war. Today we have the wonders of much better balancing technology. If this was my McCauley prop, first I would check the plane's paperwork and see if there is a 377 or one-time STC for this prop to be on this airframe and see if acceptable static rpm limits are given. If so, I would get a Helmuth-Chadwick balance done (with proper spinner mounted) and worry no further, presuming present static rpm is within the stated range. If not, I would seek such a one-time STC, etc. listing acceptable static rpm limits of 1975-2140 rpm (approximately one-third towards 71" prop values from 73" prop values in the Aircraft Type Certificate No.718). I also might have the prop overhauled to make sure it is in perfect condition while having the pitch tweaked a bit as necessary to get the static rpm as close to the 2140 rpm maximum (for maximum climb performance with this prop) or as far towards the 1975 rpm minimum as the engine can reach 2575 in level flight at full throttle (for maximum cruise performance with this prop). Then do a Helmuth-Chadwick (or equal) balance job. Of course it is a "given" that McCauley's attorneys would not be happy with any of these courses of action (any of which would void their warranty). Regards, William R. Bayne -- On Jan 5, 2005, at 11:59 AM, Skyport Services wrote: > >>That's on the climb side of climb. What is your static RPM? (Have > you had your tach checked?) > > My mistake! I missed the fact that the length was 72". First, > measure the actual length. If it is greater than 71", get it cut down > ASAP. (read "before further flight) The length limit is for vibration > reasons. Bad vibrations can result in bad things, like broken > crankshafts... > > > John Cooper > Skyport Services ========================================================================== ==== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
<<attachment: winmail.dat>>
