TACH MARKINGS
From: Steven Roth (StevenRoth@aol.com)
My tach needs to be changed. I have a C-125 converted to a C-145/O-300A. It has no color markings and records tach speed too quickly. It needs to be set for cruise at 2500. Any ideas what the color bands should be for this engine in a Swift? Green zones for at certain altitudes? Red line (based on altitude?)? My C-172 with O-300A has green area from around 2100 RPM to 2450-2600 RPM (depends on altitude). Should I mark my tach the same as my C-172 tach? Also, airspeed indicator is not marked. I need to mark flap operation area in white, gear speed and red line (VNE). On my C-172, the bottom on the white zone (flaps) is the stall speed at max gross. Similar for Swift? Should it have a yellow zone as my C-172 airspeed indicator which is called maneuvering speed (I believe) where operation in turbulence is to be avoided?
Steve,
The tach markings for a 145 are open to a lot of discussion. The Swift Assn. STC is a paperwork exercise, where they actually de-rate the O-300A to 125 hp. They oversimplified this by just requiring a placard “do not exceed 125 hp, 2270 rpm at any time” What they left unsaid was “at full throttle”. 125 hp is 86% power for a 145. I worked out a power chart which was published in the May issue of the “Vintage Airplane” magazine. Here at Lake Elmo, MN (932 msl) it is not possible to exceed 86% power! (125hp = 2500 rpm x 26.7″ mp) Even at sea level, 2700 rpm x 25″ mp = 125 hp, can you turn that? I doubt it. Ok – to get back to the markings A red line at 2700 rpm should be sufficient. The O-300 has no range markings to avoid. If you want to mark it like a 172 tach I see no reason not to. But, the 172 was designed not to have a manifold pressure gauge, so that’s why they have all those markings. The external glass can be marked, if a “slippage mark” is also applied, (in case the glass rotates) If you want to make your tach record properly at 2500 rpm, that can be done, but is not required.
