The International Cessna^® 170 Association

I dug out the PROP (Hartzell HC-C2YK1BF) TCDS, and it hows "Avoid continuous operation between 2000 and 2250 RPM."

So it must be the the RPM restriction comes from the PROP, not the ENGINE. That's a new one on me. I always thought it was something to do with counterweights (or lack thereof) in the engine. I'm not a mechanic.

I still need to determine the bottom of the green arc (which the red overlays in the middle range above). The top obvioulsy is 2700RPM. I would think the bottom also would would come from the engine, being the RPM below which you should apply carb heat maybe, but the engine TCDS is silent on that.

Does anybody know where the bottom of the green on the tach should be? Probably somewhere around 1800 or so?

PS: I'm still interested in obtaining a copy of the Doyn STC, including the AFM supplement!

LakeHood170B wrote:...The top obvioulsy is 2700RPM. I would think the bottom also would would come from the engine, being the RPM below which you should apply carb heat .....

LakeHood170B

If your running a Lycoming (as you are) you don't put carb heat on at a certian RPM. All the Lycoming powered aircraft (mostly Piper Cherokees) say only to apply carb heat when indicated or in other words when you get ice or think you may get ice.

The Lycoming design with the carb mounted directly to the hot oil sump is less subject to carb ice than a continental and that is at leas one reason for the difference.

Before anyone gets there pant in a bind I'm not suggesting Lycomings won't ice up and that use of the carb heat is not necessary, just that it is not called for in the flight manuals like a continental powered aircraft is.

As for the green arc I would suggest you look at other aircraft with the same engine/prop combination like a Cherokee for a guide.

Lakehood, I have the 76" Hartzel on my conversion also. The RPM restriction is just "avoid continous operation between 2000 and 2200 RPM" for my STC there is just a placard set over the Tach with redline noted on the Tach at 2700. I think it's very typical of these to see a yellow arc from 1950rpm to 2250rpm, this gives a bit a buffer for tach error or eyeball to tach angle error. A red arc for that range doesn't sound right since you obviously have to cross it every time you fly, unless there's some stipulation in the STC I'd just put a yellow arc there, or placard it.

Was told many years ago that the rpm restriction was not a factor at mp of less than either 23" or 24" don't remember. A fellow was doing offshore shark spotting and needed to use that low rpm to get more fuel endurance and back tracked the restriction to see where it came from. Somebodies covering their butts.