A minor RPM “oops”

Jmcmanna

I flew my stepson up to Milwaukee this morning to drop him off with his biological dad. On final, tower sent us around...no big deal...climb power set, gear up...I realized about 30 seconds later that the prop was set at 2400 RPM still (I was still far enough out that I hadn’t set the prop forward yet) and I was right in the middle of the red arc on the tach...oops. I pushed the blue knob forward and got out of the danger zone, but it got me thinking...what happens in that red arc? What kind of damage can happen and how long does it take before something bad happens?

arrow76r

Jim...Do you mean that in spite of setting the prop to 2400 it was actually in the middle of the red band? Why would it have changed, I wonder.

Scott Sherer

On some of the older airplanes, like Arrows, there is a danger zone for extended use in the middle rpm's on constant speed props. I've never heard of a prop failure but apparently they do happen. Newer props don't have that danger zone.

arrow76r

The placard in my Arrow reads "Avoid continuous operation between 2000 and 2350 RPM". The issue, as I understand it, it due to harmonic vibrations set up in the 2-bladed Hartzell propeller. Replacing that with either the 2-blade "scimitar" or 3-blade ($$) prop removes the placarded prohibition. So define "continuous"??? I don't know...I just blindly follow the placard but obviously your prop goes through that zone frequently so "continuous" ????

Jmcmanna

When I overhauled my engine, I put on a brand new prop which had a new limitation. “Avoid continuous operations between 2350 and 2550 RPM above 24” MP”

arrow76r

Wow! What kind of prop is it?

arrow76r

Jim...Doesn't that restriction significantly affect takeoff/climb performance? I haven't seen 24 inches MAP for a long time! 23 is the best I can do here. During the break-in period for my new engine last December I flew "low" down along the Arkansas River valley to get as close to 25 MAP as possible.

Jmcmanna

I have the Hartzell Scimitar prop. With the C1C6 engine there is no red arc, but I have the C1C and thus have that limitation. Down here in the lowlands I get 27+ inches of MP for takeoff. I can’t do the 25”/2500 RPM for climbing, so I climb at 25” and 2600 RPM, then cruise around 22” and 2400 RPM.

arrow76r

Interesting! I just checked my new engine logbook and this is an IO-360-C1C. What is the difference between a C1C and a C1C6? I had thought about the Scimitar prop but the one I have only has about 500 hours on it.

Harley Kleemann

Jim, the C1C6 engine has counterweights on the crankshaft to eliminate harmonic overtones, the C1C does not.

arrow76r

Thanks! I wonder why those are not "standard".

Jmcmanna

I think the C1C6 came with the Arrow IIIs and later. I’m not sure what the limitations are that preclude it from being used in an older Arrow, but it wasn’t offered as an option when I did my reman last summer.

MikeJJ

My Cherokee 180 had the same RPM restriction from 2150 to 2350 RPM. On a visit to the Sensenich factory years ago, I asked the same question about how long one could be in this area. It was explained to me in graphic detail, literally.

The answer was "the effects [stress] are cumulative. The less time [in the restricted range] the better." I was given a copy of a graph showing RPM points where vibration set up standing waves in the blade causing the tips to bend rapidly! It was explained the prop and the crankshaft are a system with respect to these vibrations, and the hollow crankshaft of the O-360-A3A was an issue! It was hollow to accommodate a constant speed prop which the Cherokee did not have.

Eventually, an AD was issued to require periodic inspection of the hollow crank interior for corrosion pits. I eliminated both the RPM restriction AND the crank inspections by switching to an O-360-A4A at overhaul time. The O-360-A4A, unlike the A3A, has a solid crankshaft, and the type certificate for the Cherokee 180 also specifies this alternative engine!

Regards,

Mike

Scott Sherer

Thanks Mike, most informative.