https://www.savvyaviation.com/what-we-have-here-is-a-failure-to-rotate/#tbofallacy
There have been various threads on setting power for Carson speeds, maximum range.
While these operating parameters may be benign using Lycomings, it may be worth considering the implications of operating below 60% using six cylinder Continentals.
Continental has an SB CSB09-11 advising against operating below 2300 RPM on their larger bore models. These engines have had examples failing due to crankshaft counterweights being released in flight.
In addition lower power settings may result in rotocoils (Continental term, Lycoming call them rotator caps), not functioning correctly and the exhaust valves either not rotating, or rotating sluggishly with a consequent development of hot spots.
For better or worse these engines (relatively low compression, large bore, air cooled, horizontally opposed) were designed for operation between 65% and 75% power. The famous Rare Bear Unlimited Reno Races team always ran their company hack, a Warrior, full throttle – which at Reno DA is around 65%-75% – getting typically over 3,500 hours from the little four banger Lycoming.
So glad that I got a Lycoming engine 👍
This is not a new thing. Look at this IO540 chart and the shaded portions are not recommended, due to, I am told, crankshaft flange stresses due to the counterweights not working correctly.
There is more reading here on the topic of flying at “too low power” and getting glazed cylinders. It happened to me when I did a succession of high altitude (say 40% power in cruise) flights.
Peter wrote:
shaded portions are not recommended
Isn’t this just to avoid overtorque? I mean it starts at 27 “Hg and above, and just covers low RPM regimes at or above that manifold rating. 27 to 29.8 is basically high power rating, definitely above cruise, even more than I set for climb, which is 25”. Or are you referring to something different?
I am not aware of any limitation of the IO540 that would affect normal operation.
A limit that I have to look into sometimes is the maximum MP for 1800 RPM, which is my typical descend RPM.
By the way, a good point about descending on such a low RPM (besides the greatly improved glide behaviour, i.e. reduced fuel flow) is that I’ve already been addressed several times that my aircraft is barely audible on approach. Noise is always an issue, but when you just don’t generate the noise or avoid it as much as possible, it’s much better acceptable.
I’ve never seen an authoritative writeup on the reason for this. The data Lyco would have supplied to the FAA to get the engine TC will be commercially confidential.
RobertL18C wrote:
In addition lower power settings may result in rotocoils (Continental term, Lycoming call them rotator caps), not functioning correctly and the exhaust valves either not rotating, or rotating sluggishly with a consequent development of hot spots.
An important distinction here is that big Continentals have an actual rotator cap, a little springy thing which actively rotates the valve with each actuation. Lycoming engines do not have this – they only have a cap on the end of the valve stem which allows the valve to be spun by the outflowing exhaust gasses.
