Clay (and popcorn enthusiasts
),
I picked up on your original thought…that when flying rocket ships like RVs, why not used reduced power TOs to save $ and engine life…we do have a lot of margin in RVs, and it works at work, right?
I tossed out in my first post in this thread, a query to see if anyone has hard data on reduced power TOs being less wear on a recip. I don't think it is, but would be interested in engine mfgr findings…if there are any.
My gut says that its so critical to jet engines, because the heat and rotational forces are so high (as you know). 5,000 RPM for low speed compressors and turbines, 15,000 RPM for hi speed comp/turb (the limits are in that range). All that speed and heat trying to pull the blades out of the "sockets", etc. With the hours of service and history, they know that heat is the engine killer. Keep the heat down until the bypass air is flowing, and you save engine life…and loads of engine lease dollars. I recently saw a note that indicated that "the economic benefits of reduced power takeoff and initial climb diminish or vanish at 105 seconds after power-up". So the leasing companies have it figured out to the gnats ars, based on maintenance history. At that point we actually power back
up for the rest of (the majority of) the climb. I've joked with FOs, "now that we're safely away from the ground, let's go to full power"…I'm sure many have. But there is is plenty of power to spare, as you know. However, I don't think reduced power takeoffs burn more fuel, as long as the operator doesn't waste time climbing to cruise (I think that's what Sig was referring to)
Bring it back to recips and RVs: The rotational forces are not as extreme, and the temps may not be as extreme (I'm not sure about temps though…it may be close.) It could be as simple as thinking it in terms of relative reduction of RPM: by that I mean that reducing a turbine 10-15 percent off of 5000/15000 RPM has a real impact, where reducing a recip an equal 10-15 percent of of 2500-2700 RPM may have a much smaller impact on heat generation. Also, the comments about reduced power takeoffs/climbs hurting recips are probably related to the carb or FI enrichment system. It may have been explained earlier, but at full power, a carb or FI system will meter extra fuel for cooling. Just off full power, that system does not operate. Interestingly the enrichment system is also known as the economizer system…which is not a good description of its impact on FF!
I'd still be interested in engine guru analysis or data on the impact to recip engine wear. My pref is to do full power takeoffs, and if noise abatement is a factor, I'll roll it back to 2500 RPM in the climb. However, I don't touch nuttin' till 1000' or so (for field return), as I've also heard what others have said: failures may be more prone to occur when the first change is made. I think that relates to an underlying problem lurking at a steady state, then popping its head up when the steady state is changed. Kinda like the zero oil pressure procedure in a single engine jet (TA-4 was my experience). Set XX%, and don't touch nuttin' till ya got the field made.
Again, just one technique (wrt RPTOs).
Cheers,
Bob