I would think derating would involve decreasing boost or decreasing CR. Just lowering the redline RPM doesn’t really derate it OMO ?? You get less power, but more torque at WOT, considering fixed pitch (I would think). Maybe the fuel consumption gets much better for a relatively “small” decrease in power ? Less noise also.
Jujupilote wrote:
Perhaps fictionally derating an engine (just lowering the redline) doesn’t affect TBO. Like the 235hp of the 182 vs the 250hp of ie TB20
I guess with load of engine monitor data in GA now freely available, one can easily answer by a crystal clear if “de-rating increases actual TBO” ?
LeSving wrote:
Just lowering the redline RPM doesn’t really derate it OMO ?? You get less power, but more torque at WOT, considering fixed pitch (I would think)
While lowering RPM near max RPM tends to increase torque slightly (assuming max RPM is past peak torque), it does increase it nowhere near as much [roughly flat —> power roughly linear with RPM] as the propeller torque requirement increases (with the cube of RPM).
If it were possible to get the same MAX power (WOT, best power mixture) at a lower RPM, we would all have lower revving engines…
160 hp at 2400 rpm vs 180 hp at 2700 rpm gives the exact same torque. The exact same pressure in the pistons, the same forces within the engine. The velocities are higher at 2700 rpm of course, but 2700 rpm is still pretty low as far as engines go.
Crankshaft torque is the same, but it does not mean all other forces in the engine are the same. Dissipated heat power is certainly not the same, and since the thermal resistance of engine components is never zero, it means the innards of the engine are hotter at higher RPM even if CHT is the same.
I guess with load of engine monitor data in GA now freely available, one can easily answer by a crystal clear if “de-rating increases actual TBO” ?
I doubt that. Almost nobody flies at +75% power for significant lengths of time, especially not the atmospheric engined airplanes.
LeSving wrote:
160 hp at 2400 rpm vs 180 hp at 2700 rpm gives the exact same torque
If the torque curve if flat at that point, yes, but even if it isn’t this won’t be far off.
So if you fit a fine propeller that has that torque requirement at 2,700 RPM, the engine delivers 180HP. Now fit a coarse prop that requires the same torque at 2,400 RPM, and the engine will only develop 160hp.
As @Ultranomad writes, thermal stresses will be lower, but I am not sure how much that will be a factor in an engine well away from metallurgical limits.
But everything that is moving moves 11% less per hour, so I would expect wear during cruise to be less, and there are fewer power pulses.
So it all depends how much engine wear is coming from starting the engine (where the rating makes no difference) and how much from climb / cruise. Hence all things being equal, an engine operated at lower RPM (and lower power) should last longer
But I agree that there is probably not much scientific calculation that went into the actual TBO increase. I guess that the main reason those de-rated installations with a lower certified TBO exist is that somebody did the paperwork, while for others they could not be bothered.
Cobalt wrote:
As @Ultranomad writes, thermal stresses will be lower, but I am not sure how much that will be a factor in an engine well away from metallurgical limits.
Unfortunately, some engines operate fairly closely to metallurgical limits – for example, Walter/LOM inline engines have duralumin connecting rods, which lose their strength when overheated. Furthermore, carbon deposits and oil deterioration are also temperature-dependent.
Sure, and we all know how the constant thermal stress by flying high with higher power settings eats up the cylinders in turbocharged engines [regardless of whether shock cooling exists…].
All I meant is that for a normally aspirated 180HP engine with low CHTs even if operated at its limits this may not be the deciding factor; but I may well be wrong.
Surely the difference between 1800hrs and 2000hrs is just how you log the flight 
Legally you should be logging airborne time and this is for both EASA-reg and N-reg. Brakes off to brakes on in the pilot logbook.
Most people don’t know this and log the brakes off to brakes on in their maintenance logs, so they reach TBO on average 10-20% sooner.
