Jacko wrote:
People do say all sorts of things, but assigning any significance to +/- 20F for a component designed to operate up to about 1,450F looks to me like the sort of OWT which we should hesitate to promulgate on EuroGA.
It’s not the EGT temperature itself that matters. (Which, incidentally, is why most analogue EGT indicators doesn’t have any figures on the scales.) The +/-20F is an indicator of what is happening inside the engine and that can certainly matter.
Graham wrote:
Isn’t slowing down the thing to do in the circumstances described in the original post?
Does that not depend upon the Airspeed/Headwind ratio?
In my aircraft (PA31) there is a large improvement in SFC when LoP. FF falls about 20% and speed about 5%.
But my POH (unusually) actually recommends it. And they are ordinary Lycoming TIO540s.
And, contrary to the OP, LoP is better for engines, with lower internal cylinder pressures and CHTs.
Mirage used to run 24gph at 205 knots ROP vs 13.5gph at 180 LOP. LOP clearly helped.
@Airborne_Again wrote:
The +/-20F is an indicator of what is happening inside the engine and that can certainly matter.
Really? That’s something I certainly didn’t know, so please do explain how.
We are discussing EGT (as measured by a probe) and the likely effect of +/- 20F on an exhaust valve and pipework. We know that the temperature of the gas, probe, valve and manifold are all different, and indeed that the temperature of the valve and pipework is not uniform (the hottest part of the valve being the centre of the head). However, any increase in the temperature of those components can not significantly exceed the increase in the temperature of the gas which is heating them.
Do you really believe that the materials of which aircraft exhaust valves and pipes are made are susceptible to a difference in temperature from, say, 1400F to 1420F?
Jacko wrote:
Really? That’s something I certainly didn’t know, so please do explain how.We are discussing EGT (as measured by a probe) and the likely effect of +/- 20F on an exhaust valve and pipework.
No, that’s not at all what EGT settings are about. As I said, the absolute values of EGT are irrelevant. What’s relevant is the difference from the peak value as this gives you an indication of the fuel/air mixture which in turn affects temperatures and pressures in the cylinders.
Do you really believe that the materials of which aircraft exhaust valves and pipes are made are susceptible to a difference in temperature from, say, 1400F to 1420F?
No one believes that, so your question is moot.
Good, we agree that +/-20F exhaust probe temperature does not have any effect on exhaust valves and exhaust systems (contrary to the OWT to which Bosco alluded, albeit with implied reservation).
If I understand it correctly your point is a completely different, but valid, one – namely that 20F either side of peak EGT affects cylinder temperature and pressure. But exactly how much difference in CHT do you actually see at “cruise” power of, say, 70% of rated hp?
In my O-360-C1F cylinder head temperature varies insignificantly (less than 10F) when I lean from 20 ROP to 20 LOP while maintaining constant power output (i.e. adjusting MAP as required to maintain constant airspeed in level flight).
Do others see a greater variation in CHT?
If so, does anyone have evidence that running a cylinder 10 or 20 degrees F hotter (say 380F instead of 360F) will materially affect the service life of a Lycoming cylinder or any other engine component?
My cylinders go from about 420°F to about 380°F when run LoP. That is definitely significant.
I am no metallurgist myself, but everything I have read on the subject leads me to believe that it also has significant benefits for longevity.
But each to his own. If you prefer RoP, or even going fully rich, fill yer boots. It’s your money.
In my aircraft (PA31) there is a large improvement in SFC when LoP. FF falls about 20% and speed about 5%.
Versus full-rich, no doubt. Much more, probably.
But not versus peak EGT.
If so, does anyone have evidence that running a cylinder 10 or 20 degrees F hotter (say 380F instead of 360F) will materially affect the service life of a Lycoming cylinder or any other engine component?
Not from 360 to 380, but quite possibly from 450 to 470, since the strength drops off quick quickly then.
Does that not depend upon the Airspeed/Headwind ratio?
Yes; for best range you want to fly a bit faster into a headwind, and a bit slower with a tailwind.
Yes, the logic being if you have a headwind you want a faster TAS so you spend less time exposed to it. And if you’ve a tailwind, you slow down so that you spend longer benefiting from it.
But generally most MPG is quite a lot slower than headline cruise speed, is it not?
