All WOT means is you're going to get the most MP you can at your
current altitude. With a fixed-pitch prop, it spins up faster at high
altitude because of the propeller characteristics in the thinner air.
Consequently, you can make more power for a given MP setting (i.e.
22"@3000'=2300RPM, but 22"@7000'=2400RPM=more power).

According to the Lycoming leaning doc, at 75% power or less (WOT
at 8000' DA) leaning to peak EGT (usually about where roughness sets in
on a carb'd engine) will NOT harm the engine. Everything is specified on
percent power, not throttle setting. Another way to look at it is that
with two different settings but the same percent power (constant speed
prop running, say 25"/2100 and 20"/2600 approx 70% power). With high
MP/low RPM, you've got more torque for longer time per firing. That would
tend to imply less wear at higher speeds. I suspect they kinda average
out so that percent power is a good metric for engine wear, rather than a
particular RPM/MP setting.

Where was I going with this again? Must be tired...

-Cory


Roger Long om wrote:
: I was always taught that WOT was a "renter's setting" and that you should
: back off to 2300-2400 if you want to get good life out of your engine. I
: keep running across the Advanced Pilot Seminar guys asking, "Why EVER be
: partial throttle in cruise?

: Of course, they are probably also talking about advanced engines, with GAMI,
: analyzers, matched injectors, etc. What about those of us flying behind the
: glorified lawn mower engines of simpler aircraft. I've asked the question
: over at the CPA forum but I'd like some opinions from this group.

: Should I take our 172 N above altitudes where the POH says max power is 75%,
: put the throttle all the way in, lean till it gets a bit rough, and then
: enrich until it's smooth? As long as CHT and oil temps remain in an
: acceptable range, can I then cruise along confident that I'm getting there
: fastest and getting the best life from the engine?

: I've often suspected this was the case. Turning faster seems intuitively to
: be the same as the engine working harder but the trade off is that it doesn'
: t work as long. Speed of metal surface over metal surface (with oil film)
: within normal RPM ranges doesn't seem as significant a wear factor as the
: total number of firing cycles and revolutions. It seems like those should be
: about the same whether you fly at 115 knots or 90.

: Figuring the RPM's out from the POH:

: At 4000 feet, WOT, 2500 RPM there will be 129,591 revolutions per 100 miles.
: At 2400 RPM, 130,896. At 2300, 131,100. Going WOT instead of 2300 REDUCES
: firing cycles 1.15%!

: If the faster speed saves a bladder break, you'll get a huge savings in
: engine wear avoiding a thermal cycle and restart.

: On the other hand, marine engine factory reps, who I have more frequent
: contact with, tell me that the only significant indicator of engine life (as
: long as temperatures remain in normal range) is the total amount of fuel
: that goes through it. Seems like that should be true for aviation engines as
: well.

: Slowing down from WOT to 2300 in my 172 N should reduce fuel consumption
: 14.5%. That's pretty significant as well as probably saving a fuel stop
: somewhere on a long trip.

: --
: Roger Long



--
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