"Roger Long" om writes:

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?

With a fixed-pitch prop, what you care about is your power setting --
the RPM to get that setting varies depending on your density
altitude. For example, 2400 fpm is around 75% power in a 172P at sea
level, but only 55% power at 10,000 ft DA.

From what I've seen, it's actually renters who tend to leave the
mixture full rich and close the throttle, gumming up the plugs and
valves, while owners tend to leave the throttle futher open and lean
the mixture according to POH instructions.

Some owners are getting even more aggressive, leaving the throttle
wide open and using only the mixture to set power, as long as they can
do so without the engine running rough -- that's probably what you're
hearing about. My O-320D3G runs beautifully WOT down to at least 65%,
especially in cold weather, and I've from other small engine owners
who see the same results. Larger carbureted engines like the O470
don't tend to do as well, from what I've heard so far.

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.

Try it and see. Look up the RPM for your preferred power setting
at, say, 5500 ft density altitude, fly up there, then leave your
throttle wide open and slowly pull back the mixture until you get your
desired RPM. If the engine runs smoothly, then you're fine. If it
runs rough, try turning on carb heat (which might smooth out the
distribution) and then try one more time. Lycoming put out a bulletin
warning against LOP operations for engines with constant-speed props,
because there is no way to be sure that you're not at too high a
power setting, but even they admitted that you cannot hurt an engine
with a fixed-pitch prop as long as you stay at or below the RPM for
75% power when you're running lean.

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.

A lower power setting is a great way to save fuel. For any given
power setting, however, you'll burn even less fuel running WOT with a
leaner mixture than you will with partly-closed throttle and a richer
mixture -- it's just a matter of how far you can lean before your
engine starts running rough.

I'm getting 7.5 gph or less in my 160 hp Warrior II (comparable to a
172N) at 75% power and better than 120 ktas, though I still flight
plan for the book number to give myself a safety margin. It was great
landing after a non-stop 400 nm cross-country at 75% power and still
having half-full tanks.


All the best,


David

--
David Megginson, , http://www.megginson.com/