Lean of Peak Test Flight

I flew our 172 N for two hours today at various altitudes between 3500 and
8500 experimenting further with LOP. Here's what I learned:

1) Forget "Peak" and "LOP". The buzzword applicable to a fixed pitch carb
engine with single probes is "Richening" instead of "Leaning". The engine
behaves differently when you enrichen it than when you lean it.

2) You don't need to find peak or even use the EGT.

3) At least on an O-320 H2AD, there probably isn't any such thing as partial
carb heat. Listening to our engine and watching our very accurate digital
tach, I think our carb heat is pretty much on / off with all of the action
happening in the last 1/4 or less of travel. Looking at how the system is
set up, this is actually what I would expect. I wonder if some of the
anecdotal belief that partial carb heat is dangerous is actually the result
of its being no carb heat at all. We don't have a carb temp gauge and I'd
love to hear more from someone with a similar engine who does.

Here's what works on our engine:

Push the throttle all the way in and then pull out about 1/4", just enough
to tip the throttle plate slightly to set up some swirling in the air flow
and turn off the auto enrichment feature of the carburetor. I couldn't
actually demonstrate a difference between WOT and slightly cracked today but
this seems like a good idea. I'll have to look at this more closely next
time.

Put on full carb heat.

Lean steadily but briskly until the engine sags 100 - 200 RPM. It will get
quite rough but let it run that way. In 10 - 20 seconds, you will see the
EGT drop 100 - 150 degrees. As soon as you see the drop, turn the mixture
knob. Almost immediately, about 1/8 turn on our vernier knob, the engine
will smooth out and speed up.

As the plane picks up speed, the EGT will rise and the engine will get rough
again, give another turn. Repeat as the plane picks up speed until you get
to a stable point where the engine remains acceptably smooth as the airspeed
stabilizes. Sometimes, and more often at lower altitudes, this stable point
will be about 25 LOP. In the 6000 - 8000 foot range, I could only get our
engine to stabilize at peak. I fiddled around to find where peak was but
that was only to learn how the engine was operating. I now don't consider
finding actual peak part of the standard procedure.

I tried finding the stable and minimally smooth point leaning from the ROP
side and consistently ended up running richer and not noticeably smoother.
Enrichening 50 - 100 degrees from that point will make it sound better,
however. If this was a musical instrument, I'd always run it that way but it
's an engine and it's smooth enough running leaner and cooler.

At 3500 feet and below, I generally found that I ended up LOP but running
slightly above the RPM for 60% power. A slight pull of the throttle took
care of that and EGT then went down a hair.

At 4500 feet, the engine seemed to end up at the 60% power point by itself
as well as 25 degree LOP. This seems to be the optimum altitude for the
technique.

5500 feet and above, I ended up with lower RPM than 60% power. Turning the
mixture knob in until I got to 60% power made the engine slightly smoother
and brought the airspeed up to where I usually cruise.

At 8500 feet, the stable point was at peak. I project that it will be ROP at
higher altitudes if you want to get to 60% power (or maximum at those
altitudes). I expect that you will still end up leaner and cleaner finding
the sweet spot enrichening from the lean side instead of leaning from the
rich.

It was 40 F. on the ground today. Warmer temperatures should make all this
work even better by promoting better fuel vaporization.

Basically, I'm finding that the throttle just isn't used for level flight
and climb above about 3500 feet. Set just cracked from WOT. The mixture then
becomes the primary control. Turn it all the way in for climb and then lean
to the EGT you noted at about 1000 feet AGL on take off. (Keep an eye on CHT
if it's hot) Lean as above for level flight. For descent, pull the throttle
but don't touch the mixture except maybe to set it to minimally leaned in
the pattern in case a go around is necessary.

Anecdotal evidence is that there is a lot of variation among engines, maybe
within the same model and airframe. I hope some other 172 drivers will try
these methods and let me know how it works out.


--
Roger Long