Low power cruise in a Cherokee 6

On Jan 22, 12:48*am, JOM wrote:
Now, I know that 55% power is 55% power, however I wonder if things like
prop effeciency and thrust could make some changes at different mp and
rpm settings?

Yes. Also drag losses. In general, your best efficiency will be at
the lowest RPM and highest manifold pressure that provides smooth
operation.

There are several things you should know. First, your Six came with
both an AFM (FAA approved) and an owner's manual (not FAA approved,
but way more useful). The owner's manual contained tables of power
settings and speeds at different altitudes. It probably got lost
along the way, but you can probably get a copy from your type club.
That will give you the acceptable power settings.

The settings will probably be given for 50 degrees ROP, but since you
are planning to fly at best economy (55% power give or take) you can
simply lean until roughness, enrich just enough for smooth operation,
and call it good. Using just a touch of carb heat can allow you to
lean that much more at full throttle, but at low altitudes, you won't
need it, because you throttle won't be all the way open at 55%, even
at the lowest allowable RPM, until you hit 8000 ft or so. Carb heat
and throttle do about the same thing - induce mixing in the intake air
which will give you a little bit better fuel distribution.

Don't forget that if your engine has good fuel distribution, you will
be operating LOP anyway if you lean aggressively. Don't worry - your
power setting is too low to hurt anything, even without
instrumentation. However, you may notice the manifold pressure
dropped after you got done leaning. No worries - open the throttle
back up to get it back, then relean. Or don't, if you are satisfied
with the speed you are getting.

You are correct that minimum power is not necessarily the best economy
- but this is only true once you start gettting slow. What is slow?
Best glide speed, plus some extra (best range speed is always a little
more than best L/D in prop planes), plus half the headwind (if any),
roughly. Best glide in your airplane ought to be about 85 kts, I
think. That's indicated. Realistically, as long as you are doing
about 95 kts indicated plus half the headwind, you are not too slow.

Best range airspeed is the one that gives you best fuel economy, and
for an airplane like yours it will probably be around 45% power. If
you do run that low a power setting, run with the lowest practical RPM
(like maybe 1900-2000) and lean as much as possible - as long as the
engine is reasonably smooth you are not leaning too much. Really you
can't hurt your engine by overleaning below 60% power anyway.

Michael