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

This comment about lower Tach time is interesting. I always noticed about a 10% variation Hobbs time to tach in just regular flying. I have always pointed out that electronic tachs do not have this error and can therefore cost thousands of dollars more in terms of reduced TBO. I know that some airtaxi operators use a Hobbs with an "airspeed switch" that doesn't activate untill something like 40 knots to prolong TBO. FAA regs only require actual flight time to count as engine time. Engine Overhauls are expensive, and 10% longer tbo is 10% less cost over the life on the engine, right?

When I can get enough cruising info put togather I will post it all. I asked this same question about low rpm cruise in a couple of other forums and it looks like cruising at about 100 mph at the lowest possible rpm will give the greatest mpg, but that is terribly slow, and really low mp can cause sticking valves and excessive oil useage (according to Lyc.) and it is important to maintain a reasonable oil temp. I am wondering if there are any ferry pilots out there that might have some input on stretching range and fuel economy?

Originally Posted by Tony

John, if you plot a graph of mpg vs percent power, you'll probably
find a pretty broad area where you're within a few percent of optimum.
A couple of knots one way or the other is picking at nits. In real
life, slight changes in rigging on individual airplanes, and cg
locations will have that sort of effect, won't they? I don't know this
for sure, but think as cg goes aft the airplane becomes a little more
effective since the stab doesn't add as much aerodynamic 'weight'
pushing the tail down to overcome the load moment.

Ditto, plot mpg vs RPM for a given percentage power, and pretty often
that's at the lower engine speeds. Since TBO is determined by tach
hours, we tend to fly our IO360 at lower RPMs anyhow. Average RPM of
2200 instead of 2500 is about a 14% difference in clock hours for time
between overhauls, and as Ben Franklin might have said, an hour saved
is an hour earned.




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? What about fuel flow? Does anyone see much difference
when flying a PA32-260 at 18.0/2400 vs 20.2/2100, or at any of the
other settings?
I do have wheel pants, but no other speed mods. I really appreciate
all the input, thanks.

John

cwby-flyer;595035 Wrote:



On Jan 18, 6:22 pm, JOM wrote:-
I have very recently purchased a Cherokee 6-260 and was wondering if
anyone has any sugestions regarding the most economical cruise
settings? Speed is not so important as MPG. Most of the distances I
fly are not long, so high altitude stuff doesn't really factor in.
Most flights will be 100 to 300 miles and maybe 5000'. This is a
carburated engine, so GAMI's and lean of peak doesn't fit the
picture.
I have heard of lean of peak with a carb and some carb heat, but I
usually fly 60% power at peak as per the Lyc operators manual.

I have been told I can get 11-12 gph at 120 knots, but I seem to use
more fuel. Any suggestions?

Thanks,

John

--
JOM-

John,

I looking at a POH from a 1968 PA32-260 and at 7,000 ft density
altitude the cahrts are indicating about 140mph (121 kts) true. The
power settings for 55% power at this altitude a 20.2/2100,
19.3/2200, 18.3/2300, or 18.0/2400.

Obviously, make sure that you're using true airspeed and not
indicated. Do you have the wheel pants installed? Another huge
problem with getting book speed out of a cherokee is the rigging of
flaps - if you gowww.pipermods.comthere's a drawing of a tool to
check for proper rigging.

Mike

--
JOM

Michael,

Thanks for the input, I have ordered a POH, since there is not another 'Six within 100 miles of my home airport so that I can borrow one.
I am just wondering if anyone has had any real world examples of what they use in a PA32-260 for low power cruise, and if they have any problems regarding oil temps, or sticking valves, etc?

John

Originally Posted by Michael[_1_]

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

"JOM" wrote in message ...

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? What about fuel flow? Does anyone see much difference
when flying a PA32-260 at 18.0/2400 vs 20.2/2100, or at any of the
other settings?
I do have wheel pants, but no other speed mods. I really appreciate
all the input, thanks.

John



Best propeller efficiency is at lower RPM, so your best bet is to choose highest manifold pressure/lowest propeller RPM
that yields 55% power while staying within the ops limitations of the POH...

This is how Lindbergh trained the pilots in the Pacific to get the best range from their p-38s, etc...