Cruise RPM setting

Shortly after getting my private license I went to a different FBO and
got checked out in their aircrafts - C172 and Piper Cherokee.

I talked with the CFI and asked what RPM settings he would recommend.
I'm renting dry, so one way to reduce cost is to slow down and thus
save fuel. At the present time I'm just building hours towards my
commercial ticket. Rarely am I in a hurry to get anywhere. He
mentioned that if I needed to get somewhere quickly, to keep it around
2400. If I'm just building time then to reduce to 2300. It will save
fuel as well as reduce the sound level.

The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.

Thanks,

John

The best setting is whatever gives you the best miles per gallon. You
should find that at around 40% power. The more drag the airframe has
the less it will matter, you'll find yourself saving an extra tenth of a
mpg. Not worth the effort. The 182 I used to have would indicate 135
mph in the summer at 23" and 2450 rpm and 13 gph(10.4 mpg). At 20
squared(about 45%) it would indicate about 100 mph while burning about
9(11.1 mpg). My Bonanza indicates 190 mph at 23/2500 burning 14.5
gph(13.1 mpg). At 19/2100(also 45%) I indicate 160 mph, burning 8(20
mpg). Using true airspeeds would change these slightly but you get the
idea.

john wrote:

Shortly after getting my private license I went to a different FBO and
got checked out in their aircrafts - C172 and Piper Cherokee.

I talked with the CFI and asked what RPM settings he would recommend.
I'm renting dry, so one way to reduce cost is to slow down and thus
save fuel. At the present time I'm just building hours towards my
commercial ticket. Rarely am I in a hurry to get anywhere. He
mentioned that if I needed to get somewhere quickly, to keep it around
2400. If I'm just building time then to reduce to 2300. It will save
fuel as well as reduce the sound level.

The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.

Thanks,

John

Since you're building time, why wouldn't you fly at the lowest fuel
consumption settings you're comfortable with that are consistant with
the POH?

You log calendar time for logbook purposes, and done correctly at low
RPM and well leaned you'll really cut back on fuel. In our Mooney,
which gets pulled around by an IO360, we often cruise at 1950 RPM --
allowed in the POH -- and fuel flow is in the 7 or 8 gph.

As an owner, we like low RPM as well, since that extends the real time
between overhaul -- tach hours assume 2400 RPM or so.


, On Oct 13, 11:16 pm, john wrote:
Shortly after getting my private license I went to a different FBO and
got checked out in their aircrafts - C172 and Piper Cherokee.

I talked with the CFI and asked what RPM settings he would recommend.
I'm renting dry, so one way to reduce cost is to slow down and thus
save fuel. At the present time I'm just building hours towards my
commercial ticket. Rarely am I in a hurry to get anywhere. He
mentioned that if I needed to get somewhere quickly, to keep it around
2400. If I'm just building time then to reduce to 2300. It will save
fuel as well as reduce the sound level.

The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.

Thanks,

John

"Newps" wrote

The best setting is whatever gives you the best miles per gallon. You
should find that at around 40% power.

For just building time, MPG is not significant. He wants to stay in the air
for the least gas. For him, I think the old joke, " if that last engine
quits, we'll be up here all day" is probably the "best" he could do. g

I guess the best for him is the slowest speed that he can go, and not put up
with too much aggravation flying the plane. Whether or not that harms the
engine, I'll leave to others much more qualified than me.

P.S. Long taxi times help out, too! g
--
Jim in NC

john wrote:
The CFI no longer flies out of that airport, so I don't have contact
with him. I'm interested in others opinions related to rpm settings.
Is there any harm to the engine with reducing the RPM even lower, say
2200 or even 2100 RPM. I will confess to bringing it back to as low
as 1800 for a short X-C and found it to be good slow flight practice.,
which gave my one leg a work out in order to keep the bubble in the
middle. I wouldn't want to make many flights at 1800 though.

In the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.


The 1981 C-172P POH I just looked at listed 2100-2450 as the normal operating
range at sea level. The redline climbs with altitude to as much as 2700 rpm at
10,000 feet but it doesn't drop below 2100 on the lower end.

One thought about continued flight at 1800 rpm: if you were running with the
carb heat on to avoid icing, then you're ingesting unfiltered air. Best to keep
that at a minimum if you can.



--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com

John,

n the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.


First of all, the power settings in the POH mostly make a lot of sense.
Secondly, if you pay by Hobbs time, many people use what is referred to
as "rental cruise", aka full throttle full rich. You probably save
money by that, the rental outift probably won't.

Your objective of saving fuel and prolonging flight time is unusual for
a renter. Still, if I were to build time with a rental aircraft and
cost would be the same regardless of power settings, I would GO to
exciting new places at the fullest speed available rather than dawdling
around in the area I know. With respect to time building for a
commercial, that tactic would be immensely valuable since it would
expose you to new terrain, different weather and unknown fields. So,
again, if cost remains the same regardless of power setting, IMHO you
might want to rethink how you build time. I would even suggest a
multi-day cross country to an area of the country you always wanted to
go to.

As a general rule, I normally use the maximum recommended cruise power
setting from the POH, since I normally want to go somewhere. That's
mostly 75 percent. I use best economy mixture if I have a tail wind and
best power if I have a headwind. Also, I use as low an RPM setting as
possible for noise reduction, but I have a choice, since my airplane
has a constant speed prop.

--
Thomas Borchert (EDDH)

Thomas Borchert wrote:

Your objective of saving fuel and prolonging flight time is unusual for
a renter. Still, if I were to build time with a rental aircraft and
cost would be the same regardless of power settings, I would GO to
exciting new places at the fullest speed available rather than dawdling
around in the area I know. With respect to time building for a
commercial, that tactic would be immensely valuable since it would
expose you to new terrain, different weather and unknown fields. So,
again, if cost remains the same regardless of power setting, IMHO you
might want to rethink how you build time. I would even suggest a
multi-day cross country to an area of the country you always wanted to
go to.

As a general rule, I normally use the maximum recommended cruise power
setting from the POH, since I normally want to go somewhere. That's
mostly 75 percent. I use best economy mixture if I have a tail wind and
best power if I have a headwind. Also, I use as low an RPM setting as
possible for noise reduction, but I have a choice, since my airplane
has a constant speed prop.


I would say this is very good advice. I would much prefer to go
somehwere and visit new and different airports to gain experience
not just mindlessley logging time for your logbook. The goal of
the 250 hours for the commercial is for you to gain flying
experience and build skills.

A few nights ago I flew with a student in a 152 on a night
cross country and didn't get back till 11PM. He chose the 152
because it was cheaper per hour. When we got back I was able to
demonstrate that by flying the 172 which is faster and rents at
a higher rate per hour would have been pretty close to the same
amount of money. And no, we didn't need to go slow just to make
XC for his logbook.

If John needs to log 100 more hours to get his Commercial, and he's
paying $100 an hour dry Hobbs time, the airplane cost is $10,000. If
he burns 12 gallons an hour of $4 avgas he'll spend $4,800 on fuel --
if he does it at 7 gph $2,800. Crank in your own numbers, of course.
As those Wentworth commercials on TV say, "It's your money, use it
when you want to." Saving a couple of thousand dollars might be a
worthwhile consideration.

With respect to going to new and interesting places? Sure, but in
terms of learning good flying techniques, don't you think slower is
better than faster? Cross winds, airplane control, all of that matters
at least as much as flying into a new airport. My favorite pilot,
during his BFRs and check rides with his friends, doesn't often
demonstrate straight and level, but slow flight (I hate a Mooney's
attitude when he practices slow flight at 25 square, hanging that
thing on its prop.)


On Oct 14, 7:26 am, kontiki wrote:
Thomas Borchert wrote:
Your objective of saving fuel and prolonging flight time is unusual for
a renter. Still, if I were to build time with a rental aircraft and
cost would be the same regardless of power settings, I would GO to
exciting new places at the fullest speed available rather than dawdling
around in the area I know. With respect to time building for a
commercial, that tactic would be immensely valuable since it would
expose you to new terrain, different weather and unknown fields. So,
again, if cost remains the same regardless of power setting, IMHO you
might want to rethink how you build time. I would even suggest a
multi-day cross country to an area of the country you always wanted to
go to.

As a general rule, I normally use the maximum recommended cruise power
setting from the POH, since I normally want to go somewhere. That's
mostly 75 percent. I use best economy mixture if I have a tail wind and
best power if I have a headwind. Also, I use as low an RPM setting as
possible for noise reduction, but I have a choice, since my airplane
has a constant speed prop.

I would say this is very good advice. I would much prefer to go
somehwere and visit new and different airports to gain experience
not just mindlessley logging time for your logbook. The goal of
the 250 hours for the commercial is for you to gain flying
experience and build skills.

A few nights ago I flew with a student in a 152 on a night
cross country and didn't get back till 11PM. He chose the 152
because it was cheaper per hour. When we got back I was able to
demonstrate that by flying the 172 which is faster and rents at
a higher rate per hour would have been pretty close to the same
amount of money. And no, we didn't need to go slow just to make
XC for his logbook.- Hide quoted text -

- Show quoted text -

"Thomas Borchert" wrote in message
...
John,

n the 172 I have reduced fuel from over 7.5 gph at 2400rpm to about
5.5 gph at 2200-2300.


First of all, the power settings in the POH mostly make a lot of sense.
Secondly, if you pay by Hobbs time, many people use what is referred to
as "rental cruise", aka full throttle full rich. You probably save
money by that, the rental outift probably won't.

Your objective of saving fuel and prolonging flight time is unusual for
a renter. Still, if I were to build time with a rental aircraft and
cost would be the same regardless of power settings, I would GO to
exciting new places at the fullest speed available rather than dawdling
around in the area I know. With respect to time building for a
commercial, that tactic would be immensely valuable since it would
expose you to new terrain, different weather and unknown fields. So,
again, if cost remains the same regardless of power setting, IMHO you
might want to rethink how you build time. I would even suggest a
multi-day cross country to an area of the country you always wanted to
go to.

As a general rule, I normally use the maximum recommended cruise power
setting from the POH, since I normally want to go somewhere. That's
mostly 75 percent. I use best economy mixture if I have a tail wind and
best power if I have a headwind. Also, I use as low an RPM setting as
possible for noise reduction, but I have a choice, since my airplane
has a constant speed prop.

--
Thomas Borchert (EDDH)

Remember, he's renting dry.

Admittedly, that has been a little unusual for some time; but, with
fluctuating fuel prices, who knows?

In addition to asking the owner, I would ask around the local flying
clubs--since many flying clubs have tended to operate their equipment in a
similar manner.

Peter

Tina,

and he's
paying $100 an hour dry Hobbs time,

As I said, my remarks hold only if cost is independent of power setting.
Dry rentals are very uncommon, AFAIK. And I overread his remark that he
is indeed renting dry blush. Sorry!


Sure, but in
terms of learning good flying techniques, don't you think slower is
better than faster?

Not at all. Both is important, IMHO.

--
Thomas Borchert (EDDH)