Reduce RPM to "coast" in long descent?

Answer to this question isn't in the flight manual for my '53 Cessna
180. Hope to get some expert advice.

I am planning a long flight Labor Day Weekend and calculate no wind
reserve based on my previous fuel burn rate of about 50 minutes. Book
says even more.
My last leg will be about 170 N mile and as much as 20 miles off the
coast of Georgia. Not in a rush. I want to save gas.

I expect to be descending from either 9,500 or 7,500 feet at about 200
feet per minute to a sea level landing. This will take about half an
hour. My economy cruise will be max 22.5" MP or WOT and 2200 RPM. Any
advantage to reducing the RPM to 2000 or lower during the long
descent? Does it save gas? Is it better to reduce MP and leave RPM at
2200? I know the prop acts like a brake a high RPM-fine pitch. Is low
RPM and coarse pitch-more like feathering when you are going downhill?

Thanks in advance -

Stuart Grant wrote:

Answer to this question isn't in the flight manual for my '53 Cessna
180. Hope to get some expert advice.

I am planning a long flight Labor Day Weekend and calculate no wind
reserve based on my previous fuel burn rate of about 50 minutes. Book
says even more.
My last leg will be about 170 N mile and as much as 20 miles off the
coast of Georgia. Not in a rush. I want to save gas.

I expect to be descending from either 9,500 or 7,500 feet at about 200
feet per minute to a sea level landing. This will take about half an
hour. My economy cruise will be max 22.5" MP or WOT and 2200 RPM. Any
advantage to reducing the RPM to 2000 or lower during the long
descent? Does it save gas? Is it better to reduce MP and leave RPM at
2200? I know the prop acts like a brake a high RPM-fine pitch. Is low
RPM and coarse pitch-more like feathering when you are going downhill?

Thanks in advance -
You should not linger between altitudes. Everybody, other pilots and
ATC, expect you to be at the correct altitude. Safety, not economy,
demands this.

William W. Plummer wrote:

Stuart Grant wrote:

Answer to this question isn't in the flight manual for my '53 Cessna
180. Hope to get some expert advice.

I am planning a long flight Labor Day Weekend and calculate no wind
reserve based on my previous fuel burn rate of about 50 minutes. Book
says even more.
My last leg will be about 170 N mile and as much as 20 miles off the
coast of Georgia. Not in a rush. I want to save gas.

I expect to be descending from either 9,500 or 7,500 feet at about 200
feet per minute to a sea level landing. This will take about half an
hour. My economy cruise will be max 22.5" MP or WOT and 2200 RPM. Any
advantage to reducing the RPM to 2000 or lower during the long
descent? Does it save gas? Is it better to reduce MP and leave RPM at
2200? I know the prop acts like a brake a high RPM-fine pitch. Is low
RPM and coarse pitch-more like feathering when you are going downhill?

Thanks in advance -

You should not linger between altitudes. Everybody, other pilots and
ATC, expect you to be at the correct altitude. Safety, not economy,
demands this.

I don't recall a VFR flight being REQUIRED to descend at a minimum
speed/rate. See and avoid applies regardsless of the hemispheric rule.
If he wants to come down at 200/min instead of 500 or 1000/min, thats
his call. My personal pref is around 500/min but again, its not
mandated. When under IFR I believe there is a requirement to notify ATC
if you cant meet a minimum rate.

Dave

I don't recall a VFR flight being REQUIRED to descend at a minimum
speed/rate. See and avoid applies regardsless of the hemispheric rule.
If he wants to come down at 200/min instead of 500 or 1000/min, thats
his call. My personal pref is around 500/min but again, its not
mandated. When under IFR I believe there is a requirement to notify ATC
if you cant meet a minimum rate.

Dave


Yes I am sure you are correct. I know its proper/expected IFR piloting
to descend at a minimum 500 fpm but there are issues of rapid cooling
the engine, and 200-300 fpm allows me to make more gradual changes in
engine power and airspeed. A fringe benefit of VFR flight I enjoy is a
more gradual descent.

This descent will be mostly on an airway and I hope with Flight
Following. I guess that it is safer to be in a constant descent for 12
minutes rather than 25 even if it is out over the Atlantic Ocean.

I think I DID find the answer in my pilot handbook. The range curves
show significant increase in range with the same MP and lower RPM.
Manifold pressure 22" and 1800 RPM for example. In descent the reduced
power would be made up for by gravity for a more constant airspeed. I
think I will try this.

Still like to hear what other people do.

I think I DID find the answer in my pilot handbook. The range curves
show significant increase in range with the same MP and lower RPM.
Manifold pressure 22" and 1800 RPM for example. In descent the reduced
power would be made up for by gravity for a more constant airspeed. I
think I will try this.

Still like to hear what other people do.

I fly 24 square. when decending I push the stick forward. I watch my airspeed
so not to get to far into the yellow arc. Watch my
MP so that it does not go much above 24.
HANK

"Stuart Grant" wrote in message
om...
[...]
I think I DID find the answer in my pilot handbook. The range curves
show significant increase in range with the same MP and lower RPM.
Manifold pressure 22" and 1800 RPM for example.

Your handbook is telling you something different. That is, that the engine
is generally more efficient for a given percentage horsepower when the lower
RPM is selected.

That will always be true, but it doesn't mean there's a reduction in
airframe drag, which is what you asked about. As Dale said, if the engine
is driving the prop rather than the other way around, reducing prop pitch
isn't going to change the drag of the prop.

Now, if you have an engine failure, or are gliding with the power completely
reduced, that's a different story and reducing prop RPM will increase your
glide range. But that's not what you asked.

In descent the reduced
power would be made up for by gravity for a more constant airspeed. I
think I will try this.

I don't understand "more constant airspeed". In a stabilized descent, the
airspeed should always be constant. You are certainly right that in a
descent, gravity adds thrust (equivalent to adding power), so you can reduce
power and maintain the same airspeed.

This is, in fact, a technique that is usually taught to every pilot during
their initial training: to descend while maintaining your current airspeed,
simply reduce power. The airplane will remain at (or near) its trimmed
airspeed, and will descend at that airspeed.

Pete

William W. Plummer wrote:

You should not linger between altitudes. Everybody, other pilots and
ATC, expect you to be at the correct altitude. Safety, not economy,
demands this.

Yeah, you need to get down smartly to an altitude where everybody else is. It's so much
safer.


Jack

William W. Plummer wrote:

You should not linger between altitudes. Everybody, other pilots and
ATC, expect you to be at the correct altitude.

Uh, no we don't. You are where you are.

In article ,
(Stuart Grant) wrote:

Answer to this question isn't in the flight manual for my '53 Cessna
180. Hope to get some expert advice.

I am planning a long flight Labor Day Weekend and calculate no wind
reserve based on my previous fuel burn rate of about 50 minutes. Book
says even more.
My last leg will be about 170 N mile and as much as 20 miles off the
coast of Georgia. Not in a rush. I want to save gas.

I expect to be descending from either 9,500 or 7,500 feet at about 200
feet per minute to a sea level landing. This will take about half an
hour. My economy cruise will be max 22.5" MP or WOT and 2200 RPM. Any
advantage to reducing the RPM to 2000 or lower during the long
descent? Does it save gas? Is it better to reduce MP and leave RPM at
2200? I know the prop acts like a brake a high RPM-fine pitch. Is low
RPM and coarse pitch-more like feathering when you are going downhill?

Thanks in advance -

You might save a small bit of fuel with lower RPM, depending on the MP.

As long as the engine is driving the prop I don't believe you will
notice any advantage to a lower RPM as far as "coasting" is concerned,
prop drag comes into play when the engine is no longer providing power.

Flying jump planes I use 2100 RPM and bottom of the green MP, but my
rate of descent is more like 3000-3500fpm due to the airspeed I use.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

Dale wrote in message ...
In article ,
(Stuart Grant) wrote:
You might save a small bit of fuel with lower RPM, depending on the MP.

As long as the engine is driving the prop I don't believe you will
notice any advantage to a lower RPM as far as "coasting" is concerned,
prop drag comes into play when the engine is no longer providing power.

Usually that transition point is about 1500 RPM. Below that the air
tends to drive the prop.