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

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

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 . com...
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 -

A long slow let down is better on the engine and allows you to "milk"
back the speed you lost in the climb. In my Mooney I'll often begin my
let down 40-50 miles out (although I'm flying a bit higher than you
mention here in this post). In the Mooney its fun because you can keep
up with the biz jets as long as you don't level off.

-Robert

Stuart Grant wrote:

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?

A 200 rpm reduction will save you 2 gallons per hour in the 470. So you
will save 1 gallon.



Is it better to reduce MP and leave RPM at
2200?

Every two inch reduction in MP also saves you 2 gallons per hour.


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?


In my 182 I always fly balls to the wall on a cross country. WOT minus
a little to tickle the needle and 2450 RPM. When it comes time to
descend I do not touch power and just roll in a turn of trim, that gets
me 500 fpm down and an extra 15-20 knots. I will reduce MP as necessary
to stay at the top of the green, but you don't have to that living out
here in the west where 26" is all there is anyways.

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.

Is it better to reduce MP and leave RPM at
2200?

In my Mooney I like VFR decents at whatever speed I had been cruising. It's my
habit in cruise to keep the RPM low, almost always going a little 'oversquare'
(MP in inches a bit higher than RPM in hundreds). I don't like speeding up in
the decent, and consider the altitude energy in the bank, I spend it on haaving
the MP lower.