Reduce RPM to "coast" in long descent?

[Stuart Grant]

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 -

[William W. Plummer]

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.

[Dave S]

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

[Dale]

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

[Stuart Grant]

>
> 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.

[Hankal]

>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

[Robert M. Gary]

(Stuart Grant) wrote in message >...
> 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

[Newps]

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.

[Robert M. Gary]

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.

[AJW]

>
> 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.

[Peter Duniho]

"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

[Jack]

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

[Newps]

> 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.

[WIACapt]

>From: (Robert M. Gary)

>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

Those Mooneys must really be fast

[Roger Halstead]

On 29 Aug 2004 04:14:44 GMT, (WIACapt) wrote:

>>From: (Robert M. Gary)
>
>>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

I was lined up on the glide slope at Port Columbus. The instructor
told me to take the foggles off and look to my left. There off our
wind was a 737 and it was staying in position, that is until we hit
the outer marker and I lowered the gear. <:-))

>>up with the biz jets as long as you don't level off. :)
>>
>>-Robert
>
>Those Mooneys must really be fast

Figure 200 MPH. That is 3.33 miles per minute. At a 500 fpm rate of
descent you need 3.33 miles for each 500 foot, so IF you want to
descend from 7000 to 3000 it will take 4000/500 = 8*3.33, or 26.64
miles. From 9000 to 3000 is 6000/500 = 12*3.33, or basically 40
miles. BUT you also have to slow down. I have to slow from 200 MPH to
120 MPH and that takes a good mile in level flight. A lot more while
coming down hill. OTOH leveling off AND a power reduction will get me
there in 2 miles, but 3 gives a comfortable margin.

The FAF for the VOR - A into 3BS is 5.5 NM out and figuring I need to
be down and slowed down comfortably before the FAF I'd figure a
minimum of 3 miles. So 5.5 + 3 + 40 would mean I'd have to start down
48.5 miles out. OTOH 180 is 3 miles per minute so from 9000 to 3000 =
12 * 3 = 36 miles to get down and 5.5 + 3 or 44.5 miles.

Figuring 120 MPH and 2 miles per minute would change that to 12 * 2 =
24 miles to get down and 3 + 5.5 or 32.5 miles including the
approach.

At 200 MPH I'd still have to start down soon enough to be at pattern
altitude and slowed to pattern entry speed to safely enter the
pattern. That would only let me nock off 3 or 4 miles compared to the
instrument approach.

The Mooney OTOH has to slow even more to get the gear down so although
he can go like a bat coming down hill it's going to take him longer to
get that slippery airplane slowed to gear down speed. So he is going
to need to start down even farther out than I do.

They aren't kidding when they say you have to learn to think much
farther ahead when flying a high performance retract.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Robert M. Gary]

(AJW) wrote in message >...
> >
> > 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.

But your giving up all the speed you should have gotten back from the
climb. You fly slower in the climb but get it back when you start
down. Pulling power back just gives it all up.

[Robert M. Gary]

Newps > wrote in message >...
> Stuart Grant wrote:
>> 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 pull the throttle back just enough to maintain the same MP as I come
down (adjusting every 2000 feet or so). If you forget to roll the
throttle back, you'll end up pulling 30" in the pattern. :)

-Robert

[AJW]

>
>> > 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.
>
>But your giving up all the speed you should have gotten back from the
>climb. You fly slower in the climb but get it back when you start
>down. Pulling power back just gives it all up

I admit to not running the numbers on that. I figured I chose an airspeed that
made sense, save a little fuel on the decent, and even if I come down from
11500 at 500 fpm it's what -- 21 minutes to pattern altitude. At 150 kts +
wind, that means I start down about 50 miles out. If I come down at an average
of say 165 ktsI'd be starting down maybe 8 miles sooner. Maybe it's as much
this as anything -- I mostly do XC under IFR, and like to keep whatever is the
filed airspeed.

[Peter Duniho]

"Robert M. Gary" > wrote in message
om...
> But your giving up all the speed you should have gotten back from the
> climb. You fly slower in the climb but get it back when you start
> down. Pulling power back just gives it all up.

It's just a matter of what you want to get back. You are right, you are
sacrificing speed (energy banked during the climb), but you get in return
fuel efficiency (energy banked during the climb).

Which is, essentially, what AJW wrote in the first place.

You don't lose the energy. You just use it differently, depending on your
power setting. Thermodynamically speaking, the lower power setting also
produces the more efficient use of the energy, but of course flying isn't
always about what's most efficient. :)

Pete