WOT in cruise?

I was always taught that WOT was a "renter's setting" and that you should
back off to 2300-2400 if you want to get good life out of your engine. I
keep running across the Advanced Pilot Seminar guys asking, "Why EVER be
partial throttle in cruise?

Of course, they are probably also talking about advanced engines, with GAMI,
analyzers, matched injectors, etc. What about those of us flying behind the
glorified lawn mower engines of simpler aircraft. I've asked the question
over at the CPA forum but I'd like some opinions from this group.

Should I take our 172 N above altitudes where the POH says max power is 75%,
put the throttle all the way in, lean till it gets a bit rough, and then
enrich until it's smooth? As long as CHT and oil temps remain in an
acceptable range, can I then cruise along confident that I'm getting there
fastest and getting the best life from the engine?

I've often suspected this was the case. Turning faster seems intuitively to
be the same as the engine working harder but the trade off is that it doesn'
t work as long. Speed of metal surface over metal surface (with oil film)
within normal RPM ranges doesn't seem as significant a wear factor as the
total number of firing cycles and revolutions. It seems like those should be
about the same whether you fly at 115 knots or 90.

Figuring the RPM's out from the POH:

At 4000 feet, WOT, 2500 RPM there will be 129,591 revolutions per 100 miles.
At 2400 RPM, 130,896. At 2300, 131,100. Going WOT instead of 2300 REDUCES
firing cycles 1.15%!

If the faster speed saves a bladder break, you'll get a huge savings in
engine wear avoiding a thermal cycle and restart.

On the other hand, marine engine factory reps, who I have more frequent
contact with, tell me that the only significant indicator of engine life (as
long as temperatures remain in normal range) is the total amount of fuel
that goes through it. Seems like that should be true for aviation engines as
well.

Slowing down from WOT to 2300 in my 172 N should reduce fuel consumption
14.5%. That's pretty significant as well as probably saving a fuel stop
somewhere on a long trip.

--
Roger Long

All WOT means is you're going to get the most MP you can at your
current altitude. With a fixed-pitch prop, it spins up faster at high
altitude because of the propeller characteristics in the thinner air.
Consequently, you can make more power for a given MP setting (i.e.
22"@3000'=2300RPM, but 22"@7000'=2400RPM=more power).

According to the Lycoming leaning doc, at 75% power or less (WOT
at 8000' DA) leaning to peak EGT (usually about where roughness sets in
on a carb'd engine) will NOT harm the engine. Everything is specified on
percent power, not throttle setting. Another way to look at it is that
with two different settings but the same percent power (constant speed
prop running, say 25"/2100 and 20"/2600 approx 70% power). With high
MP/low RPM, you've got more torque for longer time per firing. That would
tend to imply less wear at higher speeds. I suspect they kinda average
out so that percent power is a good metric for engine wear, rather than a
particular RPM/MP setting.

Where was I going with this again? Must be tired...

-Cory


Roger Long om wrote:
: I was always taught that WOT was a "renter's setting" and that you should
: back off to 2300-2400 if you want to get good life out of your engine. I
: keep running across the Advanced Pilot Seminar guys asking, "Why EVER be
: partial throttle in cruise?

: Of course, they are probably also talking about advanced engines, with GAMI,
: analyzers, matched injectors, etc. What about those of us flying behind the
: glorified lawn mower engines of simpler aircraft. I've asked the question
: over at the CPA forum but I'd like some opinions from this group.

: Should I take our 172 N above altitudes where the POH says max power is 75%,
: put the throttle all the way in, lean till it gets a bit rough, and then
: enrich until it's smooth? As long as CHT and oil temps remain in an
: acceptable range, can I then cruise along confident that I'm getting there
: fastest and getting the best life from the engine?

: I've often suspected this was the case. Turning faster seems intuitively to
: be the same as the engine working harder but the trade off is that it doesn'
: t work as long. Speed of metal surface over metal surface (with oil film)
: within normal RPM ranges doesn't seem as significant a wear factor as the
: total number of firing cycles and revolutions. It seems like those should be
: about the same whether you fly at 115 knots or 90.

: Figuring the RPM's out from the POH:

: At 4000 feet, WOT, 2500 RPM there will be 129,591 revolutions per 100 miles.
: At 2400 RPM, 130,896. At 2300, 131,100. Going WOT instead of 2300 REDUCES
: firing cycles 1.15%!

: If the faster speed saves a bladder break, you'll get a huge savings in
: engine wear avoiding a thermal cycle and restart.

: On the other hand, marine engine factory reps, who I have more frequent
: contact with, tell me that the only significant indicator of engine life (as
: long as temperatures remain in normal range) is the total amount of fuel
: that goes through it. Seems like that should be true for aviation engines as
: well.

: Slowing down from WOT to 2300 in my 172 N should reduce fuel consumption
: 14.5%. That's pretty significant as well as probably saving a fuel stop
: somewhere on a long trip.

: --
: Roger Long



--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

I'm a WOT guy, with one exception. You back off the throttle until the
MP gauge needle tickles, then leave it. This way you come out of the
economizer mode and use a lot less gas.

Roger Long wrote:
I was always taught that WOT was a "renter's setting" and that you should
back off to 2300-2400 if you want to get good life out of your engine. I
keep running across the Advanced Pilot Seminar guys asking, "Why EVER be
partial throttle in cruise?

Newps wrote:
: I'm a WOT guy, with one exception. You back off the throttle until the
: MP gauge needle tickles, then leave it. This way you come out of the
: economizer mode and use a lot less gas.

This works for me in my 180hp O-360 Lycoming. At 9500' WOT gave
EGTs of +-50 degrees. Pulled it about 1/4-1/2" MP lower and got the
spread +-10 degrees. I can't lean beyond peak, however, as it starts to
run rough.

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

I didn't mean to imply that it has anything to do with EGT's. Although
I have a 6 channel EGT there is no other less useful instrument in the
plane. Other than the gee wiz factor it was a waste of money.

wrote:
Newps wrote:
: I'm a WOT guy, with one exception. You back off the throttle until the
: MP gauge needle tickles, then leave it. This way you come out of the
: economizer mode and use a lot less gas.

This works for me in my 180hp O-360 Lycoming. At 9500' WOT gave
EGTs of +-50 degrees. Pulled it about 1/4-1/2" MP lower and got the
spread +-10 degrees. I can't lean beyond peak, however, as it starts to
run rough.

-Cory

Newps wrote:

I'm a WOT guy, with one exception. You back off the throttle until the
MP gauge needle tickles, then leave it. This way you come out of the
economizer mode and use a lot less gas.

That's a good tip. I use that in my PA-30 and save between .75 to 1gph
depending on the power setting.

-Ryan

"Roger Long" om writes:

I was always taught that WOT was a "renter's setting" and that you
should back off to 2300-2400 if you want to get good life out of
your engine. I keep running across the Advanced Pilot Seminar guys
asking, "Why EVER be partial throttle in cruise?

With a fixed-pitch prop, what you care about is your power setting --
the RPM to get that setting varies depending on your density
altitude. For example, 2400 fpm is around 75% power in a 172P at sea
level, but only 55% power at 10,000 ft DA.

From what I've seen, it's actually renters who tend to leave the
mixture full rich and close the throttle, gumming up the plugs and
valves, while owners tend to leave the throttle futher open and lean
the mixture according to POH instructions.

Some owners are getting even more aggressive, leaving the throttle
wide open and using only the mixture to set power, as long as they can
do so without the engine running rough -- that's probably what you're
hearing about. My O-320D3G runs beautifully WOT down to at least 65%,
especially in cold weather, and I've from other small engine owners
who see the same results. Larger carbureted engines like the O470
don't tend to do as well, from what I've heard so far.

Of course, they are probably also talking about advanced engines,
with GAMI, analyzers, matched injectors, etc. What about those of
us flying behind the glorified lawn mower engines of simpler
aircraft. I've asked the question over at the CPA forum but I'd
like some opinions from this group.

Try it and see. Look up the RPM for your preferred power setting
at, say, 5500 ft density altitude, fly up there, then leave your
throttle wide open and slowly pull back the mixture until you get your
desired RPM. If the engine runs smoothly, then you're fine. If it
runs rough, try turning on carb heat (which might smooth out the
distribution) and then try one more time. Lycoming put out a bulletin
warning against LOP operations for engines with constant-speed props,
because there is no way to be sure that you're not at too high a
power setting, but even they admitted that you cannot hurt an engine
with a fixed-pitch prop as long as you stay at or below the RPM for
75% power when you're running lean.

Slowing down from WOT to 2300 in my 172 N should reduce fuel
consumption 14.5%. That's pretty significant as well as probably
saving a fuel stop somewhere on a long trip.

A lower power setting is a great way to save fuel. For any given
power setting, however, you'll burn even less fuel running WOT with a
leaner mixture than you will with partly-closed throttle and a richer
mixture -- it's just a matter of how far you can lean before your
engine starts running rough.

I'm getting 7.5 gph or less in my 160 hp Warrior II (comparable to a
172N) at 75% power and better than 120 ktas, though I still flight
plan for the book number to give myself a safety margin. It was great
landing after a non-stop 400 nm cross-country at 75% power and still
having half-full tanks.


All the best,


David

--
David Megginson, , http://www.megginson.com/

In article ,
Roger Long om wrote:
I was always taught that WOT was a "renter's setting" and that you should
back off to 2300-2400 if you want to get good life out of your engine.

If you want to stick to 75/65/55% power, check the POH table which
relates power to altitude. If you hang back at 2400RPM in a 172 as
you climb, your power is actually going down and down. By about 7000'
you can only get 75% with the throttle all the way in. That will be
a lot higher than 2400 RPM.

--
Ben Jackson

http://www.ben.com/

Roger,

Should I take our 172 N above altitudes where the POH says max power is 75%,
put the throttle all the way in, lean till it gets a bit rough, and then
enrich until it's smooth? As long as CHT and oil temps remain in an
acceptable range, can I then cruise along confident that I'm getting there
fastest and getting the best life from the engine?


Well, are you getting more than 2400 rpm at 75 percent? I don't think so. So
you will be WOT at those altitudes anyway, right?

--
Thomas Borchert (EDDH)