Max RPM for constant speed

"Robert M. Gary" wrote:

Somebody at the Mooney factory is smoking some powerful stuff.

Not at all. There is a mountain of engineering data concluding that
reducing RPM reduces fuel flow and cooling. In fact, I just spoke with
one of the test pilot the other day who talked about flying the
instrumention for that. Its too bad Cessna doesn't do the same level
of
engineering.

This is not making sense, to me. Of course increasing fuel flow by
richening the mixture results in cooler CHT's, but how does increasing
fuel flow by increasing RPM give cooler CHT's? My engine runs hotter at
higher RPM.

--
Dan
C172RG at BFM

Confusion reigns, Dan... Lets assume a 2700 rpm continuous engine...
Extra fuel for cooling the exhaust valves comes when the throttle is
firewalled.... Pull it back a quarter inch and you lose the cooling...
Prop speed control comes from the setting of the prop governor
adjustment screw and/or the prop lever position... This also controls
the engines power...

So, if a 2700 rpm engine is adjusted to run 2700 at full throttle and
full prop lever then you climb at full throttle and full prop lever,
gaining the benefits of fuel cooling at max continuous power...
If the engine governer is set at 2750 and the pilot pulls the throttle
back to reduce power to max continuous, then he loses the benefits of
extra fuel flow for valve cooling... He would do less damage by letting
the engine overspeed 50 rpm at full throttle and get the extra fuel...
What he should do to control the max continuous power is leave the
throttle full forward and pull the prop rpm back to 2700 - which is
where it would have been if the governor screw was set up properly in
the first place...
For bush pilots, setting the 2700 prop adjustment to 2800 is the thing
to do for short field takeoffs... They, being smarter than the average
urban pilot, know that after takeoff they reduce the propellor RPM to
the 2700 continuous setting and LEAVE the throttle full forward to gain
the extra cooling the engineers intended...

Recently I flew a C-180 for a biennial... The owner is an API and
CFI... As is usual he had extra rpm adjusted into his airplane and was
alwyas proud of how short it will takeoff... After takeoff I would
reduce the prop to max continuous RPM... Each time he would grimace and
glare at me and reach over and pull the throttle back a half inch...
"Never reduce the prop without reducing the throttle or you will ruin
the engine.", he would thunder... I would just shrug and do it the same
way on the next takeoff... He gave me heck but signed off the
biennial... He just had to change two jugs on a mid time engine...
Coincidence? I don't think so...

denny...

"Denny" wrote in message
ps.com...
Confusion reigns, Dan... Lets assume a 2700 rpm continuous engine...
Extra fuel for cooling the exhaust valves comes when the throttle is
firewalled.... Pull it back a quarter inch and you lose the cooling...
Prop speed control comes from the setting of the prop governor
adjustment screw and/or the prop lever position... This also controls
the engines power...

So, if a 2700 rpm engine is adjusted to run 2700 at full throttle and
full prop lever then you climb at full throttle and full prop lever,
gaining the benefits of fuel cooling at max continuous power...
If the engine governer is set at 2750 and the pilot pulls the
throttle
back to reduce power to max continuous, then he loses the benefits of
extra fuel flow for valve cooling...

It's really got nothing to do with the RPM setting, per se, which
Robert's post implied. It's purely a matter of throttle and/or mixture
setting: one can run max rich at lower RPM.

--
Dan
C172RG at BFM

Dan, either we are miscommunicating or there is confusion...
At full throttle you cut in an extra X% of fuel flow, beyond 'max
rich'... It is for cooling the exhaust valves at take-off power...
Pull the throttle back, even a small amount, from full forward and you
lose that extra X% of cooling... It is the same for carburetor and
injected engines...
The RPM at which you can run full throttle continuously is in the
POH... Reduce power in climb by reducing the prop to that RPM and leave
the throttle full forward and your engine will be a happy camper with
cool valves... My API / CFI buddy still hasn't figured it out...

On my Super Viking Take-Off RPM was limited to 5 minutes... But you can
pull the RPM back to Max-Continuous power, still at full throttle, and
run it all day and have the benefits of the valve cooling - hard on the
fuel supply though... It is a separate metering circuit on the
injector body for full throttle cooling... Pull the throttle back as
little as a quarter inch and the valve cooling metering circuit shuts
off... Leave the throttle retarded slightly in a long hot climb and you
will likely have valve damage...

Same thing applies to fixed pitch propellprs... Leave the throttle
firewalled in climb for the cooling - assuming the POH allows it... As
soon as you pull the throttle you lose the cooling... Now, be aware
that as you climb at full throttle you are losing manifold pressure
with each thousand feet gained, which is automatically reducing the
engines output power, reducing the strain, and improving the cooling by
allowing the mixture to go even richer than it was a thousand feet
lower... Airplanes, at lower altitudes, are best climbed at full
throttle... Funny how those dumb engineers at the tractor engine
factory figured that out all on their own without any help from us god
like pilots...

cheers .../ denny

Denny wrote:
: soon as you pull the throttle you lose the cooling... Now, be aware
: that as you climb at full throttle you are losing manifold pressure
: with each thousand feet gained, which is automatically reducing the
: engines output power, reducing the strain, and improving the cooling by
: allowing the mixture to go even richer than it was a thousand feet
: lower...

... so leaning to a constant EGT in a climb is a great way to keep the *same*
overrich mixture without going too rich.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

"Denny" wrote:

Dan, either we are miscommunicating or there is confusion...
At full throttle you cut in an extra X% of fuel flow, beyond 'max
rich'... It is for cooling the exhaust valves at take-off power...
Pull the throttle back, even a small amount, from full forward and you
lose that extra X% of cooling... It is the same for carburetor and
injected engines...
The RPM at which you can run full throttle continuously is in the
POH... Reduce power in climb by reducing the prop to that RPM and leave
the throttle full forward and your engine will be a happy camper with
cool valves...

That's what I'm saying. It really has nothing to do with the RPM setting.
In fact, the higher the RPM, the higher the horsepower and the higher the
CHT.

--
Dan
C-172RG at BFM

"Denny" wrote in message
oups.com...

At full throttle you cut in an extra X% of fuel flow, beyond 'max
rich'... It is for cooling the exhaust valves at take-off power...
Pull the throttle back, even a small amount, from full forward and you
lose that extra X% of cooling... It is the same for carburetor and
injected engines...

On our Turbo Saratoga's TIO540S1AD, the throttle is nowhere near full
forward at takeoff when we set max manifold pressure. Full forward will
significantly overboost. The throttle has to be continually moved forward
as altitude increases, reaching full forward at critical altitude.
Throttle-linked wastegate, no turbocharger governor.

Stan

That's true and it is because the cheap sob's who built your plane
refused to spend the money for proper boost control... That's a
business decision unrelated to proper engine management... Assuming tha
tthe fuel engineer knew his business you should have adequate fuel flow
for valve cooling built into the fuel controller at part throttle...
However, my experience is that turbo airplanes are hard on exhaust
valve...

denny