Constant Speed Props

[Colonel Sanders]

If you have more than 500TT, stop reading
now.

Constant speed props are often a source
of mystery to people who haven't used them
before. They look at all sorts of complicated
diagrams and try to memorize arbitrary rules.

It's really pretty simple. From a pilot's standpoint
you have another knob - often blue - between
the throttle and the mixture control.

All the knobs into the dashboard for takeoff.
You can think of the prop control as the stick
shift of a manual transmission in a car. When
it's all the way forward, it's in first (low) gear,
for accelerating from a dead stop, and when
it's all the way back, it's in fifth (high) gear,
for cruising at high speed.

When you want to increase power (eg to
takeoff, or climb), you go from right to left -
mixture forward, prop forward, throttle
forward.

When you want to decrease power (eg
to level off) you go from left to right -
throttle back, prop back, mixture back.

The setting of the throttle is shown on
the Manifold Pressure gauge, and the
prop knob controls the RPM.

There are a million combinations of throttle
and prop which will make the same power,
but as a rule of thumb, the higher the MP and
the lower the RPM, the lower your fuel burn will
be, and the quieter it will be, too. Lycoming
says a maximum spread of 5 between the
MP and the RPM, for the little engines. For
example, with 2000 RPM (20 "turns" is very
salty) you can run max 25 inches of manifold
pressure.

It's important to realize that although you have
a variable-pitch prop - that is, the blades of the
prop rotate in the hub in flight - you really have
a constant-speed prop, courtesy of a magical
lump of metal, bolted onto the engine, called
a "governor". The prop control actually moves
an arm on the governor, which meters oil
pressure to the prop.

You don't actually directly control the prop
blades. What you do, is request that the
governor maintain a particular RPM, and it
will do it's best to do so, within the limits
of the prop blade movement.

If you reduce the power (with the throttle)
too much, what you will notice is that the
RPM falls. The governor has the blades at
their finest pitch, and what you have now
is essentially a fixed-pitch (climb) prop, which
you control with the throttle, just like you
would on a 172. You can ignore the MP
now - just look at the tachometer for
small power adjustments, as you are likely
to do on approach.

And, now that the RPM has fallen, you can
safely advance the prop control all the way
in, as part of your pre-landing check. It
won't make any difference to the engine,
because the prop is already at it's max
fine pitch.

One tiny little detail ... I mentioned that the
prop is controlled by the governor, by the
oil pressure from the engine.

On 99% of piston singles, if you lose oil
pressure, the prop is built so that it goes
to full fine pitch, as you would have at
takeoff or short final.

However, on a piston twin, or an aerobatic
airplane, "counterweighted" props are used,
and when they lose oil pressure, they go to
the opposite stop, at full coarse pitch.

Hope this helps. People make this stuff 'way
more complicated than it needs to be.

[triplese7en]

Really nicely written CS!

To add to it: When you apply takeoff power from a static start, the RPM will be a bit less than maximum. The governor is trying to give you maximum RPM (because you asked for it by putting the prop level full forward) by putting the blades right to the low pitch stops. As you start accelerating down the runway, the airflow will help 'windmill' the propeller and will increase the RPM to maximum. Say it reaches max RPM at 23 knots. Any speed higher than 23 knots (with the prop lever full forward) and the prop blade angle is progressively increasing.

This is also a good information manual (in more detail) on prop systems: http://www.mccauley.textron.com/tech_guide.pdf

Edit: Crap.. I forgot about that 500 hr note! Sorry...

[Colonel Sanders]

I forgot about that 500 hr note

No problem ... my point is that this "flight training"
forum all-too-often turns into quite spirited and
very esoteric discussions which I fear are of
dubious value to the newbies (less than 500TT)
whom are terrified to post here because they
are certain to get royally roasted if they screw
up and say something wrong.

It's nice to have something directed at (and hopefully
useful for) the lower-time lurkers here.

That's one of the biggest challenges for someone
that's been flying a while - trying to remember what
it's like for people just starting out.

[Cavok416]

Nice post, thanks for sharing!

[photofly]

Lycoming
says a maximum spread of 5 between the
MP and the RPM, for the little engines

Do you have a pointer to where they say this?

Does Continental offer any advice on the same subject?

Maybe you could say something about the validity or invalidity of the "oversquare rule", and what - if any - are the downsides of low rpm & high mp combinations.

[Colonel Sanders]

See "Lycoming Flyer - Operations" in the
"Governed Engines" section. The section
"Explanation of Power Settings" starts like
this:

A letter received here at the factory asked a question we have heard quite often:

“Is it a fact, or is it fiction, that engines with constant speed props should not use power settings where inches of mercury exceed RPM in hundreds? I am referring, of course, to
non-turbocharged engines in general .”

The answer to this question is easily found in cruise power charts of the airframe Pilot’s Operating Handbook. Whatever the combinations of RPM and MP listed in the charts — they have been flight tested and approved by the airframe and power plant engineers. Therefore, if there are power settings such as 2100 RPM and 24" MP in the power chart, they are approved for use.

The confusion over so-called “squared” power settings (i.e., 2400 x 24" MP), appears to have been a carry-over from some models of the old radial engines which were vulnerable to excessive bearing wear where an MP higher than “squared” was used. More pressure on the bearings with the higher than “squared” MP was the cause of their problem. However, changes in design, metals and lubricants permit changes in operation in the more modern flat-opposed power plants.

Let’s look at the power charts in a couple of the Pilot’s Operating Handbooks of two different aircraft manufacturers, but where both are using the four-cylinder 200 HP Lycoming engine.

Cessna’s Model 177 RG, using the Lycoming IO-360-A1B6D, in the cruise range at 6,000 feet, lists a cruise power-setting range at that altitude of anywhere from 2100 RPM to 2500 RPM with variations all the way from 18" MP to 24" MP. They list a recommended power-setting for 66% power at 2100 RPM at 24" MP

[Colonel Sanders]

For Continental, this is interesting:

http://www.tcmlink.com/pdf2/CSB09-11.pdf

Effective immediately, TCM strongly recommends the following limitation be observed on all the models affected above:

Engine cruise RPM settings should be no lower than 2300 RPM.

Like Lycoming, Continental has a specific engine
operator's manual which has all sorts of good
information in it. Well worth acquiring the one
for the aircraft you fly. I'm sure Essco stocks
them.

Here is the performance chart for the TCM IO-360A
in the Maule:

[triplese7en]

A link to the Lycoming document: http://lycoming.textron.com/support/tip ... ations.pdf

Page 43 is where CS quoted from, and page 65 has additional information.

There's a lot of good information in that PDF.

Here is another Lycoming document showing the max MAP and RPM combinations - does 44" and 2500 RPM seem high?!

http://lycoming.textron.com/support/pub ... I1257G.pdf

In summary - operate it in accordance with the POH! "Over-square" is only a coincidence since MAP is measured in inches of mercury. If it were measured in PSI that would be a different story!

[pelmet]

Used to run a couple of IO-360's at 24" and 2200 rpm. Nice and quiet and a decent power output. I think with the rpm stuff, there is a spring of a certain strength always trying to change the blade angle which is held in position by the amount of oil pressure allowed in by the pilot, through the prop control unit(governor). But the Colonel knows better than me.

[Colonel Sanders]

The "Lycoming Flyer - Operations" PDF contains a wealth
of information, that every pilot flying a Lycoming should
be aware of.

For example, a pet peeve of mine is people pushing the
mixture full forward for descent. A little of bit of me dies
inside, every time you do that. Let's see what Lycoming
says:

Rapid cooldown during initial descent can damage the engine. Gradual cooldown is preferable.

The descent power reduction should be accomplished in several steps. Ideally, the descent should begin by nosing the aircraft over slightly while engine power and mixture remain at the cruise setting. The added speed will initiate a gradual cooldown.

When the CHT has stabilized, reduce the manifold pressure to 25" Hg, and relean the mixture to maintain 1,350F exhaust gas temperature, which will prevent rapid cooldown.

After a period of at least one minute, a further reduction of manifold pressure to 20" Hg and 2000 RPM can be made, if necessary. Again, mixture should be leaned to maintain 1,350F exhaust gas temperature.

Cowl flaps should not be used as an aid in slowing the aircraft during descent. Descent power settings at greater than 20" Hg manifold pressure should be utilized for the greatest possible time to avoid accelerated piston ring wear.

Couple of points there. Note that Lycoming wants you
to lean the mixture in descent, to try to keep the engine
warm. They said it twice, for each power reduction.

Also note that the MP wasn't changed at this silly "inch
per minute" - it's changed 5 inches at a time, just like I
do when I fly the GTSIO-520 in the C421B's, an engine
which is notorious for cylinder cracking (but not when I
fly it).

Note that Lycoming says, wrt the above quoted advice:

These tips are directed specifically at TI0-540-J series engines, but they may be applied to other Lycoming engines as well.

PS I can't make the Lycoming URL to the PDF work
for me, right now. Use this dropbox link instead for
now:

https://dl.dropboxusercontent.com/u/165 ... ations.pdf

PS This ought to be required reading for every
pilot/owner and CPL who flies a Lycoming. Gotta
do something during that 80 hrs CPL gnd school.

PPS For the handful of keeners here, when you have
read that, download and read this:

https://dl.dropboxusercontent.com/u/165 ... Radial.pdf

Between those two PDF's, you will learn more about
engine operation than in a lifetime of hanging around
the airport.

[white_knuckle_flyer]

Would the same thing go for your basic 172 with constant pitch prop (i.e.) nose over, leaning mixture or at least not enriching immediately ?

[Colonel Sanders]

basic 172 with constant pitch prop

Sure. In a plain-jane 172 (4 cyl Lyc) with a fixed-pitch
prop, lean the mixture for max RPM in the descent.

For example, if you are cruising at 2300 RPM, and it's
bumpy out (typical summer day) I would pull the throttle
back to 1800 RPM and descend at your cruise speed. I
am lazy - I don't like to mess with the trim. After setting
1800 RPM, lean the mixture for max RPM. You simply
can't hurt it, at that low power setting. Look at the CHT's
and EGT's to verify for yourself.

In fact, I strongly recommend leaning the mixture for
max RPM on the ground immediately after start and
landing. If you have to wait, as Lycoming says, idle
at 1200 RPM. This is to avoid fouling the bottom plugs.

FTU's never lean the mixture on the ground, and
sit and idle for long periods of time at low RPM, and
sure enough, they're always having big mag drops
during run-up, caused by lead-fouling the bottom
spark plugs. A self-inflicted wound.

PS If you have a multi-probe EGT, you can tell
which spark plugs is fouled - the one with the highest
EGT, on both mags. This is because combustion is
not as complete with only one spark plug firing, so
the mixture leaving the cylinder through the exhaust
valve is hotter.

[Big Pistons Forever]

In the spirit of supplying an alternate opinion, this is what I teach for descents for simple fixed gear fixated pitch prop airplanes.

I tell students to leave the power at the cruise value and slightly enrichen the mixture then pitch down untill a descent rate of 500 FPM is achieved. Trim to hold that and the extra speed will make up for most of the time you lost on the climb.

Note as you descend the engine RPM will increase so you will have to periodically retard the throttle to maintain your cruise RPM. The reason you want to enrichen the mixture a bit as you start the descent is to make sure you don't go too lean if you get distracted and let the RPM creep up.

[Colonel Sanders]

the extra speed

That works if the air is smooth. These parts, the wind
just keeps howling, day after day, and it's so bumpy
people don't fly at all.

I don't like descending faster than Va in the bumps,
even if it's in the green arc. You're probably flying an
airplane older than you are. Be nice to it. Decades
ago, I flew with the T-34 people in Atlanta that did
air-to-air, and they didn't give a sh1t about the yellow
arc or Va or bumpy air or anything. Sure enough, a
few years later, the very aircraft I flew shed a wing
and killed two guys, and all hell broke loose on the
T-34 spar AD. They're dead and I'm alive.

Of course, if the air is smooth, you can do a bananas
Pitts approach and push everything forward and try to
get 200 mph on short final. With a bit of luck, tower
will ask you to slow down for the four-bars in their
kerosene burners.

[photofly]

One thing I have noticed is that a suitable manifold pressure is (unsuprisingly) dependent on outside air temperature.

Cruising around low-level in Ontario I know that in winter I achieve about 120 KIAS lean of peak at 9.5gph with 2200 rpm and about 23" of mp. That's with an OAT of about 0 celsius.

Yesterday at 20 degrees I was cruising at the same speed with the mp two inches higher at 25.

If you work it out 25" of pressure at 20 degrees contains about the same concentration of oxygen as 23" at 0 degrees. If the mp is that high when the air is cold (other parameters the same) the engine stumbles because it's too lean.

I also picked up a couple of knots yesterday by slowing the prop to 2100rpm but keeping the fuel flow and mp the same.

[white_knuckle_flyer]

My local FTU revamped their checklists a year ago to include leaning for ground idle. It seemed like an additional hassle for a noobie to learn and I wondered why they did it.

Now I know.

[Colonel Sanders]

I never thought I would live long enough, to
find an FTU that leaned the mixture on the
ground.

[Shiny Side Up]

I have to say that since the sarcasm generally runs so thick through anything on the flight training forum I genuinely can't tell if you approve or disapprove of said practice.

[I_Drive_Planes]

[/img]

I never thought I would live long enough, to
find an FTU that leaned the mixture on the
ground.

The FTU here in YXS does too.

[Cessna driver]

I never thought I would live long enough, to
find an FTU that leaned the mixture on the
ground.

The FTU i did all of my training at leaned the mixture on the ground, the one i work at now as well leans the mixture on the ground....

[Colonel Sanders]

Amazing! Slow, but steady progress. It's funny how
people defend their poor practices by saying,

"TC won't let us operate the engines as the manufacturer recommends!"

(see many, many previous discussions here over
many years) then gosh, it turns out they can!

Maybe some day, I can stop people from shoving the
mixtures full forward in the descent. I know, it's a lot
to ask. Very, very complicated.

I fantasize about people re-leaning after carb heat is
applied but I know that will never happen.

And in my dreams, people wait for the RPM to fall,
before they advance the prop control to full fine on
approach. See "de-tuning counterweights" and the
neighbours shaking their fists at the airport.

Tip Of The Day: Change the RPM of an aircraft
piston engine slowly.

The needle on the tach should move steadily, not
flick back and forth. If your engines go

"ZING! ZING! ZING!"

as they bounce off the redline at low power settings
on approach, your prop governors frantically working
to keep the blades off the fine pitch stop, you could
probably improve your technique.

[jump154]

I forgot about that 500 hr note

No problem ... my point is that this "flight training"
forum all-too-often turns into quite spirited and
very esoteric discussions which I fear are of
dubious value to the newbies (less than 500TT)
whom are terrified to post here because they
are certain to get royally roasted if they screw
up and say something wrong.

It's nice to have something directed at (and hopefully
useful for) the lower-time lurkers here.

That's one of the biggest challenges for someone
that's been flying a while - trying to remember what
it's like for people just starting out.

It is much appreciated, thanks.

[Shiny Side Up]

Amazing! Slow, but steady progress.

(see many, many previous discussions here over
many years) then gosh, it turns out they can!

Maybe some day, I can stop people from shoving the
mixtures full forward in the descent.

I fantasize about people re-leaning after carb heat is
applied but I know that will never happen.

And in my dreams, people wait for the RPM to fall,
before they advance the prop control to full fine on
approach.

Well if it helps, I was taught all those things by my instructors, pass it on to my own acolytes and see people do this on a reasonably regular basis, that obviously there's some hope out there that stuff is being done as you dream. Of course I'm aware that when someone evil does something good then it's sort of a wash, so I don't really count. Sort of like the reasoning that Hitler was a big advocate of brushing your teeth and was very kind to dogs. So if Shiny says you should lean your mixture on the ground then you always have to wonder about his evil intent with the purpose and can't take that as sound reasoning.

I'll just be quiet now.

[burninggoats]

You only want to lean the mixture on the ground to save fuel and engine costs to maximize profit.... because you are part of the greedy FTU scam rampant across Canada.

SHAME ON YOU!

[Cessna driver]

You only want to lean the mixture on the ground to save fuel and engine costs to maximize profit.... because you are part of the greedy FTU scam rampant across Canada.

SHAME ON YOU!

I hope your just being sarcastic, i've been in carb'd aircraft numerous times in the last couple months and we get a mag drop of ~ 200. Cause - fouled plugs from NOT leaning on the ground. I always lean on the ground and never have a problem, wish students (that arn't mine) would do the same