Why do constant speed power setting charts limit RPM?

[Ben Jackson]

A typical power chart for a constant speed prop will limit the RPM to
2400 at all altitudes and power settings. The 75% power table will
just end when it is no longer possible to make enough manifold pressure
to get 75% HP out of the engine at 2400 RPM.

In contrast, a fixed pitch prop will turn faster and faster to make 75%
at high altitudes. I think some Cherokees call for up to 2650 RPM
cruise settings.

Another way to look at it is that a plane with a constant speed prop
may take off with full throttle and full RPM, reach a cruising altitude
of 8000' and then pull back to 2400 RPM (leaving the throttle full forward)
while a fixed pitch prop plane would just accept a few hundred RPM rise
at full throttle and 8000'.

Is the answer that the constant speed prop is slowed down because we
*can* and the fixed pitch prop is just suffering all the ill effects
you'd expect, like higher wear, more noise and frictional losses?

--
Ben Jackson
>
http://www.ben.com/

[Peter Duniho]

"Ben Jackson" > wrote in message
news:yLqec.18880$rg5.39150@attbi_s52...
> A typical power chart for a constant speed prop will limit the RPM to
> 2400 at all altitudes and power settings. The 75% power table will
> just end when it is no longer possible to make enough manifold pressure
> to get 75% HP out of the engine at 2400 RPM. [...]

What do you mean "typical"? There are plenty of constant-speed
installations that don't limit published power settings to 2400, and of
course there are others for which the aircraft manual doesn't bother to
publish much of anything with respect to power settings. Documentation is
all over the map, so I don't see how you can talk about "a typical power
chart".

For a specific installation, I'm sure there's a specific answer. But
without knowing more about the specific installation, seems to me the reason
could be any number of things. Possibly the manufacturer just didn't want
to flight test higher RPM conditions. Or possibly there's a published RPM
limit for the installation. Perhaps for that installation, you're just not
going to get more than 2400 RPM above the altitude in question and power
really IS limited.

For that matter, your comment about fixed-pitch installations is similarly
overly optimistic in its generalization. I don't doubt that there are some
that can make 75% power at 8000', but I certainly doubt that they all do.
For those that don't, their power charts (assuming the aircraft manual has
one) will also stop showing 75% power at a lower altitude.

Not all engines are the same, nor are all props the same. They all come
with their own variety of operating and performance limitations. If you are
flying behind a constant-speed prop, the aircraft limitations don't say
anything about limiting RPM, the manual doesn't show settings beyond 2400
RPM and 75% power above a certain altitude, but you think you can get 75%
power above a certain altitude simply by using a higher RPM setting, by all
means...go for it.

Pete

[Julian Scarfe]

"Ben Jackson" > wrote in message
news:yLqec.18880$rg5.39150@attbi_s52...
> A typical power chart for a constant speed prop will limit the RPM to
> 2400 at all altitudes and power settings. The 75% power table will
> just end when it is no longer possible to make enough manifold pressure
> to get 75% HP out of the engine at 2400 RPM.

I don't think that is by any means universal. The Mooney 201 POH has (IIRC)
settings for 2600 and 2700 (red line).

> Is the answer that the constant speed prop is slowed down because we
> *can* and the fixed pitch prop is just suffering all the ill effects
> you'd expect, like higher wear, more noise and frictional losses?

It is, but you should still get more power with the CS prop at higher RPM.
It just won't necessarily deliver that power efficiently.

Julian Scarfe

[Dylan Smith]

In article <yLqec.18880$rg5.39150@attbi_s52>, Ben Jackson wrote:
> Is the answer that the constant speed prop is slowed down because we
> *can* and the fixed pitch prop is just suffering all the ill effects
> you'd expect, like higher wear, more noise and frictional losses?

Yep. Just like if you've got a car with a 5th (or 6th) gear, you'd use
it on the freeway instead of 4th. You can still make the same power in
4th but your car will be noisier and use more fuel.

The Bo I used to fly I would typically use 2300 RPM which seemed to
provide the best compromise of power (and cruising speed) to fuel flow
and noise. The difference in noise was dramatic in that plane between
2600 and 2300.

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

[EDR]

In article <yLqec.18880$rg5.39150@attbi_s52>, Ben Jackson >
wrote:

> A typical power chart for a constant speed prop will limit the RPM to
> 2400 at all altitudes and power settings. The 75% power table will
> just end when it is no longer possible to make enough manifold pressure
> to get 75% HP out of the engine at 2400 RPM.
>
> In contrast, a fixed pitch prop will turn faster and faster to make 75%
> at high altitudes. I think some Cherokees call for up to 2650 RPM
> cruise settings.
>
> Another way to look at it is that a plane with a constant speed prop
> may take off with full throttle and full RPM, reach a cruising altitude
> of 8000' and then pull back to 2400 RPM (leaving the throttle full forward)
> while a fixed pitch prop plane would just accept a few hundred RPM rise
> at full throttle and 8000'.
>
> Is the answer that the constant speed prop is slowed down because we
> *can* and the fixed pitch prop is just suffering all the ill effects
> you'd expect, like higher wear, more noise and frictional losses?

Get a copy of the book, "FLYING THE BEECH BONANZA" by John C Eckalbar.
He provides and excellent discussion on engine operation, including the
formulas. I am still trying to find sources of propellor efficiency
charts.

[Bushleague]

The simple answer is that the manufacturer does not want you to exceed
the maximum sustained horsepower rating at any given rpm/mp/altitude.
For example if you run the book MP during a climb however increase the
prop rpm because you are heavy, hot, etc then you are exceeded the
maximum rated hp for the climb.

I got a chance to witness this years ago with with turbopropeller
driven airplane: The Captain on this particular Regional was having
difficulty climbing during the Summer months with a full load
and looking at the charts kept the maximum torque (a setting used for
turboprops) setting which was designed for a maximum climb HP
at a specific rpm. To expedite the climb he pushed the propeller rpm
up which helped, however this combination exceeded the maximum rated
horsepower of the engine (limited by the reduction gearbox), not the
turbine and the gearbox exploded resulting in several injuries to
the passengers. Follow the book.

Have a great one!

Bush

On Mon, 12 Apr 2004 06:34:06 GMT, (Ben Jackson) wrote:

>A typical power chart for a constant speed prop will limit the RPM to
>2400 at all altitudes and power settings. The 75% power table will
>just end when it is no longer possible to make enough manifold pressure
>to get 75% HP out of the engine at 2400 RPM.
>
>In contrast, a fixed pitch prop will turn faster and faster to make 75%
>at high altitudes. I think some Cherokees call for up to 2650 RPM
>cruise settings.
>
>Another way to look at it is that a plane with a constant speed prop
>may take off with full throttle and full RPM, reach a cruising altitude
>of 8000' and then pull back to 2400 RPM (leaving the throttle full forward)
>while a fixed pitch prop plane would just accept a few hundred RPM rise
>at full throttle and 8000'.
>
>Is the answer that the constant speed prop is slowed down because we
>*can* and the fixed pitch prop is just suffering all the ill effects
>you'd expect, like higher wear, more noise and frictional losses?

[Orval Fairbairn]

In article >,
Bushleague > wrote:

> The simple answer is that the manufacturer does not want you to exceed
> the maximum sustained horsepower rating at any given rpm/mp/altitude.
> For example if you run the book MP during a climb however increase the
> prop rpm because you are heavy, hot, etc then you are exceeded the
> maximum rated hp for the climb.
>
> I got a chance to witness this years ago with with turbopropeller
> driven airplane: The Captain on this particular Regional was having
> difficulty climbing during the Summer months with a full load
> and looking at the charts kept the maximum torque (a setting used for
> turboprops) setting which was designed for a maximum climb HP
> at a specific rpm. To expedite the climb he pushed the propeller rpm
> up which helped, however this combination exceeded the maximum rated
> horsepower of the engine (limited by the reduction gearbox), not the
> turbine and the gearbox exploded resulting in several injuries to
> the passengers. Follow the book.


That isn't the problem faced with piston engines. Many types are "hp
limited," due to engine/airframe certification requirements (usually to
limit horsepower, so new certification doesn't have to be done). An
example is the IO-360 Continental on some Cessnas, in which an engine is
limited, while the same engine/prop are not limited on other planes. The
only difference is the max RPM allowable.

Sometimes you can -- sometimes you can't.



> On Mon, 12 Apr 2004 06:34:06 GMT, (Ben Jackson) wrote:
>
> >A typical power chart for a constant speed prop will limit the RPM to
> >2400 at all altitudes and power settings. The 75% power table will
> >just end when it is no longer possible to make enough manifold pressure
> >to get 75% HP out of the engine at 2400 RPM.
> >
> >In contrast, a fixed pitch prop will turn faster and faster to make 75%
> >at high altitudes. I think some Cherokees call for up to 2650 RPM
> >cruise settings.
> >
> >Another way to look at it is that a plane with a constant speed prop
> >may take off with full throttle and full RPM, reach a cruising altitude
> >of 8000' and then pull back to 2400 RPM (leaving the throttle full forward)
> >while a fixed pitch prop plane would just accept a few hundred RPM rise
> >at full throttle and 8000'.
> >
> >Is the answer that the constant speed prop is slowed down because we
> >*can* and the fixed pitch prop is just suffering all the ill effects
> >you'd expect, like higher wear, more noise and frictional losses?
>