Constant speed prop question

"Mike" wrote in news:WPHgk.144$DS3.119@trnddc01:

"Terence Wilson" wrote in message
...
In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___
rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing because it makes several
references to rpm but doesn't clarify whether it is engine or prop
rpm. The blanks were inserted by me. Can someone help me out?

Thanks in advance.

As others have said, unless you have a gearbox (not many planes do),
they are one and the same.

You may also want to ditch your Jepp book as the "theory" they are
describing really doesn't apply to most small piston aircraft. The
old "don't run oversquare" mentality which has been taught for years
originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.


Actually, they dont, since most military aircraft,even smaller ones,
were supercharged and they ran well oversquare.. A 985, for instance, is
around 37 inches max and a typical cruise MP might be in the order of
25 inches with a cruise rpm of something like 1850, depending on how
fast you want to go and how much you want to burn.
The geared engines were even less relevant to this argument, since
almost everything larger than about 1500 c.i.d. was geared. The
indicated RPM was usually engine rpm and max for somthing like an 1830
was around 2400 and max MP for takeoff was about 43 IIRC and cruise was
around 30/2,000.
The practice originates from a perceived need to simplify for light
aircraft pilots new to variable pitch props.


Bertie





Bertie

"Bertie the Bunyip" wrote in message
...
"Mike" wrote in news:WPHgk.144$DS3.119@trnddc01:

"Terence Wilson" wrote in message
...
In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___
rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing because it makes several
references to rpm but doesn't clarify whether it is engine or prop
rpm. The blanks were inserted by me. Can someone help me out?

Thanks in advance.

As others have said, unless you have a gearbox (not many planes do),
they are one and the same.

You may also want to ditch your Jepp book as the "theory" they are
describing really doesn't apply to most small piston aircraft. The
old "don't run oversquare" mentality which has been taught for years
originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.


Actually, they dont, since most military aircraft,even smaller ones,
were supercharged and they ran well oversquare.. A 985, for instance, is
around 37 inches max and a typical cruise MP might be in the order of
25 inches with a cruise rpm of something like 1850, depending on how
fast you want to go and how much you want to burn.
The geared engines were even less relevant to this argument, since
almost everything larger than about 1500 c.i.d. was geared. The
indicated RPM was usually engine rpm and max for somthing like an 1830
was around 2400 and max MP for takeoff was about 43 IIRC and cruise was
around 30/2,000.
The practice originates from a perceived need to simplify for light
aircraft pilots new to variable pitch props.

And that need is even more of a necessity in military trainers which have
considerably more power and are much more easily red lined. That's why I
always assumed the mentality came primarily from military instructors giving
instruction in training aircraft. At any rate the myth still persists to
this day even with instructors who should know better.

"Mike" wrote in news:qNIgk.158$oU.42@trnddc07:

"Bertie the Bunyip" wrote in message
...
"Mike" wrote in
news:WPHgk.144$DS3.119@trnddc01:

"Terence Wilson" wrote in message
...
In the course of trying to understand how a constant speed prop
works I came across the following passage in one of the Jeppesen
books:

"If the throttle is advanced without decreasing the pitch of the
prop blades to increase ___ rpm, the manifold pressure increases as
the prop mechanism attempts to keep ___ rpm constant by increasing
the blade angle. The combination of high manifold pressure and low
___ rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing because it makes several
references to rpm but doesn't clarify whether it is engine or prop
rpm. The blanks were inserted by me. Can someone help me out?

Thanks in advance.

As others have said, unless you have a gearbox (not many planes do),
they are one and the same.

You may also want to ditch your Jepp book as the "theory" they are
describing really doesn't apply to most small piston aircraft. The
old "don't run oversquare" mentality which has been taught for years
originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.


Actually, they dont, since most military aircraft,even smaller ones,
were supercharged and they ran well oversquare.. A 985, for instance,
is around 37 inches max and a typical cruise MP might be in the
order of 25 inches with a cruise rpm of something like 1850,
depending on how fast you want to go and how much you want to burn.
The geared engines were even less relevant to this argument, since
almost everything larger than about 1500 c.i.d. was geared. The
indicated RPM was usually engine rpm and max for somthing like an
1830 was around 2400 and max MP for takeoff was about 43 IIRC and
cruise was around 30/2,000.
The practice originates from a perceived need to simplify for light
aircraft pilots new to variable pitch props.

And that need is even more of a necessity in military trainers which
have considerably more power and are much more easily red lined.
That's why I always assumed the mentality came primarily from military
instructors giving instruction in training aircraft. At any rate the
myth still persists to this day even with instructors who should know
better.


Well, outside of the T-34 I can't think of anything that would fit the
"square" scenario, and military instructors would not have taken any
sort of soft route with the students in any case. For instance, I happen
to know any Navy student would have to have memorised a very lengthy
series of checklists at the primary student stage for a T-28, for
instance. That's ALL of the checklists. Ever single one, emergencies and
all. And having seen them I know they were very, very complicated
indeed.
They also had to be able to touch every single switch, dial, and lever
in the airplane blindfolded. I can't see them going soft on a little
thing like not having to memorise a given MP RPM combo. Now, during
aerobatics, it would make sense to have a nominal max MP a bit shy of
normal max, as you say, but for operations outside of that, they
certainly would not have done that.
No, the only place I've ever seen he practice touted s by FBOs renting
airplanes or using them for comercial instruction.


Bertie

,

On Jul 20, 10:48 am, Bertie the Bunyip wrote:
"Mike" wrote innews:qNIgk.158$oU.42@trnddc07:



"Bertie the Bunyip" wrote in message
.. .
"Mike" wrote in
news:WPHgk.144$DS3.119@trnddc01:

"Terence Wilson" wrote in message
...
In the course of trying to understand how a constant speed prop
works I came across the following passage in one of the Jeppesen
books:

"If the throttle is advanced without decreasing the pitch of the
prop blades to increase ___ rpm, the manifold pressure increases as
the prop mechanism attempts to keep ___ rpm constant by increasing
the blade angle. The combination of high manifold pressure and low
___ rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing because it makes several
references to rpm but doesn't clarify whether it is engine or prop
rpm. The blanks were inserted by me. Can someone help me out?

Thanks in advance.

As others have said, unless you have a gearbox (not many planes do),
they are one and the same.

You may also want to ditch your Jepp book as the "theory" they are
describing really doesn't apply to most small piston aircraft. The
old "don't run oversquare" mentality which has been taught for years
originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.

Actually, they dont, since most military aircraft,even smaller ones,
were supercharged and they ran well oversquare.. A 985, for instance,
is around 37 inches max and a typical cruise MP might be in the
order of 25 inches with a cruise rpm of something like 1850,
depending on how fast you want to go and how much you want to burn.
The geared engines were even less relevant to this argument, since
almost everything larger than about 1500 c.i.d. was geared. The
indicated RPM was usually engine rpm and max for somthing like an
1830 was around 2400 and max MP for takeoff was about 43 IIRC and
cruise was around 30/2,000.
The practice originates from a perceived need to simplify for light
aircraft pilots new to variable pitch props.

And that need is even more of a necessity in military trainers which
have considerably more power and are much more easily red lined.
That's why I always assumed the mentality came primarily from military
instructors giving instruction in training aircraft. At any rate the
myth still persists to this day even with instructors who should know
better.

Well, outside of the T-34 I can't think of anything that would fit the
"square" scenario, and military instructors would not have taken any
sort of soft route with the students in any case. For instance, I happen
to know any Navy student would have to have memorised a very lengthy
series of checklists at the primary student stage for a T-28, for
instance. That's ALL of the checklists. Ever single one, emergencies and
all. And having seen them I know they were very, very complicated
indeed.
They also had to be able to touch every single switch, dial, and lever
in the airplane blindfolded. I can't see them going soft on a little
thing like not having to memorise a given MP RPM combo. Now, during
aerobatics, it would make sense to have a nominal max MP a bit shy of
normal max, as you say, but for operations outside of that, they
certainly would not have done that.
No, the only place I've ever seen he practice touted s by FBOs renting
airplanes or using them for comercial instruction.

Bertie

,

The whole idea of don't run oversquare is not a military technique
taught, but rather a technique taught to radial pilots back in the
day. These pilots then moved over to our flat engines, and decided
running oversquare would still be a bad idea. I agree with the other
guys-and having been through an Advanced Pilot Seminar, I can
certainly say-you will know much more about your engine after going
through the seminar. There is another seminar coming up later this
year, and I'm thinking of attending it again.
Also those pelican perch articles are fantastic, and if Deakin ever
shows up to a seminar, you get to meet the author.

Terence,

whether it is engine or prop
rpm


They are connected at a fixed ratio (1:1 on direct drive engines), so
the distinction doesn't matter.

May I recommend John Deakin's engine management columns at avweb.com to
shed light on this? They are priceless in understanding your engine,
specifically these four:

Pelican's Perch #15:
Manifold Pressure Sucks!

Pelican's Perch #16:
Those Marvelous Props

Pelican's Perch #18:
Mixture Magic

Pelican's Perch #19:
Putting It All Together

http://www.avweb.com/news/pelican/182081-1.html
http://www.avweb.com/news/pelican/182082-1.html
http://www.avweb.com/news/pelican/182084-1.html
http://www.avweb.com/news/pelican/182085-1.html

--
Thomas Borchert (EDDH)

Terence Wilson wrote in
:

In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___ rpm
can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing

I'm not surprised. It's a **** poor explanation and actually misleading,
not to say wildly inaccurate in some places. "High internal manifold
pressures" WTF is that? The author has no understanding whatsoever ofwhat
he's talking about.


Bertie

Bertie the Bunyip wrote:
Terence Wilson wrote in
:

In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___ rpm
can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing

I'm not surprised. It's a **** poor explanation and actually misleading,
not to say wildly inaccurate in some places. "High internal manifold
pressures" WTF is that? The author has no understanding whatsoever ofwhat
he's talking about.


Bertie


I've yet to see a normally aspirated horizontally opposed engine "blow
up" due to "high manifold pressure". I've seen governors fail (which
caused an engine overspeed), but all things being "normal", there is no
way you're going to damage your engine running with the throttle wide
open and your prop on "fine"... One of my engine checks (pre-flight) is
to cycle the prop. Is the idiot that wrote this article saying that I'm
damaging the engine when I do that???

Frank Olson wrote in
news:F1zhk.23979$nD.19490@pd7urf1no:

Bertie the Bunyip wrote:
Terence Wilson wrote in
:

In the course of trying to understand how a constant speed prop
works I came across the following passage in one of the Jeppesen
books:

"If the throttle is advanced without decreasing the pitch of the
prop blades to increase ___ rpm, the manifold pressure increases as
the prop mechanism attempts to keep ___ rpm constant by increasing
the blade angle. The combination of high manifold pressure and low
___ rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing

I'm not surprised. It's a **** poor explanation and actually
misleading, not to say wildly inaccurate in some places. "High
internal manifold pressures" WTF is that? The author has no
understanding whatsoever ofwhat he's talking about.


Bertie


I've yet to see a normally aspirated horizontally opposed engine "blow
up" due to "high manifold pressure". I've seen governors fail (which
caused an engine overspeed), but all things being "normal", there is
no way you're going to damage your engine running with the throttle
wide open and your prop on "fine"... One of my engine checks
(pre-flight) is to cycle the prop. Is the idiot that wrote this
article saying that I'm damaging the engine when I do that???



Well, you will do damage with a low rpm and high enough MP to raise the
BMEP to the point where knocking occurs. Knocking will apply loads that
will exceed the ability of the oil to keep the metal parts seperated and
wear the engine or even overstress parts to the point of breaking.
Observing manufacturer's limitations should ensure that knocking doesn't
occur and while there is obviously a margin included in those to account
for conditions instrument error and so forth, I know i would take care
if it were my engine!

Bertie

On Jul 23, 1:08*am, Frank Olson
wrote:
Bertie the Bunyip wrote:
Terence Wilson wrote in
:

In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___ rpm
can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing

I'm not surprised. It's a **** poor explanation and actually misleading,
not to say wildly inaccurate in some places. "High internal manifold
pressures" WTF is that? The author has no understanding whatsoever ofwhat
he's talking about.

Bertie

I've yet to see a normally aspirated horizontally opposed engine "blow
up" due to "high manifold pressure". *I've seen governors fail (which
caused an engine overspeed), but all things being "normal", there is no
way you're going to damage your engine running with the throttle wide
open and your prop on "fine"... *One of my engine checks (pre-flight) is
to cycle the prop. *Is the idiot that wrote this article saying that I'm
damaging the engine when I do that???

Unless there's a good reason to do otherwise, it would be good
practice to stay within the limits of the POH. Ours lists many
manifold pressures, (measured in inches of Hg) greater than RPM. Most
times in cruise at 5000 to 12000 feet we have rpms cranked pretty far
down with the throttle at max. 1950 rpm with our IO 360 is pretty
common.

On Jul 20, 6:24*am, Terence Wilson wrote:
In the course of trying to understand how a constant speed prop works
I came across the following passage in one of the Jeppesen books:

"If the throttle is advanced without decreasing the pitch of the prop
blades to increase ___ rpm, the manifold pressure increases as the
prop mechanism attempts to keep ___ rpm constant by increasing the
blade angle. The combination of high manifold pressure and low ___ rpm
can cause damage due to high internal manifold pressures."

Go to you tube. There are a couple of videos out there taken from "How
its made" that shows a prop being made. They don't show the gov but
they do a great job of showing you the inside of the hub.

-Robert