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Constant speed prop question



 
 
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  #11  
Old July 20th 08, 04:10 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
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Posts: 2,969
Default Constant speed prop question

John Smith wrote in
:

In article ,
Thomas Borchert wrote:

originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.


Actually, it couldn't have been all of the military. Many major
cities in my home country of Germany would contain many more historic
buildings today, if those B-17s hadn't run oversquare and lean of
peak. The B-17s would never have reached them.


We can all say thank you Charles Lindberg for teaching us this
technique.



Well, the 38 pilots in the pacific could, but leaning techniques as well as
a lot of other things about how engines work, were well known to both
manufactuers and pilots in the 30s. Some, strangely, have been lost in the
mists of time only to be "rediscovered" when things go wrong. Remember the
Lycoming crank problem? Moisture in the hollow portion of the shaft just
behind the prop?
i have several manuals, one form the 1920's , that address this problem and
guess what? The same cure recommended in the SB for the lycomings was in
those manuals.


Bertie
  #12  
Old July 20th 08, 04:14 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
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Posts: 2,969
Default Constant speed prop question

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
  #13  
Old July 20th 08, 04:25 PM posted to rec.aviation.piloting
Mike[_22_]
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Posts: 466
Default Constant speed prop question

"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.

  #14  
Old July 20th 08, 04:28 PM posted to rec.aviation.piloting
Steven P. McNicoll[_2_]
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Posts: 721
Default Constant speed prop question

Terence Wilson wrote:
On Sun, 20 Jul 2008 08:31:45 -0500, "Steven P. McNicoll"
wrote:

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."

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.


It's engine rpm


Just to clarify, it's engine rpm for all the blanks?


Yes. Or propeller. Either would be correct as there is a fixed ratio
between engine rpm and prop rpm. In a direct drive engine that ratio is 1.


  #15  
Old July 20th 08, 04:48 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
external usenet poster
 
Posts: 2,969
Default Constant speed prop question

"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

,
  #16  
Old July 20th 08, 05:36 PM posted to rec.aviation.piloting
RST Engineering
external usenet poster
 
Posts: 1,147
Default Constant speed prop question

Those cities would also have contained many more historic buildings if your
elected leaders hadn't invaded Poland, France, Czechoslovakia, and a few
other minor countries.

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


"Thomas Borchert" wrote in message
...

Many major cities in my
home country of Germany would contain many more historic buildings today,
if
those B-17s hadn't run oversquare and lean of peak.



  #17  
Old July 20th 08, 05:42 PM posted to rec.aviation.piloting
Thomas Borchert
external usenet poster
 
Posts: 1,749
Default Constant speed prop question

Terence,

Just to clarify, it's engine rpm for all the blanks?


There is no difference between the two. Change one, and the other
changes in the same way.

--
Thomas Borchert (EDDH)

  #18  
Old July 20th 08, 05:42 PM posted to rec.aviation.piloting
Thomas Borchert
external usenet poster
 
Posts: 1,749
Default Constant speed prop question

RST,

Those cities would also have contained many more historic buildings if your
elected leaders hadn't invaded Poland, France, Czechoslovakia, and a few
other minor countries.


No doubt about it.

It seems your wink-o-meter might need adjustment.

--
Thomas Borchert (EDDH)

  #19  
Old July 20th 08, 06:11 PM posted to rec.aviation.piloting
Mxsmanic
external usenet poster
 
Posts: 9,169
Default Constant speed prop question

Thomas Borchert writes:

It seems your wink-o-meter might need adjustment.


The wink-o-meter tends to cloud up when it winks at 100 million dead.
  #20  
Old July 20th 08, 06:13 PM posted to rec.aviation.piloting
[email protected]
external usenet poster
 
Posts: 20
Default Constant speed prop question

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.
 




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