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Constant speed prop question



 
 
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  #1  
Old July 20th 08, 02:24 PM posted to rec.aviation.piloting
Terence Wilson
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Posts: 28
Default Constant speed prop question

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.

  #2  
Old July 20th 08, 02:31 PM posted to rec.aviation.piloting
Steven P. McNicoll[_2_]
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Posts: 721
Default Constant speed prop question

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, but in direct drive engines prop rpm and engine rpm are the
same.


  #3  
Old July 20th 08, 03:00 PM posted to rec.aviation.piloting
Jay Maynard
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Posts: 521
Default Constant speed prop question

On 2008-07-20, Steven P. McNicoll wrote:
Terence Wilson wrote:
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?

It's engine rpm, but in direct drive engines prop rpm and engine rpm are the
same.


....and in geared engines, or engines with some other propeller speed
reduction unit (such as the Rotax 912), they're directly proportional. Thus,
it doesn't matter whether it's engine or prop rpm.
--
Jay Maynard, K5ZC http://www.conmicro.com
http://jmaynard.livejournal.com http://www.tronguy.net
Fairmont, MN (FRM) (Yes, that's me!)
AMD Zodiac CH601XLi N55ZC (got it!)
  #4  
Old July 20th 08, 03:19 PM posted to rec.aviation.piloting
Mike[_22_]
external usenet poster
 
Posts: 466
Default Constant speed prop question

"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. I run oversquare as
much as possible simply because it's more efficient. Many turbo engined
pilots routinely run 12" oversquare with no ill effects.

Here's an excellent group of articles that explains the principles involved
in easy to understand language:

http://www.advancedpilot.com/downloads/prep.pdf

  #5  
Old July 20th 08, 03:20 PM posted to rec.aviation.piloting
Thomas Borchert
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Posts: 1,749
Default Constant speed prop question

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)

  #6  
Old July 20th 08, 03:25 PM posted to rec.aviation.piloting
Thomas Borchert
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Posts: 1,749
Default Constant speed prop question

Mike,

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.

--
Thomas Borchert (EDDH)

  #7  
Old July 20th 08, 03:38 PM posted to rec.aviation.piloting
john smith
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Posts: 1,446
Default Constant speed prop question

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.
  #8  
Old July 20th 08, 03:55 PM posted to rec.aviation.piloting
Terence Wilson
external usenet poster
 
Posts: 28
Default Constant speed prop question

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?

Thanks.
  #9  
Old July 20th 08, 04:03 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
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Posts: 2,969
Default 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

  #10  
Old July 20th 08, 04:06 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
external usenet poster
 
Posts: 2,969
Default Constant speed prop question

Thomas Borchert wrote in
:

Mike,

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.


Irrelevant. They didn't run "oversquare" They ran where they were suposed
to. And they didn't run lean of peak, that engine had autolean and that
setting ran well rich of peak.





Bertie
 




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