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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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