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  #11  
Old 08-09-2014, 06:01 AM
rzbill's Avatar
rzbill rzbill is offline
 
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Quote:
Originally Posted by crabandy View Post
Just after takeoff and a safe altitude you pull your manifold pressure back to 24 inches
Ummm......why?

Quote:
Originally Posted by crabandy View Post
, then you pull the Rpms back to 2500
If still climbing and wanting best performance,......why?

I know this is a drift and I am undeservedly jumping on Andy who is trying to set up an example (sorry) but I don't have to deal with the manifold pressure change due to rpm drop. Except in unusual circumstances where I am flying very low altitude (MP above 26 in), my throttle is wide open from TO to destination. Power level is controlled by prop and is (by choice to follow Lyc docs) set to 75% or below at cruise by using the power indication on the GRT engine screen (which reads a mapping I made myself rather than the one provided by GRT which does not represent an IO-360-M1B). The MP probably does rise a bit as the RPM comes down but I don't use it for power setting since the MP and RPM combinations change with altitude.

For those that shiver at oversquare operation, I am operating at all times within the power chart settings published by Lycoming which show considerable oversquare settings.
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Bill Pendergrass
ME/AE '82
RV-7A: Flying since April 15, 2012. 850 hrs
YIO-360-M1B, mags, CS, GRT EX and WS H1s & A/P, Navworx
Unpainted, polished....kinda'... Eyeballin' vinyl really hard.
Yeah. The boss got a Silhouette Cameo 4 Xmas 2019.

Last edited by rzbill : 08-09-2014 at 04:21 PM.
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  #12  
Old 08-09-2014, 03:01 PM
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Jerry Cochran Jerry Cochran is offline
 
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Location: Sherwood, Oregon
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Default HP formula

I'm wondering if knowing the simple formula for horsepower would help this thread. The formula is: HP=Torque x RPM. One can think of torque as a result of manifold pressure (MAP) and RPM is self-evident. If you raise either side or both sides of the equation it will raise HP and the reverse is true for lowering them.

It works every time it's tried...

Jerry
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  #13  
Old 08-09-2014, 03:12 PM
BillL BillL is offline
 
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Quote:
Originally Posted by Jerry Cochran View Post
I'm wondering if knowing the simple formula for horsepower would help this thread. The formula is: HP=Torque x RPM. One can think of torque as a result of manifold pressure (MAP) and RPM is self-evident. If you raise either side or both sides of the equation it will raise HP and the reverse is true for lowering them.

It works every time it's tried...

Jerry
No. (min length satisfied)
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and express it in numbers, you know something about it; but when you
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  #14  
Old 08-09-2014, 06:21 PM
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N546RV N546RV is offline
 
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Originally Posted by rzbill View Post
Ummm......why?
I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.
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  #15  
Old 08-09-2014, 06:28 PM
usmcmech usmcmech is offline
 
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Location: San Antonio TX
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There are a lot of misconceptions about MAP, and Deakins Avweb article should be required reading for anyone flying behind a CS prop.


Misconception #1 MAP is the amount of power the engine is producing.

Not true in the least. The highest MAP you will see is when the engine is turned off producing 0 Hp. Even in flight if you pull the mixture to idle cut off or turn the mags off, the engine will keep spinning and MAP will be unchanged.

What MAP is in fact is a crude way to know your throttle setting. This combined with RPM tells you home much air your engine is breathing, nothing more.

Another key is the word "throttle" and it's many definitions. To "throttle" something (or someone) is to choke off it's air supply. "Full throttle" is actually dead idle, while at full power the engine is not being throttled at all.
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  #16  
Old 08-09-2014, 06:43 PM
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rzbill rzbill is offline
 
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Quote:
Originally Posted by N546RV View Post
I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.
Ha Ha. Exactly! Of course, I should give credit in case it is due. Hartzell could have issued a timed RPM restriction (2700 for 5 min for instance). I don't remember. The Blended Hartzell/M1B combo has no restrictions. I flew Arrows too and it has taken me a long time to quit fooling with the prop knob on initial climb.
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Bill Pendergrass
ME/AE '82
RV-7A: Flying since April 15, 2012. 850 hrs
YIO-360-M1B, mags, CS, GRT EX and WS H1s & A/P, Navworx
Unpainted, polished....kinda'... Eyeballin' vinyl really hard.
Yeah. The boss got a Silhouette Cameo 4 Xmas 2019.

Last edited by rzbill : 08-09-2014 at 06:46 PM.
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  #17  
Old 08-09-2014, 06:56 PM
Ron Ron is offline
 
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Default Fixed Pitch

So I guess MP has no value to us with fixed pitch?
Just curious...
Ron
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  #18  
Old 08-09-2014, 06:56 PM
luddite42 luddite42 is offline
 
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Quote:
Originally Posted by N546RV View Post
I'm pretty sure the practical reason is "because that's how it gets taught." Instructors I've had in the Arrow really drill in that power/RPM reduction, the same way they drill in the whole oversquare stuff.
And I've never understood WHY that was taught. If for some reason you want to make less noise on climb out, I can see backing off on the RPM slightly, but not the power. We don't back off on the power with fixed pitched props. Climbing out at full power/2500 RPM is the same thing whether flying FP or CS.
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  #19  
Old 08-09-2014, 07:13 PM
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N546RV N546RV is offline
 
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Quote:
Originally Posted by luddite42 View Post
And I've never understood WHY that was taught. If for some reason you want to make less noise on climb out, I can see backing off on the RPM slightly, but not the power. We don't back off on the power with fixed pitched props. Climbing out at full power/2500 RPM is the same thing whether flying FP or CS.
I found it kind of odd when I first starting doing my complex. 500' AGL in a new plane, and you want me to direct my attention to the MAP gauge so I can precisely reduce power to 25"? Okay...

I mean, it's not like it's a particularly huge deal or anything, but it seems like an unnecessary thing to me. But now, after 10 hours dual in the Arrow, I still do it, because it's just ingrained. It got turned into a habit and now I'm probably going to start teaching myself not to do it, and then I'll get used to not doing it and go for a BFR or something and get **** about it.
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-8 fuselage in progress (remember when I thought the wing kit had a lot of parts? HAHAHAHAHA)
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  #20  
Old 08-09-2014, 08:14 PM
BillL BillL is offline
 
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This has wandered from the OP, but the MAP and RPM reduction right after TO came up from a Bo owner in another thread too. I know that both originated with the engine manufacturers for OEM product. As compression, timing etc were pushed to publish better performance numbers, and achieve the best TO power, the margin for cooling, piston temps, and detonation are reduced relative to cruise. I asked this of the engine engineers (that I worked with) way back and they would give me chapter and verse for why - cylinder pressures etc. for a particular engine model. The engine thrust/fuel flow is better at the lower speed and rpm too. Not absolute performance, but efficiency - as evidenced in what we call peak torque. There was a classic engine power rating schedule of continuous vs TO power as well. The piston speeds (RPM) and power settings (MAP) are lower for lower forces/stresses and lower heat flux (temperatures) on all components at the continuous ratings. This has sort of disappeared over the decades, but had a historical influence on the operating limitations/guidelines. Think how long your engine would last running 2700 - 29" all the time vs. 2400 - 24". Now make an equation for each TBO and time fraction for how the engine is operated. The TBO is somewhat variable by choosing the time at each for a mission profile, maintenance being the same.

Now we, in the RV crowd, are going faster, climbing faster and with better cooling instrumentation than the OEM's. Also, most RV pilots are much more well informed and attentive to these issues than the general population, so temps are not exceeded. For extending life, these lower numbers may well provide more hrs to TBO, or and maybe not. We just don't have good definitive data on limitations of the 4/6 cylinder, normally aspirated, Lycomings.

Sorry, no hard and fast answers here, but some history on why the OEM operating recommendations make some sense even if we don't have the numbers (and I can not remember the numbers either).
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RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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