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  #31  
Old 10-02-2009, 07:40 PM
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dwschaefer dwschaefer is offline
 
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Default Training

So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS



Quote:
Originally Posted by Ironflight View Post
I'm with you guys! C/S Prop needs to be no more complicated than Pierre describes. I also pull the RPM's back a little after I've reached a safe altitude, while leaving the throttle full in (I don't believe in the "Oversquare Bogeyman" when it comes to small Lycomings...). I generally don't push the prop forward in the pattern until I know that the power is low enough so that it won't surge and make noise for the neighborhood below.....

Paul
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  #32  
Old 10-02-2009, 07:41 PM
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Flyfalcons Flyfalcons is offline
 
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Nothing wrong with your technique, DWS.
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  #33  
Old 10-02-2009, 07:54 PM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by dwschaefer View Post
So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS
No re-education required - looks fine to me. The thing is...."square" mens nothing. It's an accidental relationship brought about by the units used. if you were reading RPM in Radians per Second, and manifold pressure in psi, no one would think of them as square. the numbers are fairly arbitrary - the point of Pierre's post is that the operation is simple if you want it to be!

Paul
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  #34  
Old 10-02-2009, 09:01 PM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by dwschaefer View Post
So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS

By "sq", are you saying that you pull the MP back to 25 inches as well ("25 squared" = 2500 RPM/25 inches)?
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  #35  
Old 10-02-2009, 11:55 PM
Flying Scotsman Flying Scotsman is offline
 
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Quote:
Originally Posted by Ironflight View Post
No re-education required - looks fine to me. The thing is...."square" mens nothing. It's an accidental relationship brought about by the units used. if you were reading RPM in Radians per Second, and manifold pressure in psi, no one would think of them as square. the numbers are fairly arbitrary - the point of Pierre's post is that the operation is simple if you want it to be!

Paul
Paul is dead-on here...if you stop and think about it, the 25" MP is measured in *inches* of *mercury*. What the...? Why on earth should that have anything to do with the proper stoichiometric ratio and spark advance timing to get proper burn and such on an engine? It just so happens that the two measurements (25" and 25 hundred RPM) *sound* close together (they're not even that, really...25" and 2500 RPM are different by two orders of magnitude).

So how did this old wive's tale about running "square" or avoiding "over-square" come about?

Again...shouldn't this be more analytical in nature? There should be a chart that says the max efficiency for speed, or climb is at X" and Y RPM at Z Density Altitude and S speed, etc.?

I suspect this is what goes into the algorithms and control loops for a FADEC, no? Along with other things, such as pitch/ROC/etc.?

I confess to always setting "25-squared" at 1000' AGL upon departure, but now I'm rethinking this seriously...just keep everything where it is but ensure that RPM is below redline, and let 'er climb, then reconfigure at cruise altitude? (Until the -7A is done, this is for Archers and Arrows, though ).

But since I expanded this thread to max efficiency and whatnot in the first place, I wanted to commend Paul on pointing out what should be obvious...the units chosen are affecting the operational decisions (or in other terms, the "flight rules"), and that shouldn't be the case.
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  #36  
Old 10-03-2009, 06:30 AM
Gary 40274 Gary 40274 is offline
 
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Quote:
What we want is powertrain efficiency. In general, WOT improves engine efficiency, as that minimizes pumping losses, while lower RPM improves engine efficiency by minimizing fricitional losses. Propeller efficiency is generally improved at lower RPM, due to minimizing the Mach number out at the tip. Thus, depending upon your altitude, desired % of power, and tolerance for "oversquare" operation, WOT and lower RPM are your best bet. All of these factors pale in comparison to what's going on with the mixture knob
.


I fly a 10. Shortly after take off, I lower the RPM to 2200 and fly WOT until I reach the altitude I want. It is nice and quiet and I feel that it is easier on the engine than at full throttle. I get a lot of flak when I mention this to others, but no one has indicated that it is hard on the engine.

Gary Specketer
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  #37  
Old 10-03-2009, 06:55 AM
pauldan181 pauldan181 is offline
 
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Default RPM Restriction

I don't think it's been mentioned yet but don't forget about the Hartzell recomendation to reduce RPM to 2600 or less "as soon as practical after take-off" or some such wording. This is only if you have electronic ignition or high compression pistons on an O-360. The BA blade removes a 2050-2250 avoid slot but the high RPM restriction remains. This was due to vibration studies done by Hartzell.

As others have stated, I also reduce MP and RPM at about pattern altitude after take-off to 25... umm, to some settings that are easy to remember for climb. I'll cruise at 2300-2400 and leave it there until just before landing. I once forgot to run the blue lever up before landing and had to go around. As I crammed all three levers up to full the resulting surge and noise must have sounded like a real amateur at the controls.

Paul Danclovic
Jamestown NC
RV-8A N181SB

Last edited by pauldan181 : 10-03-2009 at 07:13 AM.
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  #38  
Old 10-03-2009, 07:24 AM
wv4i wv4i is offline
 
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Default carb enrichment feature and related pubs

The enrichment valve in the carb at full power actually helps to cool the engine, and will result in lower CHT's.
Vic


Somehow, what Vic said above got buried in this thread, but is absolutely correct, especially here in South FL during the hot season. To prove it, try one takeoff where, at a safe altitude, you pull back to 25" then 2500 rpm, then one where you simply pull back to 2500 rpm. If method B does not result in lower CHTs after prop RPM reduction, than A, there is something wrong.

One other note is that fuel flow, and percent power, is a function of RPM and MAP. I reduce the RPM in cruise to lower fuel flow and go faster. But when I stop going faster, I stop reducing. My O-360/carb/CS RV-6 also has a 2050-2250 RPM range to avoid, but about 2300 RPM, WOT, at 8k', seems about the best, re min fuel flow and best TAS.

If you're really into this stuff, the best two pubs I''ve read on it are:

EI pub "The Pilot's Manual...":

http://buy-ei.com/Information/Pilots%20Manual.pdf

And Lyc pub "Key Reprints":

http://www.lycoming.textron.com/supp...nts/index.html
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  #39  
Old 10-03-2009, 07:49 AM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by wv4i View Post
but about 2300 RPM, WOT, at 8k', seems about the best, re min fuel flow and best TAS.
I have played around with just about every permutation of cruise power settings and have also found that 2300/25" works the best for cruise.
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  #40  
Old 10-03-2009, 09:22 AM
eh009us eh009us is offline
 
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Seems to me Dan C used to play around a lot with C/S prop settings, LOP stuff and got pretty good economy with his big engine......perhaps he'll pontificate on the matter. Presently I'm flying an Arrow (200 hp) and in instrument mode for training purposes I'll generally run 2400/21-22". Almost never "square" . But again, that's an Arrow.
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