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  #11  
Old 07-08-2017, 01:02 PM
rv7charlie rv7charlie is online now
 
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Quote:
Originally Posted by TThurston View Post
I hope I didn't come across as suggesting that one ought to try and control power exclusively with mixture. I'm just trying to make sure I really understand the issues about why it may not be good practice. I think I understand the issues better now than I did when I first asked the question.

I should also note that my home airport is at 4500 MSL, so my normal flying is at 7000 MSL or above, usually with substantial leaning. In fact, with my limited instrumentation, my normal practice is to start leaning as soon as I leave the pattern. I pull the mixture until I see a slight RPM drop (usally a LONG pull on the mixture), and then enrich just a tad. I normally repeat the procedure again on reachng cruise altitude, and maybe mid-climb I'm climbing a long way. If the OAT is high, I might actually perform the procedure before takeoff, pushing in a little more (more rich) until I leave the pattern.

When I reach cruise, I normally not only lean, but also pull the throttle just a bit so that I'm 100-200 RPM less than full throttle. I guess mostly what I've been wondering is, what I used mixture to get that 100-200 RPM drop at cruise instead of using the throttle? Would I hurt my engine? Would I get better fuel economy? What I was trying to achieve a real maximum range fuel economy?

I can't imagine I would ever (short of an emergency) ever use the mixture for power reductions for normal descent, approach, and landing. But I really do want to make sure I understand the issues.

But it's still interesting to me that the cruise performance charts in the CAFE reports of the carburated RV9A all use wide open throttle, and control the fuel flow with mixture. The first chart starts at 10.0 gph, shows a peak EGT at 7.5 gph, but continues leaning to 6.1 gph, and shows that as the point with best mpg and best CAFE score. To me, that seems to be a wide range of fuel flows to be controlled exclusively with mixture. I assume the CAFE people knew what they were doing when they ran the tests.
The technique you're describing goes back (at least in aviation lore) to Lindbergh teaching fighter pilots how to extend their range in the Pacific during WW2. If you carefully peruse the Lyc operator's manuals, they will show that at some altitude (typically around 8k'), you need 2700 rpm to make 75% power; something many (most?) pilots don't seem to understand. You constantly hear things like, "Yeah; I only burn 7 gph at 75% power up at 8500 feet, turning 2400 rpm." Yeah; no they don't.

My understanding is that once you're high enough that it requires 2700 rpm to make 75% power, the most efficient way to operate the engine is to control with mixture. It reduces 'pumping losses' in the engine. Probably could be applied to lower altitudes, as well, as long as actual egt's are monitored & kept under control.

Charlie
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  #12  
Old 06-21-2018, 06:29 PM
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Blain Blain is offline
 
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Not meaning to hijack the thread but the question is related. When departing an airport with a DA of say, 4,000' would you lean to best power on runup and depart or or go full rich?

I was shown the lean to best power by an experienced pilot but I know the Lycoming manual says full rich for any power changes. Injected by the way.
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  #13  
Old 06-22-2018, 08:04 PM
rightrudder rightrudder is offline
 
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Another option with a runaway engine would be to switch off the magnetos/ignition, and switch it back on briefly if you need a burst of power. That would be my first instinct in a stuck throttle scenario.
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  #14  
Old 06-22-2018, 08:50 PM
David Paule David Paule is offline
 
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That might work in some engines. One of mine has a failure mode if the engine backfires, an intake balance tube blows off and the engine won't run until it's put back on.

So before accepting that as a valid response, ensure that it really is. Which it might be on a Lycoming. Best to check, though.

Dave
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  #15  
Old 06-23-2018, 07:26 PM
lr172 lr172 is online now
 
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Quote:
Originally Posted by penguin View Post
Dude, Go read the 'Pelican's Perch' articles by John Deakin on AvWeb. Search for the ones on learning (there are about 100 and only 5 or so that are relevant). What you did is fine for an emergency, but could potentially be quite damaging to your engine (burnt holes in pistons) if you were to do it regularly. The engine will only run within a certain range of mixtures (say about 1 fuel to 15 air, but I can't remember exactly), too rich and it will quit, too lean and it will also quit. Why do you think every one else uses the throttle? If you want to use the mixture go buy a diesel engine! Pete
Please explaine how too lean of a mixture can burn a hole in the piston.

Larry
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  #16  
Old 06-23-2018, 07:32 PM
lr172 lr172 is online now
 
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Quote:
Originally Posted by David Paule View Post
That might work in some engines. One of mine has a failure mode if the engine backfires, an intake balance tube blows off and the engine won't run until it's put back on.

So before accepting that as a valid response, ensure that it really is. Which it might be on a Lycoming. Best to check, though.

Dave
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RV-3B skinning the fuselage
Please explain how the mixture setting alone (within the range provided by your servo) at any meaningful rpm level can induce a backfire
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Last edited by lr172 : 06-23-2018 at 07:34 PM.
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  #17  
Old 06-23-2018, 10:38 PM
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erich weaver erich weaver is offline
 
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Quote:
Originally Posted by lr172 View Post
Please explaine how too lean of a mixture can burn a hole in the piston.
Donít expect any reply. Iím guessing you didnít notice that the post that you are responding to is from 2008

Erich
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  #18  
Old 06-23-2018, 10:52 PM
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airguy airguy is offline
 
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Quote:
Originally Posted by erich weaver View Post
Donít expect any reply. Iím guessing you didnít notice that the post that you are responding to is from 2008

Erich
And that this basic engine operation knowledge falls under the title of "RTFM". The knowledge base has already been called out, read it and the question answers itself.
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  #19  
Old 06-24-2018, 06:37 PM
gkimbrough gkimbrough is offline
 
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FWIW, I had the problem the OP's instructor described in real life during my phase one testing.
I killed one mag in the pattern, flew it down to (very) short final, killed the other mag, and landed.
Turned out a nut on the cable bracket at the carb had loosened itself.
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  #20  
Old 06-24-2018, 07:11 PM
David Paule David Paule is offline
 
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Quote:
Originally Posted by lr172 View Post
Please explain how the mixture setting alone (within the range provided by your servo) at any meaningful rpm level can induce a backfire
My posting wasn't about mixture adjustment but about shutting off the ignition. It followed (but didn't reference) one that mentioned the concept of shutting down the ignition and turning it back on as a method of reducing power. A similar concept was used on rotary engines in WW I.

On my carbureted C180 shutting off (or even checking) the mags in flight are specifically prohibited in the manual, for the reasons I described.

Dave
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