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  #31  
Old 07-31-2018, 04:51 PM
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MikeyDale MikeyDale is offline
 
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Wait Larry, I have a CS prop running 2400 RPM.
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  #32  
Old 07-31-2018, 06:29 PM
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rv6ejguy rv6ejguy is offline
 
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With a fixed pitch prop, excluding prop efficiency factors, max fuel economy will be primarily a composite of L/D ratio vs. fuel burn which is itself, governed by many engine factors. Speed and altitude come into play with the L/D ratio. Thinner air equals less drag up to a point where L/D starts to degrade with too high alpha.

C/S props give you many more options, allowing more efficient engine operation over a wider range of altitudes and speeds.

When it comes to the engine, it is most efficient at WOT and relatively low rpms. WOT reduces pumping losses, low rpm (below torque peak rpm somewhat) reduces frictional losses. Volumetrically, the engine is most efficient at torque peak.

Theoretically the engine produces the best BSFC well lean of peak as shown below:



I've posted this graph before and it shows best BSFC occurs around 18 to 1 (.055 here) AFR. Peak EGT on avgas is around 15.2 AFR by comparison (.065 here). Best power is around 13 to 1 AFR (.077 on the graph).

So from this graph we'd lose around 8% hp at best economy mixture but drop fuel flow 25%.

We see some incredible MPG figures from Dave Anders flying high (17.5 is typical) at WOT and very low rpms, at peak or LOP.

If someone is flying with a wideband AFR meter, it would be interesting to equate AFR with EGT degrees ROP and LOP and do a flight comparison to see how the theory compares. It's important to keep in mind that EGT is affected by ignition timing as well so it would be necessary to keep that set the same on EIs when doing this testing.
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Last edited by rv6ejguy : 07-31-2018 at 06:33 PM.
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  #33  
Old 07-31-2018, 09:14 PM
Bicyclops Bicyclops is offline
 
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Howdy Guys,

For sure I can do better at higher altitudes. Drag falls off faster than power. It is also difficult to make it detonate at high altitudes. When I speak of runaway CHTs, I'm talking about blowing right through 400* on it's way to oblivion. If I'm running at 360*-380* and stable, I really doubt that it is detonating. Lycoming's power setting charts are based on "best power" - just ROP - right where the red box is the largest. Yup, 27" and 2300 RPM at 30* ROP is not a good place to be. That's why I don't do that.

Detonation is promoted by high pressure and high CHTs and causes CHTs to rise even more by disrupting the thermal boundary layer next to the cylinder, head, and piston. With the boundary layer disrupted, localized heating will happen and that's where damage can occur. This extra heat will be seen in rising CHT, (and possibly oil temp). If it is bad enough, the CHT will rise very rapidly, the detonation will get a lot worse, and stuff can get hot enough to glow causing pre-ignition. Not good. Either running very ROP or even slightly LOP increases detonation margin by slowing the combustion event so that peak pressure occurs well after TDC, about 17*. Advanced timing at high MAP would cause the peak pressure to occur earlier and would promote detonation. Most EIs don't begin to advance until MAP falls off significantly and so should not be a problem for high power ops.

So - even with high compression, I have several ways to prevent detonation at fairly high manifold pressure. I don't have a detonation sensor, none of us Lycloning operators do, so I use CHT trends to track what's going on. At take off and full rich I see about 17.5 GPH and the CHTs climb smoothly to 325* or so. I can lean it to around 12.5 GPH at full throttle remaining ROP and the CHTs will climb to about 360* or so. So far so good. Fairly quickly pulling more mixture, right past peak EGTs will put me at 9 GPH or so and the CHTs will be about the same and not rising. The catch is that my nozzles aren't yet balanced quite well enough to smoothly get far enough LOP at WOT and 3K' where I often do some cross town commuting, so I have to use other tricks - stay ROP, cowl flaps partially open and/or reducing MAP as necessary for CHT control. What we really want to avoid is a combination of high CHTs and high MAP. And I really don't want to see a very rapid increase in CHT as that would be an indication that the combustion event is proceeding more rapidly than normal - probably detonation. Another trick is reducing RPM. Though that will decrease detonation margin by decreasing the amount of time for combustion, it also decreases power produced which tends to lower CHTs. Either way rich or LOP, never right near peak at high MAP.

Ed Holyoke
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  #34  
Old 08-01-2018, 06:53 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by Bicyclops View Post
When I speak of runaway CHTs, I'm talking about blowing right through 400* on it's way to oblivion. If I'm running at 360*-380* and stable, I really doubt that it is detonating.
"Stable" being the key.

Quote:
Lycoming's power setting charts are based on "best power" - just ROP- right where the red box is the largest. Yup, 27" and 2300 RPM at 30* ROP is not a good place to be. That's why I don't do that.
Reference please. I would place best power mixture somewhat further ROP than 30, although yes, in detonation surveys (all parameters quite hot), max power is usually just short of detonation.

Quote:
Either running very ROP or even slightly LOP increases detonation margin by slowing the combustion event so that peak pressure occurs well after TDC, about 17*.
And there is where the fun starts. Much of the GAMI/APS and Busch teaching is based on a goal of longevity, the key being avoidance of very high peak cylinder pressure. Out here in the field, none of us have the equipment to measure ICP in real time, so we're left to guess, or derive it from CHT, or as a proxy, oil temperature. When I see temperature rise without a corresponding increase in output (usually speed increase, but one might argue for high MPG), I back off to a more conservative mixture and/or timing, as I'm cautious about the mechanical stress.

Nigel's timing survey (published in Kitplanes) is a good example. Two major conclusions were that (1) advanced timing is always accompanied by higher CHT, the result of higher ICP, and (2), the same speed or efficiency increase can be found with less advance by climbing to a little higher altitude, without the mechanical stress.
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Last edited by DanH : 08-01-2018 at 06:55 AM.
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  #35  
Old 08-01-2018, 02:22 PM
Bicyclops Bicyclops is offline
 
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Just throwing a number at it. Could just as easily said 50* or 70*. I certainly don't set it up that way to measure it. ;-) Yup, stable. If the CHT is rising and doesn't show any signs of stopping by 380, I do something - richen, lean, reduce MAP, nose over for more air, cowl flaps. I agree, extra heat without extra airspeed is more stress for no good reason.

Ed Holyoke

Quote:
Originally Posted by DanH View Post
"Stable" being the key.



Reference please. I would place best power mixture somewhat further ROP than 30, although yes, in detonation surveys (all parameters quite hot), max power is usually just short of detonation.



And there is where the fun starts. Much of the GAMI/APS and Busch teaching is based on a goal of longevity, the key being avoidance of very high peak cylinder pressure. Out here in the field, none of us have the equipment to measure ICP in real time, so we're left to guess, or derive it from CHT, or as a proxy, oil temperature. When I see temperature rise without a corresponding increase in output (usually speed increase, but one might argue for high MPG), I back off to a more conservative mixture and/or timing, as I'm cautious about the mechanical stress.

Nigel's timing survey (published in Kitplanes) is a good example. Two major conclusions were that (1) advanced timing is always accompanied by higher CHT, the result of higher ICP, and (2), the same speed or efficiency increase can be found with less advance by climbing to a little higher altitude, without the mechanical stress.
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