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  #51  
Old 08-09-2011, 09:43 PM
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Toobuilder Toobuilder is offline
 
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Originally Posted by David-aviator View Post
...Engines certified with a fixed pitch prop do not have the limitation. The only time they are right of the limitation line is for take off. My engine isn't even on the chart at 28" MP and 2180 RPM, but as soon as RPM is up to 2300, it doesn't matter, it would be right of the line if there were one...
Two things I'm having a hard time getting my head around (perhaps I have misunderstood your post):

1. How does the engine "know" that the RPM is being held back by a hydraulically variable blade angle or one that is forged that way from the factory, and why does it matter to ICP?

2. Using the chart shown (for an O-320), your engine is far to the "bad" side of the line at 28 inches and 2180 RPM, and will be for some time until you climb above the MP limit or speed up to let the engine spin. In theory, operating to the bad side of the line could last the entire flight... Why does the engine not self destruct?

There are so many general guidelines out there that people have turned into "rules" or "gospel", that I think the whole concept has been corrupted. I'm beginning to think that you really only need to worry about CHT, oil pressure and oil temp, and the ol' Lycosaurus will just keep pounding away.
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  #52  
Old 08-10-2011, 07:47 AM
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Sam Buchanan Sam Buchanan is offline
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Originally Posted by Toobuilder View Post
I'm beginning to think that you really only need to worry about CHT, oil pressure and oil temp, and the ol' Lycosaurus will just keep pounding away.
That is most likely the rule for the "simple" engines we run in our RV's. I think we spend a lot of effort and dollar$$ micro-managing the ol' Lycosaurus that could be better spent just enjoying the ride. Keep the thing well maintained and it should run happily for a long time. If problems arise, the Lyc usually gives the attentive pilot plenty of warning.

I realize those relative few who have "caught" problems with fancy instrumentation will disagree with this view, but consider how many millions of hours have been accumulated in aircraft with only oil pressure and temp gauges. But....a lot of us really enjoy watching needles, graphs, and twiddling with controls, so my opinion will increasingly be in the minority as complex engine monitors become mainstream.
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  #53  
Old 08-10-2011, 08:20 AM
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Originally Posted by dpansier View Post
"So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?"

I'll take a shot at that question:
Valve failure caused by poor valve geometry, angles not matching with the seat, valve not concentric with the seat leading to the valve not fully transferring it's heat to the head.
Bingo. Well done Don...exactly right.

Heat is transfered from the valve via two paths, the valve stem to the guide, or the valve head to the seat. Poor seat contact (for all the reasons Don stated plus seat width and flatness) results in a very hot valve and early failure.

Poor seating is common.....way too common, and the resulting poor cylinder life is not the operator's fault. Early failure was delivered brand new in the box.
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  #54  
Old 08-10-2011, 08:32 AM
David-aviator David-aviator is offline
 
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Originally Posted by Toobuilder View Post
Two things I'm having a hard time getting my head around (perhaps I have misunderstood your post):

1. How does the engine "know" that the RPM is being held back by a hydraulically variable blade angle or one that is forged that way from the factory, and why does it matter to ICP?

2. Using the chart shown (for an O-320), your engine is far to the "bad" side of the line at 28 inches and 2180 RPM, and will be for some time until you climb above the MP limit or speed up to let the engine spin. In theory, operating to the bad side of the line could last the entire flight... Why does the engine not self destruct?

There are so many general guidelines out there that people have turned into "rules" or "gospel", that I think the whole concept has been corrupted. I'm beginning to think that you really only need to worry about CHT, oil pressure and oil temp, and the ol' Lycosaurus will just keep pounding away.
My apologies for the confusion.

The engine doesn't know which prop is controlling RPM. Perhaps Lycoming could explain why the MP restriction line is on charts with CS props and not with FP props. I don't have the answer. (at least it appears the charts are different for that reason, for sure some 0360 charts have the restriction, some don't)

In theory the entire flight could be on the right side of the line but in fact it doesn't happen because the only way to keep it there would be to climb at a very low air speed at WOT. As soon as the nose is lowered speed builds up and so does RPM and in no time the operation is left of the line, no matter what the MP. That may be why the chart for the O360-A4M, for example, has no MP restriction line.

Also it may be because FP applications in the certified world had no MP gage. All power is a matter of setting a given RPM. The Cessna 150-172 and Cherokee 180 manuals have no reference to MP at all.
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  #55  
Old 08-10-2011, 08:36 AM
David-aviator David-aviator is offline
 
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Poor seating is common.....way too common, and the resulting poor cylinder life is not the operator's fault. Early failure was delivered brand new in the box.
Except with engines from BPE.

Allen cuts the seats at several angles, the valves sit better and last longer.
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  #56  
Old 08-10-2011, 10:45 AM
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Default The answer might be found elsewhere

"The engine doesn't know which prop is controlling RPM. Perhaps Lycoming could explain why the MP restriction line is on charts with CS props and not with FP props. I don't have the answer. (at least it appears the charts are different for that reason, for sure some 0360 charts have the restriction, some don't)"

I participated in a propeller certification process with my Rocket. The Techs doing the various tasks were candid with answers to a few questions that have been bothering me for a while - and they also knew their stuff.

While it is possible to build a 400HP 4 cyl engine, there exists no current technology to put a prop on the thing. This same problem (power stroke torque amplitude) restricts the diesel engines in that they can only have a cyl of a certain size/power output.

A PSRU has to be designed to absorb these pulses, and the clutch used by Bud Warren in his design was a clear winner in this aspect.

You might think your prop is a solid object, when in fact it is constantly flexing, something like a fishing pole. It could be that 28"/2100RPM produces enough of a power pulse to overstress the prop, assuming the crank nose is strong enough. It could also be that the reverse is happening: the prop could be stressing the crank nose section?

I would volunteer that FP props are likely stronger in the hub area when compared to a CS prop, so it follows that CS props are restricted from high MP/low RPM ops.

In any case, constant operation in that combination of MP/RPM will eventually cause failure on the tested engine/prop combination. So, the engineers tell you to not operate there. YMMV, of course.

Yes, BPE builds very good cylinders.

Carry on!
Mark
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  #57  
Old 08-10-2011, 11:34 AM
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Mark,

The restrictions on MP/RPM/propeller and engine model are understood from a resonance perspective, and we also know that they are highly influenced by the specific powerplant configuration. However, the chart discussed here is quite generic and only seems to consider detonation (a function of ICP) as the limiting factor, not damage from resonance. If this is true, then it seems that Lycoming also completely ignores the loads imposed on ICP as a function of the aerodynamic properties of the fixed propeller. This makes no sense either.

It's like the old wives tale we've all heard concerning runnning a C/S prop equipped engine "oversquare" - we've all heard it is "bad", yet the "science" quickly evaporates when you realize most F/P engines spend much of their lives in that exact (oversquare) condition.

Like I said, there is enough conflicting information out there that anyone can assemble bits and pieces to create a "position" and still be technically credible. I wish I had the resources to test all these theories for myself!
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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1940 Taylorcraft BL-65
1984 L39C
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  #58  
Old 08-10-2011, 11:58 AM
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Quote:
Originally Posted by David-aviator View Post

The engine doesn't know which prop is controlling RPM...

In theory the entire flight could be on the right side of the line...

Also it may be because FP applications in the certified world had no MP gage...
Here's my problem with all the various charts and guidelines out there - If the engine will be damaged by operation in some condition (like high indicated MP), it is not acceptable to mitigate the condition by eliminating the M/P gage!

The fact that the "restriction" only applies to an engine equipped to monitor the condition (while other engines are ignored) leads me to the logical conclusion that it's not really a restriction at all.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
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1984 L39C
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  #59  
Old 08-10-2011, 01:32 PM
sailvi767 sailvi767 is offline
 
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I suspect its a simple as Lycoming feels that if you have the choice the power and rpm combinations should be provide the very best service life. Since with a FP prop you don't have RPM options they have no choice but to allow the operations. The difference might be a very small change in the average time between overhauls.

George
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  #60  
Old 08-10-2011, 09:00 PM
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RV8R999
Quote:
I see Mr. Deakin is a very experienced pilot but what about his engineering/test background? Maybe this short bio doesn't include his technical expertise or background - anybody know?
Ken, With the power of the internet, you could learn so much. John Deakin, George Brayly and Walter Atkinson combined are the worlds greatest resource in present times when it comes to this stuff. Much of it was known in the 50's but was lost in time.

I must have said this now at least 5 or 6 times now, attend an APS seminar.
http://www.advancedpilot.com/

Anyone who missed the one this week will need to book in for the next one!

The more I read these threads, I see folk who know some, know a lot, and those who clearly know very little. A lot seem to know what to do, but do not understand why. The key to it is understanding the why, from there you can vary the way you operate and when small anomolies come along you can diagnose and correct it before trouble strikes.

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Quote:
Originally Posted by RV10inOz View Post
...And yes the low compression as you call it, or standard LYC can be run in detonation...
There are quite a few experts who would disagree with you... Not under "normal" circumstances anyway.

Also, while I'm not among the "experts", I do know that I have never seen detonation as evidenced by runaway CHT on any Lycoming I've flown - and I have done some dumb things with the mixture control over the years.

Just my (nearly) undeducated opinion here, but I'll bet if the engine has an STC or is otherwise capable of operating on the watered down Mogas, it's nearly impossible to induce detonation in that engine on 100LL.
Just because you have not seen it does not make you the expert. I have. And I can demonstrate it at will if you like. Yep happy to do it on my engine too.....just not for long!

As for this,
Quote:
There are quite a few experts who would disagree with you... Not under "normal" circumstances anyway.
Well your quite a few experts are wrong too. refer back to my post #20. I talked about rapidly rising CHT's on takeoff and it was due to mixture not hitting full rich due to cables (too short in throw from Vans) and the mount flexing. This was detonation.

You are correct though in one point.....none of this is under "normal" circumstances. But this is my whole point, you need to be aware. All cylinder monitors saved this engine from serious trouble, may have taken a few years to fail or be severly damaged, but it would have happened had we had just 1 CHT probe on a cylinder not affected and an old analogue gauge.

Someone else here has posted a similar experience, and I suggest everyone makes a point of checking their mixture cables allow the FR and ICO stops to be made with some cable travel left over for when the engine rubbers sag or pull under load.

Quote:
Just my (nearly) undeducated opinion here, but I'll bet if the engine has an STC or is otherwise capable of operating on the watered down Mogas, it's nearly impossible to induce detonation in that engine on 100LL.
Those that have a higher margin might be hard to detonate on AVGAS, but I do not know this for sure. Do you? Not by your statement above so why encourage others to do so when you do not know. I would ask George Brayly, ask about the engines STC'd for Mogas and how the would fair on 100LL on his engine test stand and see what he says. I would love to know the answer.

If you can't be bothered let me know, I will ask. Your time zone is much closer than mine

Last edited by RV10inOz : 08-10-2011 at 09:05 PM.
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