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#21
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an interesting in flight experiment
"a" wrote in message ... On Oct 18, 6:39 pm, "Peter Dohm" wrote: "a" wrote in message I was not trying to suggest that you failed to check the mags prior to takeoff, and I do not have an opinion as to whether an in-flight check would necessarily tell anything of value. The problem that I have personally observed was a case of points which had gradually "closed up" on a 65 horsepower Piper Cub until the engine could not be manually started--and then was started on the first "lave" pull after the points had been dressed and gapped. A second case, that was only confirmed much later, involved a Cessna 172 which occasionally required manual starting for an assortment of stupid reasons; but started very reluctantly in those instances... The salient point is that both aircraft passed all tests normally available to a pilot; but, based upon the number of hours that each was operated, probably had one or both mags out internal tolerances for multiple years. So there are failure modes that the pilot can not necessarily overcome--including damaged insulation on a p-lead, or a shorted mag switch, amoung others. By the way, what were the problems later identified on your aircraft? Peter My in-flight check in fact produced something of value, Peter. The engine in cruise went a little rough and stayed that way with mixture adjustments. When I went to a single bank of spark plugs the engine noise went from rough to none: I was flying on half the spark plugs. That told me two things -- to land for a repair, and what to tell the A&E. My suggestion in the OP was that pilots learn what their engine does when on a single bank of plugs when at cruise. It might be instructive, it might not be. The failure mode I experienced was in the high voltage lead between the magneto and the distributer. The voltage impulse found a gap more convenient than the one at the spark plugs, this on an engine that was only about 1100 hours (on a 2000 hour engine) since last major overhaul. I continued on my trip in less than 2 hours. Clearly the aviation gods smiled on me. It looks like you did about the only thing that can be done for that sort of problem. There is just no reasonable way, at least none that I have ever seen, to inspect for or predict an impending failure of a shielded cable--or of several other parts of magnetos and distributors. It just serves as the remaining justification for dual ignition! I'm glad that it worked out well. Peter |
#22
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an interesting in flight experiment
Jon Woellhaf wrote:
"a" wrote in message ... ... Even on run up you lose some RPMs when on a single bank of spark plugs. If you don't get an rpm drop when running on a single mag, something's probably amiss. My engine has never run rough during a mag check, except when I forgot to lean aggressively before taxi and got lead fouling. Long ago I had rented a C=150 that the run up mag check was fine before a return flight back to the home airport (about 1 hour flying). All of a sudden in cruise the engine was running rough. I did a mag check, on one side was rough and the other side smooth. I elected to run on one mag that was good and squawked the aircraft upon landing. -- Regards, Ross C-172F 180HP Sold KSWI |
#23
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an interesting in flight experiment
Ross schreef:
I ... squawked the aircraft upon landing. Ross, what do you mean by "squawking a plane upon landing?" I only know the verb in a transponder context. |
#24
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an interesting in flight experiment
Reading the posts here I believe there may be confusion between two different,
but related issues. Why do we have two mags per engine and two spark plugs per cylinder? The main reason is redundancy, and the secondary reason is performance. On the performance side, losing one mag in flight should produce a slight decrease in performance, but no really significant roughness or danger to the engine. The function of both mags is detected through the single-mag check on runup. A far more likely occurrence however is the failure of a sparkplug in one cylinder. This goes almost undetected if both mags are working, but will produce very significant roughness on the single-mag check, and will produce a considerable imbalance in operation. So on the redundancy side the mag check serves not only to detect a faulty mag, but more likely to detect a faulty spark plug. If you never did the single mag check, you could theoretically run for some time with a defective plug or even more than one. Then the day you have a mag failure you are at risk of a rapid engine failure. |
#25
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an interesting in flight experiment
One question beginning pilots frequently ask, and rightly so, is given the
obvious performance effect of two functioning spark plugs per cylinder, why do automobile engines not adopt this? The answer, in recent years, is of course that many do, but not so many years ago this was not the case. The effect of two or more spark plugs per cylinder and the development of the flame front in a combustion chamber has been the object of a great deal of research, and experimental engines have been built with up to four plugs per cylinder. The results have shown a strong performance improvement by using two plugs, and diminishing returns thereafter. Part of the performance increase must be attributed to redundancy - plugs simply do not fire every time, and doubling the number of plugs greatly enhances the probability of a fire each stroke. Redundancy is not alone though, there is an ample body of evidence for enhanced uniformity of the flame front in a combustion chamber with two sparking points instead of one. Automobile manufacturers have known this since the 1940’s, so we can only assume that economy is the driving factor, overcome in aircraft engines by safety concerns related to engine failures. |
#26
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an interesting in flight experiment
In article ,
jan olieslagers wrote: Ross schreef: I ... squawked the aircraft upon landing. Ross, what do you mean by "squawking a plane upon landing?" I only know the verb in a transponder context. Squawk in this usage is a maintenance issue. In the US, it is common for a plane to have a 'squawk sheet' aka maintenance log that pilots can note issues on. If the transponder wasn't working, you'd squawk it for not squawking. Isn't English fun? John -- John Clear - http://www.clear-prop.org/ |
#27
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an interesting in flight experiment
John Clear schreef:
In article , jan olieslagers wrote: Ross schreef: I ... squawked the aircraft upon landing. Ross, what do you mean by "squawking a plane upon landing?" I only know the verb in a transponder context. Squawk in this usage is a maintenance issue. In the US, it is common for a plane to have a 'squawk sheet' aka maintenance log that pilots can note issues on. Thank you, Sir, it is nice to learn some little thing every day. If the transponder wasn't working, you'd squawk it for not squawking. };-) Isn't English fun? Not bad, but I'll bet you never tried or even tasted French! KA |
#28
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an interesting in flight experiment
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#29
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an interesting in flight experiment
"-b-" wrote in message ... One question beginning pilots frequently ask, and rightly so, is given the obvious performance effect of two functioning spark plugs per cylinder, why do automobile engines not adopt this? The answer, in recent years, is of course that many do, but not so many years ago this was not the case. The effect of two or more spark plugs per cylinder and the development of the flame front in a combustion chamber has been the object of a great deal of research, and experimental engines have been built with up to four plugs per cylinder. The results have shown a strong performance improvement by using two plugs, and diminishing returns thereafter. Part of the performance increase must be attributed to redundancy - plugs simply do not fire every time, and doubling the number of plugs greatly enhances the probability of a fire each stroke. Redundancy is not alone though, there is an ample body of evidence for enhanced uniformity of the flame front in a combustion chamber with two sparking points instead of one. Automobile manufacturers have known this since the 1940's, so we can only assume that economy is the driving factor, overcome in aircraft engines by safety concerns related to engine failures. Aircraft engines are unique, in that the driving force for dual ignition really is redundancy and the same performance--and that, with the edxception of redundancy, similar performance could be achieved by a very slight change in timing. OTOH, Wankel rotaries are simply unable to achieve the required flame propagation at high RPM without a second starting point; and a similar problem exists in some engines with dradically peaked pistons--which can be resolved by a second ignition system or, in some cases, by a channel bored across the crown of each piston. And, yes, a lot has been known since the 1940s that was not practical to implement at that time. Some of it still is not. Peter |
#30
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an interesting in flight experiment
"-b-" wrote in message
... In article , says... Isn't English fun? Not bad, but I'll bet you never tried or even tasted French! KA What's wrong with French? It's the only place where saying "Pitot" comes naturally! ;-) I've been outdone! Peter |
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