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

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

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.

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.

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.

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/

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

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! ;-)

"-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

"-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