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At 09:40 PM 12/13/2005, WRB wrote:
is it realistic to suggest that:
1. The 95+% of coupes operating with the A-65 and C-75-85
Continentals with compression ratios of 6.3:1, correctly timed,
without significant cylinder chamber lead deposits (that would raise
said ratio), and burning average fuel are capable of cylinder
pressures high enough to endanger cylinders or induce actual
"preignition/detonation" at their "high power levels"?
No. Nor is it realistic to assume that 95+% of coupes operating with
the A-65 and C-75-85 Continentals with compression ratios of 6.3:1
are correctly timed, and are without significant cylinder chamber
lead (or carbon) deposits. Also, it is important to distinguish
between preignition and detonation. Preignition occurs when the
air/fuel charge ignites before the spark event (or the spark event
occurs prematurely). Deposits in the combustion chamber can create
hot spots that then ignite the air/fuel mixture
prematurely. Preignition can in turn cause detonation, which is when
the air/fuel mixture explodes rather than burning smoothly.
Detonation can destroy pistons in short order.
2. To infer by omission that deficiencies of single-cylinder sensing
apply to multi-cylinder information (EGT and/or CHT)?
I'm not sure what I'm supposed to defend here. My statement about
unbalanced intake charges is irrespective off ones EGT sensing system.
Can you reference any instances of engine damage on a coupe (or
aircraft with the same engine/carburetor combination and similar
performance) reasonably attributable to "misuse" of the mixture control?
No. But then it's hard to prove. Unless you count the student pilots
who pull the mixture out, kill the engine and ultimately crash the
aircraft. ;)
The issue is that above 70% or so power, the compression ratio
coupled with the minimum grade of fuel results in a decreased margin
of error and you may find that when leaned to maximum power, one
cylinder is lean enough (still rich of peak, mind you), to produce
enough heat and/or pressure to cause damage. I have seen instances
of damage to cylinders on these engines, but, of course, when you ask
the pilot what he did to cause it, the answer is always the same.
If so, do you personally consider such occurrence (or the "risk"
thereof) to be of statistical significance to an Ercoupe owner/operator?
I cannot speculate on how an individual might use or miss use the
mixture control, however, I can state that with a Stromberg carb,
there is no benefit to leaning below 5000' (DA), no effect when
leaning at idle, and no way to lean effectively during ground ops, so
why bother?
(BTW, the Stromberg NAS-3 series does not have any sort of full
throttle enrichment device, so that added safety net is not present)
Do you agree that judiciously applied carburetor heat can improve
(reduce) the "normal" (rather high) mixture ratio spread between
cylinders on our little Continentals?
Yes. The warmer air aids in atomization of the fuel. However, you
then have an overall richer mixture, so what's the gain? It's
beneficial during warm-up on cold days, but you have to balance that
against the fact that you're feeding your engine unfiltered air...
As a"point of order", per the Continental Operator's Manual, Form
X30012 Dec. '80, a pilot is advised to lean "...at altitudes of more
than 5,000 feet above sea level..." and "Prior to takeoff from
fields above 5000 feet elevation...". While I entirely agree these
references SHOULD be with reference to "density altitude" (which can
be MUCH lower in summer), in fact they are not.
Value added, no extra charge. The engine doesn't give a hoot what the
physical altitude is. It only reacts to the effect, lower air
density, which is proportional to density altitude. Then again, it
really doesn't make a whole lot of difference which you use, either.
John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865
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