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At 04:51 PM 12/14/2005, WRB wrote:
Well, at least I attempted to qualify my question to the "real world".
If your guess would be 90%, that wouldn't make a difference most of us would
quibble over. If your guess would be less than 50%, that would truly be a
remarkable (and poor) reflection as to the level of skills the average pilot
gets when he hires the average FAA-licensed mechanic.
There are many possible causes of preignition, very few of which
would be attributed to poor maintenance practices. Deposits
accumulate, magneto distributor caps deteriorate, moisture gets into
them, etc. However, the percentages are irrelevant. It only matters
if the plane you are flying is right or not.
Would you not agree that those coupes WITH significant cylinder chamber
lead deposits are probably operated by pilots of lesser skill and/or with
non-functioning mixture controls?
No. Use of leaded fuels, lower power ops, carb anomalies (they all
leak, remember) all contribute, as well as oil consumption,
compression pressures, field elevation, the list goes on and on.
My point (that Ercoupe pilots need a
functional mixture control, as installed by the factory, and know both how
and why to use it) would seem well validated.
There are tens of thousands of small Continentals flying around with
their mixture controls wired rich. In fact, some variants of the
NA-S3 carb do not even have a mixture control (parts not
installed). Many of them have reached altitudes in excess of
10,000'. I know of none that have fallen out of the sky for reasons
related to the lack of a mixture control. The Stromberg is quite
good at compensating for changes in altitude (or DA) and is capable
of feats a Marvel could never approach.
I certainly would not argue the difference, and can only presume it is
because one leads to the other (and neither is good) you elected to combine
the two in the post to which I responded. It is not clear how this further
distinction invalidates anything I said or suggested, as seems inferred.
Facts can confuse or enlighten, and we should all strive for the latter.
My point is that is it unlikely for the reasons you stated that a
small Continental will spontaneously detonate, but it is possible for
it to experience pre-ignition which can cause detonation which is destructive.
"Judicious" application of carb heat", to me, would be just enough to
narrow, insofar as possible, the readings between cylinders. Once this
(admittedly richer) balance is achieved, one would continue leaning to the
"proper" mixture (now more evenly distributed). Also, as I understand it,
operation of the carb heat butterfly results in a mixture of filtered and
unfiltered air to the carburetor (starting with very little unfiltered air).
In any case, the question of filtered versus unfiltered air in cruise at
altitude is, in the grand scale of things, no biggie unless you are filming
a volcanic explosion.
Use of partial carb heat is generally discouraged. It is difficult
to control and can lead to carb ice without enough additional carb
heat available to correct the problem. I thought this related back
to leaning on the ground, hence the concern about unfiltered air.
How many 415's are equipped with 4 probe digital EGT monitors? IMHO
that's like using an electron microscope to look at a house. Great
detail, but no real useful information. Especially when you consider
you're using a pick axe to make adjustments to the thing you're
looking at with the electron microscope.
"...the mixture should be full rich unless the engine is rough due
to a too-rich mixture."
Duh! However, if you have a Stromberg And you have to lean And
you're below 5000' DA, something else is happening. Best look into it!
Per the Ercoupe Instruction Manual, p. 6 states: "For average operations
below 5000 ft. altitude, the carburetor should be left in the full-rich
position. When at part throttle, the fuel consumption may be improved by
leaning the mixture, but in no case should the control be moved back far
enough to decrease the engine rpm."
Duh again. And again, if you're below 5000' DA and leaning DOESN'T
decrease RPM, something else is going on. Best look into it.
I believe you and I are agreed that the 5000 ft. should be density altitude.
I would also presume one would still, in the process of leaning, use rpm
drop as a tool (but not a setting). Either way, it would certainly appear
that:
1. It is possible for the engine to be "too rich" for maximum power below
5000 ft. at full throttle (requiring mixture adjustment) and
Disagree, assuming no mechanical issues.
2. It is possible for a good pilot to achieve better range ("improved", as
in decreased fuel consumption in cruise) by carefully
leaning the mixture even when operating below 5000 ft.
Unlikely.
The "Technical Manual Overhaul" for the Bendix/Stromberg Model NA-S3A1, FOrm
15-91C of 2/76, p. 3 states:
"c. When the manual mixture control is in the "Full-Rich" position,
the large holes in the upper and lower plates are aligned so that the
fuel in the float chamber is subjected to the full pressure behind the
venturi (approximately atmospheric) through the main vent channels."
It would not require "any sort of a full throttle enrichment device" to
achieve extra richness at full throttle if the large holes in said plates
were sized in design to deliver excess fuel at full throttle.
Those holes regulate the amount of air (not fuel) at (approximately
atmospheric) pressure vented to the float chamber. This flow counters
the flow of low pressure air which is the motive force that leans the
mixture. More airflow through the venturi could only reduce that
pressure which would decrease the pressure in the float chamber which
would decrease the fuel flow. In essence, the Stromberg mixture
control can only lower fuel flow, not increase it. Moreover, the
point that this passage vents to is chosen because it is not subject
to pressure fluctuations caused by changes in airflow through the
carb throat. It is possible to engineer a "back suction" mixture
enrichment, but the NA-S3 is not equipped with this feature.
I agree the mixture control has no effect at idle because the Stromberg idle
circuit is separate (not connected).
Actually, the reason is that the source of low pressure air that
reduces the fuel flow is airflow through the venturi (separate from
the pressure metered by the plates). Low airflow at idle does not
generate any reduction even if the mixture control is in the full
lean position because the low pressure port and the "almost
atmospheric" port are essentially at the same pressure.. If it did,
the control would have effect at idle as the idle circuit gets its
fuel from the same float chamber. The amount of fuel delivered is
determined by the difference in pressure between the fuel passage in
the carb and the float chamber. (Note that it IS possible to kill the
engine with the mixture control. All you have to do is pull the
mixture full lean then sharply open the throttle.)
On the other hand I have lost engine power in the landing pattern following
descent from cruise without mixture adjustment (leaned) upon throttle
reduction for further descent. Please explain how, only 1000 ft. lower,
this very same mixture control could be totally ineffective in leaning
mixture for taxiing and/or running up to clear plugs?
The problem is that the mixture control's effect is tightly coupled
to the throttle position. Unlike the Marvel, you cannot set the
mixture control to some "ground lean" position and then motor around
the airfield. The Stromberg leaning system is only practical to use
if the throttle is not being manipulated. Therefor, it is not
practical for taxi. It could be used during run-up or to clear
plugs, but, again, it is unnecessary.
John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865
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