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-----Original Message----- From: William R. Bayne [mailto:[EMAIL PROTECTED] Sent: Friday, December 16, 2005 3:19 AM To: Ed Burkhead Subject: WRB Re: [COUPERS-TECH] Leaning with Stromberg carb RLYTECH On Dec 15, 2005, at 10:58 AM, John Cooper wrote: > 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. I strongly disagree. Most of us have a professional set our timing. Would not those set incorrectly be "poor maintenance practices"?. It matters (in terms of day-to-day risk to other coupe enthusiasts) if the number of operational coupes "not right" is greater than the 5% I tossed out as a guess. It matters if each of us undertakes to better understand and intelligently operate our birds, in that, to the extent we succeed, our individual risk is lowered. It's important if you know you should not fly today because your plane is not "right". It's even important (at least to those discussing your last flight) if you did not know your plane was not right (and why). This is called "risk management", and most would deem the practice worthwhile even if results vary. > >> 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. More obfuscation. "Use of leaded fuels" has not been a historical problem of significance in engines such as ours. Burning 100LL, a grossly overleaded fuel, now is; particularly with rich or over-rich mixture ops. Lower power ops, properly leaned, will not produce deposits of significance. Carb with anomalies so severe as to cause excess lead accumulations in properly leaned operation should be rarer than the proverbial hen's teeth, and the drop-a-minute on the ramp leak that is common would not be sufficient to change plug electrode color, much less discernibly increase the accumulation of in-cylinder deposits in cruise. Those with high oil consumption or low compression are probably aware of same; expecting to do remedial work soon, if not immediately. Suggesting one's home field elevation as a factor of significance leading to preignition or detonation in an aircraft engine, I'd like to see some data on. You are suggesting that well maintained engines properly operated are just as likely to have significant cylinder deposits as poorly maintained engines operated by the less informed and proficient, and that's neither logical nor credible. > >> 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. And there are the "Special Olympics". Most prefer to meet the challenges of the sky fully functional, if optional. > 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. > Well, THAT ought to encourage our less technical readers to leave those mixture controls wired! > >> 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. I believe if you go back and check, that this threat was unlikely (if possible) was MY point! > >> "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. You're right. We were all taught that. This discussion was not a "one size fits all", though. If there is a genuine danger in such careful and minor application of carb heat in the manner described for the purpose described, what is it? > It is difficult to control ... Are you suggesting our simple push-pull knob moves untouched after adjustment, or that we need a vernier? > ...and can lead to carb ice without enough additional carb heat > available to correct the problem. Considering the specific and minor percentage necessary to achieve the desired result of more uniform mixture to cylinders, this suggestion brings "Chicken Little" to mind. > I thought this related back to leaning on the ground, hence the > concern about unfiltered air. Sorry. I assumed it obvious you don't lean on the ground to clear plugs at run up in a dust storm. > > 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. > I see. We, flying mere Ercoupes are somehow presumptive to pursue higher than the average operational efficiency of our peers, much less dare to take pride should we accomplish same. > >> "...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. You either didn't read the second sentence, or you disagree with Erco. > > >> 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. By this time the medical term "in denial" (with facts) would seem to apply. > >> 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. Did I suggest otherwise? > 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. If the holes are of a size when mixture is at "Full Rich" to deliver more fuel to the engine than it can convert to power (as a "safety factor" at take-off), they will do so. It is my present understanding that these holes were deliberately sized with this result in mind. Have you verifiable facts to support a different opinion? > >> 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.) > That's useful to know. Thanks for sharing. > >> 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. The cable controls are entirely separate. If you refer to what happens inside after leaning when the throttle is moved, more information would seem necessary for clarity. > > It could be used during run-up or to clear plugs, but, again, it is > unnecessary. > > John Cooper > Skyport Services We weren't talking necessity. We were talking about a common pilot option demonstrably useful in some circumstances that is not available with a wired Mixture Control. Regards, WRB ============================================================================== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
