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-----Original Message----- From: William R. Bayne [mailto:[EMAIL PROTECTED] Sent: Wednesday, December 14, 2005 3:50 PM To: Ed Burkhead Subject: WRB Re: [COUPERS-TECH] Leaning with Stromberg carb RLYTECH On Dec 14, 2005, at 10:44 AM, John Cooper wrote: > 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. 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. 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? 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. > 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. 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. > >> 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. > >> 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... > I have moved the sequence of the above two give-and-takes because they relate. Your original statement was essentially that monitoring the effects of mixture change with a digital indicator was ineffective. I disagree. Monitoring the effects of mixture change with just the tachometer offers sufficient "gain" for the factory to recommend the practice. A combustion reading from a single cylinder may not greatly improve an imbalanced fuel ratio ("crude") to the various cylinders (as you suggested, rather common), but I do believe some further leaning efficiency possible (per what follows). It is advantageous for a pilot to know if he is dealing with a "normal" range of induction balance (and knows what that is from past experience) or with an induction leak caused by a hose, hose clamp, gasket or seal which has ceased to do its job and requires attention. An indicator showing simultaneous readings in each cylinder, or even a single indicator with probes on all cylinders with a selector switch should make even better mixture control possible. "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. > >> 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... That sounds like a good thing...should we presume you meant a "decreased margin of safety"? > ...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) > > John Cooper > Skyport Services Per a booklet published by Skyport (in Harrisburg, PA days) entitled "Aircoupe Owner's Manual Ercoupe", p. 10, under "Maximum Performance Climb" it states "...the mixture should be full rich unless the engine is rough due to a too-rich mixture." 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." 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 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. 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. If you know someone with the company how handling Stromberg sales and service, perhaps you can find out and share with us the "real skinny" on this subject. I agree the mixture control has no effect at idle because the Stromberg idle circuit is separate (not connected). 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? Your declaration of fact and my experience would seem to disagree, yet I would hope there is but one reality here when all is said and done. Regards, William R. Bayne <____|-(o)-|____> (copyright 2004) ============================================================================== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
