The Passion of Mixture
That Lycoming chart and others like it use the fuel-air ratio as their baseline. Their perspective is that of a researcher, not a pilot. Since pilots can measure that ratio only by EGT, EGT should be our baseline. With this in mind, I have redrawn the chart (page 84).
Mixture, measured in degrees Fahrenheit below peak EGT, grows leaner from left to right. The colored lines show how fuel flow, speed and cylinder head temperature change as the mixture becomes leaner. These changes are expressed as ratios to the values at peak EGT; that is why all the lines converge at the peak EGT point. For example, at
100 degrees Fahrenheit on the rich side of peak EGT, fuel consumption is 20 percent greater than at peak EGT. That is, if you burn 15 gallons an hour at peak EGT, you will burn 18 at 100 degrees ROP.
In a graph like this, the steeper the slope of a line is, the larger and more rapid is its response to the controlling variable — in this case, EGT. Lines that are relatively flat reveal small, probably inconsequential variations. You can see at a glance that fuel consumption (blue) is very sensitive to mixture. Speed (yellow) varies little on the rich side of peak, but falls off more rapidly on the lean side.
The most interesting line, in the context of Thomas’ prediction of a general deflagration within the engine, is that of CHT (red). CHT rises gradually as the mixture is leaned, hits its maximum a little on the rich side of peak and drops thereafter. It drops much more steeply on the lean side than it rises on the rich side. Many pilots, trained to shun an overly lean mixture, would find this hard to believe, but pilots who regularly fly on the lean side of peak know it well: Cylinder heads run noticeably cooler LOP.
So how did Thomas, and others like him, get the idea that LOP operation would fry an engine?
I suppose it may be due to a number of factors. One is historical. Much of the science of high-performance aviation recips was developed before and during World War II and applied to highly supercharged engines whose design goal was a high power-to-weight ratio. These engines were difficult to cool and prone to detonation at high power but could be protected by running rich — in effect, cooling with fuel. There arose, consequently, an association of lean mixtures with possible heat damage and rich ones with safe, conservative engine management.
The story of Charles Lindbergh teaching fighter pilots in the Pacific theater to increase their range by leaning the mixture and reducing rpm illustrates that belief. You would suppose that when cruising long distances over open water one would naturally lean the mixture out of concern for one’s own skin. But pilots were apparently not doing it; I suppose because they believed richer was better.
That belief filtered down through flight instructors to their students. When fuel was cheap, flight schools didn’t mind using it for cooling. Proper leaning also requires EGT instrumentation that trainers seldom had, and an attention to engine management that students — not to mention most pilots — did not care to pay. With fixed-pitch props, furthermore, the most salient effect of leaning — only up to a point on the rich side of peak — was to increase power and speed, and that was what pilots liked.
Moreover, many engines, including most carburetor-equipped ones found in trainers, do not lend themselves to LOP operation because of unequal distribution of fuel to the cylinders. If one cylinder becomes so lean that it begins to miss while another is still running at maximum temperature, LOP cruise is impossible.
The engine manufacturers have vacillated on mixture, with Lycoming usually taking the more conservative stance. GAMI — the Ada, Oklahoma, manufacturer of custom injectors designed to even out the mixture distribution among cylinders — strongly advocates LOP operation and, as far as I know, has not had to pay for a lot of melted engines. My own practice is to lean to about 50 degrees LOP immediately after takeoff and stay there, climbing and cruising at 65 percent power or less. As you may guess, I am not uninfluenced by the price of avgas.
Thomas would, no doubt, call me “the idiot that overleans.” He wouldn’t be the first. But at least my cylinders are cool.